productivity history of the nw iberian …digital.csic.es/bitstream/10261/4305/1/tesis...
TRANSCRIPT
Patricia BERNÁRDEZ
[Vigo, May 2007]
PhD THESIS
PRODUCTIVITY HISTORY OF THE NW IBERIAN RÍAS
AND SHELF FROM THE LATE HOLOCENE TO PRESENT
The interplay between upwelling and runoff using
biogeochemical and biosiliceous markers
PRODUCTIVITY HISTORY OF THE NW IBERIAN RÍAS
AND SHELF FROM THE LATE HOLOCENE TO PRESENT
The interplay between upwelling and runoff using
biogeochemical and biosiliceous markers
Grupo de Biogeoquímica Instituto de Investigaciones Marinas de Vigo
(IIM, CSIC)
Departamento de Xeociencias Mariñas e Ordenación do Territorio.
Universidade de Vigo
PRODUCTIVITY HISTORY OF THE NW IBERIAN RÍAS AND SHELF FROM THE LATE HOLOCENE TO PRESENT
The interplay of upwelling and runoff using biogeochemical and biosiliceous markers
Patricia Bernárdez Rodríguez
PhD THESIS
Vigo, May 2007
Grupo de Biogeoquímica Instituto de Investigaciones Marinas de Vigo
(IIM, CSIC)
Departamento de Xeociencias Mariñas e Ordenación do Territorio.
Universidade de Vigo
PRODUCTIVITY HISTORY OF THE NW IBERIAN RÍAS AND SHELF FROM THE LATE HOLOCENE TO PRESENT
The interplay of upwelling and runoff using biogeochemical and biosiliceous markers
Tesis doctoral presentada en el Departamento de Xeociencias Mariñas e Ordenación do
Territorio de la Universidade de Vigo.
Memoria presentada por Patricia Bernárdez Rodríguez para optar al título de Doctora por
la Universidade de Vigo.
Realizada bajo la dirección de
Dr. Guillermo Francés Pedraz Dr. Ricardo Prego Reboredo
Vigo, 9 de mayo de 2007
Guillermo Francés Pedraz, Profesor Titular del Departamento de Geociencias Marinas y Ordenación del Territorio de la Universidad de Vigo y Ricardo Prego Reboredo, Investigador Científico del Consejo Superior de Investigaciones Científicas (CSIC) en el Instituto de Investigaciones Marinas de Vigo:
CERTIFICAN QUE:
La presente memoria titulada PRODUCTIVITY HISTORY OF THE NW IBERIAN RÍAS AND SHELF FROM THE LATE HOLOCENE TO PRESENT. The interplay of upwelling and runoff using biogeochemical and biosiliceous markers, para optar al Grado de Doctora que presenta Patricia Bernárdez Rodríguez ha sido realizada bajo nuestra dirección tanto en la Universidad de Vigo como en el Grupo de Biogeoquímica del IIM (CSIC).
Considerando que representa trabajo de Tesis, autorizan su presentación ante la Comisión de Doctorado de la Universidad de Vigo.
Y para que así conste y surta los efectos oportunos, firmamos el presente certificado en Vigo a 9 de mayo de 2007.
Los directores
Guillermo Francés Pedraz Ricardo Prego Reboredo
La doctoranda
Patricia Bernárdez Rodríguez
—din que non hai r ías má is bonitas que as nosas
—pero xa non teñen peixes
Cousas da vida
CASTELAO
AGRADECIMIENTOS
Esta Tesis Doctoral se ha podido llevar a cabo gracias a que ha existido un gran apoyo
humano y económico y es por ello que me llena de orgullo y satisfacción agradecer como se
merecen a todos aquellos, personas e instituciones, que han puesto su pequeño granito de
arena en esta duna.
Como en esta vida lo que importan son las personas, no los recursos, mis primeros
agradecimientos van a ser a para ellas. Todas y todos, personas humanas en los que se
incluyen compañeiros, amigos, becarios, alumnos, técnicos, profesores, que se hayan
cruzado conmigo en los últimos siete años han sufrido mis preguntas, necesidad de ayuda,
cabreos, caprichos, disquisiciones filosóficas y demás stuff que va asociado al inicio,
desarrollo y finalización de LA TESIS. La lista de nombres me la guardo, no vaya a ser que el
orden, la jerarquía o el rango molesten a unos o a otros, aparte del hecho de que siempre te
olvidas de alguien… Así, la enorme contribución de cada uno de vosotros me la llevo en la
cabeza y sobre todo en el pecho. Pero, ante todo, graciñas compañeiros. Sois importantes
para mí. Sin vosotros, todo esto sería menos llevadero, porque sois los que me habéis
ayudado, animado y sobre todo los que me hacéis reír, que es lo importante, porque le alarga
a una la vida y porque evita la aparición de canas, jajajaja!!
Pero sin duda, el que más me ha sufrido ha sido el que ha estado más cerca sensu
stricto. !Que pasiensiña tes rapas! A él es a quien va dedicada. Por cierto, irmao, a vida, obra
e milaghros da mosca dos collóns ghustoume moito. Xa sabes que inda que teña o PhD
diante do nome, faltame moito para chegar o teu nivel. Bótote de menos. Pai e nai, que
saibades que aprendin moito, de todo, incluso do que non é ciencia, inda que pareza mentira.
Graciñas por soportarme e apoiarme nesta decisión. Espero non trabucar.
Gracias Directores por la valía científica y humana que me habéis demostrado y que ha
contribuido a mi superación personal y científica. Soy una cabezona, lo sé, es la herencia de
la familia Vitos… así que romper el melón es difícil y no me cabe duda de que me habéis
ayudado mucho.
Agradezco al área de Paleontología de la Universidad de Salamanca y al Research
Center of Ocean Margins de Bremen por haberme acogido, por proporcionarme los medios y
por enseñarme ciencia de la mejor. Mis supervisores allí son también directores de esta Tesis.
Por supuesto todos lo firmantes y revisores de los papers que resultarán de la finalización de
esta tesis son una parte importantísima de ella. Gracias por las ideas, correcciones, datos,
aportaciones, muchísimas gracias ¡Que genial es la ciencia cuando se trabaja en equipo!
La realización de esta Tesis Doctoral ha sido posible gracias a la concesión de la beca
predoctoral AP2002-2905 del Programa Nacional de Formación de Profesorado Universitario
(FPU) del Ministerio de Educación, Cultura y Deportes, así como la beca de Estudios de
Tercer Ciclo otorgada por la Xunta de Galicia. Le agradezco a la Xunta de Galicia, en
concreto a la Consellería de Educación y Ordenación Universitaria, la concesión de una beca
de ayuda para la realización de una estancia de investigación en la Universidad de
Salamanca. A la Universidad de Vigo le agradezco la concesión de diversas ayudas de viaje
para la difusión de resultados científicos en diversos congresos nacionales e internacionales.
Sin la financiación derivada de la incentivación a la investigación concedida al Grupo de
Oceanografía Geológica y Biogeoquímica por parte de la Universidad de Vigo hubiera sido
imposible la finalización de este trabajo de investigación.
Los proyectos de investigación así como los contratos de investigación con empresas
y/o administraciones han financiado y proporcionado el material, los equipos, los resultados y
la experiencia necesarios para que este trabajo se llevara a cabo. Se agradece en concreto a
los proyectos REN2003-09394, METRIA-REN2003-04106-C03, PGIDIT05PXIB31201PR,
EVK2-CT-2000-00060, PGIDT04PXIC31204PN, PGIDT00MAR30103PR, MAR96-1782,
1FD97-0479-C03-02. Además, esta Tesis Doctoral es una contribución al programa español
del LOICZ (Land-Ocean Interactions in the Coastal Zone).
TABLE OF CONTENTS
Presentation and Thesis structure
Chapter I ________________________________________________ 1
INTRODUCTION
1. ÁREA DE ESTUDIO: MARCO CLIMÁTICO Y OCEANOGRÁFICO ......................... 3
2. MATERIAL Y MÉTODOS ........................................................................................ 9
3. TRABAJOS PREVIOS........................................................................................... 12
4. JUSTIFICACIÓN Y OBJETIVOS ........................................................................... 14
5. Referencias ........................................................................................................... 21
Chapter II_______________________________________________ 25
OPAL CONTENT IN THE RÍA DE VIGO AND GALICIAN CONTINENTAL SHELF: BIOGENIC SILICA IN THE MUDDY FRACTION AS AN ACCURATE PALEOPRODUCTIVITY PROXY
1. INTRODUCTION................................................................................................... 31
2. REGIONAL FRAMEWORK.................................................................................... 33
3. MATERIAL AND METHODS ................................................................................. 33
3.1. Sample recovering and processing
3.2. Opal analysis
3.3. Accuracy and precision of the opal determination
4. RESULTS AND DISCUSSION .............................................................................. 37
4.1. Opal distribution in surface sediments of the Ría de Vigo
4.2. Opal content in the muddy fraction of surface sediments of the Ría de Vigo
4.3. Opal content in the Galician continental shelf: An accurate paleoproductivity proxy
5. CONCLUSIONS .................................................................................................... 47
Acknowledgements
References................................................................................................................ 49
Chapter III ______________________________________________ 55
BENTHIC–PELAGIC COUPLING AND POSTDEPOSITIONAL PROCESSES AS REVEALED BY THE DISTRIBUTION OF OPAL IN SEDIMENTS: THE CASE OF THE RÍA DE VIGO (NW IBERIAN PENINSULA) 1. INTRODUCTION: BACKGROUND AND OBJECTIVES......................................... 61
i
2. STUDY SITE ......................................................................................................... 63
3. MATERIALS AND METHODS ............................................................................... 65
4. RESULTS AND DISCUSSION .............................................................................. 66
4.1. Surface sediment: biogenic silicon fluxes and opal record
4.2. Subsurface sediment: postdepositional processes
5. CONCLUSIONS .................................................................................................... 76
Acknowledgements
References................................................................................................................ 77
Chapter IV ______________________________________________ 81
PROCESSES CONTROLLING THE DIATOM PRODUCTION AND ACCUMULATION IN A WESTERN GALICIAN RÍA: IMPLICATIONS FOR PALEORECONSTRUCTIONS 1. BACKGROUND AND OBJECTIVES ..................................................................... 87
2. REGIONAL SETTING ........................................................................................... 89
3. MATERIAL AND METHODS ................................................................................. 91
3.1. Location and sampling
3.2. Procedures and analytical strategies
4. RESULTS.............................................................................................................. 93
4.1. Diatom assemblages in the water column: Seasonal patterns
4.2. Diatom assemblages in the sediment traps: Seasonal patterns
4.3. Diatom assemblages in the surface sediment: Species encountered
4.4. Other biosiliceous components
4.5. PCA analysis: Relationships among diatoms, biosiliceous compounds and geochemical features
5. DISCUSSION ...................................................................................................... 109
5.1. Water column and sediment trap data vs. diatoms record in the sediment: Implications for paleoenvironmental reconstructions
5.2. Diatom and biosiliceous compound surface sediment distribution: oceanographic and environmental controlling factors
5.3. Diatom record vs. geochemical characteristics of the sediment
6. CONCLUDING REMARKS .................................................................................. 120
Acknowledgements
References.............................................................................................................. 123
Chapter V _____________________________________________ 131
DIATOM COMMUNITY IN A SEMI-ENCLOSED RÍA AND THEIR CONTRIBUTION TO THE SEDIMENTARY RECORD 1. INTRODUCTION AND OBJECTIVES.................................................................. 137
ii
2. STUDY SETTING................................................................................................ 138
3. MATERIALS AND METHODS ............................................................................. 140
3.1. Water column sampling and processing
3.2. Surface sediment sampling and analytical procedures
3.3. Taxonomic identification
4. RESULTS AND DISCUSSION ............................................................................ 142
4.1. Planktonic diatoms composition, distribution and relation to hydrology
4.2. Distribution of diatom assemblages and biosiliceous material in the surface sediments
4.3. Paleoimplications
5. SUMMARY AND CONCLUSIONS....................................................................... 155
Acknowledgements
References.............................................................................................................. 157
Chapter VI _____________________________________________ 163
LATE HOLOCENE HISTORY OF THE RAINFALL IN THE NW IBERIAN PENINSULA—EVIDENCE FROM A MARINE RECORD 1. INTRODUCTION................................................................................................. 169
2. REGIONAL SETTING ......................................................................................... 170
3. SAMPLING AND ANALYSES.............................................................................. 173
3.1. Location of the core and sampling
3.2. Procedures and analytical strategies
AMS dating
Grain size, organic carbon, calcium carbonate, nitrogen, opal and terrigenous content determinations
Metal analyses
Preparation sample cleaning and mounting of slides for siliceous compounds counting
Diatom and biosiliceous compounds quantification
4. RESULTS............................................................................................................ 178
4.1. Sediment lithostratigraphy, chronology and age-depth model
4.2. C/N ratio and terrigenous content
4.3. Metals content in bulk sediments: Fe, Al, LSi and Ca/Al
4.4. Occurrence of diatoms in marine sediments: Freshwater and benthic assemblages
4.5. Phytoliths, crysophycean cysts and palinomorphs: Biosiliceous land-input indicators
5. DISCUSSION ...................................................................................................... 183
5.1. Period 1: 4700–3300 cal. yr BP
5.2. Period 2: 3300–1700 cal. yr BP
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5.3. Period 3: 1700–1200 cal. yr BP
5.4. Period 4: 1200–0 cal. yr BP
6. SUMMARY AND CONCLUSIONS....................................................................... 191
Acknowledgements
References.............................................................................................................. 193
Chapter VII ____________________________________________ 199
PALEOPRODUCTIVITY CHANGES AND UPWELLING VARIABILITY IN THE GALICIA MUD PATCH DURING THE LAST 5000 YEARS: GEOCHEMICAL AND MICROFLORAL EVIDENCES 1. INTRODUCTION: BACKGROUND, STUDY SITE AND OBJECTIVES................ 205
2. FIELD AND LABORATORY PROCEDURES: DATA ACQUISITION AND METHODS .............................................................................................................. 208
2.1. Core location and sampling
2.2. Procedures and analyses
Chronological control
Bulk biogenic component analyses
Analytical procedures for metal determinations: Ba, Cu, Mn, Pb, Al, Fe
Siliceous microfossils preparation: Diatom counting and relative species percentages
3. PALEOENVIRONMENTAL PROXIES. PALEOCLIMATE AND PALEOPRODUCTIVITY APPROACHES................................................................. 214
3.1. Chronostratigraphical features
3.2. Sediment composition
Bulk biogenic components
Metals content
Diatoms
3.3. Diatom assemblages: downcore variations
4. DISCUSSION ...................................................................................................... 220
5. CONCLUDING REMARKS .................................................................................. 225
Acknowledgements
References.............................................................................................................. 229
Chapter VIII: Summary and conclusions ____________________ 237
Chapter IX: Self-criticism and perspectives _________________ 249
Taxonomic appendix ____________________________________ 257
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LIST OF TABLES AND FIGURES
Chapter I ________________________________________________ 1
Figura I.1. Localización del área de estudio ........................................................................... 5
Figura I.2. Esquema de los materiales y procedimientos utilizados en cada una de las áreas estudiadas.................................................................................................................... 11
Chapter II_______________________________________________ 25
Figure II.1. Chart of the study area. Above: core CGPL00-1 location in the Galician continental shelf. Contour lines in this map show depth in m. Below: map of the Ría de Vigo showing the 51 sampling sites (circles). Samples of sediment were taken from the uppermost oxic layer (0–1 cm). Opal analyses were performed in bulk and muddy fractions for selected samples (black circles).......................... 34
Table II.1. Study of precision of the method. Table shows samples used in this work, location (latitude and longitude in UTM units), number of analysis, mean, standard deviation and relative standard deviation. Opal content determinations for each sample were done in different runs ...................... 37
Figure II.2. a) Opal distribution in the bulk sediment throughout the Ría de Vigo. b) Detailed map of the superficial sediment distribution of the Ría de Vigo (modified from Vilas et al., 1995) ..................... 38
Table II.2. Opal percentage in the bulk sediment and in the <63 µm fraction from selected surface samples from the Ría de Vigo. Table also shows a description of the sediment samples, the percentage of each fraction and the variation percentage between opal in bulk and in muddy fraction. Variation percentage is calculated following the equation (2) ................................................................ 42
Figure II.3. Plots showing the linear correlation between opal percentage in the bulk sediment and in the muddy fractions. a) Superficial sediment samples from the Ría de Vigo. b) Gravity core CGPL00-1. Dots represent sediment samples of the upper 47 cm and crosses are the samples located in the sandy sequence. Linear correlation parameters are m (slope), a (intercept), R2 (R-squared value) ............ 44
Figure II.4. Plot of the down core variations of the CGPL00-1. Dots symbolize samples that have been analysed in the muddy fraction, and the crosses show the biogenic silica content in the bulk sediment. Relative standard deviation (10% uncertainty) for samples analysed in the <63 µm fraction (dashed line). Relative standard deviation for samples analysed in the bulk sediment (solid line) ................. 45
Table II.3. Opal percentage in the bulk sediment and in the <63 µm fraction for the core CGPL00-1. Table also shows the percentage of mud in each sample and the variation percentage between the opal content in bulk and in muddy fraction. Variation percentage is calculated following the equation (2) .. 46
Chapter III ______________________________________________ 55
Figure III.1. Map of the Ría de Vigo showing the sampling stations. Bathymetric lines every 10 m depth.................................................................................................................................. 64
Figure III.2. Contour plots of the distribution of opal percentage in the Ría de Vigo at different sediment depths. Contour lines every 0.2 wt.% opal. Colour scale from light to dark grey indicates the increase in the opal percentage ........................................................................................................ 67
Table III.1. Biogenic silicon flux data (annual mean) for both boxes and different hydrographical situations (winter, spring, summer with upwelling and summer without upwelling). Original data from
v
Prego et al (1995) is also presented in brackets (mg Si m-2 d-1) and represents the biogenic silicon flux to the sediment for each hydrographical condition ................................................................. 69
Figure III.3. Plot showing the linear correlation between the mean annual silicon flux to the seabed and the mean opal content in the first centimetre of the sediment in each box considered in Prego et al. (1995). Box 1, located in the San Simón Inlet, was not included in the regression. Equation of the linear regression also shown (BSiF represents the biogenic silicon flux .............................................. 70
Figure III.4. SEM images of some benthic diatom species found in the San Simón Inlet (surface sediment samples 49, 50, 51, 52, 53, 54). a) b) c) d) e) f) Cocconeis spp. g) h) i) Diploneis spp. j) k) Psammodyction spp. l) Paralia sulcata m) n) Achnanthes spp. o) Amphora sp. ............................. 72
Figure III.5. Plot of the percentage of opal from the top down to 15 cm in the longitudinal and cross-section of the Ría de Vigo. Stations used for the longitudinal and cross-section is indicated in the map. Sample number at the top of every contour plot .................................................................... 75
Chapter IV ______________________________________________ 81
Figure IV.1. Schematic illustration of the physiography and surface sample locations of the Ría de Pontevedra. Map shows the location of the bed sediment sampling (black circles) and grain size distribution (modified from Vilas et al., 2005). White diamonds indicate the stations were sediment traps were moored and the water column phytoplankton sampling carried out. Depth contours in metres ... 90
Table IV.1. Mooring positions and water depth ..................................................................... 91
Table IV.2. Sediment characterization at the sampling sites. Data from Dale and Prego (2002) ........ 94
Figure IV.2. Temporal variations of the standing stocks of total diatoms and abundance of the main diatom groups in the water column (cell l-1) at the three sampling sites. Abundance mean values of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom abundance ................................................................................................................... 95
Figure IV.2. (cont.) Temporal variations of the standing stocks of total diatoms and abundance of the main diatom groups in the water column (cell l-1) at the three sampling sites. Abundance mean values of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom abundance ................................................................................................................... 96
Figure IV.3. Seasonal patterns of the total diatom and vertical fluxes of the main diatom community groups in the sediment traps (diatom m-2 day-1) at the inner, middle and outer sampling sites (I, M, O). Mean values of the vertical fluxes of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom downward flux .......................................................................... 98
Figure IV.3. (cont.) Seasonal patterns of the total diatom and vertical fluxes of the main diatom community groups in the sediment traps (diatom m-2 day-1) at the inner, middle and outer sampling sites (I, M, O). Mean values of the vertical fluxes of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom downward flux .......................................................................... 99
Table IV.3a. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the diatom valves and fragments of diatoms .................................................. 101
Figure IV.4. Contour plots of the relative abundance of the main diatom species found in the superficial sediment of the Ría de Pontevedra .................................................................................. 103
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Figure IV.5. Contour plots of the abundance per gram of sediment of the crysophycean cysts, phytoliths and relative abundance of the benthic and freshwater diatom groups .......................... 105
Table IV.3b. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the porifera spicules, phytoliths and crysophycean cysts .................................. 106
Table IV.3c. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the radiolarians, silicoflagellates, the dinoflagellate A. pentasterias and palynomorphs................................................................................................................................ 107
Figure IV.6. Scatterplot with the factor loadings extracted using the R-mode principal component analysis. Each variable is represented as a point................................................................. 108
Figure IV.7. Mean relative abundance of dominant diatoms in water column (cell ml-1), sediment traps (diatoms m-2 day-1) and surface sediments (valves g-1×103) throughout the sampling period. Average value of the same taxa accumulated in surface sediments for each station ................................ 110
Figure IV.8. Distribution patterns of the factor scores obtained using the R-mode principal component analysis ..................................................................................................................... 120
Chapter V _____________________________________________ 131
Figure V.1. Locality map of the study area. The position of the bed sediment sampling (black circles) is shown. Diamonds indicate the stations where water column phytoplankton sampling was carried out. Contour plot shows the mud content in the surface sediments (modified from Cobelo-García and Prego (2004). Dotted white line represents the longitudinal axis. Water depth variation along the longitudinal axis of the ría is also shown............................................................................................ 139
Table V.1. Position and water depth of the stations sampled for diatom water column estimates .... 140
Figure V.2. Solid line indicates the Grande de Xubia River daily discharge during year 2000 (values in m3 s-1). Grey bars show the daily variations of the Upwelling index (Qx m3 s-1 km-1) at point 43°11'’ kindly given by Jose Manuel Cabanas (IEO). Dashed lines indicate the water column sampling dates................................................................................................................................ 141
Figure V.3. Temporal variations of the standing stocks of total diatoms and the main diatom groups abundance in the water column (white symbols, cell ml-1) at the three sampling sites. Bars show the relative percentage of each species or group. Note the logarithmic scale in the diagram showing the total diatom Note also that there is no data for the outer station in the sampling survey of May. ..... 143
Table V.2. Sediment characterization at the sampling sites.................................................... 148
Figure V.4. Contour plots of the abundance per gram of sediment several sediment compounds. Longitudinal axis variation of the abundance each is also shown A) A. pentasterias, B) Porifera, C) Cingulum, D) Phytoliths, E) Crysophycean cysts, F) Palinomorphs, H) Diatom valves, I) Silicoflagellates................................................................................................................................ 150
Figure V.5. Maps showing the distribution of the main diatom species found in the superficial sediment of the Ría de Ferrol as well as the variation throughout the longitudinal axis of their relative percentage................................................................................................................................ 151
Figure V.6. Average value of the relative contribution of the diatom taxa in the water column during the sampling period and percentage of the diatoms accumulated in surface sediment at sites 42 (Outer), 7 (Channel) and 11 (Middle).............................................................................................. 155
Chapter VI _____________________________________________ 163
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Figure VI.1. Localization of the study area on the northwestern Iberian margin. Simplified map showing the present surface sediments distribution of the Galician continental shelf (modified from Dias et al. 2002a) and the location of the core SMP02-3 .................................................................................... 172
Table VI.1. Radiocarbon dates and calibrated ages from core SMP02-3. Samples were pretreated and measured at the radiocarbon laboratories of Geochron Laboratories, USA and AMS 14C Dating Centre of the Aarhus University, Denmark. The age estimations were derived from the intercepts of the radiocarbon age plus and minus two times the total standard deviation of the age (2σ) with the linear interpolation of the marine calibration data set (Marine 04, Hughen et al., 2004). Conversions of radiocarbon ages to calibrated ages are worked out by using the CALIB 5.0.1 program after Stuiver and Reimer (1993, modified 2002). No local reservoir effect has been applied ................................. 174
Table VI.2. Results of the analysis of the PACS-2 (NRCC, Canada) certified reference sediment. .. 176
Figure VI.2. Sedimentological features and lithological description of the core. C: Clay. S: Silt. Sd: Sand. VF: Very fine sand. F: Fine sand. M: Medium sand. Co: Coarse sand .............................. 179
Figure VI.3. Age versus depth model for core SMP02-3 based on calibrated ages listed in Table VI.1. Solid line represents the theoretical age model assuming linear sedimentation rates between 14C dated levels (black circles). Down-core variations of the δ13C (‰) in foraminiferal tests, sedimentation rate (mm yr-1) .................................................................................................................... 180
Figure VI.4. Plot showing the down-core profiles of paleo-indicators of terrestrial input used. Note the inversed scale for Ca/Al ratio. Solid triangles indicate positions where dating samples were collected. Shaded areas indicate rainy climatic events discussed in the text............................................ 181
Figure VI.5. Summary scheme of the periods described, their relationship with the main global climatic events, the correlation with intervals/episodes defined by other proxies and other authors in the nearby area, and climate forcing mechanisms. Dark grey squares represent periods of high rainfall whereas Light grey squares are indicative of high productivity and upwelling influence. C: Cold. W: Warm. SB/SA: Subboreal/Subatlantic Transition. RWP: Roman Warm Period. DA: Dark Ages. MWP: Medieval Warm Period. LIA: Little Ice Age. GSM: Grand Solar Maximum. W: Wolf. S: Spörer. M: Maunder.... 185
Table VI.3. Pearson correlation matrix of the main riverine input proxies used in this work. Good correlations have been found between parameters, showing their potential use for terrestrial input markers ..................................................................................................................... 189
Figure VI.6. Scatter diagrams of Al, Fe, LSi, Ca/Al, C/N ratio and terrigenous content. Linear regression is shown (solid line). Dashed lines represent the prediction intervals at the 95% confidence interval. Pearson’s correlation coefficients are shown in the Table VI.3 (p<0.01). Grey areas indicate the strong rainy period described in the text ..................................................................................... 190
Chapter VII ____________________________________________ 199
Figure VII.1. Base map of the study area showing the core site (SMP02-3) and the geographical distribution of the sediments containing more than 50% of mud (modified from Dias et al., 2002a) .. 208
Figure VII.2. Sedimentary logs of the core. Distribution of depositional facies are also shown. Dated levels throughout the core and the result of the calibration is shown on the right side................... 210
Table VII.1. Comparison of the analytical results of the certified reference material PACS-2 (NRCC, Canada) with the measured data ..................................................................................... 212
Table VII.2a. Content of opal, TOC, calcium carbonate and detritics ........................................ 215
Table VII.2b. Concentrations of several metals analysed in the sediments of the core ................. 216
viii
Figure VII.3. Profiles of the chemical elements analysed in the core. Total concentration (solid circles) is shown on the left axis and metal/Al normalization (open squares) is shown on the right axis ...... 217
Table VII.2c. Al, total Ba and calculations of the Baexcess ....................................................... 218
Figure VII.4. Compilation of diatoms and geochemical proxies in the sediment core SMP02-3. Grey areas indicate climatic events discussed in the text. Lithostratigraphic units and AMS 14C datings (filled triangles) are also displayed ........................................................................................... 221
ix
PRESENTATION AND THESIS STRUCTURE
Presentación de la Tesis
Esta memoria de Tesis Doctoral se enmarca dentro del estudio de ciclo biogeoquímico
del Si, concretamente en forma de sílice biogénica, así como en el análisis de las
asociaciones de diatomeas y su registro en el sedimento como indicadoras de las condiciones
océano-hidrográficas del margen noroccidental gallego. Este estudio del ópalo biogénico, de
las asociaciones de diatomeas en la columna de agua, en trampas de sedimento y en el
registro superficial, combinado con otros marcadores (proxies) permite deducir cambios en la
oceanografía y en el clima regionales, al igual que profundizar en las causas de esos
cambios. Las rías de Vigo, Pontevedra y Ferrol y un registro Holoceno de la plataforma
continental gallega son el material empleado para la consecución del objetivo general de este
trabajo de investigación.
El estudio de las condiciones sedimentarias, oceanográficas y climáticas actuales en
las rías será de utilidad para acotar los procesos que actúan y/o intervienen en dichos medios.
Una vez finalizado este análisis de las condiciones actuales y de los procesos, le sigue un
examen y reconstrucción de la paleoceanografía, paleoclimatología y paleoproductividad
durante el Holoceno tardío en un archivo sedimentario de la plataforma adyacente a las Rías
Baixas. La plataforma continental es especialmente apropiada para el análisis paleoclimático,
puesto que en ella confluyen los procesos típicamente marinos y oceánicos. Además, suele
registrar tasas de sedimentación relativamente elevadas que permiten un estudio de alta
resolución. En la zona de estudio, la reconstrucción hidrográfica se llevará a cabo a partir de
diferentes proxies de diversa naturaleza: micropaleontológicos, biogeoquímicos y
xi
sedimentológicos, con el fin de emplear marcadores independientes que aporten mayor
solidez a las interpretaciones obtenidas.
En cualquier estudio científico se tiene que plantear una hipótesis general de trabajo,
que quedaría cubierta por un objetivo general. Aquí la hipótesis de partida inicial se basa en el
uso de las diatomeas como indicadoras de las condiciones de productividad en sistemas
afectados por la dinámica de upwelling y por los aportes continentales, procesos que en
ambos casos provocan una fertilización de la columna de agua superficial y el subsiguiente
incremento de la productividad primaria. En síntesis, se trata de dilucidar la utilidad de estos
organismos de forma combinada con otros indicadores de productividad de diversa índole,
para seguidamente elaborar una reconstrucción paleoclimática y paleoceanográfica de la
región.
Estructura de la Tesis
Esta memoria consta de nueve capítulos. De forma sucinta, el contenido de cada uno
de los mismos es el que se expone a continuación:
Capítulo I: En este capítulo, de carácter introductorio, se aborda en primer lugar el interés de
las diatomeas y de los marcadores biosilíceos para el análisis de los ambientes
actuales y su posterior aplicación en las reconstrucciones paleoclimáticas y
paleohidrográficas. Además se presentan de una forma muy general las principales
características de la región de estudio, ya que en los capítulos correspondientes se
tratarán de una forma más detallada las peculiaridades de cada área concreta. Así
mismo, se tratan diferentes aspectos relacionados con el material con que se ha
trabajado y con las técnicas empleadas. También es objeto de este capítulo el
considerar los estudios genéricos previos en el área de estudio y las acciones de
diversos organismos y centros de investigación que tradicionalmente han contribuido al
conocimiento multidisciplinar de las rías y su plataforma adyacente. Finalmente, se
enumeran los objetivos generales y específicos de la investigación y se justifica la
pertinencia de los mismos y del desarrollo del resto del trabajo presentado como Tesis
Doctoral.
Este capítulo ha sido redactado íntegramente en castellano en virtud de la normativa
propia de la Universidad de Vigo sobre la presentación de Tesis Doctorales.
El cuerpo principal de análisis de resultados y discusión se centra en varios apartados
que han sido estructurados en forma de artículos científicos, algunos ya publicados o en
revisión y otros enviados para su publicación. Por ello estos capítulos aparecen redactados en
inglés y en la forma en la que fueron aceptados o en la que han sido enviados. No obstante,
xii
se ha considerado pertinente incluir al principio de cada uno de estos capítulos un resumen
en castellano.
Capítulo II: Esta sección recoge los resultados y discusión sobre el contenido en ópalo
biogénico de los sedimentos superficiales de la ría de Vigo y de un testigo de la
plataforma continental gallega, tanto en el sedimento total como en la fracción fina.
Capítulo III: En este capítulo se describe el contenido en ópalo del sedimento subsuperficial
de la ría de Vigo, su registro sedimentario y las condiciones de preservación que
afectan al mismo. Además se efectúa una correlación entre el flujo de silicio biogénico
hacia el fondo de la ría y el contenido de ópalo de los sedimentos.
Capítulo IV: En este apartado se expone un estudio estacional completo del proceso de
producción primaria de origen biosilíceo en la columna de agua, su flujo vertical hacia
el fondo y su posterior registro en el sedimento reciente de la ría de Pontevedra.
Además, se integran los resultados de la distribución de diatomeas en los sedimentos
superficiales con los patrones de distribución y contenido de diversos parámetros
geoquímicos.
Capítulo V: En esta sección se describe el modelo de producción de diatomeas a escala
estacional y espacial en la columna de agua en la semicerrada ría de Ferrol y su
relación con los patrones de distribución de diatomeas en el sedimento superficial.
Capítulo VI: En él se presenta una reconstrucción de las condiciones hidrológicas, climáticas
y las variaciones en la precipitación sobre el noroeste de la Península Ibérica a partir
de diversos trazadores de aporte fluvial recogidos en un testigo que registra estos
cambios durante los últimos 5000 años. Específicamente nos centraremos en la
identificación de diversos periodos de mayor/menor influencia terrestre y relativamente
más/menos lluviosos. Se pondrá especial énfasis en la caracterización de los factores
climáticos de escala global y regional que afectan a la zona, y la posible influencia
antropogénica sobre el registro.
Capítulo VII: En este apartado se efectúa una aproximación a las condiciones generales de
producción primaria y los procesos que influyen en la misma en la plataforma
continental gallega durante los últimos 5000 años a partir de las asociaciones de
diatomeas y los marcadores biogeoquímicos.
Capítulo VIII: En él se presenta una síntesis general de todos los resultados obtenidos en los
seis capítulos precedentes. Además, se hacen explícitas las conclusiones generales y
particulares de esta Memoria.
xiii
Capítulo IX: En este apartado se presentan y plantean algunas ideas sobre el desarrollo de
futuros trabajos relacionados con el tema de la Tesis. Por otra parte, también se
exponen algunas inquietudes científicas y una visión autocrítica del trabajo llevado a
cabo.
Al igual que el Capítulo I, los dos últimos (Capítulos VIII y IX) también han sido
redactados en su totalidad en castellano.
Dada la estructura de memoria por la que se ha optado, se ha considerado más
oportuno reflejar las referencias bibliográficas al final de cada capítulo, en lugar de que
aparezcan en su conjunto como un apartado independiente. De esta forma, resulta más
sencillo discriminar las citas específicas utilizadas para cada uno de los diferentes aspectos
abordados en este trabajo.
xiv
[Chapter I] INTRODUCTION 1. ÁREA DE ESTUDIO: MARCO CLIMÁTICO Y OCEANOGRÁFICO
2. MATERIAL Y MÉTODOS
3. TRABAJOS PREVIOS
4. JUSTIFICACIÓN Y OBJETIVOS
Referencias
INTRODUCTION
1. ÁREA DE ESTUDIO: MARCO CLIMÁTICO Y OCEANOGRÁFICO
Las zonas costeras son lugares de especial interés para la investigación científica
puesto que son las áreas donde la densidad de población es mayor y, por ello, están sujetas a
una intensa actividad antrópica. Así, es posible obtener en el registro sedimentario marino
pistas de esa actividad, al igual que de los procesos naturales de carácter climático-
oceanográfico que ocurren.
La zona concreta en la que se ha desarrollado este trabajo se localiza en el margen
noroccidental atlántico de la Península Ibérica (Figura I.1). Galicia queda enmarcada entre 42°
y 44° de latitud Norte y, desde el punto de vista climático e hidrográfico, se encuentra bajo la
influencia del sistema del Atlántico Norte. Toda su geografía está marcada por el carácter
oceánico, con cierta influencia mediterránea que se traduce en un ambiente húmedo y
lluvioso, de abundantes precipitaciones, pero a su vez de suavidad térmica y con tendencia a
la aridez en verano (Pérez-Alberti, 1982; Martínez-Cortizas, 1999a, b). Debido a su gran
influencia marítima, Galicia tiene un clima caracterizado por variaciones de temperatura muy
ligeras, es decir, presenta inviernos suaves y veranos frescos.
En particular, las rías y la plataforma continental gallegas están afectadas de un modo
muy importante por los cambios en los sistemas de de altas y bajas presiones de las Islas
Azores e Islandia, respectivamente (Wooster et al., 1976; Haynes et al., 1993), lo que causa
que el régimen de vientos varíe estacionalmente, tanto en intensidad como en dirección.
Durante el verano el anticiclón de las Azores se sitúa sobre el Atlántico Norte, hacia el Oeste
de la Península Ibérica, mientras que la baja de Islandia se encuentra debilitada. Bajo estas
3
Introduction
condiciones, los frentes fríos discurren a latitudes más altas y no afectan a la costa
norooccidental ibérica. El gradiente de presiones atmosférico que se establece entre el
anticiclón de las Azores y la Península Ibérica se ve reforzado por el desarrollo de una baja
termal sobre la península, lo que da lugar a que se desarrollen vientos del Norte persistentes.
Este patrón de vientos genera un transporte de Ekman hacia el Oeste (McClain et al., 1986) y
el subsiguiente afloramiento de la Eastern North Atlantic Central Water (ENACW, Fraga,
1981). Este autor describe la presencia de dos masas de agua diferentes a lo largo de la
costa gallega. Ambas son similares a la Eastern North Atlantic Water (ENACW), definida por
Fiúza (1984), el cual discrimina un miembro extremo subpolar (ENACWp) y otro subtropical
(ENACWt). La subpolar se mueve hacia el Sur, mientras que la subtropical lo hace hacia el
Norte (Ríos et al., 1992), convergiendo ambas en las proximidades de cabo Fisterra. También
influyen en este proceso la circulación general de la ENACW (Ríos et al., 1992) y las
características topográficas de la línea de costa (Blanton et al., 1984). La inyección de
nutrientes por medio de la ENACWt a las aguas superficiales de la plataforma y al interior de
las rías (Álvarez-Salgado et al., 1993; Prego et al., 1999) se produce desde abril a octubre
(Fraga, 1981; Fiúza et al., 1982; Blanton et al., 1984). Este particular proceso oceanográfico
da lugar a elevadas tasas de producción primaria y biomasa fitoplantónica.
En invierno el anticiclón de las Azores se desplaza hacia una posición más meridional y
la baja de Islandia se refuerza. Los sistemas de bajas presiones que se mueven hacia el Este
pasan ahora sobre el Atlántico Norte y extienden su influencia sobre la península, dando lugar
a un régimen de vientos predominantes del Oeste-Suroeste altamente energético, que genera
alturas de olas mucho mayores y el flujo de una corriente que viaja hacia el Norte denominada
Iberian Poleward Current (IPC, Frouin et al., 1990; Haynes y Barton; 1990; Vitorino et al.,
2002a, b). Las borrascas atlánticas que discurren más al norte de nuestras latitudes también
afectan al litoral gallego a través de sus extremos o colas y, ocasionalmente, se presentan en
forma de borrascas continuadas que afectan de lleno a la región. En esta situación ciclónica
dan lugar a episodios de altas precipitaciones con vientos del Suroeste típicamente invernales
y otoñales.
4
Chapter I
Figu
ra I.
1. L
ocal
izac
ión
del á
rea
de e
stud
io.
5
Introduction
Otra situación de invierno, aunque ocurre de forma más esporádica, se produce cuando
un anticiclón se sitúa al este de las islas Británicas y una borrasca sobre el continente
europeo, dando lugar a la entrada de aire frío de componente N-NW. Es la llamada entrada de
aire polar de carácter árido y seco, que induce precipitaciones y temperaturas más bajas que
las habituales (Naranjo y Pérez Muñuzuri, 2006).
Por otro lado, la fisiografía de la costa de Galicia presenta notables particularidades de
tipo biogeográfico, derivadas de su peculiar forma. Su apertura muy recortada de cara al
océano y sus fuertes variaciones orográficas dan lugar al principal rasgo que caracteriza el
litoral gallego, que es la presencia de entrantes costeros denominados rías, que consisten en
valles fluviales y zonas deprimidas invadidas por el mar (Rey, 1993; Vilas, 2002; Evans y
Prego, 2005). Es por ello, que las rías son zonas de intercambio tierra-océano con un gran
potencial para evaluar diversos procesos que tienen lugar en este ambiente, y que serán
desarrollados a lo largo de esta Tesis. Su posición latitudinal y su evolución geomorfológica le
imprimen un patente carácter de área de transición climática muy adecuada para estudios
paleoclimáticos. Por otro lado, y tal como ya se ha puesto de manifiesto, toda la zona se
localiza en un punto de encuentro de diversas masas de aire que le confieren ser una zona
clave para el estudio de las en zonas templadas, debido a la gran variedad de ambientes.
Las áreas específicas de investigación son las rías de Pontevedra, Vigo y Ferrol y la
plataforma continental gallega adyacente a las Rías Baixas (Figura I.1). Las rías de Vigo y
Pontevedra presentan unas características fisiográficas típicas de las Rías Baixas, que son
las que se localizan en el Suroeste de Galicia (Figura I.1), como son la orientación NE-SW, su
forma de embudo en planta y la presencia en la boca de la ría de una serie de islas que
actúan de protección frente a los temporales (excepto la ría de Muros, que carece de islas en
su embocadura). Su conexión con el mar abierto se produce a través dos bocas situadas al
Norte y al Sur de cada grupo de islas, de tal manera que el intercambio de agua entre las rías
y la plataforma está parcialmente bloqueado por la presencia de las Islas Cíes en Vigo y las
de Ons y Onza en Pontevedra, que forman parte del Parque Nacional Marítimo–Terrestre de
las Islas Atlánticas. Ambas, como el resto de las Rías Baixas, reciben el aporte principal de
agua dulce a través de ríos que desembocan en la zona más interna, y que varían su caudal
estacionalmente en función del régimen de precipitaciones, el cual es más alto durante los
meses invernales. En el caso de la ría de Vigo, los aportes continentales son el resultado de
la combinación de un flujo regulado por la presa de Eiras, en el curso alto del río Oitabén y el
flujo natural de agua debido al río Verdugo, el cual confluye con con el Oitabén antes de
desembocar en la ensenada de San Simón. Los ríos Redondela y Ullo también desembocan
en la ría de Vigo, si bien sus aportes son menores (Gago et al., 2005). Para la ría de
6
Chapter I
Pontevedra, el mayor caudal fluvial y los aportes continentales son debidos al río Lérez
(Prego et al., 2001; deCastro et al., 2006a).
Todo este conjunto de características da lugar a que en el interior de las Rías Baixas
se produzca una circulación residual, forzada por el aporte de agua dulce y el régimen de
vientos. La dinámica mareal, debido a su carácter cuasiperiódico, no ejerce normalmente una
especial influencia en la dinámica de aguas. La circulación se estructura en dos capas, con
entrada de agua oceánica por el fondo y salida de agua superficial, menos salina, por la
superficie. Como regla general, la circulación residual se intensifica en invierno por causa de
un mayor flujo de agua dulce que actúa como motor del sistema, hecho que origina una
disminución del tiempo de residencia del agua en la ría. Por el contrario, durante la primavera
el flujo de agua dulce es menor y, consecuentemente, la circulación se ralentiza (Prego y
Fraga, 1992). En verano la fuerza dinámica que incrementa la velocidad residual de las aguas
es el afloramiento de aguas subsuperficiales procedentes de la plataforma continental
provocado por los vientos persistentes del norte. Sin embargo, con otras condiciones
meteorológicas, como la presencia de los vientos del sur, se bloquea la salida de aguas,
reteniéndose el agua dulce en las zonas más internas y actuando las rías como un sistema
semicerrado. También es necesario resaltar que existe un comportamiento diferencial entre
ambas bocas de acceso a las rías, de tal manera que la entrada de agua oceánica se produce
preferentemente por la boca sur, mientras que la salida es mayor por la boca norte, por la cual
se produce la salida, por ejemplo en el caso de la ría de Vigo (Souto et al., 2003). Esta
circulación diferencial a través de las aperturas oceánicas es debida al giro inducido por la
fuerza de Coriolis, lo que genera una preferencia de la salida de agua dulce por la margen
norte (deCastro et al., 2006b).
Atendiendo a la influencia continental y oceánica que reciben las Rías Baixas, éstas se
pueden subdividir longitudinalmente en diversas partes. Las zonas más internas suelen estar
más afectadas por procesos típicamente estuáricos; las zonas medias son áreas de
intercambio y en las partes externas predominan los procesos oceánicos.
Dependiendo de la situación geográfica, la configuración fisiográfica, la influencia de
agentes marinos (marea, oleaje, temporales, afloramiento), la descarga fluvial y la influencia
antrópica sobre cada una de las Rías Baixas, la composición y distribución espacial del
material sedimentado presenta diversas variaciones. Los fondos de estas cuatro rías se
encuentran tapizados por una mezcla de sedimentos siliciclásticos y carbonáticos de origen
litogénico y biogénico, respectivamente. En general, la distribución de tamaños de grano está
controlada por el ambiente hidrodinámico que, a su vez, es función de la energía del oleaje, la
topografía del fondo, las corrientes mareales y el aporte de agua dulce de los ríos. Así, a
7
Introduction
grandes rasgos, la cobertera sedimentaria actual presenta los tamaños más finos en el eje
central y profundo de las rías y en las zonas internas. A lo largo de la línea de costa y
bordeando al cinturón de fangos el sedimento se hace cada vez más grueso. Los canales de
entrada a las rías presentan fracciones arenosas en elevada proporción (ver revisión en Vilas
et al., 2005).
La ría de Ferrol presenta unas características muy diferenciadas de las dos anteriores.
Forma parte de las Rías Altas, las cuales se localizan en el noroeste de la costa gallega. En
primer lugar presenta una morfología en embudo, con un canal central muy estrecho y
relativamente profundo en el que se registran velocidades de corriente muy elevadas. El
principal curso fluvial hacia la ría de Ferrol es el río Grande de Xubia, que vierte en su
cabecera, con oscilaciones características de un régimen pluvial oceánico (Rio y Rodríguez,
1992; deCastro et al., 2004). Este pequeño aporte continental es uno de los motores de la
circulación residual estuárica que suele observarse en esta ría, aunque el principal
mecanismo que controla su circulación es la influencia mareal (deCastro et al., 2003;
deCastro et al., 2004). Así, la circulación dentro de la ría es cuasiperiódica, puesto que el
aporte fluvial es bajo. La columna de agua está muy mezclada en la parte externa y en el
canal de entrada a la ría, mientras que en las zonas internas se encuentra parcialmente
estratificada, especialmente durante el invierno, cuando el caudal fluvial es mayor. El
sedimento dominante en la boca de la ría y en el canal de entrada es arena fina, pobre en
materia orgánica y rico en material carbonatado, mientras que en el resto de la ría los fondos
presentan un contenido importante de las fracciones fangosas (López-Jamar et al., 1996;
Cobelo-García y Prego, 2004). Hay que indicar que en las zonas más internas el contenido en
fango es mayor en el margen norte.
Desde el punto de vista medioambiental, las rías y la plataforma gallegas presentan
una gran riqueza biológica debido a que en sus aguas aflora la ENACW, portando gran
cantidad de nutrientes. Toda la zona de estudio, como ya se apuntó anteriormente, está muy
influenciada por la dinámica del afloramiento o upwelling, lo que da lugar a una elevada
producción primaria. No obstante, por lo que respecta al ciclo anual de producción biosilícea y
las condiciones de producción, las rías de Vigo y Pontevedra presentan características
distintas a la de Ferrol. Uno de los factores principales que controlan el crecimiento de las
diatomeas es el contenido en nutrientes esenciales, cuya abundancia depende del aporte
fluvial y la entrada de agua central noratlántica cuando tiene lugar el afloramiento estacional.
Este proceso es precisamente el que diferencia a la ría de Ferrol de las Rías Baixas. Mientras
que en las rías de Vigo y Pontevedra el agua aflorada penetra hasta zonas muy internas
debido a su orientación NE-SW, en la ría de Ferrol está dificultada la entrada de aguas ricas
8
Chapter I
en nutrientes, dado su menor calado, y la posición y forma del Golfo Ártabro que aíslan a la
ría del afloramiento (Prego y Varela, 1998).
Por su parte, en la plataforma continental gallega la producción primaria y su registro
se ven afectados por procesos semejantes a los que acontecen en las rías. En concreto nos
estamos refiriendo al desarrollo del upwelling durante los meses de verano y al aporte fluvial,
en este caso, por parte de los ríos Miño y Duero principalmente. La plataforma continental
gallega adyacente a las Rías Baixas es estrecha, y su morfología es bastante plana. La
plataforma interna está cubierta por una capa casi continua de arena y fango de reducido
espesor cuya granulometría aumenta hasta el tamaño grava en las cercanías de la costa y de
los afloramientos rocosos (Dias et al., 2002a, b; Jouanneau et al., 2002; Ferrín, 2006). En la
plataforma media el sedimento superficial está compuesto principalmente por fango y arcilla
formando el denominado Galicia Mud Patch, un cuerpo sedimentario relativamente reciente
con orientación Norte-Sur (Drago, 1995). En la plataforma externa dominan los materiales
arenosos (Dias et al., 2002a, b; Jouanneau et al., 2002; Ferrín, 2006). La fuente de
sedimentos que da lugar al Galicia Mud Patch proviene principalmente del Sur, de la descarga
de los ríos Miño y Duero (Araújo et al., 2002; Oliveira et al., 2002a, b), que tienen una cuenca
de drenaje extensa y caudales elevados (Vitorino et al., 2002a, b). En condiciones de
elevadas precipitaciones este transporte hacia el Norte se ve favorecido por el desarrollo de
una pluma superficial poco salina llamada Western Iberian Bouyant Plume (WIBP) (Peliz et
al., 2002). El resto de ríos que vierten en la costa oeste de Galicia, como ya se ha
mencionado anteriormente, son de pequeño caudal en comparación con estos últimos citados
y desaguan en las cabeceras de las rías, de manera que estas actúan como trampas de su
carga sedimentaria. Además del aporte sedimentario de origen continental, la segunda fuente
de material es de naturaleza biogénica, la cual no es ajena a la elevada producción primaria
en la columna de agua. Esta componente biogénica, en concreto el material biosilíceo, será
uno de los principales objetos de este estudio del registro sedimentario del Galicia Mud Patch.
2. MATERIAL Y MÉTODOS
Dada la diversidad de objetivos planteados y de los materiales analizados (muestras de
columna de agua, trampas de sedimentación, sedimento superficial y subsuperficial y columna
sedimentaria), la descripción detallada del material, las técnicas de muestreo y los
procedimientos analíticos utilizados se encuentran en cada uno de los capítulos que
componen el cuerpo principal de esta Tesis. Por tanto, este apartado se limita a presentar el
esquema que aparece en la Figura I.2 con el fin de que el lector tenga desde el inicio una idea
clara del tipo de material analizado y de las variables estudiadas en cada una de las áreas de
trabajo. Es común a todas ellas el análisis de las comunidades/asociaciones de diatomeas y
9
Introduction
del ópalo, si bien en algunas también se han contemplado otros marcadores biosilíceos, como
los fitolitos, las crisófitas, las espículas de esponjas, etc. Dichos marcadores poseen un gran
interés en estudios paleoclimáticos y paleoceanográficos, como se pondrá de relieve en los
capítulos posteriores. También se han analizado aspectos sedimentológicos (litología, tamaño
de grano, estructuras sedimentarias, etc.) y biogeoquímicos (composición y contenido en
metales). Todos los resultados serán posteriormente integrados en un marco de interpretación
que, por estar basado en múltiples trazadores, harán posible la obtención de conclusiones
apoyadas en datos independientes entre sí.
A modo de resumen, el hilo conductor de toda la Tesis Doctoral se basa en el estudio
de las diatomeas y el ópalo en diversos compartimentos del medio marino: la columna de
agua, trampas de sedimentos, sedimento superficial y registro sedimentario Holoceno. Sin
embargo, es en el sedimento donde se centra gran parte de la atención y donde, además de
sus características texturales, se determinará la abundancia de diatomeas fósiles, el
contenido en otros marcadores biosilíceos y el contenido en metales, con el fin de integrar
todos los resultados dentro de un marco de interpretación basado en múltiples trazadores que
resulten en un adecuado planteamiento de las conclusiones.
10
Chapter I
Figu
ra I.
2. E
sque
ma
de lo
s m
ater
iale
s y
proc
edim
ient
os u
tiliz
ados
en
cada
una
de
las
área
s es
tudi
adas
. LM
: Mic
rosc
opía
ópt
ica.
SEM
: Mic
rosc
opía
el
ectró
nica
de
barr
ido.
CH
NS:
Aná
lisis
ele
men
tal d
e ca
rbon
o, h
idró
geno
, nitr
ógen
o y
azuf
re. L
ECO
: Aná
lisis
ele
men
tal d
e ca
rbon
o to
tal e
inor
gáni
co
y ni
tróge
no.
FAAS
: Es
pect
rom
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de
abso
rció
n at
ómic
a de
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ma.
GFA
AS:
Espe
ctro
met
ría d
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sorc
ión
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ica
en c
ámar
a de
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fito.
AM
S:
Espe
ctro
met
ría d
e m
asas
. TN
: Nitr
ógen
o to
tal.
TIC
: Car
bono
inor
gáni
co to
tal.
TOC
: Car
bono
org
ánic
o to
tal.
11
Introduction
3. TRABAJOS PREVIOS
La investigación en el medio marino es compleja y generalmente requiere una
aproximación interdisciplinar a través de la cual se aporten datos de una zona desde
diferentes especialidades. Este es el caso de las rías y plataforma continental gallegas, cuyo
conocimiento se ha enriquecido de forma espectacular en los últimos quince años. Con
anterioridad sólo existían trabajos muy locales y dispersos que, si bien en algunos casos,
aportaron informaciones de suma relevancia, no permitían establecer una visión de conjunto
sobre las condiciones ambientales regionales y de su evolución. En los últimos años gran
cantidad grupos de investigación pertenecientes a diferentes organismos han concentrado sus
esfuerzos en el conocimiento científico de la zona desde diversos puntos de vista:
oceanografía física, climatología, hidrografía, biogeoquímica, geología, etc. Ello trae como
consecuencia que el conocimiento, los temas abordados y el número de trabajos se haya
multiplicado de forma notable y que la recopilación de toda la información existente se
convierta en una tarea ingente. Por ello, en este apartado lo único que se pretende es poner
de manifiesto la gran labor científica llevada a cabo por diferentes grupos de trabajo, tanto de
Universidades como de organismos públicos de investigación, ya sean nacionales o
extranjeros. Huelga decir que existe más información bibliográfica que la referida en esta
memoria y a la que se ha procurado acceder para el conocimiento exhaustivo del medio, de
los procesos que actúan y para la interpretación de los resultados. Así, aunque se ha llevado
a cabo una intensa revisión de la literatura científica disponible, para cada capítulo se han
seleccionado aquellas citas consideradas más relevantes por la información que aportan y por
su difusión internacional. Es decir, en este momento únicamente se hace alusión a las
instituciones y proyectos que más han contribuido al conocimiento de las rías y de la
plataforma continental de Galicia, dejando para los capítulos posteriores la reseña de los
trabajos específicos relacionados con las zonas y los aspectos concretos que se abordan en
cada uno de ellos.
Durante los últimos 50 años en el Instituto de Investigaciones Marinas de Vigo (IIM-
CSIC) se han realizado innumerables estudios hidrográficos y biogeoquímicos de gran
relevancia internacional. Este Instituto es pionero en el conocimiento de las rías y sistemas
costeros gallegos desde un punto de vista oceanográfico (Guerra y Prego, 2003). Todo este
estado de conocimiento ha resultado de especial relevancia para abordar este estudio. La
colaboración de este centro con diversos departamentos de la Universidad de Vigo a través,
por ejemplo, de la creación de unidades asociadas ha contribuido además a implementar,
actualizar y aumentar todo este conocimiento científico.
12
Chapter I
Otro centro de relevancia es el Instituto Español de Oceanografía y, particularmente,
sus centros de Vigo y A Coruña. Ambos trabajan intensivamente en el estudio del régimen de
afloramiento y su influencia en el sistema biológico. En concreto, es muy destacable todo el
estudio ecológico de las comunidades biológicas pelágicas y bentónicas de la región, de tal
manera que gran parte de las publicaciones derivadas de este centro han sido consultadas
con asiduidad.
Resultan de especial importancia dos proyectos de investigación sobre la plataforma
continental, el Omex I y II, en los que han trabajado diversos grupos de investigación de la
zona y de los países vecinos, Portugal y Francia. Ambos proyectos abordan el conocimiento
regional desde diversos puntos de vista: biológico, químico y sedimentológico, resultando en
una importante cantidad de información disponible de gran valor. En continuación con estos
estudios, actualmente se está llevando a cabo y desarrollando el proyecto Galiomar, cuyos
investigadores principales pertenecen al Research Center of Ocean Margins (RCOM) y a la
Universidad de Bremen. En una línea continuista, se pretende elucidar la arquitectura
sedimentaria y los procesos de sedimentación que afectan la plataforma continental gallega
frente a las Rías Baixas y hasta Finisterre y al talud continental.
Desde el punto de vista de la geología y, más concretamente, de los aspectos
relacionados con el relleno sedimentario, una importante contribución al conocimiento de las
Rías Baixas gallegas y de la plataforma se debe al Departamento de Geociencias Marinas y
Ordenación del Territorio de la Universidad de Vigo. A través de diversos proyectos
desarrollados por los especialistas de dicho departamento y utilizando técnicas estratigráficas,
sedimentológicas, sísmicas, geoquímicas y micropaleontológicas se ha profundizado en el
conocimiento de los sistemas de rías, en la cartografía de los sedimentos superficiales, en las
características del relleno sedimentario y en los factores locales, regionales y globales que
han determinado la evolución durante el Cuaternario de las rías y de la plataforma continental
del Galicia. También algunos investigadores de la Universidad de A Coruña han contribuido a
la información que hoy se dispone sobre algunos de las cuestiones citadas anteriormente.
Aunque en este apartado se ha puesto el acento principalmente en el trabajo de
investigación llevado a cabo mayoritariamente por organismos localizados en Galicia, no hay
que olvidar la contribución de especialistas y grupos de otros centros de investigación,
fundamentalmente mediante colaboraciones con los grupos gallegos que más activamente
han trabajado en la región.
En definitiva, se puede afirmar que aunque toda esta zona se encuentra muy estudiada
desde temáticas muy diversas, aún se pueden realizar más trabajos y proyectos de
investigación que aborden otros aspectos, fundamentalmente aquellos de carácter integrador
13
Introduction
y con una perspectiva multidisciplinar. En este sentido, es cuantioso el número de Tesis
Doctorales derivadas de todos los proyectos y líneas de investigación que se llevan a cabo en
los centros citados anteriormente, entre las cuales se incluye la que ahora se presenta y en la
que confluyen la tradición y la experiencia de líneas de trabajo llevadas a cabo por el grupo de
Bioqueoquímica del IIM-CSIC de Vigo y por el Departamento de Geociencias Marinas de la
Universidad de Vigo.
4. JUSTIFICACIÓN Y OBJETIVOS
Por todo lo anteriormente expuesto respecto al área de estudio, la metodología
empleada y los antecedentes regionales de la investigación marina, en este apartado se van a
plantear de forma concreta la motivación de esta Tesis Doctoral, las hipótesis de trabajo y los
objetivos generales y específicos del estudio.
El establecimiento de zonas con diferente productividad primaria en una región o a
escala global es un reflejo de la circulación oceánica y, en zonas costeras, de los aportes de
nutrientes desde el continente a través fundamentalmente de los ríos. Puesto que parte de la
materia, ya sea orgánica o mineral, generada por la proliferación de organismos
fitoplanctónicos se acumula en los sedimentos de fondo, la medida de la concentración y la
abundancia relativa de los compuestos generados por estos mecanismos en sedimentos del
pasado puede ser utilizada como un indicador de la paleoproductividad, la paleohidrografía y
la paleocirculación oceánica. Debido precisamente a este potencial para la reconstrucción
paleambiental, los estudios sobre la productividad de la columna de agua han sido tratados
ampliamente en las últimas décadas.
Uno de los indicadores más comúnmente utilizados como marcador de
paleoproductividad es el ópalo biogénico generado por diferentes grupos fitoplanctónicos,
como por ejemplo, diatomeas y silicoflagelados. Estos organismos son capaces de extraer de
las aguas el ácido silícico para formar sus esqueletos, por lo que dicho compuesto es para
ellos un nutriente esencial. Las diatomeas proliferan en zonas de alta productividad, en las
cuales las probabilidades de que una parte se preserve en el registro sedimentario son más
altas. Son excelentes indicadores de las características físico-químicas y de los procesos
oceanográficos que ocurren en la columna de agua. Además constituyen un grupo fósil
relativamente abundante en las muestras de sedimento objeto de estudio. Estas zonas de
mayor productividad primaria, además de las regiones caracterizadas por upwelling
permanente, son los sistemas costeros, donde se encuadra esta investigación. Así, la
producción primaria está basada en gran medida en la producción biosilícea y, por tanto, el
seguimiento de sus patrones de crecimiento y conservación en el registro sedimentario sirve
14
Chapter I
para caracterizar las interacciones entre los sistemas atmosférico, oceánico y geológico. Es
por este motivo que el registro de ópalo en los sedimentos se utiliza como un marcador de
paleoproductividad. Si bien el contenido de ópalo y las asociaciones de diatomeas en los
sedimentos pueden ser utilizados como trazadores de la paleoproductividad y de la
paleohidrografía, hay que tener en cuenta que para cada región y para cada caso de estudio
existen una serie de procesos que determinan su abundancia y su distribución espacial y
temporal. Las características hidrodinámicas de la zona, los aportes fluviales de silicato
disuelto, los efectos antropogénicos, los procesos de transporte y resuspensión, o la
remineralización en la columna de agua podrían enmascarar el registro del material biosilíceo
y en concreto de las diatomeas en el sedimento como indicador directo de las condiciones de
producción en la columna de agua. Estos tipos de procesos son complejos y a veces no están
bien descritos, de tal manera que es necesario un estudio integral de los mismos. Por todo
ello, existe una necesidad de calibración de la información oceanográfica que nos dan estos
marcadores biosilíceos. En este sentido, el presente trabajo está dirigido a la utilización del
ópalo biogénico y las asociaciones de diatomeas como trazadores de la productividad en una
región costera, unido al uso de otros marcadores biogeoquímicos y sedimentológicos.
Consecuentemente, y con el fin de evaluar la productividad primaria en la zona de
estudio y de sentar las bases de los factores que determinan sus variaciones, en este trabajo
se propone analizar diferentes aspectos relacionados con la concentración de sílice biogénica
en el sedimento, tanto de carácter metodológico, como de naturaleza ambiental. Así pues, se
analizarán no sólo los patrones de productividad y su expresión sedimentaria, sino también
factores distintos a la productividad que afectan a la concentración de ópalo y diatomeas en el
sedimento, como son la preservación y la dilución. Por otro lado, basándose en marcadores
biogeoquímicos, también se valorarán algunos aspectos relacionados con causas antrópicas.
El conocimiento de estos aspectos y de su influencia en el registro reciente es el paso previo
para, en un futuro, aplicar los resultados obtenidos a una interpretación más exhaustiva del
registro fósil.
El clima es consecuencia de la interrelación que existe entre los diferentes
compartimentos que forman la Tierra, la atmósfera, los océanos, el hielo o criosfera, los
organismos vivos o biosfera, y los sedimentos y rocas o geosfera y, a tenor de los datos
recientes, de lo que se está dando en llamar la antroposfera. Los factores que controlan esa
variabilidad climática se relacionan principalmente con procesos de carácter natural
relacionados con la dinámica océano-atmosférica. Sin embargo, en las últimas décadas se
están detectando cambios relacionados con las actividades humanas, por ejemplo el
incremento del CO2 atmosférico (Intergovernmental Panel on Climate Change, IPCC 2007).
15
Introduction
Además, actualmente, existe un fuerte consenso científico respecto al hecho de que el clima
global se verá alterado significativamente en los próximos años. Estos efectos de las
actividades antropogénicas se superponen con la variabilidad natural del clima, la cual varía
en escalas de tiempo muy diversas. Así, para poder comprender algunos de los procesos
relacionados con el cambio global se debe primero comprender el sistema globalmente y
observar cómo funciona.
Este estudio del clima y de su variación natural o antropogénica es un trabajo
multidisciplinar en el que hay que considerar muchas cuestiones, aspectos y metodologías de
trabajo que pertenecen a campos de la Ciencia muy diversos, y que tradicionalmente se
encuentran muy fragmentados: Oceanografía, Química-Física, Geología, Biología,
Meteorología, Economía, etc. Además, existe una necesidad de trabajo en conjunto de todos
los organismos implicados en el problema: administraciones y organismos internacionales,
centros de investigación, asociaciones sociales, etc. Para ello es necesario analizar cada uno
de los compartimentos interrelacionados y en este sentido, este trabajo se centra en uno de
ellos, el océano y el registro en el sedimento de todos los procesos que actúan.
Consideraremos los sedimentos marinos como archivos naturales de la actividad biológica y
de las condiciones hidrográficas en las que han sido formados. Este estudio del registro actual
y reciente, así como su correlación con el medio ambiente permite calibrar las posibilidades
de interpretación paleoambiental del registro y poder utilizarlo como una ventana al pasado.
Es por eso que esta Tesis Doctoral sólo pretende abarcar un estudio muy concreto de las
condiciones actuales y su aplicación a un registro sedimentario Holoceno como parte de ese
conocimiento global necesario para la resolución de problemas relacionados con el clima.
El periodo Holoceno, dentro del cual se enmarca nuestro estudio paleoclimático, hasta
hace unas décadas se consideraba como una fase climáticamente estable y caracterizada por
un nivel del mar alto. Sin embargo, los datos recientes revelan la existencia de una cierta
variabilidad climática durante el Holoceno. De hecho, el interés de la comunidad científica por
reconstruir la variabilidad climática reciente de nuestro planeta ha ido in crescendo de manera
significativa, puesto que la evolución climática dentro de este periodo ha determinado y, muy
probablemente, determinará el progreso y futuro de la humanidad. En este sentido, el
conocimiento de estas variaciones regionales en nuestro pasado reciente servirá para
caracterizar esta zona climática. Lo que se plantea, en definitiva, es prever la posible
evolución futura del sistema, y aportar luz a todo el conocimiento sobre el cambio global que
se extrae en otras zonas.
De forma concreta, el presente trabajo tiene como objetivo general el estudio, desde
una perspectiva sedimentológica, biogeoquímica y micropaleontológica, de los sedimentos
16
Chapter I
recientes depositados en los ambientes de ría, así como calibrar su dependencia de los
patrones hidrográficos y de producción primaria como base para una reconstrucción climático-
oceanográfica precisa del registro sedimentario Holoceno.
Este objetivo general se desglosa en tres grandes apartados en los que se agrupan
diversos objetivos específicos o concretos que aluden a aspectos de diversa índole, los
cuales se especifican a continuación:
Objetivo I. Caracterizar el registro de ópalo y su preservación en los sedimentos
superficiales y subsuperficiales de la ría de Vigo y calibrar los valores obtenidos con el flujo
de sílice biogénica hacia el fondo. Ello supone:
1. Realizar una puesta a punto del método de análisis de ópalo en los sedimentos
de Galicia, en concreto en muestras de la ría de Vigo y de la plataforma
continental adyacente, así como la estimación de la precisión del mismo en los
rangos existentes.
2. Determinar y elaborar mapas de la distribución espacial del contenido en ópalo
en los sedimentos superficiales y subsuperficiales de la ría de Vigo.
3. Estudiar la relación del ópalo con otras variables composicionales del
sedimento, como el contenido en materia orgánica y en carbonatos.
Igualmente, determinar la influencia de las características texturales del
sedimento en la concentración de ópalo medida.
4. Evaluar los diversos procesos que afectan a la sedimentación de sílice
biogénica y a su preservación.
5. Establecer las diferencias espaciales en la productividad de la ría de Vigo
atendiendo a la presencia de ópalo en el sedimento, y relacionar éstas con los
flujos de silicio biogénico desde la columna de agua hacia el fondo. En este
sentido, se busca valorar la utilización del porcentaje de ópalo en el sedimento
como indicador directo de la productividad en la columna de agua y,
eventualmente, determinar qué otros factores influyen en dicho porcentaje.
Objetivo II. Caracterizar la dinámica estacional de las poblaciones de diatomeas
actuales en dos rías (Ferrol y Pontevedra), cuyas características hidrográficas son muy
diferentes, lo que incluye estimar el papel de la variabilidad estacional y espacial de los
patrones atmosféricos, oceanográficos e hidrográficos sobre el flujo de organismos biosilíceos
hacia el fondo marino y, de forma inversa, la aplicabilidad del registro sedimentario para la
17
Introduction
interpretación de las condiciones oceanográficas e hidrográficas en el pasado. Más
concretamente, los objetivos específicos de este bloque son:
1. Analizar la variación estacional de la abundancia de diatomeas en la columna
de agua en dos rías gallegas, Pontevedra y Ferrol, y su relación con los
procesos hidrográficos, oceanográficos y biogeoquímicos que ocurren en
ambas rías.
2. Cuantificar los flujos de diatomeas y su exportación al fondo de la ría de
Pontevedra, así como explicar sus variaciones espaciales y temporales.
3. Comparar el porcentaje relativo y la abundancia de cada una de las especies
de diatomeas que proliferan en la columna de agua con su concentración en el
sedimento, con el fin de reconocer variaciones en la preservación y registro de
cada una de ellas.
4. Cuantificar la abundancia relativa de las especies de diatomeas y de otros
restos silíceos encontrados y elaborar mapas de su distribución en el
sedimento superficial, así como contrastar los resultados con los patrones
hidrográficos y de producción primaria característicos de las rías de Ferrol y
Pontevedra para poder explicar los procesos y factores que controlan su
distribución.
5. Verificar las utilidades y limitaciones de la distribución de los componentes
biosilíceos en el registro reciente como trazadores paleoambientales,
paleohidrográficos y paleoclimáticos en los ambientes de rías y sentar las
bases para futuras reconstrucciones.
Objetivo III. Realizar una reconstrucción de la evolución oceanográfica y climática de
la zona durante el Holoceno basada en los marcadores biogeoquímicos y biosilíceos.
1. Reconstruir la historia paleoceanográfica, paleohidrológica y paleoclimática de
la región, con especial énfasis en las condiciones de paleoproductividad.
2. Examinar las variaciones de la productividad primaria a partir de las
asociaciones de diatomeas y parámetros geoquímicos en la plataforma
continental de Galicia, atendiendo a los procesos que controlan la misma, los
ciclos de upwelling y el aporte de nutrientes por los ríos.
3. Evaluar el aporte de material de origen continental o litogénico al registro
sedimentario y su relación con cambios en el régimen de lluvias sobre el
continente y con el patrón de vientos dominantes.
18
Chapter I
4. Analizar el posible impacto y registro en los sedimentos de la plataforma
continental de diversas actividades antropogénicas.
5. Identificar los mecanismos climáticos de escala regional y local que controlan
los parámetros biológicos, sedimentológicos y geoquímicos registrados. En
este sentido, se busca correlacionar el registro Holoceno con otros obtenidos
en la misma zona climática y con diversos eventos globales ya descritos.
6. Estudiar la influencia de procesos diagenéticos/tafonómicos que afectan al
registro sedimentario Holoceno.
19
Chapter I
Referencias
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23
[Chapter II] OPAL CONTENT IN THE RÍA DE VIGO AND GALICIAN CONTINENTAL SHELF: BIOGENIC SILICA IN THE MUDDY FRACTION AS AN ACCURATE PALEOPRODUCTIVITY PROXY∗
1. INTRODUCTION
2. REGIONAL FRAMEWORK
3. MATERIAL AND METHODS
3.1. Sample recovering and processing
3.2. Opal analysis
3.3. Accuracy and precision of the opal determination
4. RESULTS AND DISCUSSION
4.1. Opal distribution in surface sediments of the Ría de Vigo
4.2. Opal content in the muddy fraction of surface sediments of the Ría de Vigo
4.3. Opal content in the Galician continental shelf: An accurate paleoproductivity proxy
5. CONCLUSIONS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Ricardo Prego, Guillermo Francés, Raquel González-Álvarez, 2005. Continental Shelf Research 25, 1249–1264. doi:10.1016/j.csr.2004.12.009
Abstract. Biogenic silica content was determined in both superficial marine sediments from the Ría de Vigo and gravity core samples (core CGPL00-1) from the adjacent continental shelf. Samples were processed following the alkaline leaching procedure. The standard deviation for opal rich samples is very low (±0.2), whereas for opal poor samples (<1.3 wt.%) the relative standard deviation can reach up to 16%.
Opal percentages in superficial dry bulk sediments and gravity core samples range between 0.2–5.1 wt.% and 1.1–2.0 wt.%., respectively. Maximum opal percentages are found in the inner part of the ria around San Simón Inlet. Values of 2–3 wt.% typify the inner-central part of the ria. Throughout the ria longitudinal axis opal content is about 2 wt.%. Smaller values are found in the margins at the mouth of the ria. Opal distribution throughout the core is irregular, but there is a general tendency for higher values in upper muddy level and lower values in sandy sequence.
Opal analyses were performed for the total and <63 µm fractions of both ria and core sediment samples. For the core samples, there is no correlation between opal content in the fine and bulk fractions, but opal percentage in the muddy fraction is an useful parameter to standardize results and to apply as a paleoproductivity proxy. For the ria surface sediments there is a good correlation between biogenic silica content in both fractions (R2=0.90). This fact suggests that the information provided by total and fine fraction analysis is similar, as a result, the opal content analysis in the fine fraction does not supply any new information concerning diatom productivity.
Keywords: opal/paleoproductivity/grain size/ria/Galician continental shelf/Spain
Resumen. El contenido en sílice biogénica de sedimentos marinos superficiales de la ría de Vigo y del testigo CGPL00-1 obtenido en la plataforma continental gallega adyacente ha sido determinado. Las muestras han sido procesadas siguiendo un método de digestión alcalina. La desviación estándar obtenida para las muestras más ricas en ópalo es muy baja (±0.2) mientras que para las muestras pobres en ópalo (<1.3%) la desviación estándar relativa puede incluso llegar al 16%.
El porcentaje de ópalo en el sedimento total seco para las muestras de sedimento superficial de la ría de Vigo varía entre 0.2 y un 5.1%. Los porcentajes máximos de ópalo se encuentran en las partes más interna de la ría y en la ensenada de San Simón. Las partes central e interna se caracterizan por valores entre el 2 y 3%. A lo largo del eje longitudinal de la ría el contenido de ópalo se encuentra en porcentajes cercanos al 2%. Los valores más bajos se localizan en los márgenes y en la boca de la ría.
Para las muestras del testigo de gravedad el rango de valores encontrado es de 1.1–2.0%.El perfil de distribución de ópalo a lo largo del registro es irregular, pero existe una ligera tendencia a registrarse valores más altos en el intervalo fangoso superior y más bajos en la secuencia arenosa basal.
Los análisis de ópalo fueron realizados además en la fracción <63 µm tanto para las muestras de la ría como del testigo. No existe correlación entre el contenido en ópalo en la muestra total y en la fracción fina para los sedimentos del testigo. Así, el contenido en ópalo en la fracción fangosa es un buen parámetro para estandarizar resultados y aplicarlo como un indicador de paleoproductividad. Sin embargo, se observa una buena correlación entre el contenido en sílice biogénica entre ambas fracciones (R2=0.90) para los sedimentos superficiales de la ría. Este hecho sugiere que la información obtenida por ambos análisis es similar y que la determinación de ópalo en la fracción fina no es necesaria para inferir la producción de diatomeas.
Palabras clave: ópalo/paleoproductivvidad/tamaño de grano/ría/plataforma continental gallega/España
OPAL CONTENT IN THE RÍA DE VIGO AND GALICIAN CONTINENTAL
SHELF: BIOGENIC SILICA IN THE MUDDY FRACTION AS AN ACCURATE
PALEOPRODUCTIVITY PROXY
1. INTRODUCTION
Silicid acid is one of the major nutrients in the marine environment since marine
planktonic microorganisms (diatoms, radiolaria and silicoflagellates) build amorphous silica
shells from this nutrient, which they must extract from surface seawater undersaturated with
respect to solid amorphous silica. A large fraction of biogenic silica production in superficial
waters is recycled via dissolution within the upper 100 m of the water column (Nelson et al.,
1995; Tréguer et al., 1995; Ragueneau et al., 2000), though dissolution can continue on the
sea floor (Willey and Spivack, 1992; Tréguer et al., 1995; Rickert et al., 2002). As a result,
only a small fraction of the original opal reaches the sediment surface (Nelson et al., 1995;
Tréguer et al., 1995; Ragueneau et al., 2000). In spite of this fact, biogenic silica (BSi)
accumulation in the sediments still reflects the general pattern of primary productivity in the
overlying waters (Lisitzin, 1972; Banahan and Goering, 1986; Leinen et al., 1986) and can be
used as a proxy for paleoproductivity studies (Charles et al., 1991; Mortlock et al., 1991;
Ragueneau et al., 1996; De La Rocha et al., 1998; Masqué et al., 2003).
Spatial distribution of opal content has been widely studied, both in coastal-shelf areas,
(DeMaster, 1981; Kamatani and Oku, 2000; Emelyanov, 2001; Gehlen and van Raaphorst,
2002; Liu et al., 2002) and in the deep ocean (Schlüter and Rickert, 1998; Dixit et al., 2001;
Ragueneau et al., 2001; Rathburn et al., 2001; van der Weijden and van der Weijden, 2002).
However, only a few studies have been done in sediment cores, with the purpose to determine
31
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
changes in the biosiliceous paleoproductivity of the superficial water column (Charles et al.,
1991; Mortlock et al., 1991; Abrantes, 1996; Mortyn and Thunell, 1997; Anderson et al., 1998;
De La Rocha et al., 1998; Weber and Pisias, 1999; Gorbarenko et al., 2002; Masqué et al.,
2003).
Numerous techniques and modifications have been used in the determination of
biogenic silica in sediments. In general, they can be divided into four broad areas. Some
procedures are based on structural analysis of the solid phase e.g., direct IR spectroscopy of
amorphous opal (Chester and Elderfield, 1968; Fröhlich, 1989) and X-ray diffraction, direct
diffraction of amorphous opal (Eisma and van der Gaast, 1971) and the conversion of opal to
cristobalite at high temperature (Bareille et al., 1990). Other methods are based on point
counts of siliceous microfossils (Leinen, 1985; Pokras, 1986), and on normative calculations
estimating biogenic silica by subtracting mineral silicates calculated from the Al and Mg
concentration from the total Si content, to determine the excess of biogenic silica (Leinen,
1977).
However, the potentially most sensitive technique for determining biogenic silica is wet-
chemical leaching (Müller and Schneider, 1993). This technique is based on the assumption
that the biogenic silica and aluminosilicates have different dissolution rates, even in a weak
alkaline solution, with fast-dissolving amorphous biogenic silica and slow-dissolving non-
biogenic silica phases, such as clays, feldspars and quartz. The advantages and limitations of
these various methods have been discussed by DeMaster (1991), who basically proposed the
use of the wet alkaline extraction technique as the most versatile procedure for opal
measurements of samples with different origins and compositions. These procedures are also
simple and economical for processing (Kamatani and Oku, 2000). Accordingly, wet-alkaline
methods are the most used techniques (Hurd, 1973; Eggimann et al., 1980; Kamatani, 1980;
DeMaster, 1981; Shemesh et al., 1988; Mortlock and Froelich, 1989; Gehlen and Van
Raaphorst, 1993; Müller and Schneider, 1993; Kamatani and Oku, 2000; Fabres et al., 2002;
Koning et al., 2002; Liu et al., 2002).
The alkaline leaching technique has diverse problems, and several factors may affect
the precision and accuracy on BSi content measurement. The recovery of biogenic silica
depends on extraction conditions, leaching procedure, sediment composition and dissolution
by non-biogenic silica compounds. Therefore, variability in opal measurements should be
recognized and discussed.
The main objectives of the present study are 1) to establish the precision of the
Mortlock and Froelich (1989) method for the opal percentage range found in Galician sediment
samples, 2) determine the distribution of the biogenic silica content in surficial sediment of the
32
Chapter II
Ría de Vigo and in core samples of the Galician continental shelf, 3) determine the influence of
grain size distribution in opal determination, and 4) discuss the importance of the use of
biogenic silica percentage in fine fraction as a paleoproductivity proxy.
2. REGIONAL FRAMEWORK
The study area is located in the northwestern Iberian Peninsula and embraces two
zones, the Ría de Vigo, one of the chief embayments of the Galician coast, and the Galician
continental shelf (Figure II.1).
The ria can be divided into several zones according to the degree of continental or
oceanic influence. The innermost zone includes the San Simón Inlet, that is under a strong
tidal influence (average tidal range ~3m) and freshwater supply from the Verdugo-Oitavén
River (annual average flow of 27.5 m3s-1, Río and Rodríguez, 1992). The middle zone is
influenced by both continental and oceanic contributions, whereas in the outermost zone
freshwater input is negligible (Nogueira et al., 1997). The ria behaves as a partially mixed
estuary (Beer, 1983) with a two-layered positive residual circulation pattern, maintained in
winter by the freshwater flow and in summer by upwelling (Prego and Fraga, 1992).
Off the ria, the Galician continental shelf is relatively narrow: the 200 m isobath lying at
15–30 km from the coastline (Figure II.1). Sediment supply comes mainly from the Miño River
(the rias act as sediment traps (Prego, 1993)), and defines a sedimentary body at 110–120 m
depth known as the Galicia Mud Patch (Dias et al., 2002). The hydrography of the ria-shelf
region is strongly influenced by a seasonal wind-driven upwelling (Fraga, 1981; Blanton et al.,
1984; Álvarez-Salgado et al., 1993), SW storm surges (Vitorino et al., 2002) and the presence
of a poleward current during winter months (Frouin et al., 1990; Haynes and Barton, 1990).
3. MATERIAL AND METHODS
3.1. Sample recovering and processing
Fifty-one sediment samples from the uppermost oxic layer (0–1 cm) were collected in the
Ría de Vigo by a Van Veen grab on board the R/V Mytilus during a research cruise in October 1999
(Figure II.1). After collection, surface sediment samples were dried in an oven below 40°C and kept
in plastic storage for subsequent analyses. Afterwards, bulk sediment was size-fractionated by dry
sieving into mud, sand and gravel through sieves of 63 and 2000 µm mesh size. Opal
determinations were done on bulk sediment for all samples, and for selected sites, analyses were
carried out also in the <63 µm fraction (Figure II.1).
33
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
Figure II.1. Chart of the study area. Above: core CGPL00-1 location in the Galician continental shelf. Contour lines in this map show depth in m. Below: map of the Ría de Vigo showing the 51 sampling sites (circles). Samples of sediment were taken from the uppermost oxic layer (0–1 cm). Opal analyses were performed in bulk and muddy fractions for selected samples (black circles).
34
Chapter II
Opal analyses were also carried out on samples from a 96 cm gravity core (CGPL00-1)
retrieved from the Galician continental shelf (42°5’15.115’’N, 9°3’46.380’’W, 130.8 m water
depth) on board the R/V Mytilus during a cruise in May 2000 (Figure II.1). The core was sealed
just after collection and kept in storage at 4°C until analyses were performed in the laboratory.
The core was split longitudinally in two sections: one was used for determination of the
biogenic silica percentage and correlative subsamples of 2 cm thickness were taken for grain
size analyses. After removing the organic matter with H2O2, the coarse fraction was separated
from the fine fraction by wet sieving through a sieve of 63 µm mesh size. The >63 µm residue
was dry sieved through 2, 1, 0.5, 0.250, 0.125 and 0.063 mm sieves and weighed. Mud
content was determined following the method outlined by McManus (1991). X-Ray
nephelometry (Micromeritics, Sedigraph 5100) was used to separate silt and clay. Finally,
determinations of opal percentage were carried out on the dry bulk sediment and in the <63
µm fraction. Measurements of the biogenic silica content in the bulk sediment were done every
2 cm, whereas in the muddy fraction opal analyses were done every 5 cm.
3.2. Opal analysis
Opal concentration was measured using the alkaline leaching technique outlined by
Mortlock and Froelich (1989). Sediment samples were kept in storage at 4°C and softly dried
to constant weight at 40°C. Preceding leaching, samples were crushed with a mortar and
pestle and homogenised.
At least two replicates of each sediment sample were treated in each leaching
experiment. In addition, a blank leaching solution was included in all runs. Dried sediment was
weighted (approximately 200 mg) into a polypropylene centrifuge tube. Carbonates and
organic matter were removed by hydrochlorhydric acid 1M and peroxide (pharmaceutical
grade). 40 ml of 2M Na2CO3 solution was added to the samples. The tube was closed with
caps having pin-holes for ventilation, sonified, homogenised, and placed in a covered
constant-temperature water bath preheated to 85°C for 5 h. The tubes were agitated
vigorously to suspend the solids at 2 and 4 h and incubated again in the water bath. All steps
after removal of the tube from the hot bath were done quickly to minimize irreversible loss of
dissolved silica to solid surfaces. After a total of 5 h, samples were agitated and then removed
from the water bath and centrifuged at 5000 rpm for 5 min at 22°C. Immediately after
centrifugation, approximately 10 ml of the leaching solution were transferred to plastic tubes
and stored for subsequent analysis. The resulting extract was measured for the dissolved
silicate concentration by the molybdate blue spectrophotometric method according to Hansen
and Grashoff (1983) using an AutoAnalyser Technicon II. The sample was previously acidified
35
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
with HCl to neutralize the Na2CO3 leaching solution and segmented by air bubbles to enhance
mixing with reactive agents (ascorbic acid, oxalic acid and molybdate).
The analyser was calibrated and checked for linearity by running the blank leaching
solution and the working standard solution. Once linearity was checked, an appropriate
working standard solution was run (106.58 µM Si(OH)4) before and after the sample series.
Precision of the analytical system is based on replicate measurements of silicon standard
solutions. Dissolved silicate content for each sample was calculated taking into account the
dissolved silicate concentration of the working standard solution. Finally, the percentage of
opal in dry sediment is expressed by:
KP
CCopal m ×
×−=
100)(% 0 (1)
Cm is the dissolved silicate concentration of the sample in µM, C0 is the dissolved
concentration of the blank in µM, P is the dry sediment weight in µg and K (2.7 gmol-1l) is a
constant value comprising the digestion volume (l), the molecular mass of Si (gmol-1) and a
correction factor that depends on the opal water content (10% in this case because of their
recent origin).
3.3. Accuracy and precision of the opal determination
Due to the absence of certified BSi standards, establishing absolute accuracy from
existing methodologies is not possible. As there is not an amorphous silica standard or purified
BSi available to add to the sediment matrix, some authors have established accuracy by
comparison with other techniques (Mortlock and Froelich, 1989; Conley, 1998; Liu et al., 2002)
or analysis of artificial sediment standards that contain a known fraction of pure biogenic opal
(Mortlock and Froelich, 1989; Müller and Schneider, 1993; Koning et al., 2002). However, it is
difficult to prepare an artificial matrix similar to natural sediments, to mix various sedimentary
components in the laboratory or to manufacture an internal laboratory standard. Thus, it is
accepted the assumptions pointed out by Mortlock and Froelich (1989) about accuracy and the
opal measurements done are considered exact.
The precision of the overall method was estimated from analyses of selected superficial
samples from the Ria de Vigo (samples 10, 49, 26, 39 and 31) with opal contents covering the
entire opal range found in the ria and shelf sediments (Table II.1). The standard deviation of
the analysed samples ranges between 0.14 and 0.22 and increases slightly as the opal
content diminishes, but the general trend is nearly constant. Precision of the method is about ±
0.2 for the opal range studied. It decreases when opal percentages are relatively low, but
according to the stability of the standard deviation value (± 0.2) precision is satisfactory for all
36
Chapter II
sediment samples analysed, regardless of their opal content. When the relative standard
deviation (RSD) about the mean is examined, this value rises when the biogenic silica content
diminishes down to 1.3 wt.%. Mortlock and Froelich (1989) analysing sediments from Atlantic,
Antarctic, Pacific and Indian Oceans obtained similar RSD results, i.e. around 8% in samples
with opal concentrations lower than 15 wt.%.
Table II.1. Study of precision of the method. Table shows samples used in this work, location (latitude and longitude in UTM units), number of analysis, mean, standard deviation and relative standard deviation. Opal content determinations for each sample were done in different runs.
SAMPLE CODE LONGITUDE
(UTM) LATITUDE
(UTM) NUMBER
OF ANALYSIS
OPAL MEAN (wt.%)
STANDARD DEVIATION
RELATIVE STANDARD
DEVIATION (%)
10 530005.31 4683159.26 6 5.01 0.18 3.49
49 528921.71 4682062.81 6 4.12 0.19 4.73
26 516124.18 4676928.81 6 2.97 0.14 4.88
39 523588.86 4677136.80 8 2.14 0.21 9.95
31 518330.63 4678200.05 6 1.37 0.22 16.30
4. RESULTS AND DISCUSSION
4.1. Opal distribution in surface sediments of the Ría de Vigo
Biogenic silica analysis of bulk sediment samples from the Ría de Vigo shows that, as a
general tendency, maximum opal percentage is found in the inner part of the ria (Figure II.2A),
clearly decreasing towards the mouth of the ria. Higher percentages (about 4–5 wt.%) are
found in the San Simón Inlet. In the inner part of the ria and near the Rande Strait adjacent to
the northern coast opal percentage is around 2–3 wt.%. Similar values are registered in the
middle zone of the ria (about 2 wt.%), especially throughout the longitudinal axis. In contrast,
lower percentages (<0.5 wt.%) are found in the ria margins, particularly, at the ria mouth. In
this sector, sediment structure (Figure II.2B) comprises coarser grain sizes such as quartz
sand and biogenic carbonates (Nombela et al., 1987; Vilas et al., 1995).
It is interesting to compare our results with previous works by different authors in the
surrounding area. Prego et al. (1995) studied the biogeochemical silicon cycle in the Ría de
Vigo and also described a decrease in opal content from the inner part to mouth in the
superficial sediments. According to these authors, opal distribution shows high percentages
along the central axis (>2.5 wt.%), especially in the central zone, whereas low opal contents
typify the margins to the ria. Compared to this study, we have increased the number of
37
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
sampling stations, and the innermost part of the ria (San Simón Inlet) has also been surveyed.
This higher sampling resolution permits us to establish a more accurate account of opal
distribution of the ria and, in fact, remarkable differences with the Prego et al. (1995) results
are found. For example, the highest values (even 5 wt.%) are located in the inner part of the
ria (San Simón Inlet) and in the inner-central zone.
Figure II.2. a) Opal distribution in the bulk sediment throughout the Ría de Vigo. b) Detailed map of the superficial sediment distribution of the Ría de Vigo (modified from Vilas et al., 1995).
38
Chapter II
Subsequently, Prego and Bao (1997) evaluated biogenic silica content in superficial
sediment samples recovered from the whole Galician continental shelf, and Barciela et al.
(2000) extended this study to two other rias, Pontevedra and Arousa. Recently, Dale and
Prego (2002) have determined the opal content in the uppermost oxic layer (1 cm of sediment)
in sampling stations of the Ría de Pontevedra. This work shows that higher opal percentages
are recorded in the inner ria and towards to the northern coast, whereas, at the ria mouth,
biogenic silica content is relatively low, mainly due to the coarser grain size distribution in this
zone.
Comparisons with Ría de Pontevedra and Arousa illustrate that opal content is similar
between them, ranging from 0.5 to 5 wt.%. Biogenic silica distribution is analogous between
the Ría de Vigo and Pontevedra, higher percentages occurring in the inner part and towards
the north shore in the inner-central zone. This particular trend is due to a dilution effect caused
by residual organic matter in the sediments of the southern margin of the Ría de Vigo (Vilas et
al., 1995) and by discharges of particulate organic matter from a nearby paper mill (Arbones et
al., 1992) and urban wastewater in the Ría de Pontevedra (Barciela et al., 2000; Dale and
Prego, 2002). On the other hand, biogenic silica distribution of the Ría de Arousa is quite
singular because higher opal percentages in the north coast are not detected and opal in this
ria shows a general decreasing trend from head to mouth. This pattern is explained by the
higher fertilization by silicate provoked by the freshwater input of the Ulla River (Vergara and
Prego, 1997).
Taking into account these previous studies and the results of the present work, it is
possible to discriminate three areas in the Ría de Vigo in relation to opal content of seabed
sediment: the inner ria (San Simón Inlet), a central area where opal content is still relatively
high, but less than in the internal zone and, finally, an outer area characterized by lower
biogenic silica percentages.
Data concerning primary productivity in the Ría de Vigo are relatively scarce and have a
wide range of variability depending on the ria sector and the sampling period considered.
Mean annual value of net primary production throughout the ria, at an annual scale, is about
350 mgCm-2d-1 (Prego, 1993), although 40% of this quantity is remineralized. During the
upwelling season in 1997 Gago et al. (2003) found values of net ecosystem production around
790 mgCm-2d-1, but 68% of production is trapped in the sediments. Elevated values from
spring to autumn range between 700 and 1200 mgCm-2d-1 in the inner part of the ria (Vives
and Fraga, 1961), even 2400 mgCm-2d-1 in summer (Tilstone et al., 1999). Fraga (1976) found
values of about 2800 mgCm-3d-1 in the inner zone. A mean seasonal water column gross
primary production of 2100–2700 mgCm-2d-1 (Moncoiffé et al., 2000) is also found in the inner
39
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
ria, but only 33% is transferred to sediment on the shelf or to superior trophic levels. Thus,
elevated values of primary production are found when upwelling occurs form April to October.
As a general rule, productivity in the Ría de Vigo shows a decreasing trend from head
to mouth (Fraga, 1976) with the highest values in the inner-central part (adjacent to Rande
Strait). Primary production is 3–4 times higher in the inner zone than in the outer zone (Vives
and Fraga, 1961). Prego (1993) and Gago (2000) also registered this spatial and temporal
differences in their calculations about carbon fluxes to the sediment.
Considering data about carbon primary production in the ria, the opal content in the
sediment is correlated with the productivity in the water column, principally diatoms, typical
biosiliceous organisms in upwelling areas (Bao et al., 1989; Lisitzin, 1996; Bao et al., 1997).
However, in San Simón Inlet the opal content found in the sediment is higher than
expected, and it cannot be explained only by water column productivity. This semi-enclosed
area is characterised by shallower depths, which permits seabed illumination, tidal mixing, and
introduction of dissolved silicate input from river discharge (Prego and Fraga, 1991; Vergara
and Prego, 1997). Thus, proliferation of benthic diatoms in this sector is enhanced. Moreover,
Varela (1984) and Varela and Penas (1985) pointed out the increasing in the
phytomicrobenthic production in intertidal environments, and the higher frustule resistance of
benthic diatoms to dissolution. Pazos et al. (2000) revealed the presence of green algae in
particulate matter of the Verdugo-Oitavén River. Margalef (1956) and Bao et al. (1989) found
freshwater diatoms in the San Simón Inlet and in the vicinity of Rande Strait. Therefore, the
high opal percentage found on the sediment of the San Simón Inlet could be due to the growth
of benthic diatoms and the input of freshwater diatoms.
Looking at the sediment cartography of the Ría de Vigo (Vilas et al., 1995) (Figure
II.2B), the distribution of biogenic silica in the bed-sediment appears closely correlated with
the sediment structure throughout the ria. Seafloor sediment is composed of mixed siliciclastic
and skeletal gravels in both the outer area and the margins of the ria. Grain size distribution in
the entrance channels, north and south of the Cíes Islands, is characterised by a high sand
proportion, especially in the northern mouth, whereas along the longitudinal axis, sediment is
finer. Towards the shoreline, the sediments become coarser, grading through various
intermediate sediment types into clear carbonate skeletal sands or mixed carbonate
siliciclastic sands. In some places, especially in the outer and northern parts of the ria,
patches of the calcareous algae Lithothamnion corralloides and Phymatolithon calcareum are
found (Nombela et al., 1987; 1995; Vilas et al., 1995). The central and inner parts of the ria are
dominated by clay and silt fractions. Particularly in the inner parts, fine-grained sediments
persist up to the shoreline. Opal content appears to be related to the fine fractions distribution,
40
Chapter II
showing higher percentages throughout the longitudinal axis and in the inner and central parts
of the ria (Figures II.2A and II.2B).
Carbonate content also shows a good agreement with the grain size distribution.
Maximum percentages are recorded in the south entrance channel, whereas minimum
carbonate content is recorded in the central axis of the ria. Organic matter content appears to
be related to the fine fraction distributions, showing its maximum percentages in the inner and
middle part of the ria (Vilas et al., 1995).
Opal distribution all along the ria appears to be influenced by the primary production in
the water column, as well as the biogenic carbonate and lithogenic fractions of the sediment,
resulting of the ria hydrodynamic (Prego and Fraga, 1992; Taboada et al., 1998) and
sedimentation processes (Prego, 1993; Dale and Prego, 2002).
4.2. Opal content in the muddy fraction of surface sediments of the Ría de Vigo
Grain size distribution can affect opal content in sediment because coarser grains
provide a dilution effect that must be eliminated, as it was demonstrated by Luoma et al.
(1990) in the heavy metal case. In order to remove this, we evaluated the opal percentage in
the <63 µm fraction because we assumed that most opal, principally diatoms, is retained here.
Therefore, important information can be obtained when opal is normalised and quantified in
this fraction. This hypothesis is supported by two conditions:
1) Fertilisation by nutrients during upwelling process leads to an increase in the total
primary production, particularly in the biosiliceous production. Tilstone et al. (2000) in a study
in this area show that biosiliceous production is due to a larger contribution by diatoms. 2)
Diatom size depends on a few parameters, such as the characteristic size of the species,
water column temperature (Margalef, 1956; 1959), and the essential nutrient supply (Burckle,
1998). Nevertheless, it is unlikely that diatom frustules can reach sizes >63 µm.
However, other biogenic silica contributions in the >63 µm fraction would be possible,
e.g., sponge spicules remains but this type of biogenic silica is quite refractory and the
dissolution in the alkaline leaching solution is difficult (Müller and Schneider, 1993).
In order to test the above hypothesis, a comparison between opal percentage in total
and muddy fractions was carried out for nineteen sediment samples, with an opal content in
the bulk sediment of 1–5 wt.% and a mean value about 2.9 wt.% (Table II.2). In these
samples, opal percentage in the fine fraction ranged between 1.9–5.9 wt.%, with a mean value
of 3.5 wt.% (Table II.2).
41
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
Tabl
e II.
2. O
pal p
erce
ntag
e in
the
bulk
sed
imen
t and
in th
e <6
3 µm
frac
tion
from
sel
ecte
d su
rface
sam
ples
from
the
Ría
de
Vigo
. Tab
le a
lso
show
s a
desc
riptio
n of
the
sedi
men
t sam
ples
, the
per
cent
age
of e
ach
fract
ion
and
the
varia
tion
perc
enta
ge b
etw
een
opal
in b
ulk
and
in m
uddy
frac
tion.
Var
iatio
n pe
rcen
tage
is c
alcu
late
d fo
llow
ing
the
equa
tion
(2).
42
Chapter II
To quantify the difference between the opal content in both fractions, the variation
percentage is calculated using the expression:
10063 %
%63 % ×<−<
=mopal
bulkopalmopalPercentageVariationµ
µ (2)
Variation percentage (Table II.2) shows that the opal percentage in the bulk sediment is
relatively lower than in the <63 µm fraction, except for sample 20 (variation percentage -2.5%).
Samples 44 and 37 contain similar opal percentages in the bulk and muddy sediment. Mean
variation percentage for all samples is approximately 17%, therefore differences between opal
percentage in the bulk and muddy fractions are remarkable. Variation percentage can even
reach 40%, demonstrating that biogenic silica is more concentrated in the muddy fraction.
Figure II.3A shows the linear correlation between the opal content in the bulk sediment
and in the finer fraction. The correlation coefficient is high (R2=0.90). This excellent
relationship shows that variations in the opal content in both sediment fractions are analogous.
Thus, we conclude that analysis of opal in the muddy fraction is unnecessary, and
variations in biosiliceous productivity in the recent sedimentary record of the ria can be
detected by the analysis of opal in the bulk sediment. This methodological information can be
also applied to the other Galician Rías or similar environments as estuaries or embayments.
4.3 Opal content in the Galician continental shelf: An accurate paleoproductivity proxy
To establish the temporal variability of the sedimentary record of biogenic silica, a
gravity core CGPL00-1 was retrieved from the Galician continental shelf.
Lithology of the core is characterised by the presence of two well-differentiated
sections: the lower half of the core consists of laminated glauconitic sand overlying a bioclastic
gravel interval 2 cm thick, whereas the upper one is characterised by green mud deposits
(González-Álvarez et al., 2005).
According to the interpretation of González-Álvarez et al. (2005), the sandy sequence
recorded in the lower part is defined as a nearly instantaneous deposit caused by successive
storm events during the Subboreal/Subatlantic transition, and the upper muddy interval
corresponds to the last 3000 years. Therefore, to establish changes in the paleoproductivity for
the last 3000 years we will focus on the biogenic silica profile recorded in the muddy sequence
(level 47 cm to top of the core) because the sandy interval has a reworked origin. Opal
measurements in bulk and fine fractions were done throughout both sandy and muddy
sequences of the core.
43
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
Figure II.3. Plots showing the linear correlation between opal percentage in the bulk sediment and in the muddy fractions. a) Superficial sediment samples from the Ría de Vigo. b) Gravity core CGPL00-1. Dots represent sediment samples of the upper 47 cm and crosses are the samples located in the sandy sequence. Linear correlation parameters are m (slope), a (intercept), R2 (R-squared value).
As a general pattern, the trend of opal abundance in the bulk sediment throughout the
whole core shows two different modes (Table II.3, Figure II.4). In sandy sequence, opal
distribution shows a very irregular profile, whereas in the muddy sequence biogenic silica
values stabilize around 1.5 wt.%.
44
Chapter II
Profile of the biogenic silica percentage in the <63 µm fraction displays a relatively
regular tendency. From the bottom of the core to 60 cm, opal content increases progressively.
Values became stable from 60 to 25 cm, averaging 1.6 wt.%, and from this level to the core
top, opal percentage decreases considerably (Table II.3, Figure II.4). This trend closely
resembles the bulk sediment fraction profile in the muddy sequence, whereas in the sandy
sequence both profiles differ.
Figure II.4. Plot of the down core variations of the CGPL00-1. Dots symbolize samples that have been analysed in the muddy fraction, and the crosses show the biogenic silica content in the bulk sediment. Relative standard deviation (10% uncertainty) for samples analysed in the <63 µm fraction (dashed line). Relative standard deviation for samples analysed in the bulk sediment (solid line).
Considering the relative standard deviation for each analysis, variations in biogenic
silica content in both fractions are within the uncertainty range of 10% in the muddy sequence
of the core (excluding sample 15–16), although bioturbation signatures would blur down core
variations and homogenize the opal concentrations. However, in the sandy sequence
(excluding samples 80–81 and 85–86), dynamic range in opal concentration shows major
variations between both fractions (Figure II.4). Discrepancies in the concentration of biogenic
45
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
silica in bulk and muddy fractions reveal that dilution by coarser sediments is important. It is
also supported by the high values of the variation percentage calculated for samples of sandy
sequence in contrast to those calculated for samples of muddy sequence.
Table II.3. Opal percentage in the bulk sediment and in the <63 µm fraction for the core CGPL00-1. Table also shows the percentage of mud in each sample and the variation percentage between the opal content in bulk and in muddy fraction. Variation percentage is calculated following the equation (2).
SAMPLE CODE
(depth, cm)
<63 µm FRACTION
(%) OPAL <63 µm
(wt.%) OPAL BULK
(wt.%) VARIATION
PERCENTAGE
0-1 – 1.47 1.36 7.28 1-2 74.502 1.56 1.39 10.92 5-6 77.883 1.35 1.57 -16.44
10-11 – 1.39 1.40 -0.45 15-16 86.394 1.41 1.74 -22.91 20-21 83.791 1.35 1.39 -2.81 25-26 85.464 1.60 1.61 -0.50 30-31 82.633 1.59 1.47 7.61 35-36 76.472 1.54 1.57 -1.99 40-41 67.968 1.53 1.49 3.09 45-46 65.863 1.59 1.56 1.47 50-51 62.462 1.53 1.45 5.28 55-56 26.294 1.48 1.26 14.98 60-61 39.014 1.53 1.61 -4.70 65-66 52.166 1.26 1.60 -26.81 70-71 40.413 1.33 1.60 -20.09 75-76 27.410 1.03 1.35 -31.63 80-81 30.846 1.36 1.33 2.77 85-86 27.912 1.22 1.21 0.90 90-91 21.926 1.22 1.93 -58.08 95-96 – 1.09 1.60 -47.07
A linear correlation between opal percentage in the bulk sediment and in the muddy
fraction is effectuated to contrast changes in the opal content between both fractions. Figure
II.3B shows that there is no correlation between both analyses, in contrast to the results found
for Ría de Vigo. Assuming that the biogenic silica (largely diatoms) is presented mainly in the
muddy fraction, we conclude that opal concentration in bulk sediment does not reflect changes
in the biosiliceous productivity of the uppermost seawater. Coarser biosiliceous material, as
sponge spicules remains would be masking the contribution of the phytoplanktonic organisms.
Therefore, opal content measurement in the finest fraction is a better tool to ascertain changes
in paleoproductivity. However, biosiliceous productivity reconstructions are traditionally done
using the opal content in the bulk sediment (Charles et al., 1991; Mortlock et al., 1991;
46
Chapter II
Abrantes, 1996; Mortyn and Thunell, 1997; Anderson et al., 1998; De La Rocha et al., 1998;
Weber and Pisias, 1999; Gorbarenko et al., 2002; Masqué et al., 2003). As we have
demonstrated in this study, sediment samples with a high content of >63 µm fraction should be
studied carefully, and comparisons of opal in the bulk and in the muddy fraction must be
testing. We recommend using this parameter instead of biogenic silica percentage in the bulk
sediment in temporal records of other world areas that present major variations in sediment
structure.
Using opal in the muddy fraction as a paleoproductivity proxy, González-Álvarez et al.
(2005) have described the productivity conditions of the Galician continental shelf during the
last 3000 years. In this way, relatively stable biosiliceous production (averaging 1.6 wt.% opal
in the finest fraction, Table II.3) is recorded between 885 cal BC and 1420 AD (47 to 25 cm).
At 1420 AD a strong decrease in the opal content is detected. Taking into account other
parameters analysed in that study, a short but intense upwelling of intermediate cold waters is
registered at this level.
5. CONCLUSIONS
Biogenic silica determinations in sediment samples of the Ría de Vigo and the Galician
continental shelf have been carried out satisfactorily. Standard deviation of the method in
these samples is calculated in ±0.2. Relative standard deviation ranges between 4 and 10%
for sediment samples higher than 1.37 wt.%. For opal poor samples precision of the method is
lower, but satisfactory.
High-resolution sampling permits us to establish an accurate description of opal
percentage variations in the Ría de Vigo. Biogenic silica determinations were also carry out in
areas of the ria not previously studied. Opal content distribution shows that maximum values
are found in the San Simón Inlet. High values are found in the northern coastline in the inner
ria. Smaller percentages typify the margins of the outer ria and the entrance channels.
Biogenic silica percentage in the sediments throughout the ria is controlled by biosiliceous
production of the overlying water column as well as the dilution effect caused by coarser
sediment fractions and carbonate and organic matter content.
In order to remove the dilution effect, especially by coarser sediment structure,
analyses in the <63 µm fraction were carried out. Elevated linear correlation (R2=0.90)
between the opal analyses in both fractions leads to conclude that biogenic silica percentage
in bulk sediment is a helpful parameter to ascertain spatial changes in the biosiliceous
production in the water column of the Ría de Vigo.
In the core CGPL00-1, opal content measured in the muddy fraction is a sensible
parameter to establish paleoproductivity changes in the seawater column. Biogenic silica in
47
Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy
the <63 µm fraction allows us to standardize results and to eliminate interferences due to
coarser grain sizes, because coarser biosiliceous compounds could mask the diatom record.
This new contribution to the study of biogenic silica content in spatial and temporal records is
keen interesting and permits us to evaluate adequately the use of opal as a paleoproductivity
proxy.
Acknowledgements
Authors would like to thank Marta Elena González and Daniel Caride for their technical assistance in laboratory processing. We are indebted to M. Leeder, M. Pérez-Arlucea and P. Diz for their helpful comments to this paper. P.B. and R.G.-A. acknowledge Xunta de Galicia (Secretaría Xeral de Investigación e Desenvolvemento) and Ministerio de Educación, Cultura y Deporte (Secretaría de Estado de Educación y Universidades) for the doctoral fellowships. REN2002-04629-C03, EVK2-CT-2000-00060, and PGIDT00MAR30103PR projects supported this work. We are also indebted to two anonymous referees for their constructive comments that greatly improved the quality of the paper.
48
Chapter II
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[Chapter III] BENTHIC–PELAGIC COUPLING AND POSTDEPOSITIONAL PROCESSES AS REVEALED BY THE DISTRIBUTION OF OPAL IN SEDIMENTS: THE CASE OF THE RÍA DE VIGO (NW IBERIAN PENINSULA)∗
1. INTRODUCTION: BACKGROUND AND OBJECTIVES
2. STUDY SITE
3. MATERIALS AND METHODS
4. RESULTS AND DISCUSSION
4.1. Surface sediment: biogenic silicon fluxes and opal record
4.2. Subsurface sediment: postdepositional processes
5. CONCLUSIONS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Guillermo Francés, Ricardo Prego, 2006. Estuarine, Coastal and Shelf Science 68, 271–281. doi:10.1016/j.ecss.2006.02.008
Abstract. Opal concentrations in dry bulk sediment were measured in forty three sampling stations in the Ría de Vigo at 0–1, 1–2, 4–5, 10–11 and 14–15 cm sediment depths. The amount of opal was correlated with the biogenic silicon flux to the sediment obtained by means of a box model. The biosiliceous flux to the sediment (annual mean) is in good agreement with the content of opal found in the uppermost oxic layer, revealing that pelagic primary production is the main factor controlling the biogenic silica content in surface sediments. This correlation is illustrated by the equation: Opal (wt.%)=0.103×BSiF+0.414, (R2=0.95) where BSiF is the biogenic silicon flux to the sediment in g Si m-2 year-1. In the innermost part of the ría, i.e. San Simón Inlet, the content of opal is higher than predicted by the equation, since other biogenic silica sources to the sediment may be involved, as benthic diatoms proliferation or freshwater diatoms input. Elevated percentages recorded in the inner ría in the subsuperficial sediment respond to the establishment of suboxic-anoxic conditions that enhance the preservation of opal. The input of faecal pellets to the surface sediment derived from mussel rafts also controls the opal distribution and concentration in the ría.
Keywords: opal/surface and subsurface sediments/biosiliceous flux/ría/Galicia
Resumen. Se ha analizado la concentración de ópalo en muestras de sedimento seco de cuarenta y tres estaciones a lo largo de la ría de Vigo a diferentes profundidades de sedimento (0–1, 1–2, 4–5, 10–11 y 14–15 cm). El contenido en ópalo en el sedimento superficial ha sido comparado con el flujo de silicio biogénico hacia el fondo obtenido a partir de un modelo de cajas. El flujo de material biosilíceo al sedimento en media anual se correlaciona con el contenido de ópalo en el centímetro superior de sedimento, lo que indica que la producción primaria de carácter pelágico es el factor principal que controla la cantidad de sílice biogénica que queda depositada en el sedimento. La ecuación lineal obtenida es ópalo (%)=0.103×BSiF+0.414, (R2=0.95), donde BSiF es el flujo de silicio biogénico al sedimento en Si m-2 año-1. En la parte más interna de la ría, en la Ensenada de San Simón, el porcentaje de ópalo es mayor del que predice la ecuación, lo que muestra que existen otras fuentes de sílice biogénica en o al sedimento que no dependen de la producción biosilícea en la columna de agua, como son la proliferación de diatomeas bentónicas o la entrada de diatomeas de agua dulce a la ría aportadas por la descarga del río Verdugo-Oitabén. Además, el aporte de pellets fecales al sedimento superficial derivado del cultivo de mejillón en bateas también controla la distribución de ópalo y su concentración en la ría. Los elevados porcentajes que se hallan en la parte interna de la ría en los sedimentos subsuperficiales responden al establecimiento de condiciones de anoxia-suboxia que dan un lugar a unas condiciones adecuadas para la preservación del ópalo.
Palabras clave: ópalo/sedimento superficial y subsuperficial /flujo biosilíceo /ría/Galicia
BENTHIC–PELAGIC COUPLING AND POSTDEPOSITIONAL PROCESSES
AS REVEALED BY THE DISTRIBUTION OF OPAL IN SEDIMENTS: THE
CASE OF THE RÍA DE VIGO (NW IBERIAN PENINSULA)
1. INTRODUCTION: BACKGROUND AND OBJECTIVES
Silicon is an essential parameter in the ocean because is a major nutrient required by
phytoplanktonic primary producers. The biogeochemical cycle of silicon is driven by the
biomineralization of opal by diatoms and other organisms in ocean surface waters followed by
dissolution of the biogenic silica after the organisms die. Remains of this organisms sink to the
sediment providing to opal a high potential as a proxy of paleoproductivity. However, a small
fraction of the biogenic silica is buried and preserved in marine sediment. In coastal areas
biogenic silica content is normally low as a result of the high terrigeous input, so its utility has
been questioned (Nelson et al., 1995). Recent progress performed in the study of the silicon
marine cycle suggest that the use of opal as a paleoproductivity proxy can be looked at in a
more optimistic view. There is a crucial need for a better calibration of this marker in order to
improve our capacity to interpret the opal burial in the sediment in terms of paleoproductivity
and paleoceanography. The main factors that involve the linking between surface production
and the biogenic silica burial are the spatial and temporal variations of preservation efficiency,
lateral advection, sediment redistribution and the uncoupling between Si and C
biogeochemical cycles (Ragueneau et al., 2000). These factors create discrepancies between
export and sedimentary siliceous fluxes at a global scale (Nelson et al., 1995).
Controls on biogenic silica dissolution and preservation in marine environments are
complex. The transfer processes of biogenic silica from surface waters to the sediment are
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Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
controlled by the competition between physical export and biogeochemical recycling. Several
factors such as availability of dissolved silicate for growth, temperature (DeMaster, 1981),
trace element chemistry of the seawater (van Bennekom et al., 1991), silica recycling in the
water column (Nelson et al., 1995), seasonality (pulses of primary production), supply rate
(Pokras, 1986), lateral transport and resuspension events, selective grazing or the formation of
aggregates (Pasow et al., 2003) and faecal pellets play also an important role. Dissolution of
biogenic opal starts within the water column, and continues in the sea floor, where the
processes related to preservation of the settling material to the sediment are numerous and
poorly understood, namely: post-depositional dissolution and early diagenesis in the sediment
matrix (Ragueneau et al., 2001), sedimentation rate (Pokras, 1986), benthic activity, and
bioturbation that enhances the mixing of pore fluids and increases the removal rate of
dissolved silica to the overlying water column, abundance of the lithogenic particles and the
corresponding influence of aluminium concentrations in opal solubility (Van Cappellen and
Qiu, 1997), and the kinetic and thermodynamic conditions affecting the opal solubility.
Despite these inconveniences, the information derived from content of biogenic silica in
the sediment combined with other productivity markers in a multi-proxy approach is useful.
Research is commonly focussed on surface sediments to ascertain spatial changes in pelagic
primary production. Opal-rich sediments are associated with high primary productivity in
coastal areas however, data on biogenic silica content (BSi) in estuarine and coastal
sediments are relatively scarce.
One of these areas is the Galician coast and adjacent shelf. Previous studies on opal
distribution in the rías (Barciela et al., 2000; Dale and Prego, 2002; Cobelo-García and Prego,
2004) and continental shelf (Prego and Bao, 1997) were limited to the surface sediment (<2
cm depth, uppermost oxic layer), and have the limitation of a low resolution mapping. The Ría
de Vigo has been targeted as the subject of research aimed at understanding its hydrography,
nutrient cycles, primary production and plankton communities. Although, the Ría de Vigo has
been researched thoroughly, little information is available about the record of productivity in
the sediments since a very limited number of studies have been carried out to date. Prego et
al. (1995) evaluated the silicon cycle focusing in the water column processes, including
phytoplankton taphocenosis in the surface sediments in relation to upwelling non-upwelling
events. Also, Bernárdez et al. (2005) have reported values of biogenic silica in the uppermost
oxic layer, which are strongly correlated to spatial variations in primary productivity.
Thus, on the basis of a high resolution mapping and a description of the distribution of
the amount of opal in a Galician Ría, the aims of this study are (1) to map the opal content and
its local changes in the surface and subsurface sediments in the Ría de Vigo and (2) to
62
Chapter III
correlate the opal record in the surface sediments with the biogenic silicon fluxes from the
surface waters to the seabed.
2. STUDY SITE
The Ría de Vigo is the southernmost one of a set of four incised valleys (Rías Baixas,
NW Iberian Peninsula), where long term climatic variability has changed the extension of the
estuarine zone through time (Evans and Prego, 2003). The physiography of the ria shows a
funnel-like shape in plan view, gradually widening seawards, and is partially enclosed by the
Cíes Islands resulting in relatively calm conditions in the ría (Figure III.1). The most important
freshwater input comes from the Verdugo-Oitabén river that flows into San Simón Bay, at the
landward head of the ría. The annual freshwater flux is 26 m3 s-1, but the freshwater
contribution varies monthly from 30 to 56 m3 s-1 during the rainy season (November–March)
until 2 to 12 m3 s-1 in the wet season (June–October) (Pérez-Arlucea et al., 2000 and
references therein).
The Rías Baixas are influenced by a seasonal quasi-permanent upwelling, usually
occurring from spring to September or October (Fraga, 1981). This process is driving by the
northerly trade winds where the resulting baroclinic pressure gradients in the rías are
compensated by intrusions of the cold, nutrient-rich, sub-surface oceanic East North Atlantic
Central Water, ENACW. Therefore, residual circulation is strongly influenced by coastal
upwelling dynamics (Fraga 1981). The ría is characterised by a two-layered residual
circulation pattern, with freshwater outflow at the surface balanced by the inflow of saltier
water in the lower layer. In winter, river discharge favours the positive estuarine circulation, but
during summer, stratification is maintained by the vertical distribution of the temperature in
spite of the river flow (Prego and Fraga, 1992). The density-driven flow is influenced by wind
regime rather than by the other weather conditions such as continental runoff and heat
exchange with the atmosphere (Álvarez-Salgado et al., 1993). Southerly winds promote
coastal downwelling, and thus, the surface inflow of surface oceanic water with relatively high
thermohaline variability and low nutrient content.
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Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
Figure III.1. Map of the Ría de Vigo showing the sampling stations. Bathymetric lines every 10 m depth.
64
Chapter III
The annual cycle of primary production in this region is controlled by the inflow pulses
of ENACW during upwelling, with peaks of biomass during late spring and summer. Due to this
upwelling situation, the Rías Baixas are among the most productive oceanic regions in the
world being subjected to important human activities of economic interest such as mussel
production. Consequently, the ría sediments have typically high contents of organic matter and
opal (Prego et al., 1995; Bao et al., 1997; Barciela et al., 2000; Dale and Prego, 2002;
Bernárdez et al., 2005). In the Ría de Vigo, mean annual values of net primary production are
about 350 mg C m-2 d-1 (Prego, 1993), but during the upwelling season in 1997 Gago et al.
(2003) reported a net ecosystem production of 790 mg C m-2 d-1. High values of around 700
and 1200 mg C m-2 d-1 were also found in the Box 2 and 3 (Figure III.1), and especially from
spring to autumn with peaks ranging from 2100 to 2800 mg C m-2 d-1 (Fraga, 1976; Tilstone et
al., 1999; Moncoiffé et al., 2000) as a result of the fertilization of surface waters when
upwelling occurs. Several authors have also reported important spatial variations in primary
productivity within the ría, suggesting a general decreasing trend from head to mouth (Prego
1993 and references therein).
The grain-size distribution of the seabed consists of mixed siliciclastic and bioclastic
gravels in both the outer area and the boundaries of the ría, and a major axial deposit of
cohesive sediments in the middle ría, whereas the inner areas are dominated by clay and silt
(Vilas et al. 2005). Elsewhere, particularly in the inner area, fine-grained sediments also
persist up to the shoreline. Organic matter content is related to mud distribution, showing
higher percentages in the central axis and in the inner and middle ría areas (Diz et al, 2006).
Elevated values are also found in the southern margin of the inner ría, probably related to the
urban wastewater (Vilas et al., 2005).
3. MATERIALS AND METHODS
Forty three surface and subsurface sediment samples, taken on a spatial high-
resolution basis, were analysed for biogenic opal content. All samples were collected with a
box-corer on board the R/V Mytilus (IIM-CSIC) in September 1998 (Figure III.1). Each box core
was completely sampled at 1–centimetre intervals from the 0–1 down to 4–5 cm, and also at
2–cm intervals from 5 to 15 cm. Opal determinations were carried out in 43 samples for the
uppermost centimetre, 42 samples at the 1–2 and 4–5 cm, 35 samples at the 10–11 cm and in
33 samples at the 14–15 centimetre sediment depth. Additionally, in some stations located in
the longitudinal axis, depth-resolution sampling is even every centimetre.
Sediment samples were stored in plastic bags at 4°C until opal analyses were
performed in the laboratory. The bulk sediment (approximately 200 mg) was dried using an
65
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
oven at a temperature lower than 50°C, and treated with 5 ml of HCl (1M) and 5 ml of H2O2
(pharmaceutical grade) in order to eliminate carbonate and organic matter. The determination
of the amount of opal contained in the bulk sediment was carried out following the wet alkaline
leaching procedure devised by Mortlock and Froelich (1989). Biogenic silica was extracted into
a 2M Na2CO3 solution at 85°C for 5 h. Finally, the dissolved silicate concentration present in
the extract was measured by means of molybdate blue spectrophotometry using a continuous
flow analyser AutoAnalyser Technicon II. Precision of the biogenic silica measurement was
evaluated from replicate analyses of selected surface samples from the Ría de Vigo, which
have different opal content that cover the entire opal range found in the ría. Standard deviation
for 6 to 8 parallel extractions was ±0.2, indicating good reproducibility, though for the opal poor
samples (<1.3 wt.%) relative standard deviation reaches up to 16% (Bernárdez et al., 2005).
4. RESULTS AND DISCUSSION
4.1. Surface sediment: biogenic silicon fluxes and opal record
The amount of opal in the surface sediment ranged from almost undetected to a
maximum percentage of 3.9 wt.% with a mean value of 1.70 wt.% (Figure III.2). Highest
concentrations were found in the San Simón Inlet, with values around 3–3.5 wt.%. In this area,
the maximum values (around 3.5 wt.%) were located in the innermost part (Box 1, Figure III.1),
i.e. nearby the Verdugo-Oitabén mouth, and the lower ones (3 wt.%) in the vicinity of the
Rande Strait. In the Box 2 (Figure III.1), biogenic silica content decreases to percentages
about 2–3 wt.%, however, higher values were found on the northern shore. Concentrations
about 1.5–2 wt.% were found in Boxes 3 and 4, along its central axis in a lengthwise direction.
Lower biogenic silica values were located at the ría mouth channels and persisted up to the
shoreline, where the sediments are rich in carbonates and sands. As expected, the amount of
opal clearly decreases from the head to the mouth of the ría.
Opal distribution appears well correlated with the sediment structure in the Ría de Vigo.
The amount of biogenic silica showed a distribution similar to the muddy fraction and also to
low energy processes. Low biogenic silica percentages were found in the entrance channels
and the shoreline, where high energy and erosive processes are of remarkable importance
and coarser sediment fractions are dominant. That grain size effect in opal distribution has
been discussed in Bernárdez et al. (2005) leading to the conclusion that for muddy sediments
of the ría the analysis in the finer fraction (<63 µm fraction) is unnecessary. The percentage of
biogenic silica in bulk sediment (without correction of grain size) was chosen to evaluate the
biosiliceous production in the water column.
66
Chapter III
Figure III.2. Contour plots of the distribution of opal percentage in the Ría de Vigo at different sediment depths. Contour lines every 0.2 wt.% opal. Colour scale from light to dark grey indicates the increase in the opal percentage.
67
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
Data about the silicon and carbon fluxes in the Ría de Vigo has been studied in terms of
biogeochemical processes and water circulation (Prego, 1993; Prego et al., 1995; Gago et al.,
2003), but the benthic-pelagic coupling concerning silicon in the ría has not been developed
yet. Further information about this relationship improves our ability to discern seasonal and
spatial changes in paleoproductivity from the geological record.
A correlation between the silicon and carbon production rates in the water column and
the opal content record in the surface sediment was found. Elevated silicon and carbon fluxes
to the sediment, and thus, high primary productivity in Box 2, matched well with elevated
concentrations of biogenic silica in surface sediments. Mean annual value of photosynthesis in
the five boxes (Figure III.1, Prego, 1993) also shows that highest values are located in the Box
2 and 3 (between Vigo and the Rande Strait, Figure III.1), where high values of opal are
recorded in the uppermost oxic layer. Enhanced productivity driving by the upwelling in these
boxes was already pointed out (Fraga, 1976). Lower values of primary productivity, found
especially in the outer zone, are correlated with a decrease in the amount of opal in the
sediment. Thus, as a general view, opal concentrations in the surface sediments are
consistent with the spatial changes in productivity along the ría (Bernárdez et al., 2005). Our
findings are supported by the amount of organic carbon in the superficial sediment reported by
Diz et al. (2006). In the outer area (limit in the station 26, Box 4 and 5, Figure III.1) values are
around 2%, whereas in the middle-inner part (Box 3 and 2) organic carbon ranges between 3–
4% being especially high in the innermost stations (Box 1, San Simón Inlet).
We used the dataset reported by Prego et al. (1995) for the calibration of the opal in the
sediment as a productivity proxy applying the biogenic silicon deposition in the sediment in
each zone of the ría and for different hydrographic and production periods (Table III.1). Due to
the spatial and temporal heterogeneity of biogenic silicon net deposition data presented in
Prego et al. (1995) some calculations were carried out: (i) first, according to the four typical
behaviours of the silicon cycle along the year described by Prego et al. (1995) we calculated
the biogenic silicon flux to the seabed for both hydrographical situation and boxes; (ii) second,
taking into account the mean upwelling index for each month, defining the seasonality in the
area, we extrapolated the biogenic silicon flux obtained for each hydrographical situation to the
months when these distinctive conditions occur. Thus, values of the biogenic silicon net flux to
the sediment (mg Si m-2 d-1) for each box were converted to annual values (g Si m-2 year-1),
and in addition, we have calculated a mean annual flux deposition of silicon in the five boxes
described in Prego et al. (1995). The mean opal content in the first centimetre in each box was
estimated with regard to the area covered by each opal isopleths on the contour map (Figure
68
Chapter III
III.2). Therefore, we can compare the annual biogenic silicon flux to the seabed (Table III.1)
with the biogenic silica content in the surface sediment.
Table III.1. Biogenic silicon flux data (annual mean) for both boxes and different hydrographical situations (winter, spring, summer with upwelling and summer without upwelling). Original data from Prego et al (1995) is also presented in brackets (mg Si m-2 d-1) and represents the biogenic silicon flux to the sediment for each hydrographical condition.
Period Time spans (months) Box 1 Box 2 Box 3 Box 4 Box 5
Winter 4 (13) 1560 (-605*) 0 (0) 0 (-4) -480 (6) 720
Spring 3 (19) 1710 (49) 4410 (17) 1530 (11) 990 (14) 1260
Summer with upwelling 2 (159) 9540 (82) 4920 (172) 10320 (46) 2760 ND Summer without
upwelling 3 (8) 720 (124) 11160 (5) 450 (14) 1260 ND
Total BSi flux (g Si m-2
year-1) 13.53 20.49 12.3 4.53 1.98
Figure III.3 shows the linear correlation between the annual biogenic silicon flux and the
percentage of opal for each box (excluding Box 1):
Opal (wt.%) = 0.103BSiF + 0.414 (1)
where BSiF is the biogenic silicon flux to the sediment in g Si m-2 year-1
The high correlation between both parameters, excluding Box 1, demonstrates the
coupling between the benthic (bottom sediment) and the pelagic environment. These
quantitative findings are supported by the correlation between the opal record in the surface
sediment of the ría and the sink to the bottom of the particles produced by silica-producing
organisms. However, since the content of opal is integrated over the full 1 centimetre, this
parameter does not reflect short-time temporal variations, being an excellent proxy of long-
term spatial and temporal changes in productivity (Rathburn et al., 2001). According to the
reported sedimentation rates (Desprat et al. 2003), one centimetre spans approximately five
years. The appearance of opal when the silicon deposition flux is zero (see equation 1) is
explained by several factors: (i) The seasonal pulses are not sufficient enough to change the
background of opal, (ii) Lithogenic silica leaching lead to a slight overestimation of the
biogenic silica, because correction for non-biogenic constituents were not applied in the
Mortlock and Froelich (1989) technique, (iii) The decrease in the residence time of water when
upwelling occurs lead to a transport and export of the biogenic particulate material from the
high productivity areas in the inner ría to the outer zone. This lateral transport of biogenic silica
due to positive residual circulation might increase the opal percentage in the sediment.
69
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
However, slow transport currents (Souto et al., 2003) and the low water column depth allow
the sedimentation of most of the biosiliceous compounds in the production area.
Figure III.3. Plot showing the linear correlation between the mean annual silicon flux to the seabed and the mean opal content in the first centimetre of the sediment in each box considered in Prego et al. (1995). Box 1, located in the San Simón Inlet, was not included in the regression. Equation of the linear regression also shown (BSiF represents the biogenic silicon flux.
Data of the biogenic silicon fluxes and the opal content in the Box 1 need special
attention. Contrary to what may be expected, the highest values of biogenic silica recorded in
the Box 1 (San Simón Inlet) are not related to an enhanced net primary production. Biogenic
silica percentage in the first centimetre in the San Simón Inlet is considerably higher than
predicted by the linear correlation. Therefore, at least, a second source of biogenic silica to the
bottom sediment must be invoked. The most reliable explanation is the presence of a
important contribution of opal due to the siliceous benthic production. Scanning electron
microscope (SEM) images of the most common benthic diatom groups and species
(Cocconeis, Diploneis, Psammodyction, Achnanthes, Amphora and Paralia sulcata) were
obtained (Figure III.4). These groups and species have a high abundance in the San Simón
Inlet surface sediment samples, instead of planktonic forms such as Chaetoceros resting
spores, Thalassionema nitzschioides or Thalassiosira, commonly found commonly found in the
sediments of the medium and outer ría (Bao et al. 1989).
70
Chapter III
The contribution of the benthic siliceous production was calculated using the equation
(1). The opal content expected in the Box 1 concerning only the input of pelagic biosiliceous
flux (13.53 g Si m-2 year-1, Figure III.3) is 1.80 wt.%, according to the equation (1). The
difference between the opal percentage found (3.16 wt.%, Figure III.3) and expected (1.80
wt.%) is the contribution owing to the benthic diatoms. Around 43% of the biogenic silica found
in the surface sediments of the San Simón Inlet is due to the benthic biosiliceous production:
%43100%)16.3/%36.1(% =×=lBenthicOpa
The high opal content due to microbenthic production is related to an increase of the
intertidal environments, the resistance of the benthic diatoms to dissolution (Varela and Penas,
1985), the nutrient inputs (Gago et al., 2005), and the shallower depths. However, the
contribution of the benthic biosiliceous organisms may be overestimated due to the biogenic
silica supply from other sources, e.g. freshwater diatoms. The abundance of the freshwater
group is low in comparison with the benthic assemblage as detected by means of scanning
electron microscopy. Low sediment suspended loads in the Verdugo-Oitabén river were found
(Pérez-Arlucea et al., 2005), and the river-borne suspended matter entering the Inlet is
dominated by clay minerals, quartz and feldspar; organic matter consists mainly of Fe-rich
algae and occasional diatoms (Pazos et al. 2000). However, several freshwater species were
identified by Bao et al. (1989), even in the medium ría.
There are also a large number of mussel rafts in the outer part of the Inlet. These rafts
contain strings of mussels (Mytilus galloprovincialis), that concentrate fine-grained sediment to
produce faecal mud, which is ultimately deposited in the adjacent ría floor. San Simón Inlet
acts a sediment trap to muddy sediments (Nombela et al., 1995). Thus, high concentrations of
the diatoms frustules in the faecal pellets raise the opal content in the Inlet.
4.2. Subsurface sediment: postdepositional processes
Linkages between biological, hydrological and sedimentological processes in the recent
sedimentary record are important to understand the limitations and usefulness of the opal
productivity proxy in the geological record. A detailed study of the biogenic silica content in the
subsurface sediment was carried out for understanding post-depositional processes that could
have an effect on the opal record.
Results of opal concentration at different sediment depths are shown in Figure III.2.
Contour plot of opal at 1–2 cm depth showed the same distribution and analogous values as
compared to the pattern found in the surface sediments. However, in the Box 2 (Figure III.1), a
distinctive trend was found, characterised by higher opal percentages close to the northern
shoreline (ca. 2.8 wt.%). This pattern was also identified in the surface sediments.
71
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
Figure III.4. SEM images of some benthic diatom species found in the San Simón Inlet (surface sediment samples 49, 50, 51, 52, 53, 54). a) b) c) d) e) f) Cocconeis spp. g) h) i) Diploneis spp. j) k) Psammodyction spp. l) Paralia sulcata m) n) Achnanthes spp. o) Amphora sp.
72
Chapter III
Several differences were found at 4–5 cm depth with regard to previous results. First,
the mean opal content (1.9 wt.%) is higher than in surface sediments. The decrease in the
percentage of opal from the Verdugo–Oitabén mouth to the Rande Strait area, as recorded in
the upper centimetres, is blurred. Second, the higher percentages of biogenic silica detected
for the previous centimetres in the Box 2, close to the northern shore, was not identified at this
sediment depth. The amount of biogenic silica in the central axis of the ría becomes higher,
reaching values up to 2 wt.%.
Biogenic silica distribution showed a distinguished pattern at 10–11 cm depth,
characterised by elevated values in the vicinity of Rande Strait and in the outermost area of
the San Simón Inlet, ranging from 3 to 5 wt.%. Moreover, higher biogenic silica percentages
were found in the longitudinal axis, up to 2.5 wt.%.
The contour map at 14–15 cm depth closely resembles the one found for the previous
depths studied. The highest percentages of opal were found in the vicinity of Rande Strait.
However, opal content in this area was slightly lower than the at 10–11 cm depth (with values
around 4.2 wt.%). In the longitudinal axis in Box 4 (Figure III.1), biogenic silica percentage
ranged between 2–2.4 wt.%.
According to the amount of opal, and as observed in the longitudinal axis section, two
areas can be clearly distinguished (Figure III.5). The limit is located between the stations 36
and 41 and represents the high-low productivity boundary. In the outer area (Box 4 and 5),
opal values are lower, especially in the two upper centimetres (Figure III.2). Generally
speaking, opal profiles show lower values in the first or even the second centimetre. Due to
the lack of data about benthic silicon dissolution rates along the profiles in the Ría de Vigo,
this fact could be interpreted as a fast remineralization of the “fresh” biosiliceous material at
the oxic sediment-water interface. The presence of the fluffy layer and the associated lower
sediment density can also play a role in this trend.
In Box 2, opal concentration in the subsurface sediments is high, especially in the
station 45. The high concentration of opal registered at 10–11 cm depth in the vicinity of
Rande Strait can be explained by the combination of several factors: 1) A rise in the primary
productivity at the period of burial 2) A decrease in the input of lithogenic particles, and 3) An
enhanced preservation efficiency of the opal due to the establishment of suboxic-anoxic
conditions during burial. The latter is particularly important in areas of an elevated high
primary production, and it is linked to oxygen consumption because of the remineralization of
organic matter during burial. Although areas of highest opal concentration seem to be coupled
with the biosiliceous pelagic productivity, the hindered diagenetic processes and oxic-anoxic
conditions in the sediments controls the opal accumulation in the sediment. Dissolved oxygen
73
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
values near the seabed in the Ría de Vigo shows that the sediment-water interface is oxic
(Doval et al., 1998), but in muddy sediments without bioturbation, as in these stations, oxic-
anoxic limit is very close to the surface sediment Abella et al (1998). Recent studies in the Ría
de Arousa indicated strongly reducing environments in these type of sediments, with mean Eh
values that ranging between -120±20 and -252±60 mV and pH values varying from 8 to 8.7
(Otero et al., 2006). Diz et al. (2006) also shows that in the most inner settings of the Ría de
Vigo degradation processes of organic carbon could cause low oxygen and/or reducing
conditions in the sediment affecting negatively the benthic foraminifera populations. Lack of
burrowing and a reduced ventilation of the sediment also increase the opal preservation
efficiency. Thus, preservation of the opal in this area is strongly influenced by the arrival of
large amounts of organic matter and the establishment of reducing conditions during burial.
As explained previously, the most common opal-depth profile is characterized by a
decrease in the amount of opal in the upper centimetres, but other trends can be found locally.
Some stations in the northern shore in the Box 2 showed a higher amount of opal in the two
upper centimetres (Stations 38 and 34, Figure III.5). The anti-clockwise circulation in this area,
cross to the ría axis (Montero et al. 1999) seems to explain the more intense deposition of fine
particles, as diatom frustules, in the northern shoreline, and the reduction of the sedimentation
of the biosiliceous compounds in the southern margin. The high input of faecal pellets with
elevated concentrations of diatom frustules derived from the nearby mussel rafts could also
increase the amount of opal in the sediments in this zone due to the high number of these
structures in the area.
In the cross-section of the outer area lower opal concentrations were found, even in the
subsuperficial sediment (Figure III.5). This pattern is linked to the coarser sediment structure
at the margins as well as higher bottom currents (Diz et al., 2004). Values around 2.2–2.4
wt.% at the station 26 between 5 and 11 cm sediment depth are associated to the
establishment of a lower energy environment and the deposition of muddy sediments in the
central axis of the ría.
74
Chapter III
Figure III.5. Plot of the percentage of opal from the top down to 15 cm in the longitudinal and cross-section of the Ría de Vigo. Stations used for the longitudinal and cross-section is indicated in the map. Sample number at the top of every contour plot.
75
Benthic–pelagic coupling and postdepositional processes as revealed by the distribution of opal in sediments: the case of the Ría de Vigo (NW Iberian Peninsula)
5. CONCLUSIONS
The benthic-pelagic equation (1) presents an evaluation of the linking between
biosiliceous production and the record in the sediments. The biogenic silicon flux to the
seabed follows the same trend as the opal content in the surface sediments, confirming the
usefulness of opal as a proxy of paleoproductivity in ría environments, as well as estuarine,
coastal and shelf areas with high primary productivity. Elevated values of opal found in the
San Simón Inlet are mainly due to the growth of benthic diatoms. The input of freshwater
diatoms from the Verdugo-Oitabén River and the concentration of diatom frustules in faecal
pellets also increases slightly the biogenic silica content in that area. The anti-clockwise
circulation in the inner areas could enhance the deposition of biosiliceous particles in the
northern margin. In the outer area, lower values of opal are related to a decrease in the
primary productivity and the coarser sediment structure.
Different patterns of opal accumulation were recognized in the subsurface sediment,
responding to changes in the opal accumulation, postdepositional processes or to the input of
diatom frustules in the faecal pellets derived from the mussel rafts. Typical opal-depth profiles,
particularly those of stations located in the longitudinal mud patch, are characterized by a
slight decrease in the amount of opal in the upper centimetres. High values of biogenic silica in
the inner area (Box 2) in the subsuperficial sediment (11–15 cm) are related to the
development of a reducing environment that enhances the opal preservation due to the
oxidation of organic matter in this high productivity area.
Acknowledgements
The authors would like to express their gratitude to Roberto Bao, Antonio Cobelo-García and Carlos Souto for the valuable comments on the manuscript and for helping improve and correcting the English. We are also indebted to M. A. Barcena and O. E. Romero for their help with diatom identification. Marta Elena González and Daniel Caride are sincerely acknowledged for their technical assistance in laboratory processing. This work was supported for the METRIA REN2003-04106-C03, REN2003-09394, EVK2-CT-2000-00060, PGIDT04PXIC31204PN and PGIDT00MAR30103PR projects. P.B. would like to thank the Xunta de Galicia (Secretaría Xeral de Investigación e Desenvolvemento) and Ministerio de Educación, Cultura y Deporte (Secretaría de Estado de Educación y Universidades) for financial support.
76
Chapter III
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79
[Chapter IV] PROCESSES CONTROLLING THE DIATOM PRODUCTION AND ACCUMULATION IN A WESTERN GALICIAN RÍA: IMPLICATIONS FOR PALEORECONSTRUCTIONS∗
1. BACKGROUND AND OBJECTIVES
2. REGIONAL SETTING
3. MATERIAL AND METHODS
3.1. Location and sampling
3.2. Procedures and analytical strategies
4. RESULTS
4.1. Diatom assemblages in the water column: Seasonal patterns
4.2. Diatom assemblages in the sediment traps: Seasonal patterns
4.3. Diatom assemblages in the surface sediment: Species encountered
4.4. Other biosiliceous components
4.5. PCA analysis: Relationships among diatoms, biosiliceous compounds and geochemical features
5. DISCUSSION
5.1. Water column and sediment trap data vs. diatoms record in the sediment: Implications for paleoenvironmental reconstructions
5.2. Diatom and biosiliceous compound surface sediment distribution: oceanographic and environmental controlling factors
5.3. Diatom record vs. geochemical characteristics of the sediment
6. CONCLUDING REMARKS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Manuel Varela, Yolanda Pazos, Ricardo Prego, Guillermo Francés. To be submitted.
Abstract. Seasonal variability of diatom populations from the Ría de Pontevedra (NW Iberian Peninsula) was measured fortnightly on the basis of water column abundance, vertical fluxes and preservation in the underlying sediment. Depending on the oceanographic situation, samples were divided into low, moderate and high productivity periods, according to diatom abundance. Seasonal variations of the diatom species in the water column were indicative of different environments and oceanographic characteristics, nutrient levels, river influx and water column stratification.
Additionally, analysis of diatom distribution and biosiliceous compounds has been carried out in 27 surface sediment samples. These organisms are used as tracers of the hydrographical and biogeochemical processes occurring in the Ría de Pontevedra. The marine planktonic assemblage in the surface sediments is located in the outer area of the ría. It is mainly composed by Chaetoceros resting spores (R.S.) together with T. nitzschioides, L. danicus R.S. and P. sulcata. Higher abundances of freshwater assemblages, as well as the terrestrial input proxies (crysophycean cysts and phytoliths), are limited to the inner ría, where the input of freshwater release of the Lérez River has its strongest influence, especially in the northern shore. Benthic group is also restricted to the innermost areas where low water depths allow for growth. Principal component analysis was successful to distinguish between estuarine-freswater and ocean-dominated upwelling areas in the Ría de Pontevedra. Thanatocoenosis seabed distribution was closely related to oceanographic features of the area and the riverine input.
A good agreement between the biocoenotic and the thanatocoenotic diatom community for the most important species was found. Biogenic silica flux and accumulation point to the utility of some species and assemblages in the reconstruction of hydrodynamic and production characteristics of the ría and estuarine domains in the marine paleo-records. However, lack of preservation of some species due to their low preservation efficiency results in difficulties in interpreting the sedimentary record.
Biological response, production patterns and diatom succession related with their ecological preferences in the Ría de Pontevedra can also be applied to other rías and temperate coastal environments.
Keywords: diatom assemblages/sediment traps/diatom fluxes/surface sediments/Ría de Pontevedra/NW Iberian Peninsula/Leptocylindrus danicus resting spores/Chaetoceros resting spores/Thalassionema nitzschioides/Paralia sulcata
Resumen. La variación estacional de la población de diatomeas en la ría de Pontevedra (NO de la Península Ibérica) fue monitorizada quincenalmente a partir de la abundancia de diatomeas en la columna de agua, su flujo vertical hacia el fondo y preservación en el sedimento. Se diferenciaron diversos periodos de producción alta, media y baja en base a la abundancia de diatomeas en la columna de agua. Las variaciones estacionales de las especies de diatomeas presentes en la columna de agua son indicativas de las condiciones oceanográficas, la abundancia de nutrientes, el aporte fluvial y la estratificación de la columna de agua.
Además, se ha determinado la distribución de diatomeas y componentes biosilíceos en veintisiete muestras de sedimento superficial a lo largo de la ría. Estos organismos se utilizan como trazadores de los procesos hidrográficos y biogeoquímicos que tienen lugar en la ría de Pontevedra. La asociación planctónica marina se localiza en la parte externa de la ría. Las especies principales que se encuentran en esta zona son esporas de Chaetoceros spp., T. nitzschioides, L. danicus R.S. y P. sulcata. La asociación de diatomeas de agua dulce limita su aparición a la ría interna, y especialmente hacia la orilla norte, así como los trazadores de aporte terrestre (quistes de crisófitas y fitolitos), donde se encuentra el mayor aporte fluvial (Río Lérez). Los taxones de origen bentónico se restringen también a las zonas más internas, donde la baja profundidad de la columna de agua permite su crecimiento. El análisis de componentes principales (ACP) nos ha permitido distinguir entre las zonas de la ría de Pontevedra influenciadas por los procesos estuarinos y de aporte de agua dulce de las dominadas por los procesos oceánicos y de upwelling.
Se ha observado una buena correlación entre la comunidad de diatomeas presentes en la columna de agua y la que se encuentra en el sedimento para aquellas especies más importantes. Del estudio de los flujos de sílice biogénica y su acumulación para estas especies y asociaciones se deriva que estos organismos son útiles para la reconstrucción de las características hidrodinámicas y de producción de la ría y ambientes estuáricos a partir de paleo registros. Sin embargo, la no presencia en el sedimento de algunas especies debido a su baja eficiencia de preservación nos pone de manifiesto también las dificultades en la interpretación del registro sedimentario.
La sucesión de diatomeas, su respuesta biológica y los patrones de producción que se relacionan con sus preferencias ecológicas, también pueden ser empleados en otras rías y sistemas costeros templados.
Palabras clave: asociaciones de diatomeas/trampas de sedimento/flujo de diatomeas/sedimento superficial/Ría de Pontevedra /NO Península Ibérica /esporas de Leptocylindrus danicus /esporas de Chaetoceros /Thalassionema nitzschioides/Paralia sulcata
PROCESSES CONTROLLING THE DIATOM PRODUCTION AND
ACCUMULATION IN A WESTERN GALICIAN RÍA: IMPLICATIONS FOR
PALEORECONSTRUCTIONS
1. BACKGROUND AND OBJECTIVES
The coastal ocean and upwelling-influenced continental margins are known to be
among the most productive biological systems in the ocean, accounting for high percentage of
the primary productivity in the marine environment (Walsh, 1991). Generally, upwelling areas
are characterised by the dominance of diatoms, but only a small quantity of their silicified
valves proliferating in the water column are preserved in the sediments (Nelson et al., 1995;
Tréguer et al., 1995; Ragueneau et al., 2000). Time-series sediment trap data are a helpful
tool to clarify the behaviour of biosilica particulate material, since it allows collection of the
settling particles directly, making possible the study of the ecological features and
characteristics of the diatoms settled. Trap and water column studies have shown that,
although there is a good correlation between primary production and downwards fluxes to the
sediment there are different probabilities of being part of the sediments depending on the
species (Sautter and Sancetta, 1992; Romero et al., 2000; Koning et al., 2001; Abrantes et al.,
2002; Bárcena et al., 2004). Little information exists about the dynamics and ecological
characteristics of the particle fluxes, and there is a lack of knowledge about the export of the
biogenic material from the water column to the sediment, especially in estuaries and rías.
These microsiliceous organisms represent a great potential for studies in palaeoclimate
and palaeoceanography research in the coastal marine domain of upwelling areas (Sancetta,
1982) due to their sensitivity to environmental variables (Schuette and Schrader, 1981a, b;
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Hobson and McQuoid, 2001). In fact, the variations observed in the diatom assemblages of
surface coastal sediments can be used to distinguish conditions of upwelling from conditions
of production mediated by continental supply. Moreover, diatom taxa variations in recent
sediments are useful to explain the differences in characteristics of upwelling water masses,
nutrient distribution, currents, rainfall, land-freshwater influence or regional climatic processes
(Sepúlveda et al., 2005).
The high primary productivity of the Galician Rías is attributed to nutrient enrichment
from coastal upwelling which favours phytoplankton growth. Studies on phytoplankton
succession in the Galician Rías (Margalef et al., 1955; Figueiras and Niell, 1987; Varela, 1992;
Varela et al., 2004; Varela et al., 2005) describe a predominance of diatoms due to upwelling
influence. However, their contribution to phytoplankton dynamics and impacts on regional
biogeochemistry are poorly understood yet. Despite of this large body of knowledge of
phytoplankton ecology in the rías, very little is known about the diatom abundance and
biogenic silica distribution patterns in the sediment reflecting present day hydrography and
productivity conditions (Margalef, 1958, Bao et al., 1989; Bao, 1991; Bao et al., 1997; Prego
and Bao, 1997; Bernárdez et al., 2005; Bernárdez et al., 2006). There is a need of connecting
all the information about the composition and structure of the pelagic community, their vertical
flux to the seabed and their record in the sediment. Studies addressed to quantify the record of
the biosiliceous material and its relationship with the pelagic downward fluxes has not been
conducted in a Galician ría yet.
In this paper the contribution of the pelagic biosiliceous particles to the sediment using
the material collected in the traps was estimated. Detailed quantitative analyses were carried
out on particulate material collected in the water column and sediment traps to asses the
seasonal and spatial variability of the diatom assemblages. As a general aim, we study the
present-day primary productivity and silica production, biosilica downward fluxes, and opal
accumulation on the seafloor, in order to ascertain the preservation biases of the assemblages
during settling and within the sediments.
The comparison among the three compartments highlighted the abundance of the
siliceous compounds in the superficial sediment and its relation with the hydrography. The
specific aims of this paper are as follows: (1) To compare the diatom content in the traps and
water column with those preserved in the surface sediments and attempt to extrapolate results
for paleoceanographical and paleoproductivity interpretations. (2) To contrast the results with
the hydrography and primary production patterns also described by other authors. The study
compiles the extents and limitations of fossil diatom distribution in surface sediments as
tracers of hydrography and oceanography focusing on the interpretation of these organisms as
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Chapter IV
paleoclimatic tracers down-core. Pelagic origin of some silica compounds was assessed to
check the importance of differential preservation of the diatom species. (3) To test the power
of the diatom taxa recorded in the surface sediments as proxies for paleo-conditions and later
reconstructions. To our knowledge, this is the first study carried out in a ría regarding the
composition of the biosiliceous elements in the surface sediment and its relation with diatom
composition in the water column and sediment traps.
2. REGIONAL SETTING
The Galician rías (NW Iberian Peninsula) are a set of estuary-type coasts best
described as drowned river valleys formed by sea flooding during Pleistocene-Holocene
transgression (Vilas, 2002), where the estuarine zone may migrate longitudinally depending on
the extent of climatic changes (Evans and Prego, 2003). The Ría de Pontevedra, belonging to
the group of Rías Baixas, is one of these funnel and V-shaped embayments. It is oriented in a
SW–NE direction and widens progressively from the Tambo Island toward the mouth (Figure
IV.1). The ría exhibits different characteristics according the degree of estuarine-marine
influence, hydrodynamic and sedimentologic characteristics, and can be divided in several
sectors. The outer zone is communicated with the shelf by two entrances due to the presence
of Ons Islands at the open sea entrance. This area presents mostly a sandy cover, with high
abundance coarse carbonate-rich sediments (Vilas et al., 2005) (Figure IV.1).
The inner part can be considered as an estuary from both hydrographic and
sedimentological considerations, with the main estuarine processes being confined to the
inner relatively small brackish water zone (Evans and Prego, 2003). In a sedimentological
point of view, this zone is characterized by the presence of fine-grained and organic-rich
sediments also present in the longitudinal axis even in the outer ría (Vilas et al., 2005).
At the ría head Lérez River provides the main freshwater runoff which flows into the
estuary, depending on the rainfall pattern (monthly average discharge 2–80 m3 s-1, peaks ~120
m3 s-1 (Prego et al., 2001; deCastro et al., 2006a).
Hydrography of the ría is very well known nowadays, acting as a partially mixed estuary
with a double-layered residual pattern (Prego et al., 2001; Gómez-Gesteira et al., 2001; Ruiz-
Villarreal et al., 2002; Álvarez et al., 2003; Gómez-Gesteira et al., 2003; deCastro et al., 2004;
Dale et al., 2004). As with the rest of the Rías Baixas, circulation is driven by the entrance of
oceanic water masses when northerly winds are persistent and upwelling occurs in spring and
summer (Fraga, 1981), the freshwater input at the head of the ría during rainy periods, tidal
forcing and the wind regime, which is mainly SW or NE (Gómez-Gesteira et al., 2001). Tidal
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
forcing on the ría circulation is only observed in the innermost ría (deCastro et al., 2000;
Gómez-Gesteira et al., 2001; Prego et al., 2001) resulting in a inversed circulation pattern.
Figure IV.1. Schematic illustration of the physiography and surface sample locations of the Ría de Pontevedra. Map shows the location of the bed sediment sampling (black circles) and grain size distribution (modified from Vilas et al., 2005). White diamonds indicate the stations were sediment traps were moored and the water column phytoplankton sampling carried out. Depth contours in metres.
Nutrient fluxes into the ría were strongly driven by the incoming oceanic flow throughout
the year (Dale and Prego, 2005), exhibiting a pronounced seasonal variation. In late spring
and summer, upwelling favourable winds are usual, and Eastern North Atlantic Central Water
(ENACW) is upwelled into the ría, renewing nutrients (Tenore et al., 1982). At other times of
the year terrestrial runoff is the dominant nutrient supply. The ecological impact of fertilization
by nutrients in the ría is high primary productivity (Tilstone et al., 1994).
Phytoplankton blooms in the rías occur in spring (mainly of large diatoms) and autumn
(mixed populations of diatoms and dinoflagellates) (Figueiras and Niell, 1987). However, the
highest phytoplankton biomass is usually observed during summer due to the effect of
upwelling (Campos and Mariño, 1984; Figueiras and Niell, 1987; Varela et al., 2001; Varela et
al., 2004). Most of the phytoplankton growth inside the rías is induced by regenerated nutrients
from organic matter in shelf waters that re-enter in the rías when upwelling occurs (Fraga,
1981; Prego, 1994; Álvarez-Salgado et al., 2000). Primary production in the Ría de
Pontevedra varies considerably depending on the balanced influence of upwelling and runoff.
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Chapter IV
It is characterized by the export of large quantities of organic carbon and organic materials
(Varela et al., 2004), and their record in the sediment is susceptible of reflecting
oceanographic conditions.
In this way, the Ría de Pontevedra constitutes an interesting area of study due to the
following reasons: i) the ría, like most of the estuaries in the western coast of the Iberian
Peninsula, is under the effect of strong upwelling or downwelling episodes, which reinforce or
stop the estuarine positive circulation and the primary productivity, ii) Seasonal changes in
hydrography are closely related to changes in productivity, iii) The river flowing at the head
leads to a large mixing zone between continental and oceanic waters.
3. MATERIAL AND METHODS
3.1. Location and sampling
Water column sampling was carried out fortnightly between February and June 1998 on
board the RV ‘Mytilus’. Seawater samples for diatom cell counts were collected at Stations O,
M and I with Niskin ‘General Oceanic’ bottles at discrete depths of 0, 5, 10, 20, 30 and 50 m
(depth permitting and 5 m above the seafloor) (Figure IV.1).
Water column particulate material was collected with a multitrap collector system
(Knauer et al., 1979) deployed 5 m above the sea bottom and anchored to the seafloor at the
same water column stations (O, M, I) for periods of ~24 h (Table IV.1). The trap system
consisted of 4 bound Plexiglas tubes (6 cm diameter) filled with preservative-free filtered
seawater. NaCl (35 g) was added to each tube to raise the salinity and prevent the exchange
of material with surrounding water and particle loss (Knauer et al., 1979; UNESCO, 1994).
Degradation of the trapped material is supposed to be limited due to the short period of time
developed. Trap deployment was intended to span the whole survey period, although logistical
problems limited the collection period to 5 months (February to June 1998) covering the most
important oceanographic periods, winter mixing, spring bloom and upwelling.
Table IV.1. Mooring positions and water depth.
Sediment traps Longitude W Latitude N Water depth (m)
Inner I 8.739 42.392 45 Middle M 8.792 42.365 36 Outer O 8.868 42.350 25
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
For the determination of the biosiliceous material abundance in the sediment, fieldwork
consists of a sediment sampling cruise covering all the ría. 27 surface sediment samples were
collected using a Shipek grab and were taken from the uppermost oxic layer with the help of a
syringe (Figure IV.1).
3.2. Procedures and analytical strategies
Diatom identification and cell counting of the material collected in the traps and in the
water column were performed in samples preserved with Lugol’s solution to be examined
under a Nikon Eclipse TE 300 inverted microscope following the technique described by
Utermöhl (1958). Organisms were identified and counted using a magnification of 40×, 100×
and 250× and 1000×. The nomenclature and taxonomic identification for species followed that
of Tomas (1997). Results are expressed as diatoms m-2 day-1, for the sediment traps, and cells
(diatoms) l-1, for water column. Results are also expressed as relative abundance of each
species averaging the complete sampling period.
The procedure for the treatment of raw material for biosiliceous counts follows the
method devised by Abrantes et al. (2005). A microscope (LEICA DMLB and a Nikon
microscope) with phase contrast optics and a magnification of up to 1000× was used for
qualitative and quantitative analyses and diatom identification. Several non-overlapping
transverses covering both central and marginal zones of the cover slip were examined
depending on the diatom abundance. In general, at least 300 diatom valves were identified in
each sample to ensure proper assessment of diatom abundance (valves g-1) and composition
when possible, and raw counts were then converted to percent abundance. In each slide
specimens were identified to the lowest taxonomic level possible. In a few cases, difficulties
arose in distinguishing between two or more species and in consequence, species were
combined in one counting group or genera group. Taxonomic identification and grouping is
based on ecological characteristics as stated in well-known bibliographies and was made
using several floras (Hustedt, 1930; Hustedt, 1959; Round et al., 1990; Hartley, 1996; Hasle
and Syvertsen, 1996; Witkowski et al., 2000; see Taxonomic appendix). Schrader and
Gersonde’s (1978) recommendations were followed for diatom counts and total number
estimates, therefore only diatoms that were essentially whole were counted.
The numerous taxa were gathered into various groups, which include three different
types: dominant taxa, species of the same genus living in similar environments; and species
showing comparable ecology and distribution, e.g. freshwater and benthic (see Taxonomic
appendix). The freshwater diatoms appearance is due to the transport from the main
tributaries draining the ría. The benthic group included epiphytic, episammic, epilithic and
epipelic species that live attached to a substrate at depths under the euphotic zone. Their
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Chapter IV
occurrence reflects inner ría conditions, that is, relatively low salinity and water depths. Paralia
sulcata was not included in this group because this species is tychopelagic. Number of diatom
fragments, silicoflagellates, sponge spicules, palynomorphs, phytoliths, crysophycean cysts
and radiolaria per gram of sediment was also assessed.
Siliceous microfossils and compounds distribution were plotted using the Golden
Software Surfer 8.0 package for Windows, using the Kriging method for data interpolation.
Geochemical data of the superficial sediment, as well as the experimental procedures
were obtained from the paper published by Dale and Prego (2002), including the content of
total nitrogen (TN), total organic carbon (TOC), total inorganic carbon (TIC), calcium
carbonate, and opal (biogenic silica or BSi) (Table IV.2).
Statistical analysis of the dataset obtained was carried out using the software package
SPSS 13.0.1 (LEAD Technologies) for Windows.
4. RESULTS
4.1. Diatom assemblages in the water column: Seasonal patterns
Seasonal variations of diatom groups are shown in Figure IV.2. Value represented is an
average of the absolute abundances at different depths. Several stages of high, moderate and
low abundance of diatoms have been recognized in the period studied. High diatom
productivity was recorded on February 25th, May 12th and June 23rd. Especially high diatom
abundance was recorded in May 12th, with values higher than 20×106 diatoms l-1 in the three
stations. Periods of moderate diatom abundances included February 11th, March 3rd and 24th
and April 27th. Low production periods coincided with April 13th, May 26th and June 10th. On
average, diatom abundance varied between 4.8×106 cells l-1 for the outer station, 4.7×106 for
the middle station and 10.5×106 in the inner station. Higher concentration in the sampling
period was found in the inner ría, being always higher in the different productivity periods
studied. For the stations O and M, diatom abundance during high production period are 2.6 to
2.2 times higher than the mean, but for the innermost station this value only reaches 1.6. High
diatom abundances persisted in the inner zone even during low production periods.
Due to the large quantity of data, we have considered only the most representative
species and taxa.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Tabl
e IV
.2. S
edim
ent c
hara
cter
izat
ion
at th
e sa
mpl
ing
site
s. D
ata
from
Dal
e an
d Pr
ego
(200
2).
94
Chapter IV
Figure IV.2. Temporal variations of the standing stocks of total diatoms and abundance of the main diatom groups in the water column (cell l-1) at the three sampling sites. Abundance mean values of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom abundance.
Chaetoceros spp. are the dominant taxa, accounting 60–70% of the assemblage in the
three stations studied. Low abundances are found on February 11th, March 11th and June 10th
for the outermost stations and also in February and June at the innermost station. Relative
abundance increases during high production periods, reaching the 90%. In the innermost
station relative abundance is low during the high production stages in comparison with the
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
moderate and low production periods. The contribution of the Chaetoceros resting spores
(R.S.) in the water column is very low at all stations, excluding June 23rd and February 25th
(high production periods).
Figure IV.2. (cont.) Temporal variations of the standing stocks of total diatoms and abundance of the main diatom groups in the water column (cell l-1) at the three sampling sites. Abundance mean values of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom abundance.
Leptocylindrus danicus is one of the main components of the assemblage, peaking on
June and ranging from 20 to 83% in all sites and on May 26th at the O and M stations. This
species represents an important contribution to the assemblage during low diatom production
periods at all stations, reaching the 50% of their abundance in the outer station.
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Chapter IV
Thalassionema nitzschioides shows a low percentage over the complete period, except
for April at the stations O and M, when a significant percentage, varying from 3 to 16% was
found. At the sampling site I, relative high percentages are found on February 10th (6.3%) and
April 13th (4.6%). Percentage of this species in the low production periods is higher than in the
productions peaks, especially in the outermost stations.
Skeletonema costatum always peaks during high production periods on May and June
23rd, and also on February 10th at the stations M and I. This species constitutes a high fraction
of the total assemblage during elevated diatom production, ranging from 10.1 to 14.3%, and
being almost absent the rest of the period studied.
Rhizosolenia spp. only peak in the outermost stations (O, M) during the February 10th
representing around 18–22% of the total assemblage. On June 6th it represents a relative high
percentage (around 7-8%) in all sampling sites. Averaging all the sampling period, relative
contribution is very low, down to 2.3%. Relative contribution is constant and non-dependent of
the diatom production.
Thalassiosira spp. do not show high abundances. Relative percentage is about 4–8%
on February 10th and especially, during a high diatom production sampling date, on May 12th.
This genus presents lower abundances in the water column at the innermost sampling site in
comparison with the stations O and M.
Freshwater diatoms, the benthic assemblage and the tycoplanktonic species, Paralia
sulcata, appear in winter, in February, and also during April, coinciding with periods of high
runoff, sediment re-suspension and high turbidity.
4.2. Diatom assemblages in the sediment traps: Seasonal patterns
Downward fluxes of diatoms were highly variable during the whole sampling period
(Figure IV.3), showing a prominent peak on February 24th (2600–5700 cells m-2 day-1) and high
values on May 12th (1100–1800 cells m-2 day-1) and June 23rd (2000–3800 cells m-2 day-1).On
average, maximum vertical diatom fluxes are recorded at the innermost station (1500 cells m-2
day-1). Maximum fluxes are found during high production periods, 40 times higher than in
moderate and low stages. In general, the production periods in the water column correlate
quite well with the vertical biogenic flux to the seabed recorded in the sediment traps. The
main flux events found are due to the high proliferation of Chaetoceros spp.
Strong fortnightly variations in the diatom flux and species flora were found in the
sediment traps. Regarding species composition, no important differences in fluxes and relative
contribution were observed between stations, excepting for some distinct patterns as
explained below.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Figure IV.3. Seasonal patterns of the total diatom and vertical fluxes of the main diatom community groups in the sediment traps (diatom m-2 day-1) at the inner, middle and outer sampling sites (I, M, O). Mean values of the vertical fluxes of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom downward flux.
A few species dominate the assemblage throughout the sampling period. Chaetoceros
spp. are the main contributor of the diatom flux, as observed also in the water column.
Chaetoceros group peaks on February 25th and on March 11th and June 23rd. Its abundance is
especially low during the first week of February and April. Daily flux of this species is high
98
Chapter IV
during high production accounting ~80% of the assemblage. The main contributor of the
Chaetoceros group to the sediment traps are their resting spores, representing even the 97%
of the total diatom concentration. Very low diatom sedimentation rates were recorded on 10th
June coinciding with the disappearance of this group in the traps and the prevalence of other
species.
Figure IV.3. (cont.) Seasonal patterns of the total diatom and vertical fluxes of the main diatom community groups in the sediment traps (diatom m-2 day-1) at the inner, middle and outer sampling sites (I, M, O). Mean values of the vertical fluxes of each diatom group or species during high, moderate and low production phases throughout the sampling period are also shown. Note the logarithmic scale in the diagram for the total diatom downward flux.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
L. danicus exhibits very low abundances and fluxes during the period studied. As
observed in the water column, higher fluxes were recorded during low productivity periods.
Two peaks on March 24th and May 26th were observed at the outermost station and one peak
on May 26th at the station M. L. danicus is almost absent at the innermost station, excepting on
March 11th. In general, the contribution of this species is higher during low diatom production.
T. nitzschioides is the second component of the diatom species found in the sediment
traps. This species peaks on April 27th at the three stations, accounting between 33 and 73%
of the assemblage, and also on February 10th at the station I. The contribution of this species
to the total diatoms is high, varying between 10 to 20% when diatom abundance is low.
Vertical fluxes to the sediment are low when production decreases. In general, excluding some
peaks, the vertical flux is roughly constant through time.
S. costatum also contributes in a significant portion of the species studied, but only
during short periods of time, appearing on May, when diatom production is high. In the
innermost station its contribution is almost negligible, increasing at the outermost stations.
Rhizosolenia spp. represent on average between 3% (station O) and 10% (site I) of the
assemblage. Maximum concentrations occur on June accounting up to 33% at the station I. Its
contribution is also important on February 11th at the station O. At the station M, its mean
relative abundance is constant and non-dependent on the production period, but in general is
better represented during high production periods. Vertical fluxes coincide with high
percentages of this genus, being higher on average at the innermost station, as observed for
other species.
Thalassiosira spp. represent a significant proportion of the total assemblage. However,
daily fluxes are very low in comparison with other species and groups. Seasonal pattern shows
strong variations in abundance and flux. In the outer and middle stations their relative
abundance is significantly higher during low/moderate production periods, accounting 6–10%
during April and February 11th. Higher contribution of this species was found at the station I
during May.
P. sulcata appears occasionally in the sediment traps, especially on April, even though
its relative abundance can reach values of 14%.
Benthic group is quite well represented in the sediment traps, on average, relative
contribution is very low 0.5–1.2%. Higher contribution during low production periods in winter
(up to 10%) is due to the input of superficial sediment in the trap.
Freshwater assemblage is poorly represented, but shows two distinctive peaks on June
10th at the outermost station and on April 27th at the sampling site I. As observed for the
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Chapter IV
benthic taxa, on average, its contribution to the total assemblage is very low, as well as
downward fluxes.
4.3. Diatom assemblages in the surface sediment: Species encountered
The distribution pattern of diatom abundance (Figure IV.4) is characterized by the
elevated values in the middle an inner ría, as the number of valves per gram is especially high
in an area parallel to the northern coast in the inner ría. The outer ría is poor in diatoms, with
absolute abundances between 6.8×105 and 2.4×105 valves g-1 (Table IV.3a).
Table IV.3a. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the diatom valves and fragments of diatoms.
Station Slide fraction
Counted valves Diatom valves Diatom fragments
×105 % ×105 % 8 1/6 397 14.7 27.8 20.5 38.9 9 1/10 254 15.6 28.7 21.4 39.4 10 1/4 522 12.8 24.5 16.1 30.7 11 1/6 412 15.2 24.8 17.8 29.0 12 1/7 313 13.5 22.5 14.1 23.5 13 1/3 252 4.6 26.0 3.2 18.2 14 1 320 1.9 36.2 0.7 12.9 15 1/2 292 3.6 21.7 2.4 14.5 16 1/5 222 6.8 19.2 4.3 12.2 N1 1/3 266 4.9 26.6 8.7 47.3 N2 1/6 359 13.2 22.3 27.1 45.6 N3 1/6 328 12.1 24.3 22.5 45.2 N4 1/8 486 24.0 19.7 59.9 49.0 N5 1/8 458 22.6 43.3 14.3 27.4 N6 1/2 211 2.6 24.5 2.3 22.1 N7 1/2 290 3.5 27.4 1.9 14.6 N8 1/3 458 8.4 27.3 3.6 11.7 N9 1/3 420 7.7 50.8 1.8 12.0 N10 1/3 132 2.4 24.1 0.9 9.5 S2 1/2 345 4.2 23.9 8.5 47.9 S3 1/3 540 10.0 33.0 13.0 42.9 S4 1/8 493 24.3 32.3 32.3 43.0 S5 1/3 414 7.6 33.2 4.4 19.2 S6 1/4 484 11.9 35.0 6.2 18.3 S8 1/4 324 8.0 27.1 5.7 19.4 S9 1/5 0 0 0.0 0.1 30.8 S10 1/6 391 14.4 48.6 5.3 18.0
Mean 10.0 28.0 11.8 27.5 Max 24.3 50.8 59.9 49.0 Min 0 0 0.1 9.5
Moreover, lowest diatom concentrations were found at the stations located at the ría
mouths. Diatom absolute abundances in surface sediments ranged between 1.98×105 (station
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
14) and 24.3×105 (station S4) valves g-1 with a median value of 10×105 (Table IV.3a). These
values are in the range reported in nearby areas (Bao, 1991; Bao et al., 1997, Prego and Bao,
1997; Abrantes and Moita, 1999). The contribution of the diatoms (whole valves and
fragments) to the bulk biogenic silica content in the sediments is ~55%, ranging from around
31% to about 76% (Table IV.3a).
Diatom community is mainly composed by Chaetoceros resting spores (R.S.) ranging
from 13% (station N1) to 86% (station N5). Among the several types of Chaetoceros R.S. four
stand out as the main contributors in the sediment: R.S. C. affinis, C. diadema, C. compressus
and Chaetoceros sp1. Vegetative frustules of Chaetoceros spp. were scarce in the sediment.
Highest percentages of resting spores were located in the middle-outer areas of the ría and
low values were found at the innermost sampling stations (Figure IV.4).
P. sulcata is the second main component of the assemblage (Figure IV.4) with highly
variable percentage values between 0–1 up to 20% and presents a scattered distribution
pattern throughout the ría. Higher abundances were found especially in the external ría and
also along the longitudinal axis, whereas this species is absent at the stations located close to
the Lérez River mouth.
The third species in importance is T. nitzschioides (Figure IV.4). Relative abundance
was within the 2.3–10% range. Distribution was similar to that found for Chaetoceros R.S.
Maximum percentages were located in the outermost stations, but also, relatively high
concentrations are found in the innermost ría at the southern margin.
L. danicus R.S. have common abundances around 1–4%, peaking at the station 15
(17.8%). Its distribution is roughly uniform throughout the ría, excepting that station with the
maximum percentages. It is almost disappeared at the innermost stations and at the northern
margin of the external ría (Figure IV.4).
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Figure IV.4. Contour plots of the relative abundance of the main diatom species found in the superficial sediment of the Ría de Pontevedra.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Genus Thalassiosira was found in very low abundances (0–3.3%), with a
heterogeneous distribution throughout the ría. Higher concentrations were found at the inner
ría and in the northern shore (Figure IV.4).
S. costatum as well as Rhizosolenia spp. genera were very scarce in the surface
sediment. However, we show their distribution maps for comparison with their presence in
water column and traps. Although their spatial pattern is scattered, in general, S. costatum
was found in the southern margin of the inner ría and Rhizosolenia spp. at the northern mouth
in the external stations (Figure IV.4).
The contribution of benthic group is very important in the shallowest stations from the
inner ría (20–62%) with poor occurrences at the external ría (Figure IV.5). Pattern distribution
of this taxa mirrors that found for the freshwater flora, with the exception of the freshwater
peak off Couso Point. The shift from the planktonic-diatom taxa dominance to the freshwater-
benthic diatom assemblage is located at the station 12 (Figure IV.5). Freshwater assemblage
is restricted to the stations close to the ría head, at the Lérez River mouth, and especially in
the northern coast (~10%), decreasing the relative abundance offshore (~6%). Limit of this
influence is located around station 12. However, high percentages of the freshwater species
are found in the station 16 located in the outer ría (Figure IV.5).
4.4. Other biosiliceous components
Silicoflagellates, radiolarians, crysophycean cysts, porifera, phytoliths and the siliceous
dinoflagellate Actiniscus pentasterias were the major constituents of the other siliceous
biogenic particles found in the sediments (Table IV.3b, c, Figure IV.5). Main contributor to
biogenic silica (BSi) in the sediment are diatom valves and fragments. The porifera were the
second component, averaging 17.2%, up to 36% of the biosiliceous components. The third
contributor were the phytoliths (average 14.4%) followed by cingulum (diatom girdle bands)
(mean 11%) and crysophycean cysts (Figure IV.5). Silicoflagellates, A. pentasterias and
radiolarians were secondary components of sedimentary BSi.
Porifera abundance varies from 1.4×106 to 1.2×104 specimens g-1, with higher values in
the middle zone and at the southern mouth.
Phytoliths abundance is high, ranging from 1.9×106 to 3×103 specimens g-1. Highest
concentration was located at the innermost sampling stations from the northern coast.
Crysophycean cysts are recorded in higher abundances at the inner ría, (1.4×105 to 1.6×104)
resembling the pattern distribution found for phytoliths. Since both parameters are of
continental origin, their presence in marine sediments implies and aeolian or fluvial transport
to the oceanic domain.
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Figure IV.5. Contour plots of the abundance per gram of sediment of the crysophycean cysts, phytoliths and relative abundance of the benthic and freshwater diatom groups.
Silicoflagellates were present in low abundances, ranging from 0 to 2×104 specimens g-1.
Higher values were located in the inner-middle ría and they were not present in the
longitudinal axis at the outer ría. A. pentasterias only appeared in the inner part of the ría and
also in three stations located at the mouths. Radiolarians content was also very low,
concentrating their appearance in the northern margin of the ría and in the outer zone. Pollen
forms were scarce all over the ría, however, they were present at the ría mouth, but the
highest abundances about (2×104–4×104 specimens g-1) were found in the innermost area.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Table IV.3b. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the porifera spicules, phytoliths and crysophycean cysts.
Station Porifera Phytoliths Cingulum Crysophycean cysts
×105 % ×105 % ×104 % ×104 % 8 1.7 3.4 13.1 25.0 18.5 3.5 7.0 1.3 9 2.2 4.1 12.0 22.1 14.1 2.6 14.1 2.6 10 8.0 15.4 11.4 21.7 30.6 5.8 8.1 1.5 11 11.8 19.3 10.5 17.2 54.0 8.8 4.8 0.8 12 13.9 23.2 11.9 19.9 55.3 9.2 6.9 1.2 13 5.6 31.3 2.6 14.5 15.5 8.7 1.8 1.0 14 1.6 29.5 0.6 12.3 4.2 7.8 0.4 0.9 15 5.1 31.1 3.4 20.7 18.6 11.2 0.7 0.4 16 11.9 33.5 7.3 20.6 49.3 13.8 0.9 0.3 N1 0.6 3.6 3.5 19.0 5.3 2.9 0.7 0.4 N2 2.8 4.7 12.9 21.7 30.7 5.2 2.9 0.5 N3 3.4 6.8 8.7 17.6 26.2 5.3 4.0 0.8 N4 7.8 6.4 18.8 15.4 106.1 8.7 6.9 0.6 N5 6.1 11.7 3.2 6.1 47.9 9.2 9.3 1.8 N6 2.4 22.9 0.8 8.1 23.2 21.8 0.3 0.3 N7 3.5 26.9 1.6 12.7 22.0 16.9 0.3 0.3 N8 8.7 28.2 4.9 15.9 50.0 16.1 1.4 0.5 N9 2.4 16.1 0.9 6.4 21.6 14.2 0.3 0.2 N10 3.6 36.0 1.1 11.3 18.8 18.6 0 0.0 S2 0.5 3.3 3.3 18.9 9.1 5.1 1.4 0.8 S3 1.6 5.5 3.1 10.3 22.9 7.6 1.6 0.5 S4 4.9 6.6 4.0 5.3 87.9 11.7 4.4 0.6 S5 2.9 12.6 2.1 9.2 57.5 24.9 0.9 0.4 S6 4.9 14.5 3.7 10.9 68.8 20.2 2.7 0.8 S8 7.4 25.2 3.2 11.1 50.3 17.0 0.4 0.2 S9 0.1 30.8 0.03 7.7 1.2 30.8 0 0.0 S10 3.1 10.6 2.2 7.7 42.2 14.2 2.5 0.9
Mean 4.8 17.2 5.6 14.4 35.3 11.9 3.1 0.7 Max 13.9 36.0 18.8 25.0 106.1 30.8 14.1 0.6 Min 0.1 3.3 0.03 5.3 2.6 0 0
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Table IV.3c. Absolute abundances per gram of dry sediment and relative percentage (with respect to BSi material) of the radiolarians, silicoflagellates, the dinoflagellate A. pentasterias and palynomorphs.
Station Silicoflagellates A. pentasterias Radiolarians Palinomorphs
×103 % ×103 % ×103 % ×103
8 3.7 0.1 0 0.0 0 0.0 3.7 9 18.5 0.3 6.1 0.1 6.1 0.1 30.8 10 7.4 0.1 9.8 0.2 0 0.0 2.4 11 7.4 0.1 7.4 0.1 0 0.0 0 12 8.6 0.1 12.9 0.2 4.3 0.1 0 13 0 0.0 3.7 0.2 1.8 0.1 7.4 14 0.6 0.1 1.2 0.2 0 0.0 0.6 15 1.2 0.1 2.4 0.1 2.4 0.1 11.1 16 6.1 0.2 9.2 0.3 3.0 0.1 18.5 N1 0 0.0 0 0.0 1.8 0.1 3.7 N2 0 0.0 0 0.0 3.7 0.1 14.8 N3 0 0.0 3.7 0.1 0 0.0 7.4 N4 9.8 0.1 14.8 0.1 0 0.0 9.8 N5 14.8 0.3 4.9 0.1 4.9 0.1 0 N6 1.2 0.1 1.2 0.1 0 0.0 1.2 N7 2.4 0.2 9.8 0.8 3.7 0.3 6.1 N8 3.7 0.1 5.5 0.2 0 0.0 1.8 N9 0 0.0 3.7 0.2 0 0.0 11.1 N10 1.8 0.2 1.8 0.2 0 0.0 5.5 S2 0 0.0 0 0.0 0 0.0 6.1 S3 5.5 0.2 0 0.0 0 0.0 5.5 S4 19.7 0.3 14.8 0.2 0 0.0 0 S5 5.5 0.2 3.7 0.2 0 0.0 12.9 S6 2.4 0.1 7.4 0.2 0 0.0 0 S8 0 0.0 0 0.0 0 0.0 4.9 S9 0 0.0 0 0.0 0 0.0 0 S10 0 0.0 0 0.0 0 0.0 0
Mean 4.4 0.1 4.6 1.1 0.04 6.1 Max 19.7 0.3 14.8 6.1 0.3 30.8 Min 0 0 0 0 0 0
4.5. PCA analysis: Relationships among diatoms, biosiliceous compounds and geochemical features
To investigate the covariability between the different diatom species, biosiliceous
compounds and geochemical parameters of the sediments a principal component analysis
(PCA) was carried out. Prior to multivariate analyses, dataset was first studied on the basis of
linear correlation between variables. Close relationships were found between the benthic and
freshwater assemblages with the terrestrial input indicators (crysophycean cysts and
phytoliths). Also these variables have a high positive linear correlation with the organic carbon
and nitrogen content and negative with the carbonate content. TOC/TN ratio in the sediment is
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
also considered as a proxy of organic matter origin, and as shown by Dale and Prego (2002)
higher values were found in the inner ría. Opal content has a good correlation with P. sulcata,
porifera, diatom fragments and phytoliths. However, diatom content is well correlated with the
diatom fragments, TOC, TN and crysophytes. With respect to diatoms Chaetoceros R.S. has a
significative positive correlation with T. nitzschioides and highly negative with the freshwater
and benthic assemblage. L. danicus correlates well with P. sulcata.
Figure IV.6. Scatterplot with the factor loadings extracted using the R-mode principal component analysis. Each variable is represented as a point.
We used a R-mode factor principal components analysis (PCA) to analyze the
distribution of diatom species (3% frequency in at least one sample) and geochemical (TN,
TOC, opal and CaCO3) and some biosiliceous compounds (phytoliths, crysophycean cysts,
porifera and diatom valves abundance) among sites. Variables were standardized to mean
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zero and standard deviation one for each species and variables, having a similar weight, and
permiting the comparison of their variance.
PCA analysis returned two principal component factors, which explained 54.6% of the
data. The PC1 (contribution to the total variance is 34.09%) loads positively mostly of benthic
and freshwater assemblages, crysophycean cysts, phytoliths and TOC and TN concentrations.
Chaetoceros R.S., T. nitzschioides and carbonate content have negative factor loadings
(Figure IV.6). S. costatum is ecologically linked also to the low salinities and high nutrient
levels from river run-off, but their low abundance in the sediment leads to slight dependence
on the PC1 factor (0.246).
The second component PC2 explained the 20.51% of the variance. Opal is the first
scorer of this factor and porifera the second in importance, with a smaller contribution of
Chaetoceros R.S. and P. sulcata species (Figure IV.6).
5. DISCUSSION
Diatom water column abundance and flux data have been assessed in several sampling
periods, i.e., winter, spring bloom, upwelling and non upwelling conditions between February
and June. In terms of productivity the sampling covers the most important periods in the ría:
the spring bloom and upwelling season. Diatoms produced during the bloom periods are
preserved with a high efficiency (Nelson et al., 1995), although the biogenic silica seabed
preservation efficiency depends on variable processes such as bioturbation, sedimentation
rate, thermodynamics (Ragueneau et al., 2000). In consequence, spatial and seasonal
variability of the diatom record is blurred by these processes.
5.1. Water column and sediment trap data vs. diatoms record in the sediment: Implications for paleoenvironmental reconstructions
Mean relative abundances of dominant diatom species in the water column and in the
sediment traps were compared to the relative contribution of each species and groups in the
underlying surface sediments (Figure IV.7).
Fortnightly sediment trap records show that species abundances and assemblages
change depending on the oceanographic processes and environmental conditions. Moreover,
species composition also varies depending on the ría area where trap was placed. This fact
also has consequences in the recent sedimentary record.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Figure IV.7. Mean relative abundance of dominant diatoms in water column (cell ml-1), sediment traps (diatoms m-2 day-1) and surface sediments (valves g-1×103) throughout the sampling period. Average value of the same taxa accumulated in surface sediments for each station.
Data shows that the most important differences in the relative contribution of each
species are always related with the innermost station I. Nutrient salts were always presents in
this part of the ría due to river input and/or runoff, leading to constant and high biosiliceous
production.
The relative abundance of the main diatom species and groups and biosiliceous
components studied showed significant differences between the water column, traps and
surface sediments (Figure IV.7). Regarding species composition in the three domains studied
the following observations are of interest:
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Peaks in Chaetoceros abundance at the water column and vertical fluxes in the
sediment traps in May is due to the high input of nutrients due to river runoff in April (deCastro
et al., 2000; Álvarez et al., 2006) combined with favourable upwelling conditions during the
second week of April and May (Prego et al., 2001). Moreover, high relative contribution of this
species from the second week of February is related to the development of unusual upwelling
conditions. Also, high contribution of this species is related to the upwelling influence and the
fertilization by nutrients by ENACW, especially in summer. However, lack of northerly winds
driving upwelling in June (Prego et al., 2001) leads to a low abundance of this species and the
proliferation of other taxa.
This group has a high preference for conditions of high productivity (Rojas de Mendiola,
1981; Abrantes and Sancetta, 1985; van Iperen et al., 1987; Abrantes, 1988; Pitcher, 1990;
Sautter and Sancetta 1992; Lange et al., 1998; Abrantes and Moita, 1999; Romero et al.,
1999a; Koning et al., 2001; Abrantes et al., 2002). Chaetoceros resting spores are indicative of
late summer/fall conditions with low nutrients, a phenomenon that is also recorded in a
sediment trap study (Lopes et al., 2006). Since this species is the dominant component of the
assemblage in the water column and sediment traps during high production periods and also
in the sediments (Figures IV.2 and IV.3) we conclude that this BSi reflects the high production
conditions in the surface waters, usually due to upwelling (Bao et al., 1997). Chaetoceros R.S.
dominance reflects the importance of the high primary production associated with fertilization
by nutrients when hydrographic conditions are adequate under turbulent conditions when
stratification is broken by upwelled waters. In fact, Rodríguez et al. (2003) found the highest
abundance of diatoms between May and October during 1999 at a fixed station of the outer
ría, and the most abundant species was C. socialis.
A sharp drop in Total Chaetoceros abundance is detected in the sediment traps, being
even lower in the sediment (Figure IV.7). This genus is composed by fragile chain-forming
diatoms rarely found in the traps and sediments in the form of vegetative cells (Bao et al.,
2000). However, with nutrient deficient and stressful conditions in the euphotic zone after the
upwelling, they form resting spores (Margalef, 1978; Rines and Hargraves, 1988; Pitcher,
1990), which are very resistant to dissolution and become a major component of both the trap
and the surface sediment assemblage (Blasco et al., 1981; Koning et al., 2001). A detailed
view of the Chaetoceros composition shows that there is an increasing percentage and also
abundance of resting spores in the sediment traps reaching up to 78%. The content of resting
spores is indicative of the production of vegetative cells during fertilization by nutrients,
especially during upwelling. In the surface sediment, values were between 44–60%, being
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
highest at the station M. A slight loss of resting spores in the sediment was detected or even
an increase of the relative percentage of other species.
Thalassiosira group shows a progressive decay of their content from the surface waters
to the sediment (Figure IV.7). At the innermost station relative percentage accounts 4.9% in
the sediment traps, dropping to 1.7% in the sediment. Vertical fluxes of this genus were very
low in comparison with other species or groups, indicating the low preservation efficiency of
this group within the water column (Margalef, 1958). Moreover, burial efficiencies for this
weakly silicified group are very low or zero being present in very low percentages of the
sediment assemblage.
Thalassiosira spp. show two peaks the first week of February after the unusual winter
upwelling conditions (Álvarez et al., 2003) and in May when upwelling starts (Prego et al.,
2001), especially at the outermost stations (Figure IV.2). This group proliferates on winter
throughout the Portugal coast reflecting more persistent fertility conditions and continuous
nutrient input (Abrantes and Moita, 1999) decreasing their abundance when fertilization by
upwelling occurs and Chaetoceros spp. dominate. In this way, this species is indicative of both
processes, dissolution of frustules in the water column and constant fertilized water column,
and their use in sediments as a paleoindicator should be took carefully.
Rhizosolenia spp. show a similar percentage in the water column and sediment traps
(~2-3%), with values up to 9.8% at the sediment trap located at the innermost sampling site
(Figure IV.7), but showed a strong decrease in the top sediment (Figures IV.4 and IV.7). This
is the result of the strong dissolution of the weakly silicified frustules. Apparently the
dissolution occurs at the sediment-water interface before being buried, but strong dissolution
processes in the water column and within the sediment can not be discarded.
Ecological requirements of this genus are linked to pre-upwelling conditions by the end
of winter and spring. As observed for Thalassiosira genera the high abundance found in the
innermost station is not reflected in the surface sediments. Higher percentages on February
10th were related to the development of a winter upwelling bloom as also reported in Saanich
Inlet (Mcquoid and Hobson, 1997). This cylindrical and slightly silicified genus is related to a
pre-upwelling bloom off Somalia (Koning et al., 2001). Their dominance before the onset of the
upwelling is due to their capacity to adjust their buoyancy, responding to changes in the
thermocline and migrating vertically between the surface water and nutrient-rich deeper water
and for nutrient assimilation (Villareal et al., 1999). Dissolution of siliceous valves of this genus
within the sediments implies their limited use for paleoreconstructions. However, when found
in high percentages are good markers of enhanced preservation conditions.
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Chapter IV
S. costatum is very well represented in the water column at all stations but it sharply
dropped at the seabottom (Figure IV.7). Although this species can be found in higher
percentages during short periods of time at the water column, their abundance sharply
decreases at the sediment trap, pointing to a strong dissolution within the water column due to
the delicate structure of their frustules. High concentrations of the high dissolution-sensitive
species in surface sediments indicate high vertical fluxes of diatom valves to the seafloor and
the effective preservation (Romero and Hebbeln, 2003).
This estuarine-adapted species appears in high abundances on February 11th and May
when upwelling conditions are relatively well developed and the input of nutrients is high
(Prego et al., 2001), especially at the outermost stations. It is a good tracer of high productivity
(Romero and Hebbeln, 2003) and characterizes the spring bloom, for example in the British
Columbia fjords (Sancetta, 1989), but its low preservation efficiency hinder its use as a
paleoenvironmental indicator. The appearance of this species in high concentrations in the
sedimentary record also allow us to trace enhanced preservation conditions, that is, a sub-oxic
environment, and/or high primary production. However, S. costatum is also related to the input
of nutrients by heavy rainfall run-off or constant river flow (Mcquoid and Hobson, 1997;
Nogueira et al., 2004). In our case, the appearance of this species in winter is linked to
freshwater inputs from the river flow (Casas et al., 1999; Varela et al., 2001). Although
Rodríguez et al. (2003) found elevated percentages during a spring bloom at the end of May
1999 this species is one of the dominant during winter. In May 13th an important supply of
continental freshwater was introduced in the shelf, arriving at the ría through the southern
mouth leading to an typical early spring bloom that was linked to haline stratification (Varela et
al., 2001; Varela and Prego, 2003). The freshwater intrusion reverses the normal salinity
gradient at the southern mouth of the ría (Álvarez et al., 2006). This unusual patterns
introduce high nutrients content suggesting the existence of a bloom that penetrates the ría,
embedded in a water mass that is fresher than the estuarine one (deCastro et al., 2006a).
High abundances of S. costatum, also in the outer ría indicates that the low-salinity water
mass (Miño River intrusion) is entering the ría from shelf, and the River Lérez is not the only
supply of continental water (deCastro et al., 2006a). This fact is important for future
paleoreconstructions, because this species, if preserved in high abundances and together with
the amount of the freshwater assemblage, can be used as riverine input marker and inner
estuarine processes (Hay et al., 2003).
L. danicus abundance in the water column, the sediment traps and surface sediment
shows a peculiar pattern. Percentage of this species is high in the surface waters, averaging
around 8% for all stations. It completely disappears in the sediment traps, and re-appears with
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
high percentages, especially in the outer ría, with the form of resting spores. This pattern could
be explained by imprecisions during identification and counting spores in the traps. The non-
appearance of L. danicus in the traps in forms of vegetative cells is due to its delicate frustule.
The absence of spores does in the traps can be the result of lack of sampling in the period just
after the bloom of this species, when sporulation occurs. In a general view, the abundance of
resting spores in the surface sediment do not clearly resembles its seasonal pattern downward
fluxes, but they are indicative of the production of the vegetative cells in the water column.
Ecologically, L. danicus appears after upwelling, with the breakage of summer
stratification and the beginning of mixing period (Varela et al., 2001) and when nutrient levels
are low. This species is one of the most important contributor to the assemblage, being a good
indicator of last phase of upwelling when stratification occurs and nutrient levels drop (Casas
et al., 1999; Varela et al., 2001; Bárcena et al., 2004). During the sampling period, L. danicus
appeared in June, under water column stratification conditions, when upwelling index is close
to cero, indicating a low influx of high-nutrient oceanic water (Prego et al., 2001). This species
proliferated after the bloom of the chain-forming species Chaetoceros spp. confirming the
above interpretations. It was also identified as a fast-growing competitor during late summer-
blooms (Bode et al., 2005) and in the Ría de Pontevedra is associated to winter conditions,
that is, under a well mixed water column (Rodríguez et al., 2003). Hobson and Mcquoid (1997)
related its appearance to the stratified phase of a turbulent environment. However, our data
does not support this ecological interpretation.
T. nitzschioides mean relative contribution progressively increases from the water
column to the sediment for all stations, indicating a strong enrichment of the contribution of
this species in the surface sediment (Figure IV.7). This is due to the high preservation
efficiency of this species in the sedimentary record since its strong frustule (Koning et al.,
2001).
Peak of T. nitzschioides in April in the water column and traps (Figures IV.2 and IV.3) is
related to the high riverine run-off (deCastro et al. 2000; Álvarez et al., 2006), the input of
nutrients and weaker upwelling conditions as shown by other authors (Pokras and Molfino,
1986; Abrantes, 1988). Seasonal variation of their abundance is controlled by the input of
nutrients when river runoff is high, coinciding with rainy conditions in winter (van Iperen, et al.,
1987; Abrantes and Moita, 1999). Moreover, in the sediment this species is highly resistant,
so, its appearance in the sedimentary record, is related to high river runoff and high rainfall.
This colony-forming species is also common in winter off Portugal coast around upwelling
centres in less nutrient-rich waters indicating periods of long but weakened upwelling
(Abrantes, 1988; Abrantes and Moita, 1999) and also in the Galician shelf, Bao et al. (1997)
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Chapter IV
linked its appearance in sediments with weaker upwelling conditions and persistent nutrient
input. This neritic species is also indicative of low/weak upwelling production conditions
(Bárcena and Abrantes, 1998). Moreover it can be also found around upwelling centres
(Margalef, 1978) and together with Chaetoceros R.S. could also help to track upwelling
conditions with increased nutrient supply (Hasle and Mendiola, 1967; Schuette and Schrader,
1981a, b; Lange et al., 1998; Bao et al., 2000; Romero and Hebbeln, 2003). T. nitzschioides
tolerates variable conditions, but the persistent presence of this species throughout the
sampling period in the traps is also indicative of resistance even when nutrients content is low,
independently of the oceanographic conditions. The relative stable downward fluxes of T.
nitzschioides (excepting in April) indicate that it is better able to get preserved in areas of low
productivity/nutrient than other diatoms species, while with strong fertilization conditions it is
diluted by fast-blooming species such as Chaetoceros spp. (Lopes et al., 2006). In our case,
this species indicates high freshwater run-off, and also tracks weaker upwelling conditions and
constant nutrient supply.
Freshwater assemblage abundance in the water column and in the sediment is higher
at the innermost station (Figure IV.7). In the sediment traps its contribution is negligible
(Figure IV.3). Relative percentage is relatively important in the sediments, especially at the
station I, being indicative obviously of the river runoff. The appearance of this group (2%) in
the water column of the innermost station on April (Figure IV.2) also indicates the influence of
the riverine plume with salinities around 30‰ at the inner ría (Prego et al., 2001). Therefore,
freshwater diatoms track the intensity and direction of the river plume, making possible the
paleoreconstruction of the riverine input by the presence/lack of this group.
P. sulcata abundance is almost absent in the water column and sediment traps (Figure
IV.7). This meroplanktonic diatom (McQuoid and Nordberg, 2003) appears during fall and
winter due to sediment resuspension caused by very intense vertical mixing in the water
column (Margalef, 1958; Sancetta and Calvert, 1988; Casas et al., 1999). In the ría, mixing of
waters occurs when high freshwater input enters towards the embayment and higher current
velocities re-suspend the sediments introducing allocthonous diatoms, as in the first week of
April. In this way, high relative contribution of this species is detected during low production
periods under non-blooming events by the lack of the spring blooming species.
Benthic assemblage follows the same distribution in the three compartments as found
for P. sulcata. Percentage is between 7 to 20 times higher in the surface sediments than in the
water and traps, indicating a strong enrichment of this group (Figure IV.7). Moreover, the
appearance of e.g. Cocconeis spp., Diploneis spp., Navicula spp. in the sediment traps and
water column especially the first week of April (Figures IV.2 and IV.3) are also indicative of
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
resuspension of the sediment typical in winter with high runoff (Figueiras and Niell, 1987).
Paleo-implications of its distribution is related to the lower contribution of the planktonic
assemblage and the impossibility of reconstructing past oceanographic conditions.
Consequently, the appearance of this group in a typical marine environment is related to
transport processes of this group to the oceanic domain. In the Ría de Vigo case, benthic
diatoms appears in very high percentage at all stations located in the inner areas (Bernárdez
et al., 2006). Their occurrence in the shelf or outer ría probably reflects littoral influence and
transport from the innermost areas.
In summary, the diatom community found in the overlying waters is usually reflected in
the surface sediments, excepting the slightly silicified frustules under low preservation
conditions in the sediment. However, we find some diatom flora only present in the water
column and traps but not in the sediments, and some species found in the sediments but not in
the plankton samples such as P. sulcata.
5.2. Diatom and biosiliceous compound surface sediment distribution: oceanographic and environmental controlling factors
Estimated diatom abundances in the surface sediments of the ría fall within similar
order of magnitude (Figure IV.4) as reported for other coastal or shelf upwelling-influenced
areas (Schuette and Schrader, 1981a; Abrantes, 1988; Lange et al., 1998; Romero et al.,
1999a; Nave et al., 2001; Romero and Hebbeln, 2003; Hay et al., 2003) being higher than for
example in the Mediterranean (Bárcena and Abrantes, 1998). Values in the Rías Baixas are
around 10×106 valves g-1 with maximum concentrations in the Ría de Arousa and minima (<105
valves g-1) nearby the Finisterre Cape (Bao et al., 1997). In the Ría de Vigo values are around
10–20×106 valves g-1 (Prego et al., 1995).
Lower diatom absolute valves abundance were recorded at the entrance channels of
the ría (Figure IV.4), where grain size is coarser, reflecting a dilution effect through increased
deposition of clastic material (Vilas et al., 2005). Grain-size variations along the ría suggest
that the lower abundance of diatoms in the outer areas could be a function of transport
(diatoms being removed from coarser sediments) or preservation (with dissolution in more
porous sediments). Favourable conditions for diatom preservation were usually reflected by
high abundance estimates. The pattern of diatom content also shows an increased percentage
at the innermost parts at the northern shore, being indicative of enhanced preservation
conditions in the muddy areas. Diatom/Chaetoceros R.S. ratio also shows high values at the
innermost sites, indicating a strong potential of these sediments for diatom preservation.
Patterns of preservation were also related to the oxygenation conditions of the sediment. Oxic-
suboxic conditions are commonly found at the innermost parts of the ría, where mud content
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Chapter IV
and organic carbon are high, leading enhanced opal preservation. At the outer areas, coarser
sediments permit the oxygenation of the sediment, favouring bioturbation and permitting the
break out and fast dissolution of diatom frustules. Only the strong silicified species resist to
dissolution, as for example, P. sulcata.
High percentages of Chaetoceros R.S. found at the middle ría are triggered by the
influx of ENACW waters fertilizing the ría. Low abundances found in the inner ría are due to
the high contribution of freshwater and benthic diatoms. Also, in the northern and southern
mouth low values were found, indicating lower diatom production in this area. The high
productivity of the surface waters and the influence of coastal upwelling especially in the
middle-outer areas of the ría are mirrored by the dominance of upwelling-related Chaetoceros
R.S. as observed in the Ría de Vigo (Prego et al., 1995) (Figure IV.4). Chaetoceros R.S.
geographical distribution in the modern sedimentary record marks the area where stronger and
more important upwelling conditions occur. As reported by Lopes et al. (2006) Chaetoceros
R.S. percentages provide a better semi-quantitative index of paleoproductivity than absolute
abundance, which is compromised by dilution with terrigenous sediments near the coast.
P. sulcata is thycopelagic, highly silicified and preserves very well in the sedimentary
record. Its abundance at the ría mouth points to its resistance to dissolution and high energy
currents. The high abundances in the outer sector point to an enhancement of this resistant
diatom due to differential dissolution, indicating oxic conditions in the seabed (Figure IV.4).
This species is also a tracer of high fertilization by nutrients and upwelling (Roelofs, 1984;
Abrantes, 1988; Abrantes 1991; Bao et al., 1997; Bárcena and Abrantes, 1998; McQuoid and
Nordberg, 2003 and references therein). In fact, their appearance in high percentages at the
outer ría can also reflect the oceanic influence and the entrance of high-nutrient shelf waters
when upwelling is well developed. However, caution must take care on interpreting their
spatial and vertical distribution due to its special resistance to deterioration.
Paleoenvironmental interpretations using P. sulcata are difficult due to their characteristics
features. In our case, their relative contribution is high at the outer ría, where oceanic waters
and upwelling predominates. We must combine its employment as paleoproxy in P. sulcata-
dominated biofacies for the identification of high production areas with other biogeochemical
markers (Bao et al., 1997).
T. nitzschioides geographical distribution (Figure IV.4) should be indicative of high
riverine discharge and weaker upwelling conditions on the basis of their seasonal variations
and downward fluxes to the seabed. The constant percentage in the outer areas of the ría is
associated to the constant nutrient supply from oceanic waters. However, its geographical
distribution with lower percentages at the stations of the inner ría, do not resemble the
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
distribution pattern of, for example the freshwater assemblage (Figures IV.4 and IV.5). Hay et
al. (2003) also found elevated percentages of this species in the outer parts of a temperate
fjord linked to the entrance of offshore waters.
Thalassiosira spp. present a scattered distribution, but persists in significative
abundances at the inner ría (Figure IV.4). This fact is indicative of their resistance and
adaptation to conditions relatively low nutrients input but constant in time, supplying by for
example by the Lérez River. In this way, relative abundance is high in the northern margin due
to displacement of the surface river waters due to the Coriolis effect, an asymmetry observed
throughout the year (deCastro et al., 2006b).
The higher percentage of L. danicus in the sediments of the outer ría is related to the
lack of the benthic and freshwater assemblage and relatively low percentages of the
Chaetoceros R.S. This species reflects the areas where stratification, following upwelling,
occurs.
The freshwater assemblage showed a strong influence of the riverine plume at the
innermost ría, declining towards the middle ría. Coriolis force drives the freshwater flow to the
northern mouth, especially during the wet season (deCastro et al., 2006b) provoking the
sedimentation of the river-derived particles at the northern margin. Oceanographic surveys
done at the same period also illustrated the extent of the low salinity river lens and reduced
freshwater retention (Prego et al., 2001). Other biosiliceous continental-derived material, as
phytoliths and crysophycean cysts appears at the same stations and with the same pattern
distribution as the freshwater assemblage, demonstrating the riverine transport of these
components to the ría (Figure IV.5). Hay et al. (2003) also found the same distribution patterns
in a temperate fjord and other authors (Rebolledo et al., 2005; Lopes et al., 2006) on the
Chilean Fjords and NE Pacific respectively, indicating the important implications for
paleoreconstructions. This fact has important paleoimplications since in coastal areas with
higher riverine influence most of these particles are derived from the fluvial domain, instead of
aeolian transport as observed for example in NW Africa and Equatorial Atlantic (Romero et al.,
1999b; Romero et al., 2000; Nave et al., 2001).
The appearance of high abundances of freshwater diatoms at a station located at the
southern mouth of the ría could be related to the influence of the Miño River plume after
extremes flood events (Figure IV.5). Lower salinities recorded at the southern mouth in the
Vigo and Pontevedra rías are derived from the salinity plume located at the shelf and getting
into the rías (Álvarez et al., 2006), and transporting allochthonous freshwater diatoms floating
on the surface water. One of these events was recorded during the sampling period leading to
an early spring bloom, detected by the high abundances of S. costatum in the water column
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Chapter IV
and sediment traps, even in the outer areas, mixed with a more advanced successional stage
of phytoplankton community (deCastro et al., 2006a). This slightly silicified species is only
present in the inner ría and at station 16. Preservation of this species in the sediment is then
related to enhanced preservation efficiency in the sediment but, the low preservation efficiency
in the sediment of this species impede their use as river plume or run-off tracking.
The record of this delicate frustule species in the fossil record implies good conditions
for preservation. The non-presence of fragile species (e.g. S. costatum) in our sediment
samples suggest that dissolution after burial in this region is high as shown by Schuette and
Schrader (1981a, b) excepting the inner areas.
Benthic species extends towards the inner ría in shallow areas. Their appearance at
higher depths indicates transport from the inner areas. Therefore, benthic taxa could be used
in the paleoceanographic record as an indicative of down-slope or down-shelf transport. The
spreading of benthic and freshwater forms towards outer areas of the ría and also in the
adjacent shelf points to the existence of currents towards the outer shelf and high riverine
discharge (Figure IV.5).
5.3. Diatom record vs. geochemical characteristics of the sediment
The spatial distribution of the positive values of the PC1 factor scores is related to the
area where continental influence and estuarine processes dominates, while negative factor
scores spreading westward in the outer area (limit station 12) reflects oceanic influence
(Figure IV.8). Therefore, the influence of the river plume is clearly identified in the sedimentary
record. In coastal areas, an important contribution of land-derived material should be
expected. In this case, values of the TOC/TN ratio below 10 (Table IV.2; Dale and Prego,
2002), even in the inner station, could lead us to conclude that the main organic matter source
is marine. However, as observed especially in the freshwater assemblage distribution, the land
derived processes are important in the inner ría. TOC/TN ratios are not suitable for
quantitative approaches for the origin of the organic matter, although values <10 reflect that
the main source of organic carbon is marine (Emerson and Hedges, 1988; Meyers, 1994).
Surface river water was displaced northward due to the Coriolis effect generating a maximum
of this factor. PC1 distribution reflects the terrestrial input and riverine influence. We will
simply designate this factor as the ‘land and freshwater influence factor’.
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
Figure IV.8. Distribution patterns of the factor scores obtained using the R-mode principal component analysis.
On the contrary, PC2 is confined to open-ocean and especially in the middle ría region,
being the factor scores highly positive in the oceanic domain (Figure IV.8). This factor is
indicative of the influence of the upwelling of ENACW. ‘Marine primary production factor’ is a
suitable designation for this component.
In summary, on the basis of the PCA, the first component (PC1) illustrated the strong
separation between the inner and outer stations, representing the freshwater influence/marine
domain and a benthic taxa gradient in the assemblages. The second axis (PC2) is related to
the marine autochthonous productivity and the influence of the input of oceanic water masses
(Figure IV.7). In fact, forcing mechanisms controlling the entrance of the oceanic water
masses is highly influenced by the climatic and atmospheric conditions. Sediment diatom
assemblages and the geochemical tracers throughout the ría are hypothesized to reflect the
combined influences from the Lérez River and the hydrodynamic and oceanographic regime,
because the rías are a principal exchange of terrestrial and marine waters. Factors extracted
using PCA analysis reflect the fluvial-estuarine domain and the oceanic influence on the
diatom assemblages and geochemical characteristics of the surface sediments, allowing the
identification of changes in these environmental conditions in the sedimentary archive.
6. CONCLUDING REMARKS
This study concerns the seasonal variation in the water column and sediment traps and
the spatial distribution of the siliceous signal in the surface sediments of a Galician ría.
Chaetoceros spp. are linked to the development of high production conditions caused
by upwelling driven by northern winds. Thalassiosira spp. appear under continuous nutrient
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Chapter IV
input. Rhizosolenia species responds to the winter upwelling blooms, but they proliferate under
low nutrient contents in the surface waters. S. costatum is related to riverine and freshwater
input and inner estuarine processes and T. nitzschioides tolerates variable conditions but also
related to river runoff. L. danicus proliferates after the upwelling under low nutrient levels and
stratification. P. sulcata and benthic group only appeared in the water column with high river
runoff due to the resuspension of the sediment. Freshwater group is relatively important in the
innermost stations tracking the intensity of the Lérez River plume.
Strong dissolution of the slightly silicified species occurred mostly at the water/sediment
interface. Rhizosolenia spp. undergo the early diagenetic and dissolution processes within the
sediment, completely disappearing. However S. costatum experiences dissolution
progressively from the low-deep water column and in the sediment independently of the
location in the ría. The same behaviour was observed for the Thalassiosira species excluding
the innermost station. T. nitzschioides shows the opposite pattern, a strong enrichment of their
relative percentage, both in the traps and the sediment. Chaetoceros species are very well
preserved in the sediment as resting spores. L. danicus is also well represent below the traps
in the form of resting spores, indicating good preservation efficiency of the spores, especially
in the middle ría. Benthic and freshwater species appear preferentially in the sediments of the
innermost areas. P. sulcata is one of the robust species that only appears in the sediments,
only when the preservation conditions are inadequate.
Diatoms were the main contributors to the opal fraction in the surface sediment.
Seasonal and repeated upwelling events and the input of freshwater diatoms in the inner areas
due to Lérez River flow permit the tracking of the primary production in the surface sediments.
Freshwater, crysophycean cysts and phytoliths document the geographical extension of the
Lérez River plume and the entrance of fresher water from the southern mouth of the ría with
high riverine discharges to the shelf from the Miño River. This fact is of great importance in
estuarine areas for paleo-discharge reconstructions, since we can track the displacement of
this estuarine zone through time using these tracers. These remarks have implications for the
use of thanatocoenosis downcore and fossil diatom assemblages together with geochemical
and other paleoproxies for reconstruction of paleoceanographic conditions in the rías and
nearby areas, such as the continental shelf.
Geochemical and micropaleontological proxies and PCA analysis in the surface
sediment reflected the strong contrast between the upwelling influenced area and the
estuarine-dominated inner zone. A good correlation between present day surface water
characteristics and surface-sediment properties was observed, taking into account the
‘continental influence’ and ‘preservation’ processes that affect and alter the surface-sediment
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Processes controlling the diatom production and accumulation in a western Galician Ría: Implications for paleoreconstructions
composition. The study shows that the diatom distribution, species composition and
biosiliceous remains in the surface sediments in the Ría de Pontevedra are closely correlated
with the oceanographic conditions and environmental variables e.g. freshwater discharges,
nutrient input by upwelling and water column stratification.
Acknowledgements
We thank the crew from the Mytilus research vessel, the people who helped during the campaigns and with the laboratory and technical work. This work was supported by the Comisión Interministerial de Ciencia y Tecnología, under the project ‘Hydrodynamic and silicon cycle in the Ría de Pontevedra’, ref. MAR96-1782, and partially funded by the projects REN2003-09394, METRIA-REN2003-04106-C03, PGIDIT05PXIB31201PR, EVK2-CT-2000-00060, PGIDT04PXIC31204PN and PGIDT00MAR30103PR. P.B. was supported by a research grant jointly funded by Xunta de Galicia (Secretaría Xeral de Investigación e Desenvolvemento) and Ministerio de Educación, Cultura y Deporte (Secretaría de Estado de Educación y Universidades).
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[Chapter V] DIATOM COMMUNITY IN A SEMI-ENCLOSED RÍA AND THEIR CONTRIBUTION TO THE SEDIMENTARY RECORD∗
1. INTRODUCTION AND OBJECTIVES
2. STUDY SETTING
3. MATERIALS AND METHODS
3.1. Water column sampling and processing
3.2. Surface sediment sampling and analytical procedures
3.3. Taxonomic identification
4. RESULTS AND DISCUSSION
4.1. Planktonic diatoms composition, distribution and relation to hydrology
4.2. Distribution of diatom assemblages and biosiliceous material in the surface sediments
4.3. Paleoimplications
5. SUMMARY AND CONCLUSIONS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Ricardo Prego, Manuel Varela, Guillermo Francés. To be submitted
Abstract. The diatom abundance in the water column was assessed in several campaigns underlying different productive regimes in the Ría de Ferrol, a semi-enclosed embayment in the NW Iberian Peninsula. Modern diatom distribution patterns in surface sediment were also investigated, in order to determine the influence of hydrography, upwelling, and riverine nutrient input on the production and record of these biogenic components.
Very low abundances were found in the water column during the winter sampling date, whereas in spring and summer diatoms proliferate. Excepting for winter, constant nutrient supply by rivers located at the ría head and continuous renovation of waters lead to constant diatom production. Chaetoceros spp. were the primary components of the water column community during spring and summer, followed by T. nitzschioides and Rhizosolenia spp. N. longissima represented a significant portion of the winter assemblage, together with P. sulcata and benthic taxa. L. danicus, N. longissima and S. costatum characterize the autumn campaign, when stratification of waters occurs, being L. danicus especially abundant at the outer ría. In this way, the main factors controlling the abundance of diatoms in the Ría de Ferrol are the riverine freshwater inputs, the hydrography, water dynamics and circulation within the ría.
Hydrographic and primary productivity patters also govern the diatom abundance and assemblage composition preserved in surface sediments. A strong E-W gradient in diatom abundance and composition in the sediments can be distinguished. Samples located at inner ría and the margins have the highest abundances of diatoms, and they are primarily dominated by benthic species. Freshwater group, crysophycean cysts and phytoliths follow the influence of river runoff at the landward stations. Middle ría is characterized by Paralia sulcata, and Thalassiosira spp., with small occurrences of the benthic and freshwater group. Chaetoceros R.S., L. danicus and T. nitzschioides species typify the outer ría, demonstrating the influence of the penetration of oceanic water waters into the embayment due to enhanced tidal mixing through the narrow channel. This last assemblage corresponds to nutrient-rich and high-productivity coastal areas influenced by oceanic waters.
In general, sediment diatom assemblages reflect diatom production patterns in the water column, excepting for slight silicified species. In the inner areas caution in interpreting a paleorecord is needed due to the high contribution of allocthonous taxa, being indicative of low water depths.
Keywords: diatoms/water column/surface sediments/Ría de Ferrol
Resumen. La abundancia y composición de la comunidad de diatomeas ha sido determinada en muestras de la columna de agua a lo largo del eje longitudinal de la Ría de Ferrol a lo largo de una serie temporal que cubre diversos periodos oceanográficos. Asimismo, se ha cuantificado la abundancia y porcentaje relativo de estos componentes biosilíliceos en el sedimento superficial a lo largo de toda la ría, con el fin de comparar ambos compartimentos y calibrar el uso de estos marcadores como indicadores de diferentes condiciones hidrográficas.
Los valores mínimos de abundancia de diatomeas en la columna de agua se producen durante el invierno, mientras que en el resto de campañas la concentración es alta, debido a la constante renovación de agua en la ría y al aporte de nutrientes por parte de los cursos fluviales. Chaetoceros spp., T. nitzschioides y Rhizosolenia spp. proliferan durante la primavera y el verano N. longissima, P. sulcata y el grupo de diatomeas bentónico son indicadores de condiciones invernales de baja producción. L. danicus, N. longissima y S. costatum caracterizan la campaña de otoño, cuando ocurre la estratificación de la columna de agua. Los principales factores que condicionan esta sucesión fitoplanctónica son la hidrografía característica de esta ría, su circulación, la escasa entrada del agua aflorada debido al upwelling y el aporte de nutrientes por parte del curso fluvial en su cabecera.
El registro de diatomeas en el sedimento superficial está controlado por la productividad y la hidrografía. Los máximos de abundancia se localizan en las zonas internas, en las cuales domina el grupo de diatomeas de carácter bentónico. Las diatomeas de agua dulce, los quistes de crisófitas y los fitolitos también se localizan en esta zona, reflejando la influencia del aporte fluvial. La ría media está dominada por P. sulcata, Thalassiosira spp. y pequeños porcentajes de diatomeas bentónicas y de agua dulce. Chaetoceros y L. danicus R.S. y T. nitzschioides caracterizan las zonas más externas, lo cual se relaciona con la entrada de aguas oceánicas a la ría ricas en nutrientes.
El porcentaje de cada una de las especies de diatomeas que queda registrado en el sedimento superficial es un reflejo de la comunidad que prolifera en la columna de agua, excepto en las zonas más internas, donde el elevado número de diatomeas de agua dulce y bentónicas enmascara el registro pelágico.
Palabras clave: diatomeas/columna de agua/sedimentos superficiales/Ría de Ferrol
DIATOM COMMUNITY IN A SEMI-ENCLOSED RÍA AND THEIR
CONTRIBUTION TO THE SEDIMENTARY RECORD
1. INTRODUCTION AND OBJECTIVES
Primary production in the coast of Galicia (NW Spain) is largely influenced by coastal
upwelling (Teira et al., 2001), due to the nutrient enrichment by the Eastern North Atlantic
Central Waters (ENACW) (Blanton et al., 1984). Thus, in coastal waters, upwelling and
productivity often represent two covariant parameters that cannot be dissociated. Ría systems,
i.e. small coastal embayments characterizing the Galician coast, are fertilized by two sources
of nutrients, upwelling and continental runoff, with the first dominant (Prego et al., 1999a). In
addition, rías are characterized by changing hydrographical conditions: salinity gradients,
stratification and mixing of waters.
Much of the primary production of this nutrient-replete areas is by diatoms (Nelson et
al., 1995) resulting in significant settling fluxes of these biosiliceous organisms. Diatoms are
one of the most common microfossils in sediments, such as those affected by upwelling
(Schuette and Schrader, 1981a, b; Abrantes, 1988; Bao et al., 1997; Bárcena and Abrantes,
1998; Abrantes and Moita, 1999; Nave et al., 2001; Romero and Hebblen, 2003; Lopes et al.,
2006). The abundance of the diatoms species in the water column and sediments show
definite regional distribution patterns associated to oceanographic conditions (Pokras and
Molfino, 1986). Of special importance is the clarification of the connection between the
seasonal and spatial variations in assemblages of biosilicate organisms in the water column
and those found in the sediment (Sancetta and Calvert, 1988; Lange et al., 1998; Romero et
al., 2000; Koning et al., 2001; Abrantes et al., 2002; Bárcena et al., 2004), since biogenic silica
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
is severely affected by dissolution in the undersaturated ocean water (Nelson et al., 1995,
Ragueneau et al., 2000).
Analysis of surface sediments, representing the recent record of these primary
production and hydrographic conditions tracers, provides an opportunity to unravel how the
diatoms found water column are incorporated into the seafloor and how modern environmental
conditions are reflected on it (van Iperen et al., 1987). The knowledge and understanding of
these processes is highly interesting in the coastal regions located where hydrographic
gradients are large and where freshwater and marine conditions play a role, such as those
found in rías and estuaries. On the other hand, coastal and small embayment areas are key
zones susceptible for using the sedimentary record in studies about paleoclimate and
paleoreconstructions. In this way, the quantification of microfossil changes occurring within the
sediment record requires an understanding of the modern distributional patterns and
ecological preferences of species.
There are very few published data in the Galician Rías and specifically in the Rías
Altas, concerning the seasonal succession of the diatom community, production patterns
(Bode and Varela, 1998a, b, Varela et al., 2001; Bode et al., 2005a, b) and sediment record of
these hydrographic tracers. Consequently, in order to improve our knowledge about the
diatoms seasonal cycle in the Rías Altas and its relation to oceanographical conditions, water
column samples from different surveys were collected at three control sites in the Ría de
Ferrol. Surface sediments characterization in terms of diatoms and siliceous compound has
been never carried out in this ría. Therefore, the second objective is to quantify the abundance
of these biosiliceous components on the seabed and document the regional distribution
patterns. Finally, other questions we strive to address are: Is the surface water diatom
production correlated to their record in the seafloor sediment? How well the sediment record
reflect spatial and temporal changes in hydrography and surface water biosiliceous
productivity?
2. STUDY SETTING
Located in the Galician coast to the north of Cape Fisterra, Ría de Ferrol is one of the
small embayments commonly named Rías Altas. Therefore, it affords a representative contrast
to the large rías situated to the south, i.e. Rías Baixas, in terms of morphology, physical
oceanography, rivers contributions and primary production dynamics. Ría de Ferrol is located
between the coordinates 43°26’–43º31’N and 8°9’–8º21’W. In plan view it is funnel-shaped
covering 27 km2 with a longitudinal axis of 15 km long and a maximum width of ~3 km. Depth
varies from 32 m at its mouth to a few meters at the innermost part (Figure V.1). The water
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Chapter V
exchange between the ría and the shelf occurs through a narrow channel (2 km length, 0.35
km width, ~20 m depth). Freshwater runoff is small and it is mainly supplied by two main rivers
located at the innermost part, Grande de Xubia River and Belelle River. Approximately
average river flows volumes are 6 and 0.8 m3 s-1 respectively (Rio and Rodríguez, 1992;
deCastro et al., 2004). The annual river discharge versus the total volume of the ría is 0.8
(deCastro et al., 2004).
Figure V.1. Locality map of the study area. The position of the bed sediment sampling (black circles) is shown. Diamonds indicate the stations where water column phytoplankton sampling was carried out. Contour plot shows the mud content in the surface sediments (modified from Cobelo-García and Prego (2004). Dotted white line represents the longitudinal axis. Water depth variation along the longitudinal axis of the ría is also shown.
From a physiographic point of view, the Ría de Ferrol can be divided into several zones
(Figure V.1; deCastro et al., 2004): (1) the outermost one extends from the mouth to the
beginning of the channel (2) the narrow channel, i.e. the Strait of Ferrol (2 km long, 0.5 km
wide, 20 m depth) (3), the middle zone, extending towards the As Pias Bridge covers most of
the area of the ría, (4) the innermost zone is characterized by wide mudflats in a very shallow
environment. It extends from the As Pias Bridge to the ría head and due to this structure the
exchange of water and organic material between the medium and inner zone is reduced.
The hydrographic behaviour of this ría is distinctive among other Galician rías. The
water circulation depends strongly on tide and therefore circulation can be considered periodic
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
since the river inflow is very low (deCastro et al., 2003). The occasional existence of wind-
induced currents constitutes other factor controlling the residual circulation in the ría, but only
affects near the surface (deCastro et al., 2003). At the mouth, where large tidal velocities
enhance the mixing, a well-mixed water column is found. In contrast, the middle and inner
parts of the Ria de Ferrol seems to be partially stratified during the wet season (deCastro et
al., 2004). The intrusion of nutrient rich water (ENACW) from the shelf is hard, due the
orientation of the ría and the narrow shape of the channel (Bode et al., 2005a). The
topographic features of the Artabro Gulf, generates a shadow and thus, the ría-shelf water
exchange is confined, remaining almost isolated from upwelled seawater (Prego and Bao,
1997; Prego and Varela, 1998; Prego et al., 1999b).
From a geological standpoint, watersheds are formed by igneous rocks, dominated by
schists, gneisses and granites. The sediment contained in the ría basin is of continental and
marine origin, derived from the biological activity in its waters and the riverine discharge
supply (Lueiro and Prego, 1999). The type and grain size of sediment varies along of the ría
(Figure V.1, López-Jamar et al., 1996; Cobelo-García and Prego, 2004) and depends on the
hydrography and velocity of the currents. At the mouth and the main channel of the ría, the
sediments are predominantly fine sand, with low contents of organic material (1.7–2.5 wt.%).
Conversely, the sediments located at the inner and middle parts of the ría are composed by
muddy fractions, and higher concentrations of organic matter (3.7–13.2 wt.%). In the middle-
inner areas mud content is higher at the northern shoreline (Figure V.1).
3. MATERIALS AND METHODS
3.1. Water column sampling and processing
Four seasonal cruises were carried out in the Ría de Ferrol onboard the R/V Lura
during 2000 on 22nd February, 16th May, 11th July and 5th September, covering three different
zones of the ría (Figure V.2). Water samples were collected using Niskin bottles ‘General
Oceanics’ of 5 litres at the Station O, C and M (Figure V.1, Table V.1).
Table V.1. Position and water depth of the stations sampled for diatom water column estimates.
Water column samples Longitude W Latitude N Water depth
(m) O Outer 8º 19.59’ 43º 27.15’ 31
C Channel 8º 15.56’ 43º 28.20’ 18 M Middle 8º 12.78’ 43º 28.15’ 12
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Chapter V
Figure V.2. Solid line indicates the Grande de Xubia River daily discharge during year 2000 (values in m3 s-1). Grey bars show the daily variations of the Upwelling index (Qx m3 s-1 km-1) at point 43°11'’ kindly given by Jose Manuel Cabanas (IEO). Dashed lines indicate the water column sampling dates.
Diatom composition was determined in Lugol’s preserved samples in these stations
throughout the longitudinal axis. Samples were counted using a Nikon microscope, following
the technique described by Uthermöhl (1958). A magnification of 100× was used for big forms,
250× for intermediate and 400× and 1000× for large species.
3.2. Surface sediment sampling and analytical procedures
Surface sediment samples were taken at 35 sites from the R/V Mytilus (Figure V.1 and
Table V.2). Two Van Veen grab samplers of different size were used. Only the surface layer of
the sediment was collected (<1-cm), using polyethylene spatulas and stored 4ºC. Sediment
was dried in a oven below 50ºC.
The determination of the amount of opal contained in the surface sediments was carried
out following the method of alkaline digestion by Mortlock and Froelich (1989) and the
following analysis of dissolved silicate in the obtained extract using the method reported by
Hansen and Grasshoff (1983). The total organic carbon (TOC) was determined by the
difference between total carbon (TC) measured in a Perkin Elmer elemental CNH analyzer and
inorganic carbon (TIC) analyzed by calcination loss between 550 and 975ºC. Total nitrogen
(TN) was also determined with the same equipment at the University of Coruña. Data is
presented in Cobelo-García and Prego (2004).
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
Preparation of the sediment samples for the diatom counts was carried out following the
technique developed by Abrantes et al. (2005). For slide preparation, a known volume of
suspension was strewn uniformly onto cleaned two circular cover slips placed in a circular
evaporation tray after stirring the solution for homogenization (Battarbee, 1973). After the
dishes had been dried, smear slides were removed and assembled with Permount™ mounting
medium onto permanently labelled slides.
3.3. Taxonomic identification
The identification of diatoms found in the water column were based on a variety of
taxonomic monographs, but mostly according to Tomas (1997). Total abundance is expressed
as cells ml-1 and the relative percentage of each species and ecological groups was also
calculated.
Our taxonomic identification (genera and species) of the diatoms present in the surface
sediments is based mainly on ecological characteristics using several floras, keys and specific
bibliographies (Hustedt, 1930; Hustedt, 1959; Round et al., 1990; Hartley, 1996; Hasle and
Syvertsen, 1996; Witkowski et al., 2000 among others). Schrader and Gersonde’s (1978)
guidelines were followed for diatom counts and total number estimates, so only diatoms that
were essentially whole were counted. Absolute abundances are expressed in number of
valves per gram of dry sediment. In a few cases, difficulties arose in distinguishing between
two or more species and therefore, these species were combined in one counting group or
genera group. The taxa were grouped into three different types: dominant taxa, species of the
same genus living in similar environments; and species showing comparable ecology and
distribution that is, common ecological meanings, as freshwater and benthic. Paralia sulcata
was not integrated in the benthic assemblage since it is considered tychopelagic.
4. RESULTS AND DISCUSSION
4.1. Planktonic diatoms composition, distribution and relation to hydrology
Only the most representative species found in the water column were plotted in Figure
V.3. May and September are the sampling dates with higher diatom abundances.
Phytoplankton abundance is high all year round excepting February, when the diatom quantity
was very low, ~1000 times lower than in the other periods (82–122 cell ml-1). This fact is in
accordance with the chlorophyll-a values, displaying maximum values in September and
minimum in February (Varela et al., 2003; Bode et al., 2005a). No significant differences in the
diatom concentration were found between stations for the same sampling date, excepting for
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Chapter V
the outer station during July cruise, when the diatom abundance was between six and ten
times lower than for the other stations.
Figure V.3. Temporal variations of the standing stocks of total diatoms and the main diatom groups abundance in the water column (white symbols, cell ml-1) at the three sampling sites. Bars show the relative percentage of each species or group. Note the logarithmic scale in the diagram showing the total diatom abundance. Note also that there is no data for the outer station in the sampling survey of May.
In general, diatom abundance values are higher than those found in other rías during
spring and summer (Varela et al., 2001; Varela and Prego, 2003; Varela et al., 2005). Spring
and summer bloom and phytoplankton succession and community are most likely similar to
that found in other rías (Figueiras and Niell, 1987; Casas et al., 1999: Figueiras et al., 2002;
Varela and Prego, 2003; Varela et al., 2004; Varela et al., 2005). Continuous renovation of
water warrant that nutrients were not exhausted and phytoplankton growth will be guaranteed
(Bode et al., 2005a). In fact, excepting for the winter campaign, there is a reduced seasonal
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
variability on diatom abundance favoured by lower water residence times within the ría and
constant input of shelf waters (Bode et al., 2005a). Diatom composition and phytoplankton
abundance may be controlled by the continental supply of nutrients due to the reduced size of
this ría.
The main component of the diatoms association was Chaetoceros spp., whose
contribution ranges from 42 to 99% in May and July. In May, the riverine runoff is
approximately half of the annual mean, but coincides after a strong runoff peak during April.
Moreover, strong northerly winds were recorded and upwelling conditions in the shelf. Water
column is very well stratified, affecting upper 10 m, due to the strong runoff and solar heating.
In this way, this chain-forming species characterizes the spring and summer blooms, when
primary productivity is enhanced by the nutrient input by oceanic waters, mixing of the water
column and solar irradiance. In the Ría de Ferrol it seems to be more representative of spring
conditions, accounting almost the 100% of the diatom community. In summer (July cruise)
Chaetoceros spp. appear together with Rhizosolenia spp., Thalassiosira spp. and T.
nitzschioides. In contrast, it almost disappears in February and September, due to the low
primary production rates and the appearance of other species more adapted to this
oceanographic conditions.
Chaetoceros spp. usually display higher percentages in the O and C stations where the
direct influence of oceanic waters is observed. However, in the Ría de Ferrol case, the input of
nutrient-rich central waters (ENACW) is low even when the upwelling is well developed in the
Galician shelf. In fact, upwelling of shelf water only affected the water layer below 15 m in the
outer ría during summer (Varela et al., 2003).
Nitzschia longissima group follows the opposite trend to that found for Chaetoceros
R.S. This group accounts most of the percentage of the total diatoms during February and
September cruises, especially at the outermost station sampled. This species is well adapted
to winter conditions, under low illumination and can be considered a tracer of mixed water
conditions as observed in other rías (Prego et al., 2007).
Winter campaign is also characterized by the appearance in the water column of
benthic taxa and the meroplanktonic species P. sulcata (1.5–3%), especially at the O and C
stations. This flora spends its life as bottom forms attached to mud, sand or to a fixed
substratum (Mcquoid and Nordberg, 2003). They enter the surface waters only if they are
moved effectively from their natural habitat by currents. In this way, reworking of the surface
sediment under turbulent conditions explains this seasonal pattern. Moreover, strong tidal
currents observed in this ría can resuspend and transport these taxa from inner to outer areas.
Riverine discharge during the winter cruise is relatively low in comparison with the annual
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Chapter V
mean (Figure V.2), therefore, thermohaline stratification is weak. In fact, the water column is
very well mixed as evidenced by the homogeneity of the vertical profiles of salinity and
temperature (Varela et al., 2003; Bode et al., 2005a, b) and shelf waters are restricted to the
outer area.
L. danicus is the third component of the diatoms assemblage in the water column. It is
absent for all the sampling periods and stations, but appears in high abundances and relative
percentages during September, together with N. longissima. In this cruise L. danicus
represents around 75% of the total diatoms at the station O and ~35% for sites C and M.
Oceanographical patterns are characterized by the reduced runoff and absence of northerly
winds, which implies the stratification of the water column (Figure V.2). A small layer of
nutrient-rich shelf water enters the ría through the bottom, provoking the blooming of this fast-
growing competitor (Bode et al., 2005b) as observed for other authors in other areas (Casas et
al., 1999; Varela et al., 2001; Bárcena et al., 2004). During autumn this species could be a
high contributor of the assemblage (Varela and Prego, 2003; Nogueira and Figueiras, 2005).
The contribution of S. costatum to the diatom community is also relevant during
September (3195–12166 cell ml-1), having the highest occurrence at the innermost station
(16%). Ecologically, this species is linked to the supply of nutrients by riverine discharge,
appearing associated to low salinities (Varela et al., 2005). This ecological preference explains
its higher relative abundance in the inner station. In fact, this species is one of the dominant
species during nutrient-enrichment of the rías in winter provoked by the high freshwater input
(Casas et al., 1999; Rodríguez et al., 2003; deCastro et al., 2006; Prego et al., 2007).
However, S. costatum is absent in winter, when riverine discharge is high. The lack of
sampling in the innermost areas could affect to the non appearance of this taxa in the water
column. It also dominated biomass in the water column during spring blooms and upwelling in
the Rías Baixas (Tilstone et al., 2000; Varela et al., 2005) and A Coruña Bay (Varela and
Prego, 2003), linked to high nutrient concentrations. A winter bloom of this species was
detected in the Ría de Vigo related to upwelling-favorable period under winter thermal
inversion conditions and later coastal wind relaxation period (Álvarez-Salgado et al., 2005).
However, this species does not appear in this ría due to the hardly supply of nutrient-rich
upwelled waters. In summary, together with L. danicus, this species is indicative of late
summer blooms in September.
Thalassiosira spp. only contribute in a high percentage (39.3%) and abundance (5299
cell ml-1) during the July cruise at the station M. It is important to note that only the large
species of Thalassiosira were included in this group. In fact, most of the small centric diatoms
(<30 µm) probably belonged to this genera. Ecologically speaking, this genus appears in
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
relatively high abundances on winter season throughout the Portugal coast indicating
continuous nutrient input (Abrantes and Moita, 1999). However, in the Ría de Vigo, the
dominance of Thalassiosira spp. biomass in the water column during upwelling is associated
with high nutrient concentrations having a strong dependence on the silicate availability
(Tilstone et al., 2000). In the Ría de Pontevedra its appearance is related to continuous
nutrient input (Bernárdez et al., in prep.) but in the Ría de Ferrol case, its blooming is strongly
related to high productivity conditions. The growing of this genus can also be explained by in
situ regeneration of nutrients, especially nitrate, in the middle ría during summer (Bode et al.,
2005a).
T. nitzschioides contribution to the total diatoms is important during July, up to 4.8% for
the station M. It also displays significative percentages during the winter cruise (absolute
abundance <100 cell l-1) due to water column mixing and river-derived nutrients supply. It
completely disappeared in May and September. Therefore, this species is representative of
mid-summer blooms in middle areas. It also tracks the nutrient supply from oceanic waters in
the outer station during the summer sampling, but it is diluted by fast-blooming species such
as Chaetoceros spp. T. nitzschioides presents low percentages in the Rías Altas (Varela et al.,
2001; Varela and Prego, 2003; Varela et al., 2005), whereas in the Rías Baixas appears in
relatively high contribution associated to spring conditions and upwelling (Prego et al., 2007).
In other coastal-shelf areas is linked to nutrient input by river runoff with high rainfall
conditions and/or occasional upwelling and with nutritive waters near transported from
upwelling centres (Blasco et al., 1981; Abrantes, 1988; Bárcena and Abrantes 1988; Pokras
and Molfino, 1986). Higher percentages found towards the inner ría reflect their adaptation to
estuarine systems, a pattern also observed in the Ría de Pontevedra (Bernárdez et al., in
prep.).
Rhizosolenia spp. appear during July cruise at the station O, accounting for 4.5% of the
assemblage. In the Ría de Ferrol this genera is representative of the summer blooms at the
outer ría, but its relative percentage is diluted by the appearance of Chaetoceros spp. taxa.
However, in the Ría de Pontevedra appeared in relevant percentages during a winter diatom
bloom in the external areas, and reappears after upwelling under stratification of the water
column (Bernárdez et al., in prep.). Also, some rhizosolenoids species are dominant during
spring and autumn blooms as well as during upwelling events (Casas et al, 1999).
No evidences of freshwater diatoms in the water column were recorded throughout the
sampling period. However, thermohaline stratification induced by continental water is detected
during all cruises, being particularly important in the inner part of the ría (Varela et al., 2003).
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Chapter V
In this way, the non appearance of this group could be partially due to the reduced water
column sampling in the inner areas and the low quantity of freshwater supply at the ría head.
4.2. Distribution of diatom assemblages and biosiliceous material in the surface sediments
The diatom signal in surface sediments depicts a clear W–E pattern, and follows the
opal content distribution (Cobelo-García and Prego, 2004) (Figure V.4, Table V.2). The
distribution patterns of diatom valves abundance show generalised high values at the inner ría
and at the A Malata Bay (4.2×106 valves g-1) indicating a clear gradient inshore-offshore
(Figure V.4). On mean, the abundance is 1.1×106 valves g-1 representing the 30.7% of the
total biosiliceous compounds found in the sediment. This averaging value is representative of
the middle ría. Low abundances characterize the channel and outer ría (down to 1.4×106
valves g-1). At stations located along the middle ría connecting the inner area and the channel-
outer ones, diatom concentration is relatively constant and lower than that found at stations
located in the inner areas. Diatom valves abundance is in accordance to that found in the
surface sediments of the Rías Baixas and continental shelf (Prego et al., 1995, Bao et al.,
1997; Bernárdez et al., in prep.).The geographical pattern in the diatom distribution is also
similar to other rías, with maximum values located close to the ría head. Distribution of the
diatom fragments follow that found for valves, accounting on mean the 22.8% of the total
biogenic silica. Other fragments of diatoms, such as cingulum (diatom girdle bands) were also
found in significative abundance (averaging 5.8% of the BSi) in the medium areas of the ría
and at the northern margin (Figure V.4).
Regarding other siliceous compounds in the sediment their geographical distribution
appeared complex (Figure V.4). One of the main contributors to the biogenic silica are the
sponge spicules remains (averaging 29.9%). Abundance is low in the outer area and in some
of the innermost stations. High contents are found in the medium zone (2.0×106 specimens g-
1) and in the A Malata Bay. Silicoflagellates are present in very low concentrations (maximum
value of 2.5×104 specimens g-1), presenting a scattered distribution pattern. The amount of the
dinoflagellate Actiniscus pentasterias in the surface sediments is very low (mean 3.3×103
specimens g-1). This organism appears in the ría sediments in those stations located at the
northern margin of the outer area and at main channel.
Diatom assemblages of ría-estuarine environments are characterized by the low
percentages of open ocean taxa, in contrast to the Galician-Portuguese shelf (Abrantes, 1988;
Bao et al., 1997; Abrantes and Moita, 1999). In general, diatom assemblages resemble those
from coastal zones in temperate areas, but the Ría de Ferrol is characterized by the large
occurrences of benthic and freshwater taxa.
147
Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
Tabl
e V.
2. S
edim
ent c
hara
cter
izat
ion
at th
e sa
mpl
ing
site
s.
148
Chapter V
The benthic assemblage dominates in the superficial sediments, averaging 55.5%
(range 16.4–86.2%) (Figure V.5). Highest values are found in the inner ría and also at the
northern margin of the outer ría close to the Ferrol city. High percentages are also found
throughout the margins where low depths permit their growth and intertidal environments are
well developed. Low values characterize the main channel and the station located out of the
ría. This pattern is typical of small embayments, fjords, or estuarine environments with inner
protected areas (Hay et al., 2003, Bernárdez et al., 2005; Bernárdez et al., 2006; Bernárdez et
al., in prep.). In our case, even the main channel and the outer station have an important
contribution of the benthic assemblage (~20%), due to the transport from the innermost areas
due to the strong tidal influence and low residence time of the waters within the ría (deCastro
et al., 2003; deCastro et al., 2004). This group can be used as a reliable proxy of the low
depth and intertidal environments, especially in this enclosed and protected environment.
Therefore, percentages up to 60% found in a marine paleoclimatic record recovered in the ría
could indicate that the water column is very low (approximately below 5 m depth).
Despite the presence of allochthonous taxa in the inner ría, high efficiency preservation
conditions can be found landwards. Higher amounts of fine sediments and organic carbon
(Cobelo-García and Prego, 2004) (Table V.2) permit the establishment of reducing conditions
leading to appropriate settings for the diatom conservation.
Chaetoceros R.S. are the second component of the sediment assemblage, accounting
for 23% on average. In general, the higher values are found at the main channel and at the
outer areas, and especially from the station 8 towards the mouth of the ría (Figure V.5).
Distribution pattern is well correlated with the water column production patterns, seasonally
and spatially. Their content in the sediments reflects the summer phytoplankton blooms
influenced by the oceanic waters. In this way, downcore concentrations of Chaetoceros spp.
R.S. would accurately reflect episodes of high spring and summertime productivity.
P. sulcata represents up to 31.3% of the association, with a mean percentage of 11.1%
(Figure V.5). Its thick walls are resistant to dissolution, contributing in high abundance in
coastal sediments. It is commonly associated with the sea-floor diatom community (Zong,
1997). Very low percentages were found in the inner ría, backwards As Pías Bridge. It is very
well represented in the middle area at the southern margin, where low water depths are found,
and in a general view, in the outer ría. Spatial distribution is closely related to that found for
Chaetoceros R.S. indicating that it is more adapted to outer and middle areas of the ría where
the oceanic influence is high. This fact is also observed in the Ría de Pontevedra (Bernárdez
et al,. in prep.). In fact, this species is indicative of the influence of upwelling and high
productivity conditions (Mcquoid and Nordberg, 2003 and references therein).
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
Figure V.4. Contour plots of the abundance per gram of sediment several sediment compounds. Longitudinal axis variation of the abundance each is also shown A) A. pentasterias, B) Porifera, C) Cingulum, D) Phytoliths, E) Crysophycean cysts, F) Palinomorphs, H) Diatom valves, I) Silicoflagellates.
150
Chapter V
Figure V.5. Maps showing the distribution of the main diatom species found in the superficial sediment of the Ría de Ferrol as well as the variation throughout the longitudinal axis of their relative percentage.
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
L. danicus R.S. displays low contents in the sediment. Maximum percentages (~3.7%)
are found in the outer and middle ría, being practically absent from the stations from the
innermost areas (Figure V.5). The sediment distribution reflects its proliferation in the water
column during September, appearing only in the outer stations (Figure V.3). Ecologically, it
represents the input of nutrient-rich shelf waters and water column stratification. In this way as
pointed by Bárcena et al., (2001), this species can be used in the sedimentary record as an
indicator of the last phase of upwelling and beginning of stratification.
Freshwater taxa is restricted to this areas where the main river flowing is found (up to
13.5%) (Figure V.5). Freshwater diatoms are indicators of the sources of continental material
and reduced salinity resulting from freshwater drainage from land. In fact, the lithogenic
components are higher in this area, reflected by the mud and Fe content in surface sediments
(Cobelo-García and Prego, 2004). Throughout the rest of the ría percentages are low, around
2%. In this way, freshwater abundance in down-core sediments from the inner ría reflects river
discharge, assuming that runoff matched by an increased transport of this allochthonous taxa
into the ría. The restricted exchange between the inner ría and the middle areas due to the
presence of the As Pías Bridge impede the transport of these taxa. Moreover, strong tidal
currents during ebb tide could transport in the surface waters a few specimens to the open
ocean. When attempting to interpret the fossil assemblages it is important to know how of
much of the diatoms were tidally transported from inner areas.
The distribution of the freshwater group matches with notable occurrences of other
biosiliceous components such as phytoliths and crysophycean cysts, also considered as
tracers of run-off (Figure V.4). The content of phytoliths (silica-rods bodies derived from grass)
in the ría sediments is relatively high, accounting for the 11.5% of the total biogenic silica.
Abundance is relatively constant throughout the medium-inner ría, showing a homogeneous
distribution, with maximum values of 9.7×105 specimens g-1 in the middle ría and at the
stations located close to the northern margin. Its abundance is below 2.0×105 specimens g-1 at
the outermost sampling sites and those located in the central channel. On the other hand,
crysophycean cysts contribute with low percentages to the biosiliceous material. Maximum
abundances (~5×104 specimens g-1) were sited at the Grande de Xubia River mouth and at the
northern margin of the ría. Abundances around 1–2×104 specimens g-1 were found in the rest
of the sampling sites. Palinomorphs, one of the tracers of continental-derived material,
distribution and abundance is low, displaying a patchy distribution with higher values in
stations located the inner-medium areas in the middle of the longitudinal channel (Figure V.4).
Maximum value is 3.7×104 specimens g-1 located at the station 53.
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Chapter V
T. nitzschioides contributes on 1.4% on average to the total diatom community. Its
distribution is irregular throughout the ría (Figure V.5). Maximum values (~5%) are located at
the outermost stations and at the main channel, being indicative of the influx of nutrient-rich
offshore shelf waters as shown in other areas (Hay et al., 2003). It is also found (1–2%) in
some sampling sites close to the Grande de Xubia River at the inner ría, since in some cases
its growing is controlled by the input of fluvial-derived nutrients (Pokras and Molfino, 1986;
Abrantes, 1988). However, in the Ría de Ferrol case, its contribution to the sediment in the
innermost areas is reduced due to the high abundance of the benthic and freshwater taxa.
This fact reduces the effectiveness of this species as tracer of past changes in river discharge.
Their abundance and corrosion resistant valves merit their use as a paleoceanographic proxy
(Tanimura, 1999). In this way, T. nitzschioides were found also in greater abundance in the
sediment collected material in the Ría de Pontevedra (Varela et al., 2004).
Other diatom genera such as Thalassiosira spp., S. costatum and Rhizosolenia spp.
appear occasionally in the surface sediment and they have a scattered distribution (Figure
V.5). In general, Thalassiosira spp. appear at the middle ría, as found in the water column
(Figure V.3). S. costatum and Rhizosolenia spp. are found at the margins of the ría and at the
outermost station.
In summary, based on the recent diatom and biosiliceous components distributions
several zones were defined and recognized:
Zone 1: This zone of the ría extends from the Belelle and Grande de Xubia rivers mouth
up to the As Pías Bridge. The most representative species of this area are the benthic and
freshwater groups with small occurrences of T. nizschioides and P. sulcata. This assemblage
is associated with the nutrients input by freshwater runoff. The highest amount of diatom
valves and crysophycean cysts, phytoliths and palinomorphs were also detected in this zone
also indicating a high riverine influence. On the other hand, the high surface of intertidal
environments favours the growth of benthic diatoms.
Zone 2: It extends from As Pías bridge to site 10. This zone is principally dominated by
P. sulcata, and Thalassiosira spp. Freshwater and benthic group (~3% and~50% respectively)
have also an important contribution. Diatom valves are present in relatively low values. High
concentrations of porifera, phytoliths, and cingulum the components characterize the area.
This zone represents the combined influence of both marine and freshwater influences. In fact,
the high phytoliths concentration is related to the transport from the inner areas in the upper
surface layers with low salinity. The abundance of all these markers at sites distal from the
river indicates that this area is influenced by both upwelling and high river discharge.
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
Zone 3: This zone extends from sampling site 10 towards open sea, covering the outer
area and the main channel. Our results show that this area is characterized by Chaetoceros
R.S., L. danicus R.S. and T. nitzschioides, together with small contributions of Rhizosolenia
spp. and S. costatum. Thus, this area is linked to the high marine influence caused by the
input of oceanic waters, driving the diatom composition and record in the sediments. This zone
faces the open ocean and receives the influence of the water exchange between the ría and
the shelf. In this way, it appears more influenced by coastal processes, including the small
entrance of shelf waters due to summer upwelling. However, A Malata Bay, although located
in the outer ría presents typical characteristics of the inner areas, with high percentages of the
benthic assemblage, as well as elevated concentrations of the diatom frustules, crysophycean
cysts and phytoliths. This area is affected by the harbour area, and it is relatively protected
from the entrance of marine waters and the mixing of the water column through the channel.
4.3. Paleoimplications
Diatom percentages in the water column and the sediment were compared, using the
average percentages of each species for all sampling campaigns and the relative
concentration in the underlying sediment (Middle-St 11, Channel-St 7, Outer-St 42, Figure
V.6). Patterns of diatom distribution in sea-bottom sediments generally agree with the
proliferation of diatoms abundance in the water column. However, the sediment assemblage
differs from the pelagic one in the middle-inner areas, where the benthic and freshwater
assemblages dominate. In fact, in this area, all species are diluted by the high abundance of
the benthic taxa, accounting for 59.6% and for P. sulcata (14.4%). All the planktonic taxa is
underestimated in this area. Therefore, caution is needed when using the diatom assemblages
for paleoreconstructions as soon as allocthonous taxa appears. Only T. nitzschioides showed
comparable percentages in the water column and sediments, but higher percentages can be
found for stations C and O.
The contribution of Chaetoceros R.S. to the seabed in the main channel is slightly
higher to that found for the water column. This pattern is also detected in the outer ría. The
relative contribution of this group in the sedimentary record is always superestimated due to
their high resistance. When found in the sedimentary record in percentages lower than ca.
20% we can conclude that this sediment corresponds to the innermost areas of the ría.
A common pattern to all sampling stations is the absence in the sediments of N.
longissima. This species could be a good indicator of the winter conditions in the ría, that is,
the mixing of the water column, but this signal is completely removed from the recent
sediments. In this way, if appearing, it is indicative of good preservation conditions. Moreover,
the contribution to the sediments of low silicified species, such as S. costatum and
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Chapter V
Rhizosolenia spp., is very low, hindering their use as environmental tracers. As reported in the
case of N. longissima their occurrence reflects both the adequate oceanographic conditions for
growing and high preservation efficiency in the sediments.
Figure V.6. Average value of the relative contribution of the diatom taxa in the water column during the sampling period and percentage of the diatoms accumulated in surface sediment at sites 42 (Outer), 7 (Channel) and 11 (Middle).
In brief, many processes, such as differential dissolution, lateral advection and
transport, and, hydrographic conditions complicate the relationship between the living diatom
assemblage and that found in the underlying sediments. In this way, inferences about
paleoenvironmental conditions must be based in several species. Attempts to generate
paleoecological transfer functions, in order to characterize the relations between the modern
diatom assemblages and the hydrographic conditions of the Ría de Ferrol, must be the next
step in this study.
5. SUMMARY AND CONCLUSIONS
Diatom community production patterns follow hydrographical changes through the
sampling period. Its record was variable depending on location and season, linked to runoff
supply, water dynamics and circulation within the ría. Diatom production is high, except for the
winter season, due to the low residence time of the ría waters and constant input of river-
derived nutrients. Chaetoceros spp. dominate during spring and summer, together with T.
nitzschioides and Rhizosolenia spp. N. longissima, P. sulcata and benthic taxa characterize
winter season, whereas in autumn L. danicus becomes abundant.
The distribution pattern of diatoms in surface sediments shows a significant relation to
hydrographic conditions, freshwater input and productivity. Several paleoenvironmentally
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Diatom community in a semi-enclosed ría and their contribution to the sedimentary record
significant diatom species address these points. The sedimentary assemblage is composed by
the most solution-resistant of the diatoms found in the water column. The dominant diatom
species in the sedimentary assemblage are those associated with the benthic environment and
with spring and summer blooming conditions. The species composition allows for a
differentiation of oceanic-influenced sediments from the inner-ría ones. Diatoms are abundant
in inner sediments of the ría, influenced by the freshwater runoff and by the low depths
permitting the benthic diatoms assemblage to growth. The transition from the oceanic waters-
affected stations and the inner ones is detected in the middle ría. Diatoms are in low
abundance in offshore sediments, being the planktonic diatoms the dominant assemblage. In
this way, paleoceanographic reconstructions based on sediment cores from the inner basin
should provide detailed records of fluctuations in the ría environments and sea level changes,
whereas cores recovered in the outer ría should yield more information about general patterns
of primary productivity.
Chaetoceros R.S., Rhizosolenia spp. and L. danicus R.S. may be used as indicators of
the oceanic waters flowing into the ría. L. danicus R.S. also tracks the stratification conditions
of the water column without upwelling. N. longissima, if found in the sediments represents the
winter conditions, and well mixed water column. T. nizschioides is linked to high production
conditions during summer and nutrient supply by oceanic waters. However it is indicative also
of nutrient-enrichment by continental waters, but is getting better preserved in the sediments
of the outer areas. These remarks can be used for future paleoenvironmental reconstructions
of the coastal changes and the spatio-temporal evolution of the ría environments.
Acknowledgements
This work was supported by FEDER-CICYT under the project ‘Biogeochemical Processes in the Ferrol Ria: Fertilization by Nutrients and Spatio-temporal Variation of Metals in the Sediment’, ref. 1FD97-0479-C03-02. We are indebted to the crew of the R/V Lura and R/V Mytilus that greatly facilitated sampling and field work, and also to the technicians who participate in all the phases of the laboratory work. Patricia Bernárdez thanks the Spanish Government (FPU program) and Xunta de Galicia for financial support.
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[Chapter VI] LATE HOLOCENE HISTORY OF THE RAINFALL IN THE NW IBERIAN PENINSULA—EVIDENCE FROM A MARINE RECORD∗
1. INTRODUCTION
2. REGIONAL SETTING
3. SAMPLING AND ANALYSES
3.1. Location of the core and sampling
3.2. Procedures and analytical strategies
AMS dating
Grain size, organic carbon, calcium carbonate, nitrogen, opal and terrigenous content determinations
Metal analyses
Preparation sample cleaning and mounting of slides for siliceous compounds counting
Diatom and biosiliceous compounds quantification
4. RESULTS
4.1. Sediment lithostratigraphy, chronology and age-depth model
4.2. C/N ratio and terrigenous content
4.3. Metals content in bulk sediments: Fe, Al, LSi and Ca/Al
4.4. Occurrence of diatoms in marine sediments: Freshwater and benthic assemblages
4.5. Phytoliths, crysophycean cysts and palinomorphs: Biosiliceous land-input indicators
5. DISCUSSION
5.1. Period 1: 4700–3300 cal. yr BP
5.2. Period 2: 3300–1700 cal. yr BP
5.3. Period 3: 1700–1200 cal. yr BP
5.4. Period 4: 1200–0 cal. yr BP
6. SUMMARY AND CONCLUSIONS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Raquel González-Álvarez, Guillermo Francés, Ricardo Prego, Mº Ángeles Bárcena, Oscar E. Romero, in press. Journal of Marine Systems
Abstract. This study reconstructs climatic variability over the last 4700 yr in the NW Iberian Peninsula on the basis of lithological, sedimentological, biogeochemical, micropaleontological (diatoms and biosiliceous compounds) and AMS 14C analyses conducted in a gravity core retrieved from the Galician continental shelf. The core was recovered at the Galicia Mud Patch, a muddy sedimentary body highly influenced by the terrestrial supply of the Miño and Douro rivers, and thus controlled by the rainfall variations over the catchment area. River plume transports the lithogenic and continental-derived compounds to the shelf area allowing us to recognize several periods of terrestrial/marine influence. These periods are well correlated with the lithological units identified. Coarser sediments, high values of Ca/Al, low values of Fe, Al and lithogenic Si (LSi) are representative of the marine-influenced periods. These stages are related to dry conditions and winds coming from the NE under a NAO positive-like phase.
Terrestrial-influenced stages are characterized by muddy sediments, with high content of Fe, Al and LSi, freshwater and benthic diatoms, continental-derived organisms (crysophycean cysts and phytoliths) and high amount of land-derived organic matter as reported by the C/N ratios. The influence of NAO positive- and NAO negative-like periods and solar activity are the two mechanisms quoted to explain the climatic variability during the last 4700 years.
Proxies for the lithogenic input and terrigenous content (non-organic material) show an increase at around 2000–1800 cal. yr BP, linked to the warmer conditions and high precipitation patterns during the Roman Warm Period, and soil erosion due to forest degradation and other anthropic activities. A strong river flow event is recorded in shelf sediments during 800–500 cal. yr BP. A pervasive NAO negative-like period, and the high irradiance registered during the Grand Solar Maximum (GSM) controlled the precipitation and induced a high run-off and riverine influx during this event.
Keywords: paleoclimatology/Late Holocene/North Atlantic Oscillation/river-input/rainfall/diatom assemblages/NW Iberian Peninsula
Resumen. En este estudio se presenta una reconstrucción de la variabilidad climática en el noroeste de la Península Ibérica durante los últimos 4700 años, a partir de análisis litológicos, sedimentológicos, biogeoquímicos, micropaleontológicos (diatomeas y material biosilíceo) y dataciones de 14C llevados a cabo en un testigo de gravedad recogido en la plataforma continental gallega. El testigo fue extraído en la Galicia Mud Match, un cuerpo sedimentario fangoso que se encuentra altamente influenciado por el aporte de material terrestre de los ríos Miño y Duero, y por lo tanto controlado por las variaciones de precipitación en la cuenca de drenaje de ambos ríos. La pluma generada por el elevado aporte fluvial de los ríos transporta el material de origen litogénico y continental hacia la plataforma, lo que permite la identificación de diversos periodos de mayor o menor influencia marina y terrestre. Estos estadios se correlacionan bien con las unidades litológicas que fueron identificadas a lo largo del testigo. Los periodos de más influencia marina se caracterizan por sedimentos más gruesos, valores altos de la relación Ca/Al, valores bajos de contenido en Fe, Al y silicio litogénico (LSi). Estos estadios se relacionan con condiciones áridas y vientos procedentes del NE bajo la influencia de una fase predominante positiva de la NAO (Oscilación del Atlántico Norte).
Los periodos durante los cuales la influencia terrestre es mayor se identifican por la presencia de sedimentos fangosos, altos contenidos de Fe, Al y silicio litogénico (LSi), diatomeas bentónicas y de agua dulce, material biosilíceo aportado desde el continente (quistes de crisófitas y fitolitos) y mayores porcentajes de materia orgánica de origen terrestre derivada de la ratio C/N.
Los indicadores de aporte litogénico y terrígeno (material no orgánico) muestran un incremento alrededor de 2000–1800 cal. yr BP, que está relacionado con condiciones más cálidas y patrones de precipitación alta durante el Periodo Cálido Romano, así como un incremento de la erosión de suelos debido a la degradación de los bosques por las actividades antrópicas. Además, se ha detectado un evento de descarga fluvial muy alta durante 800–500 cal. yr BP, el cual está controlado por el desarrollo de un periodo de NAO negativa dominante sumado a la elevada radiación solar que se registra durante el Máximo Solar (GSM), los cuales inducen la precipitación y por lo tanto la descarga y el aporte hacia la plataforma.
Así, condiciones alternantes de NAO negativa y positiva y la actividad solar son los dos mecanismos que explican esta variabilidad climática y su registro durante los últimos 4700 años.
Palabras clave: paleoclimatología/ Holoceno tardío/Oscilación del Atántico Norte /aporte fluvial/precipitación/asociaciones de diatomeas/NO Península Ibérica
LATE HOLOCENE HISTORY OF THE RAINFALL IN THE NW IBERIAN
PENINSULA—EVIDENCE FROM A MARINE RECORD
1. INTRODUCTION
The view of stability of the present Holocene epoch has been questioned (Meese et al.,
1994; O’Brien et al., 1995; Bond et al., 1997; Bianchi and McCave, 1999; deMenocal et al.,
2000; Mayewski et al., 2004). The stable and warm climate has undergone a series of climate
fluctuations and reorganizations, but their worldwide distribution on a global scale in different
environmental archives and chronology is still being discussed (Bradley and Jones, 1993;
Hughes and Diaz, 1994; Stuiver et al., 1995; Keigwin, 1996; Broecker, 2001). In addition, the
forcing mechanisms of the Holocene changes are still a matter of debate (van Geel et al.,
1999; Crowley 2000; Jones and Mann, 2004).
The NW Iberian Peninsula off the Galician coast is a high sensitive area where climate
is susceptible to change due to variations of the polar front and the Canarian-Iberian upwelling
system. Moreover, this area has also a great potential for determining the hydrographic
interaction between riverine and ocean waters because of the proximity of the Miño and Douro
river mouths (Figure VI.1).
Despite increased and well-deserved attention toward climatic and oceanographic
variability during the Holocene (e.g. Bond et al., 1997; deMenocal et al., 2000), there are few
high-resolution records over this time span for the NW Iberian Peninsula. Recent studies in the
Holocene period in this region have identified several major climatic changes (Martínez-
Cortizas et al., 1999; Diz et al., 2002; Desprat et al., 2003; Abrantes et al., 2005; Álvarez et
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
al., 2005, González-Álvarez et al., 2005c; Martins et al., 2005; Bartels-Jónsdóttir et al., 2006;
Martins et al., 2006a, b; Lebreiro et al., 2006).
Climatic fluctuations have long been noted as being cyclical in nature responding to
several processes. Causes of these variations and abrupt and slight transitions at different
time intervals have been hypothesized, including solar activity, astronomical forcing, CO2,
global thermohaline circulation and oceanography, or oscillations in the atmospheric pressure
system, such as the North Atlantic Oscillation index (NAO).
Despite these climatic factors affecting the sedimentary record, anthropogenic influence
can also be documented as having strong influence. Human impacts in the last 3000 years
registered in a wide variety of temporal archives in the nearby area have been identified by
several authors, such as mining activities (Kylander et al., 2005), and changes in vegetation
(Desprat et al., 2003; Martínez-Cortizas et al., 2005) and in land use. The study of a marine
sediment core as an environmental archive has allowed us to reconstruct the climatic history
of the last 4700 years on a regional and a global scale, and discriminate the possible human
impacts on the record.
We focused on the Late Holocene climate changes and involved processes affecting
the NW Iberian margin region. This paper reports the study of a sedimentary record retrieved
from the Galician continental shelf. Many paleoenvironmental proxies in the core have been
examined, including lithological information, X-Ray radiography, sedimentary structures,
sediment composition, metal content and diatom flora and biosiliceous compounds. This
multivariable approach is needed in order to get a more comprehensive view of the physical
and biogeochemical processes involved. In the present manuscript, the main objective is to
outline the environmental evolution of climate and anthropogenic influence in the NW Iberian
Peninsula extending back 4700 cal. yr BP. This study is specifically targeted to use the
terrestrial-derived fractions as a potential index of climatic, hydrologic and past fluvial input
conditions over the NW Iberian Peninsula. We focus on the description of alternating
wetter/drier periods and marine/terrestrial influence. This paper also explores the forcing
factors of climate in the NW Iberian area and how these climatic signals are recorded in the
sediments. In addition, we address the influence of local human activities on land and its
record in the continental shelf sediments.
2. REGIONAL SETTING
This study has been carried out in the Galician continental shelf, NW Iberian Peninsula.
This coast is characterized by the presence of the Rías Baixas, small coastal embayments
drowned by the sea during the last transgression (Rey, 1993). In this area, the shelf is
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relatively narrow. The slope change is located at a distance of 15–30 km from the shoreline
(Figure VI.1). Surface sediments have been described as fine silty-clays, building up the
Galicia Mud Patch, a sedimentary body situated in the middle shelf. In deeper and coast areas
sandy sediments cover the seabed. The Galicia Mud Patch is an elongated North-South-
orientated deposit, that yields and age of 2650±280 years BP at about 80 cm depth in the
portuguese part (Drago, 1995), extending off the Miño River. The origin of the Galicia Mud
Patch is related to the abundant supply of sediment, especially during episodic flood events,
shelf morphology and hydrographic conditions that favour the accumulation of muddy
sediments (Dias et al., 2002b; Jouanneau et al., 2002; Vitorino et al., 2002a, b) (Figure VI.1).
The main sediment source of clayey-silty sediments deposited at the Galician shelf, as shown
by their geochemical properties, come from the Douro River (Araújo et al., 2002; Oliveira et
al., 2002a, b), although the contribution of the Miño River is not negligible. These fine
sediments were remobilized and transported from the adjacent Portuguese shelf towards the
Galicia Mud Patch by several oceanographical mechanisms (Vitorino et al., 2002a, b). River
influx at the heads of the Rías is very low and export of sediment from Rías to the shelf is
small (Rey, 1993).
Sediment spreading off-shelf is low, since sediment transport when river floods, storms
and riverine supplies are high, is northwards (Jouanneau et al., 2002). Sedimentation rates in
the Portuguese-Galician shelf range overall from 0.05 to 0.40 cm y-1 (Jouanneau et al., 2002).
In nearby cores sedimentation rates vary between 0.009-0.19 cm y-1 (Jouanneau et al., 2002;
González-Álvarez et al., 2005c; Martins et al., 2006a, b).
Galician-shelf area is located in the northern boundary of the Canarian-Iberian
upwelling system, where the along-shoreline winds interact with the topography to generate
upwelling-downwelling dynamics with a marked seasonal cycle (Wooster et al., 1976; McClain
et al., 1986). The position of the Azores anticyclone and the Iceland Low determines the
passage of fronts in this area, leading to two characteristic situations. During winter, the
Azores anticyclone is located in the northwest African coast and a low-pressure center over
Iceland. This situation induces prevailing southwesterly winds blowing over the shelf and
resulting in a reversal of the typical circulation and the development of a downwelling regime
(Vitorino et al., 2002a, b). On the contrary, in spring and summer the Azores anticyclone
moves to the North inducing high pressures with N-NNE winds, and generating frequent
upwelling events off the coast and the intrusion of the Eastern North Atlantic Central Water
mass (ENACW) (Fraga, 1981; Prego and Bao, 1997).
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Figure VI.1. Localization of the study area on the northwestern Iberian margin. Simplified map showing the present surface sediments distribution of the Galician continental shelf (modified from Dias et al. 2002a) and the location of the core SMP02-3.
Other typical oceanographic features in winter are the Western Iberian Buoyant Plume (WIBP)
and the Iberian Poleward Current (IPC) flowing northwards along the Galician-Portuguese
shelf border (Haynes and Barton, 1990; Frouin et al., 1990; Peliz et al., 2003; Varela et al.,
2005). The WIBP is a low-salinity surface water mass driven by the winter-intensified runoff of
the rivers flowing on the NW Iberian Peninsula (Peliz et al., 2002). The discharge regime of
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Miño and Douro rivers is influenced by Atlantic fronts which cross the Iberian Peninsula mostly
during winter. Under favourable winds and high riverine discharge, the river plume spreads
northwards of the river mouth, towards the Rías Baixas (Álvarez et al., 2006). During typical
non-upwelling winter conditions, the plume is confined to the inner shelf (Peliz et al., 2002).
Nutrient fluxes and input of detrital and terrestrial particles to the shelf from river flood events
are high during this situation. The stratification conditions at the surface layer, induced by the
presence of the WIBP, and the high nutrient levels can be suitable conditions for
phytoplankton growth.
The atmospheric sea level pressure difference between the Azores High and the
Iceland Low, defined as NAO index, also controls the regional patterns of precipitation and
aridity (Björck et al., 2006). Over the Iberian Peninsula, atmospheric storminess and increased
precipitation coincide with NAO-negative periods (Hurrell, 1995), whereas lower humidity on
the continent and therefore, a dry period, is linked to NAO-positive phases. This parameter
varies on decadal timescales, but it shows extending phases of both positive and negative
periods. In this way, this parameter should be a useful mechanism for explaining climatic
conditions and extreme flood events related with increased rainfall.
3. SAMPLING AND ANALYSES
3.1. Location of the core and sampling
The material used in this study was obtained from the gravity core SMP02-3
(42º02.207’N, 9º02.363’W, 260 cm long, 121 m below sea level), collected in 2002 at the
Galicia Mud Patch (Figure VI.1). The core was sealed just after collection and kept in storage
refrigerated at 4°C until analyses were performed in the laboratory. The core was routinely
sectioned longitudinally in two halves, visually described and photographed. After splitting, 1-
cm thick and 30-cm long slices were removed from one half of the core to perform
radiographical analyses using Cabinet X-ray System (Faxitron Series, Hewlett-Packard). The
working-halves of the gravity core were sampled into 1-cm thick horizons. Each slice was then
divided into subsamples for different studies within the project: metals and biogenic
components analyses, grain size determinations, microsiliceous fossils and siliceous
compounds counts. Content of Fe, Al and Si, Total carbon (TC), Total nitrogen (TN), CaCO3
and opal determinations were conducted every 6 cm.
3.2. Procedures and analytical strategies
AMS dating
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Age control was established using six 14C AMS (accelerator mass spectrometry) dates
on planktonic foraminifera determined on ~10 mg of carbonate at the Geochron Laboratories
(Massachusetts, USA) and at the AMS facility of the Aarhus University (Denmark) (Table VI.1).
All 14C dates were corrected for 13C and for the marine reservoir age (δR=376±46, +400 years;
∆R=0) and then converted into calendar years (cal. yr BP) using the CALIB 5.0.1 software
(modified version 2002; Stuiver and Reimer 1993; Hughen et al., 2004). In upwelling-
influenced areas, such as off Galicia, reservoir ages may be higher than the global reservoir
correction value. However, as pointed out by Abrantes et al. (2005), the local reservoir-effect
correction is negligible and was not applied. A calibrated age range within two sigma
confidence limits was obtained, being ages mentioned in this manuscript expressed in
calibrated years before 1950 (cal. yr BP).
Table VI.1. Radiocarbon dates and calibrated ages from core SMP02-3. Samples were pretreated and measured at the radiocarbon laboratories of Geochron Laboratories, USA and AMS 14C Dating Centre of the Aarhus University, Denmark. The age estimations were derived from the intercepts of the radiocarbon age plus and minus two times the total standard deviation of the age (2σ) with the linear interpolation of the marine calibration data set (Marine 04, Hughen et al., 2004). Conversions of radiocarbon ages to calibrated ages are worked out by using the CALIB 5.0.1 program after Stuiver and Reimer (1993, modified 2002). No local reservoir effect has been applied.
Lab. number
Spliced depth (cm)
Sample type 14C AMS raw age (yr BP)
δ13C (‰)
Calibrated age 2σ (yr BP)
Calendar age (AD/BC)
GX-30664-AMS 5-6 Mixed planktonic foraminifera 580±30 -1.7 126 (207) 288 1662 (1743) 1824 AD
GX-32033-AMS 91-22 Mixed planktonic foraminifera 1310±40 -1.1 748 (841.5) 935 1015 (1108.5) 1202 AD
AAR-9450 149-150 Mixed planktonic foraminifera 1714±41 -0.74 1174 (1259.5) 1345 605 (690.5) 776 AD
GX-32034-AMS 194-195 Mixed planktonic foraminifera 2840±50 -0.4 2427 (2577) 2727 778 (627) 478 BC
AAR-9451 203-204 Mixed planktonic foraminifera 3458±50 -0.61 3207 (3330) 3453 1504 (1380) 1258 BC
GX-30665-AMS 254-255 N. pachyderma (right-coiling) 4460±40 -0.3 4517 (4652) 4787 2838 (2702) 2568 BC
Grain size, organic carbon, calcium carbonate, nitrogen, opal and terrigenous content determinations
Grain size analyses were performed every 6 cm. Sample treatment consisted of organic
matter removal with hydrogen peroxide (H2O2) and dispersion with sodium
hexametaphosphate. The coarse fraction (>63 µm) was separated from the fine fraction by wet
sieving. Grain-size distribution of the coarse fraction was determined by wet-sieving, with
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sieve columns ranging from 8 mm to 63 µm, spaced at 1 phi. Fine fraction (<63 µm)
distribution was determined by a X-Ray nephelometry technique using a Micromeritics,
Sedigraph 5100.
For the geochemical analyses, sediment samples were kept in storage in plastic bags at
4ºC until analyses were performed in the laboratory. Samples were also analyzed every 6 cm,
dried and ground to powder before processing. Total carbon (TC) and total nitrogen (TN) were
measured with a LECO CN-2000 elemental analyzer at the Research Support Service (CACTI)
of the University of Vigo. TC was measured by high-temperature combustion and detection of
the gaseous by-products. Inorganic carbon (TIC) was measured with a LECO CC-100 analyzer
attached to the CN-2000. The gases produced with the CC-100 are analyzed by the CN-2000,
and then, the percentage of TIC in the sample from the CO2 released is calculated. Organic
carbon (TOC) was then determined as the difference between TC and TIC. Calcium carbonate
(CaCO3) content was calculated by multiplying the TIC using the molecular mass ratio 8.33,
assuming that all the inorganic carbon is in the form of calcium carbonate. In order to
investigate if the organic carbon is of marine or terrestrial derived origin, the TOC/TN ratio
(hereafter C/N ratio) was calculated as the percentage weight of organic carbon versus the
percentage weight of nitrogen.
The amount of biogenic silica contained in the bulk sediment was determined by direct
dissolution using a wet alkaline leaching procedure devised by Mortlock and Froelich (1989).
Dissolved silicate extracted during leaching was measured by means of the molybdate blue
spectrophotometry according to Hansen and Grashoff (1983) using a continuous flow analyser
AutoAnalyser Technicon II. The analytical deviation of the method was determined analysing
two or more replicates of each sample, striving for a deviation below ±0.2% (Bernárdez et al.,
2005).
Percentage of terrigenous content of the core sediment was calculated as the residual
after subtracting the measured contributions of the main biogenic components (i.e. TN, opal,
TOC and CaCO3).
Metal analyses
Previously to metal determinations, around 40–50 mg of dry bulk sediment were
microwave-digested in 6 ml HNO3 (65%) and 2 ml HF (48%) in Teflon® bombs using a
Milestone MLS 1200 Mega microwave oven following the EPA 3052 guideline (EPA, 1996) for
siliceous-type matrices. Handling and analysis of samples were carried out in a clean
laboratory. Plastic labware employed for sampling, storage and sample treatment was
previously acid-washed (HNO3 10%) for at least 48 h and rinsed throughout with Milli-Q water
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
(Millipore). Fe, Al and Si, were determined using flame atomic absorption spectrometry (FAAS)
technique with a Varian 220FS apparatus. The accuracy of the analytical procedure was
checked using certified reference materials (CRMs), PACS-2 from the National Research
Council of Canada. The results obtained (Table VI.2), agree well with the certified values and
the relative standard deviations (RSDs) were typically lower than 10%, excepting Si (14.2%).
Table VI.2. Results of the analysis of the PACS-2 (NRCC, Canada) certified reference sediment.
PACS-2 Al (mg g-1) Fe (mg g-1) Si (mg g-1)
Measured value 67.6±3.9 n=3
41.6±2.0 n=3
279.7±39.7 n=4
Certified value 66.1±5.3 40.9±0.6 ~276*
*Information value only
The Si percentage due to lithogenic compounds, or lithogenic Si content (LSi), was
calculated by subtracting the total Si measured by FAAS and the biogenic Si contribution (BSi)
derived from the biogenic opal, determined by applying the molar weight of amorphous opal
(SiO2) and Si.
The Ca/Al ratio was obtained after the Ca content determined using the Ca and CaCO3
weight molar ratio and assuming that all the inorganic carbon is in this chemical form, versus
the total Al content.
Preparation sample cleaning and mounting of slides for siliceous compounds counting
Preparation of the samples was done following the method proposed by Abrantes
(1988). A fixed amount of dry sediment (2 grams) was placed in 600 ml beakers and treated
with hydrochloric acid (HCl) and hydrogen peroxide (H2O2 110 vols.) for carbonate and organic
matter destruction. First, the reaction took place over a hot plate at room temperature until the
reaction stopped. Distilled water was added and left to settle for about 8 h and then, the
excess of liquid was removed by means of a vacuum pump. Clay fraction was removed by
adding pyrophosphate sodium and distilled water to the beakers, leaving for 8 h and removing
the excess of liquid with the vacuum pump. This process was repeated until no clays remained
in the suspension. For each sample, total volume and suspension volume used to mount the
slides were known. For slide preparation, suspension was strewn evenly onto cleaned 18×18
mm cover slips placed in a circular Petri dish (Battarbee, 1973) after stirring the solution for
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homogenization. After the trays had been dried, slides were removed and assembled with a
toluene-based synthetic resin mounting medium (Permount™, Fisher Scientific).
Diatom and biosiliceous compounds quantification
Qualitative and quantitative analyses were performed with a LEICA DMLB and a Zeiss
light microscope with phase contrast illumination, using a ×100/1.00 planapochromatic oil-
immersion objective. Several transverses across the cover slips were examined depending on
the diatom abundance (1/3 to 1/16). For each sample, 300–500 diatom valves were identified,
and raw counts were converted into relative abundances. Schrader and Gersonde’s (1978)
counting protocol was followed for diatom counts and total number estimates. Number of the
biosiliceous compounds of the sediment, such as diatom fragments, silicoflagellates, sponge
spicules, phytoliths, crysophycean cysts and radiolaria per gram of sediment was also
assessed, as well as palynomorphs abundance. Grass phytoliths were identified on the basis
of the classification of Twiss et al. (1969) and Madella et al. (2005).
Diatom genera identification was based on the work by Round et al. (1990) and,
whenever possible, each individual was identified to the species level following author’s
description and specific bibliography (Hustedt, 1930; Hustedt, 1959; Hartley, 1996; Hasle and
Syvertsen, 1996; Witkowski et al., 2000, and other taxonomic sources). The diatom taxa were
also grouped by their ecological significance.
Diatoms are relatively abundant in the sediment along the Galician-Portuguese margin
(Bao et al., 1997; Abrantes and Moita, 1999) reflecting water column productivity conditions
induced by upwelling. They can be used as a powerful paleoclimate indicators in the
sedimentary record. Freshwater and the benthic assemblages are the ecological groups used
in this work, since they are indicators of material transported from the coastal zone to the shelf
area. Benthic group includes forms living attached to a substratum or associated with
sediments. Since benthic diatoms are limited to the euphotic zone, high percentages of this
group at the core site indicate offshore transport. The contribution of freshwater and benthic
diatoms is especially significant, since the presence of these species in the marine domain
may help to track intensity of river flow or direction of currents that divert the river plume. In
this case we use the reworked origin of both assemblages as a proxy of riverine input.
Species and genera included in the freshwater group are Achnanthes lanceolata (de
Brébisson in Kützing) Grunow in Cleve et Grunow 1880, Aulacoseria spp. Thwaites 1848,
Ctenophora pulchella (Ralfs ex Kützing) Williams et Round 1986, Cyclotella spp. (Kützing) de
Brébisson 1838, Cymbella spp. Agardh 1830, Diatoma spp. Bory 1824, Epithemia spp. Kützing
1844, Eunotia spp. Ehrenberg 1837, Fragilaria complex, Gomphocymbella spp. Muller 1905,
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Gomphonema spp. Ehrenberg 1832, Hannaea arcus (Ehrenberg) Patrick 1966, Hantzschia
amphioxys (Ehrenberg) Grunow in Cleve and Grunow 1880, Luticola spp. (Kützing) Mann
1990, Meridion circulare (Greville) Agardh 1831, Pinnularia spp. Ehrenberg 1843, Sellaphora
spp. Mereschkowsky 1902, Stauroneis spp. Ehrenberg 1843, Stephanodiscus spp. Ehrenberg
1845, Synedra spp. Ehrenberg 1830 and Tabellaria spp. Ehrenberg 1840. The benthic
assemblage is composed by Achnanthes spp. Bory 1822, Amphora spp. (Ehrenberg) Kützing
1844, Auliscus sculptus (Smith) Ralfs in Pritchard 1864, Campyloneis grevillei (Smith) Grunow
1867, Cerautulus turgidus Ehrenberg 1843, Cocconeis spp. Ehrenberg 1837, Diploneis spp.
(Ehrenberg) Cleve 1894, Grammatophora spp. Ehrenberg 1840, Hantzschia spp. Grunow
1877, Mastogloia spp. (Thwaites) Smith 1856, Navicula distans (Smith) Ralfs in Pritchard,
1861, N. digitoradiata (Gregory) Ralfs in Pritchard 1861, N. palpebralis (de Brébisson) Smith
1853, Mastogloia spp. (Thwaites) Smith 1856, Opephora spp. Petit 1888, Pleurosigma spp.
Smith 1852, Podosira stelligera (Bailey) Mann 1907, Psammodiscus nitidus (Gregory) Round
and Mann 1980, Surirella fastuosa (Ehrenberg) Kützing, 1844, Toxarium spp. Bailey 1854,
Trachyneis aspera (Ehrenberg) Cleve 1894 and Tryblionella spp. Smith (1853) (see taxonomic
appendix).
4. RESULTS
4.1. Sediment lithostratigraphy, chronology and age-depth model
Core sediment at SMP02-3 mainly consists of terrigenous and detrital material, quartz
and glauconite grains, with a lesser biogenic fraction, containing low abundance of calcareous
and siliceous microfossils. Four sedimentary units or intervals were recognized as obtained by
visual observations, X-Ray radiographs and grain size distribution (Figure VI.2): (i) Unit 1 from
260 to 204 cm (5Y 2.5/2 Munsell Soil Colour Chart®, 4700–3300 cal. yr BP) is composed by
glauconitic sands, bioclastic remains and quartz. It is characterized by high carbonate content
and low values of opal and organic carbon. This sedimentary unit is a fining upwards
sequence and it is slightly bioturbated at the top. (ii) Unit 2, from 204 to 163 cm (3300–1700
cal. yr BP) is composed by homogeneous clayish sediments (Colour 2.5Y 4/3, Munsell Soil
Colour Chart®) with very low values of the biogenic compounds. (iii) Unit 3, from 163 to 130
cm (1700–1200 cal. yr BP) is characterized by coarser sediments than the previous one. This
unit is slightly laminated. (iv) The upper greenish muddy interval, Unit 4 (1200–0 cal. yr BP),
has the same characteristics as the present seabed in the core of the Galicia Mud Patch. This
unit registers highly organic silt and clay with sparse gastropods and bivalve broken shells. In
a general view, textural and lithological patterns indicate a progressive decrease in energy
from the bottom to the top of the core.
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Chapter VI
Figure VI.2. Sedimentological features and lithological description of the core. C: Clay. S: Silt. Sd: Sand. VF: Very fine sand. F: Fine sand. M: Medium sand. Co: Coarse sand.
The age-depth model is based on six calibrated AMS 14C dates (Figure VI.3) of the tests
of mixed planktonic foraminifera (Table VI.1). These ages provide an excellent chronology of
this sedimentary record. Ages between dated levels were obtained by linear interpolation
between the nearest AMS-dated points. The top of the core (0–1 cm) is assumed to
correspond to 0 cal. yr BP. Sedimentation rate ranges between 0.12 and 1.39 mm yr-1. A sharp
increase in sedimentation rate is identified at 150 cm, implying a major change in
sedimentation conditions at this level (Figure VI.3).
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Figure VI.3. Age versus depth model for core SMP02-3 based on calibrated ages listed in Table VI.1. Solid line represents the theoretical age model assuming linear sedimentation rates between 14C dated levels (black circles). Down-core variations of the δ13C (‰) in foraminiferal tests, sedimentation rate (mm yr-1).
4.2.C/N ratio and terrigenous content
C/N ratio and TOC show the same downcore profile, due to the stability of the values of
TN content. C/N ratio registers low values at the bottom of the core (~7.6), increasing
progressively to values around 13 at 3300 cal. yr BP. Afterwards, the profile becomes stable,
with values around 10, but at 1200 cal. yr BP a sharp rise is identified. To the top of the core,
C/N values range between 13 and 19 (Figure VI.4). Shelf sediments receive autochthonous
particulate matter, mainly plankton due to the important biological activity in its waters, and
allochthonous particulate matter derived from river runoff. Assuming that elevated C/N ratios
(>20) are of terrestrial origin (continental vegetation) and marine phytoplankton displays C/N
ratios between 5–10, the sharp shift at 1200 cal. yr BP would be related to a change in the
intensity of terrestrial input into the Galician shelf.
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Terrigenous fraction clearly dominates the composition of the sediment, accounting for
78.2 to 95.6% of the bulk sediment. The amount of detritics follows an opposite trend to the
calcium carbonate distribution, and shows a good linear correlation (r2=0.64) with the sum of
Fe, Al and LSi content (p<0.01). Lower percentages are recorded at the bottom of the core
increasing progressively up to reach the maximum value (95.6%) around 2000 cal. yr BP. A
small decrease was recorded latter, but between 870 cal. yr BP to the present, a sharp
increase in the percentage occurs, even reaching values of 94% (Figure VI.4).
4.3. Metals content in bulk sediments: Fe, Al, LSi and Ca/Al
The vertical distribution of several metals in the bulk sediment is shown in Figure VI.4.
Fe content along the core is in the range of 17.8–48.49 mg g-1. Maximum values of the metals
considered for this study are generally registered in the muddy upper sequences, especially in
the Unit 4. Concentrations are low at the bottom of the core (17.8 mg g-1, Unit 1), progressively
increasing up to 45.3 mg g-1 at 2500 cal. yr BP and ~45 mg g-1 between 1990 and 1816 cal. yr
BP. A second peak in the Fe content is recorded between 790–540 cal. yr BP. Al content
varies from 33.22 mg g-1 to 118.76 mg g-1, showing a prominent peak between 790–540 cal. yr
BP, as observed for Fe. High Al content is also detected around 2000 cal. yr BP.
LSi profile ranges between 77.01 and 358.26 mg g-1. Values steadily increase from the
bottom of the core towards the top, peaking at 870–520 cal. yr BP. Its distribution displays a
similar pattern to those found for Fe and Al, however around 2000 cal. yr BP the LSi
concentration diminishes (Figure VI.4).
Ca/Al downcore variations mirror the distribution of the terrigenous material (Figure
VI.4, note the inversed scale). Ca/Al ratio is used as a tracer of fluctuations of the terrigenous
input into the continental shelf. Lower ratios indicate stronger discharge of terrigenous material
and the subsequent dilution of marine biogenic carbonate fraction. This marker also shows
distinct changes at 3300 cal. yr BP and at 1200 cal. yr BP. Lowest values are found between
2500–1640 cal. yr BP (Figure VI.4).
4.4. Occurrence of diatoms in marine sediments: Freshwater and benthic assemblages
Diatom abundance varies depending on diatom productivity, preservation and/or
dissolution of the diatom valves, and dilution with terrigenous and/or organic supplies. The
number of diatom valves per gram of sediment (not shown) has an irregular distribution
throughout the core. Large fluctuations varying from 7.5×104–1.6×105 valves g-1 to values up
to 2.5×106 at 790–613 cal. yr BP occur. Two levels barren of diatoms (4700–3300 and 2100–
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Chapter VI
1100 cal. yr BP) point out to changes in the silica preservation conditions or low primary
production rates.
The freshwater and benthic diatom groups plotted against time are shown in Figure
VI.4. The diatom assemblage of the freshwater flora is not especially abundant throughout the
core, ranging from 1.1 to 4.8%, averaging 2.8%. Higher abundances occurs between 870–610
cal. yr BP. Although wind could also bring freshwater diatoms to the marine area, we infer that
this aeolian transport is negligible in comparison with the run-off transport. The relative high
abundance of this group likely reflects the influence of the discharge of the rivers to the
continental shelf and a subsequent northward transport.
Relative abundance of the benthic assemblage falls in a similar range that of freshwater
(2.4–5.7%), accounting averagely for 3.9%. Downcore profile also shows a small peak around
800–520 cal. yr BP (Figure VI.4).
4.5. Phytoliths, crysophycean cysts and palinomorphs: Biosiliceous land-input indicators
Phytoliths abundance displays a relatively stable profile along the core, but it gradually
increases from 3300 cal. yr BP to ca. 2000 cal. yr BP, showing a small peak at 1800 cal. yr
BP. High phytoliths content is recorded at 790–440 cal. yr BP reaching the maximum value at
790 cal. yr BP (Figure VI.4).
Crysophycean cysts per gram of sediment have very low and constant values
throughout the core. However, larger fluctuations in abundance occur in the upper part of the
core, corresponding to 790–610 cal. yr BP, reaching values between 1.18×105 and 5.45×104
phytoliths g-1 (Figure VI.4).
The amount of palinomorphs displays a constant profile, with very low values from the
bottom of the core up to 960 cal. yr BP. A sharp increase in the abundance is detected at 613–
347 cal. yr BP. Pollen influx is high in the recent period (Figure VI.4).
5. DISCUSSION
Core SMP02-3 provides an excellent record of the climatic changes occurred during the
past 4700 yr in the Galician shelf. Lithostratigraphic information provided the first approach to
the sedimentary conditions in the marine environment. Paleoenvironmental interpretation was
carried out taking into account the environmental-derived signals described for each
parameter, together with sedimentological, geochemical and micropaleontological. Four
sedimentary units are defined on the basis of lithological characteristics revealing changing
sedimentological conditions and different oceanographic situations. Thus, the
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Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
paleoenvironmental analysis of Galician continental shelf record is explained below for each of
the four defined time periods. Moreover, several periods with a distinct degree of
marine/terrestrial influence were recognized in the core as shown by the terrestrial and
lithogenic input tracers. The mechanisms put forward to explain these sedimentological and
geochemical shifts have involved several processes and driving climate forcings as discussed
below. Finally, our observations were compared with other paleoenvironmental data in the
same area.
5.1. Period 1: 4700–3300 cal. yr BP
This interval is characterized by a fining upwards sequence from the bottom of the core
up to the 204 cm depth. Relatively high sand content and high abundance of bioclast remains
(Figure VI.2) indicate energetic hydrodynamic conditions. High abundances of calcium
carbonate remains, related to the marine production, were recorded at the core bottom. Ca/Al
ratio is also in the highest values at this period.
C/N values ranging between 6.7 and 12, indicate that there is a high contribution of
marine-derived organic material. Terrigenous material, as well as low contents of Fe, Al and
LSi also show a persistent marine control. Phytoliths and crysophycean cysts present very low
abundances in this phase. The progressive increasing in the C/N values, as well as a higher
amount of terrestrial influx proxies and biosiliceous continental compounds point to a
increasing riverine input towards the end of this period (3300 cal. yr BP).
Low values of organic carbon (not shown) as well as the high energetic conditions
registered during the deposition of this sandy sequence imply oxic sediment conditions. As a
result, diatom frustules are not preserved during this interval due to the low preservation
efficiency of this biosiliceous material in sandy sediments (Ragueneau et al., 2000). Moreover,
the top of this sequence is highly bioturbated, this being indicative of dissolution of the diatoms
remains (Hay et al., 2003). The benthic foraminifera assemblage studied on a nearby core is
composed by species related to full oxygenation conditions and relatively high velocity flows
(Martins et al., 2006a). The prevalence of a winter regime, rains and downwelling conditions
under SW winds at this period favoured the deposition of coarser sediments (Martins et al.,
2006a). Our data, however, do not support their interpretation. In fact, the sediments
recovered at this period, mainly very fine sand, indicate marine influence (high abundance of
foraminiferal tests, high Ca/Al ratios) and low supply of fine-material derived from the riverine
plume when precipitation is high (Abrantes et al., 2005) (Figure VI.5).
184
Chapter VI
Figu
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185
Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
Between 4700 and 3300 cal. yr. B.P., a warm and dry period characterized by low
nutrient levels and productivity at the inner Galician shelf occurred, as also revealed by
planktonic foraminifera (González-Álvarez et al., 2005a, b). Around 3900–3600 cal. yr BP,
aeolian sediments in the Cíes beach barrier-lagoon and at a the Traba coastal wetland were
deposited, induced by arid climatic conditions and N-NE winds (Bao et al., 2007; Costas,
2006). Furthermore, analyses of different paleoenvironmental records (e.g. peat bogs) in NW
Iberian Peninsula suggest that a series of abrupt climatic changes occurred during this period,
involving alternate episodes of cooling, drought and increased rainfall and wind intensification.
Important variations in humidity were found by Fábregas-Valcarce et al. (2003) at 4700–4000
cal. yr BP and 3400–3000 cal. yr BP in different environmental archives. However, these rainy
episodes were not detected in our marine core. Only a slight increase of the terrestrial-derived
particles at 3300 cal. yr BP in our record might be indicative of the last humid phase described
by these authors. Cold temperatures, low rainfall and elevated wind strength at 4000–3400
cal. yr BP are consistent with the paleoenvironmental conditions recorded at our record
(Figure VI.5).
In summary, this period was characterized by the reduced flux of land-derived organic
matter to the bottom. The degree of the continental influence increases progressively to the
top of this sequence. A wind pattern with prevailing NE component is invoked to be the forcing
mechanism that explains this environmental setting under a cold and arid situation.
5.2. Period 2: 3300–1700 cal. yr BP
This period spans from 3300 to 1700 cal. yr BP, although as shown in Figure VI.4 our
record lacks of data of geochemical and microsiliceous proxies in the lower part. However, we
have extended it from 2500 back to 3300 cal. yr BP based on the lithology of the core, which is
characterized by an abrupt drop in the organic material content and elevated concentrations of
fine sediments, mainly clays. The high amount of fine material suggests an increase in the
terrigenous sediment influx to the shallow Galician shelf. Variation in the fine material content
at our core site could be an indicator of the terrigenous supply due to rainfall events leading to
floods at the shelf area with a high fine suspended sediment load material.
Increasing values of Fe, Al, LSi and detrital material suggest that riverine influence is
higher in this period than in the previous one. Al follows the Fe profile quite closely (correlation
coefficient R2=0.87), indicating that Fe is also a well-accepted marker of terrestrial input. Al is
interpreted as a proxy of aluminosilicates content in the sediment and it is introduced into the
marine system mainly by river run-off. LSi is considered a good proxy of the lithogenic silicate
compounds of the sediment, mainly by quartz particles. Si enrichment of sediments over the
inner shelf may result from a preferential settling of the coarser quartz particles (Araújo et al.,
186
Chapter VI
2002). In addition, the Miño river sediments have a relatively high Si content, being
predominantly coarse quartz particles (Araújo et al., 2002). The high content of ash coals are
also indicative of the high contribution of the organic matter from the continental domain,
explained by general deforestation and occurrence of fires after 4000 cal. yr BP (Santos et al.,
2000). The occurrence of freshwater diatom flora and benthic species in this period also
suggests an increased release of freshwater to the Galician shelf. Frequent salinity changes,
as well as the establishment of a restricted environment, was identified in the Ría de Vigo from
3000 to around 1800 cal. yr BP (Diz et al., 2002). High alkenone-derived SST (Diz et al., 2002)
and a warm cocolith assemblage (Álvarez et al., 2005) in the Ría de Vigo also support the
interpretation of a warm and humid environment during this period in the Galician area (Figure
VI.5).
The most remarkable event during this period occurs ca. 2000–1700 cal. yr BP. This
event is characterized by an increase in Fe, Al, phytoliths abundance and terrigenous
percentage. The C/N ratio, however, falls in the range of marine organic matter which could
indicate a reduced supply of organic matter of continental provenance. NE winds from land
could have transported the ferroalumino-particles and phytoliths to the marine domain, but this
climatic scenario would point to relatively dry and arid conditions, an explanation that the
paleoenvironmental markers do not support. In fact, Desprat et al. (2003) identified a warm
period and the development of temperate vegetation during the Roman colonization in Galicia
(RWP, peaking at 1800 cal. yr BP), suggesting relatively humid conditions (Figure VI.5). In the
Tagus depocentre deposit an excess precipitation period was also recorded around 2000 cal.
yr BP (0–550 AD) (Abrantes et al., 2005) related with more frequent NAO negative-like
periods. Moreover, in the Ría de Muros, the northernmost of the Rías Baixas, an event
recording high values of Fe and Ti was also identified at ca. 2000 cal. yr BP (Lebreiro et al.,
2006). These authors pointed to a climatic-triggered mechanism to explain their record.
However, this increase in the terrigenous markers can also be explained by anthropic-derived
activities on land. In this way, the raise in the lithogenic indicators could be explained by the
gold mining by Romans in the Miño River catchment area (Pérez García et al., 2000). One of
the most important tributaries of the Miño River, the Sil River, underwent an intensive mining
activity around 2000 years ago, through elaborate hydraulic mining methods using water
supplied by hundreds of kilometres of canals that traversed the mountains leading to lode
deposits (Spiering et al., 2000). Martínez-Cortizas et al. (2005) identified during Roman times
a period of forest clearance. Therefore, anthropic activities on land together with a NAO
negative phase could induce an increase in soil erosion and the subsequent record in the shelf
sediments.
187
Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
5.3. Period 3: 1700–1200 cal. yr BP
During this period low values of the C/N ratio are recorded, and Fe, Al and detritic
compounds show an important decreasing trend. On the other hand, higher values of the Ca/Al
ratio as well as a slightly rise in sand content is observed. Low diatom abundances, as
consequence of strong dissolution and/or low production, result in the impossibility of define
the diatom assemblage. Paleoproxies point to a return to marine influence during this short
period, that can be considered an analog to the first described period 1 (4700-3300 cal. yr BP).
Driving climate mechanisms point to a recover in the relatively dry conditions in the
continent. This dry and cold climatic situation is triggered by N and NE winds prevailing on
land, occurring for example when NAO index is positive. Gil et al. (2006) reconstruction of the
position of the Icelandic low and Azores high during Dark Ages (DA) indicates that Azores high
was located over the Iberian Peninsula, on a typical position under NAO positive-like phase. A
cold period at the DA probably related with this atmospheric situation was also identified using
the pollen influx in the Ría de Vigo sediments (Desprat et al., 2003) (Figure VI.5).
5.4. Period 4: 1200–0 cal. yr BP
Last period is characterized by the high content of mud with the same lithological
features as those found in the present seabed at the core site. The C/N ratio with values
between 12 and 18 indicates that the organic matter is mostly of continental origin. The
presence of benthic diatoms at significant percentages in a core that is located well deeper
than the euphotic zone likely indicates transport from inner areas. Moreover, freshwater
species are also well represented in this interval. The non-organic material content remains
high and constant during this period revealing high influx of land-derived particles. All these
data point to enhanced flow of the Miño and Douro rivers and, therefore, an increase in the
precipitation over the NW Iberian Peninsula since 1200 cal. yr BP.
These terrestrial input proxies show good general agreement with changing
precipitation patterns (wet/dry) over this time interval with other archives in the same climatic
area. The high number of paleoflood records after 1300 cal. yr BP (Thorndycraft and Benito,
2006) also supports this hypothesis of increased land-material input to the Galician shelf. In
the Ría de Vigo, a period of frequent salinity changes and high runoff has been identified
during the Medieval Warm Period (MWP) until the onset of the Little Ice Age (LIA) (Álvarez et
al., 2005) (Figure VI.5).
A relevant event is recognized around 800–500 cal. yr BP. Phytoliths, crysophycean
cysts and palinomorphs abundances are also considered as terrestrial input markers due to
188
Chapter VI
their continental origin. They are transported to the ocean by river run-off and/or wind (Romero
et al., 1999). High river run-off and thus, an increase in the rainfall, or transport by N-NE winds
from the continent could explain the presence of these tracers at the core site. However, due
to the proximity of the Atlantic river mouths, we infer that riverine run-off is the main driving
process acting, and the abundance of these markers are employed as freshwater discharge
indicators. These land input indicators show an abrupt increase at this interval. The rise of
these proxies is coupled with the highest content of Fe, Al and LSi, suggesting increased
continental-derived contribution, which is in good agreement with the freshening of the surface
waters indicated by the appearance of freshwater diatoms. Scatter plots showing the linear
correlations between the geochemical tracers (Al, Fe and LSi), as well as the C/N, Ca/Al ratio
and terrigenous content of the sediment are shown in Figure VI.6. Good correlation, as shown
by the Pearson’s correlation coefficients (p<0.01; Table VI.3), point out the usefulness of all
these proxies for the interpretation of the degree of oceanic and terrestrial influence. Fe, Al,
and LSi profiles, combined with the C/N and Ca/Al ratio, suggest reduced riverine input and
regionally dry conditions when these markers are in low values, and high terrigenous input to
the shelf area when they are high. Thus, this strong terrestrial input stage and consequently,
wet period, is identifiable in the scatter plots showing the linear correlations between the most
important terrestrial input indicators (Figure VI.6).
Table VI.3. Pearson correlation matrix of the main riverine input proxies used in this work. Good correlations have been found between parameters, showing their potential use for terrestrial input markers.
Ca/Al Terrigenous Silit C/N Al Fe
Ca/Al -.966** -.734** -.617** -.800** -.769**
Terrigenous -.966** .650** .505** .775** .786**
Silit -.734** .650** .710** .731** .563**
C/N -.617** .505** .710** .646** .444**
Al -.800** .775** .731** .646** .928**
Fe -.769** .786** .563** .444** .928**
**p<0.01
In the Iberian Peninsula, paleoflood records indicate at least two phases of increased
frequency of large magnitude floods, and therefore, increased precipitation related to climatic
variability during the last ca. 1200 years, namely 975–790 cal. yr BP and 520–265 cal. yr BP
appearing to coincide with the MWP and the LIA (Thorndycraft and Benito, 2006). Martins et
al. (2006a) found an interval of increased river discharge, coinciding with the rainy event
189
Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
recorded in our core, linked to the appearance of brackish-water benthic foraminifera. A strong
humid period was also identified by several authors at the Tagus mouth matching that found in
our record (Abrantes et al., 2005; Bartels-Jónsdóttir et al., 2006; Lebreiro et al., 2006, Gil et
al., 2006) (Figure VI.5).
Figure VI.6. Scatter diagrams of Al, Fe, LSi, Ca/Al, C/N ratio and terrigenous content. Linear regression is shown (solid line). Dashed lines represent the prediction intervals at the 95% confidence interval. Pearson’s correlation coefficients are shown in the Table VI.3 (p<0.01). Grey areas indicate the strong rainy period described in the text.
The identification of this wet period during a cold stage, such as the LIA, indicates a
precipitation-influenced forcing mechanism. Increased flood magnitude and/or frequency in the
Miño and Douro rivers discharge is strongly related to increased winter precipitation in the NW
Iberian Peninsula. High rainfall is related to a low-pressure cell located in Iceland, advecting
cold fronts into the NW Iberian Peninsula with predominantly SW–NE directions. A blocked
situation of this low-pressure cell would give to persistent precipitation and increased fluvial
discharge at the river mouths. This seasonal pattern, if maintained, could be related to a
persistent NAO negative-like phase over the Iberian Peninsula (Hurrell et al., 2003).
190
Chapter VI
Periods of solar maxima are believed to be related to low values in the North Atlantic
Oscilation (NAO) index, which are associated with high winter precipitation and river flow in
the Atlantic basins of the Iberian Peninsula. Cross-correlation of the curves of our indicators
with that of atmospheric ∆14C (Stuiver et al.,1998), reflecting solar activity, is significant. The
good parallelism between our record and the solar activity, especially during the Grand Solar
Maximum (GSM), reflects the influence of the global climate shifts on the climate variability in
this area. Therefore, this wet interval can be explained by several forcing climate mechanisms:
increased precipitation and run-off to the shelf related to a NAO negative-like stage, and for
the recent rainy period (800–500 cal. yr BP), the high irradiance and warmer conditions due to
the GSM could also be involved.
6. SUMMARY AND CONCLUSIONS
Our reconstruction of paleoclimate and paleoenvironment provides evidence for a
variable Late Holocene period in the NW Iberian Peninsula. The study of a marine sediment
core allows us to identify changes in the riverine input, marine influence and human impact in
the Galician continental shelf sedimentary record for the past 4700 cal. yr BP. On the basis of
the aforementioned discussions several conclusions are drawn:
Lithostratigraphic and geochemical data allow identifying alternative periods of
terrestrial influence (Period 2: 3300–1700 cal. yr BP and Period 4: 1200–0 cal. yr BP) and
marine influence (Period 1: 4700–3300 cal. yr and Period 3: 1700–1200 cal. yr BP) in the shelf
zone. The marine-influenced periods are characterized by coarser sediments, low values of
Fe, Al, LSi, C/N, biosiliceous land indicators, high Ca/Al ratios and the lack of diatom frustules
due to the low preservation efficiency. Conversely, the terrestrial-influenced periods are
represented by muddy sequences that register high values of the lithogenic indicators (Fe, Al)
and C/N ratios, and high abundances of benthic and freshwater diatoms.
Several climatic forcing mechanisms can be invoked to explain these changes in the
high/low marine/terrestrial influence. During periods 1 and 3, prevailing dry conditions over the
NW Iberian Peninsula were triggered by winds blowing from the N-NE.
Continental-influenced periods are linked to NAO negative-like periods. In this scenario,
increased rainfall in the catchment areas of Miño and Douro rivers leads to higher run-off to
the shelf. Major peaks of terrestrial and lithogenic input proxies are interpreted as the record of
flood-like events and increased rainfall in the area.
An important riverine input peak around 2000–1800 cal. yr BP has been detected at the
top of the terrestrial-influenced period 2, being related to the warmer conditions during the
RWP. Past changes mediated by anthropic activities (forest degradation, soil erosion on land
191
Late Holocene history of the rainfall in the NW Iberian Peninsula—Evidence from a marine record
or gold mining activities in the catchment area) in NW Iberian Margin are identified in the
sedimentary record.
The strongest terrestrial signal has been identified between 800–500 cal. yr BP. This
humid period is linked to the establishment of rainy conditions in the catchment areas of Miño
and Douro rivers. The warm conditions at the Grand Solar Maximum could also be one of the
forcing factors of climate suggesting that our sedimentary record reflects global climatic
signals, as well as regional factors.
The chronology of main events in our record responds to changes in intensity and wind
direction triggered by the position of the high/low pressure systems. Therefore, the NAO drives
the rainfall patterns and river discharges to the Galician continental shelf. Within the
uncertainties of the age model, these events and their paleoclimatic implications match
relatively well with those described by several authors in the study area, as well as with global
climatic events.
Acknowledgements
The authors would like to express their gratitude to Marta Pérez-Arlucea for helping with the sedimentological features and the description of the core. We are thanked to Clemente Trujillo, Paula Ferro and Jesús Roncero for the technical assistance and the help in sample processing. We also wish to thank the referees Roberto Bao and Teresa Drago for providing insightful and helpful comments that greatly improve the final version of this manuscript. This work was supported by Ministerio de Educación, Cultura y Deporte and Xunta de Galicia under the projects METRIA-REN2003-04106-C03, REN2003-09394, PGIDIT05PXIB31201PR, PGIDT04PXIC31204PN and EVK2-CT-2000-00060, PGIDT00MAR30103PR. P.B. and R.G.-A. thank the Xunta de Galicia (Secretaría Xeral de Investigación e Desenvolvemento) and Ministerio de Educación, Cultura y Deporte (Secretaría de Estado de Educación y Universidades) for financial support.
192
Chapter VI
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[Chapter VII] PALEOPRODUCTIVITY CHANGES AND UPWELLING VARIABILITY IN THE GALICIA MUD PATCH DURING THE LAST 5000 YEARS: GEOCHEMICAL AND MICROFLORAL EVIDENCES∗
1. INTRODUCTION: BACKGROUND, STUDY SITE AND OBJECTIVES
2. FIELD AND LABORATORY PROCEDURES: DATA ACQUISITION AND METHODS
2.1. Core location and sampling
2.2. Procedures and analyses
Chronological control
Bulk biogenic component analyses
Analytical procedures for metal determinations: Ba, Cu, Mn, Pb, Al, Fe
Siliceous microfossils preparation: Diatom counting and relative species percentages
3. PALEOENVIRONMENTAL PROXIES. PALEOCLIMATE AND PALEOPRODUCTIVITY APPROACHES
3.1. Chronostratigraphical features
3.2. Sediment composition
Bulk biogenic components
Metals content
Diatoms
3.3. Diatom assemblages: downcore variations
4. DISCUSSION
5. CONCLUDING REMARKS
Acknowledgements
References
∗ This chapter is based on Patricia Bernárdez, Raquel González-Álvarez, Guillermo Francés, Ricardo Prego, Mº Ángeles Bárcena, Oscar E. Romero. To be submitted
Abstract. The Holocene paleoclimatic history of the Galician continental shelf (NW Spain) has been investigated through the analyses of diatom remains, as well as geochemical and sedimentological parameters. Diatoms, siliceous compounds, and biogenic silica (BSi) and metals content were analyzed from a gravity core recovered from the Galician continental shelf, NW Iberian Peninsula, covering Late Holocene (last 5000 years). Results were integrated together in a multiproxy study in order to determine the climatic and the oceanographic influence on the paleoproductivity conditions that have occurred in the Galicia Mud Patch.
Downcore changes in diatom assemblage composition and abundance reflect changes in diatom production related to long/short-term variations in climate, regional oceanography, upwelling strength, and riverine influx off the coast of Galicia. As displayed by the diatom taxa record, paleoclimatic variability was related to different atmospheric conditions. Metals and microflora fluctuations are interpreted as changes in the riverine influence and upwelling intensity paced by oceanographic, atmospheric and climatic changes. Lack of diatoms in stages 4700–3300 and 1800–1200 cal. yr BP could be linked to early diagenetic processes taking place in the sediment after burial.
All tracers considered, Baexcess, metals and diatom assemblages, show consistent profiles displaying a general increase of the marine productivity for the last 1200 cal. yr BP. River run-off due to high precipitation conditions in the catchment area is an additional important source of nutrients to the surface waters promoting diatom production. In this way, microflora indicates that during 800 and 500 cal. yr BP high production is triggered by influx of river-derived nutrients. The biosiliceous and geochemical signatures of sediments from the last 500 cal. yr BP indicate conditions of enhanced upwelling and increased phytoplanktonic production associated to the intensification of northerly winds. The imprint of the anthropic activities has been recorded by the increasing Pb/Al ratios for the last 400 cal. yr BP. The oceanographic changes recorded in Galician shelf are correlative with other marine and terrestrial paleoenvironmental records in the NW Iberian Peninsula.
Keywords: upwelling/ metals/ diatom assemblages/Late Holocene/paleoproductivity/paleoceanography/Galicia/NW Iberian Peninsula
Resumen. La historia paleoclimática en la plataforma continental gallega (NO de España) ha sido investigada basándose en los restos de diatomeas y en parámetros geoquímicos y sedimentarios encontrados en un registro Holoceno. Se han analizado las diatomeas y los componentes silíceos, así como el contenido en diversos metales en un testigo de gravedad de la plataforma continental gallega (NO de la Peninsula Ibérica) que registra aproximadamente los últimos 5000 años. Los resultados se integran en un estudio multidisciplinar con el fin de determinar la influencia climática y oceanográfica sobre las condiciones de paleoproductividad que tuvieron lugar en la plataforma.
La variación temporal de la composición y abundancia de la asociación de diatomeas en el registro es un reflejo de sus variaciones en la columna de agua de acuerdo con cambios en el clima, la oceanografía regional, la intensidad del afloramiento y el aporte fluvial a la plataforma. Las fluctuaciones en la composición del sedimento se interpretan como cambios en el aporte fluvial o la intensidad del afloramiento. La falta de registro de diatomeas en los periodos 4700–3300 y 1800–1200 cal. yr BP se relaciona con los procesos de diagénesis temprana que tienen lugar en el sedimento tras su enterramiento.
Todos los parámetros analizados muestran perfiles de variación que indican un incremento general de la producción primaria durante los últimos 1200 cal. yr BP. El aporte fluvial derivado de la elevada precipitación en la cuenca de drenaje adyacente es una importante fuente adicional de nutrientes a las aguas superficiales que dan lugar a un incremento de la producción de diatomeas. En este sentido, los microfósiles silíceos indican que durante el periodo 800–500 cal. yr BP existe una elevada producción primaria que se debe al aporte de nutrientes por parte del curso fluvial. Durante los últimos 500 cal. yr BP la intensificación del afloramiento da lugar a también a un aumento de la producción fitoplanctónica. El registro de las actividades antrópicas (industrialización) se detecta por el aumento de la ratio Pb/Al durante los últimos 400 cal. yr BP. Los cambios oceanográfico-climáticos que se registran en la plataforma continental gallega se correlacionan con otros registros paleoambientales de la zona.
Palabras clave: upwelling/ metales/ asociación de diatomeas/ Holoceno/paleoproductividad/paleoceanografía/Galicia/NO de la Peninsula Ibérica
PALEOPRODUCTIVITY CHANGES AND UPWELLING VARIABILITY IN THE
GALICIA MUD PATCH DURING THE LAST 5000 YEARS: GEOCHEMICAL
AND MICROFLORAL EVIDENCES
1. INTRODUCTION: BACKGROUND, STUDY SITE AND OBJECTIVES
Paleoenvironmental reconstruction allows an understanding of the processes that drive
climate and ocean changes and affect biological systems. In this way, in recent years,
increasing interest has been shown in the study of past climate variations, especially over the
Holocene period. The Galician-Portuguese region off the Iberian Peninsula is a suitable area
to assess the paleoproductivity conditions and the climatic implications. This area is, together
with the Peruvian, Californian, and northwestern African systems, one of the major coastal
upwelling regions of the world ocean (Wooster et al., 1976; McClain et al., 1986). It is under
the influence of the North Atlantic weather systems that affect the east coast of the Atlantic
Ocean. The position of both pressure systems acting, the Iceland Low and the Azores High,
determines the intensity and direction of the coastal winds leading to a marked seasonal cycle
and, therefore, a seasonally-controlled hydrography (Wooster et al., 1976; McClain et al.,
1986). Upwelling events mainly occur from April to October (Fraga, 1981) when the Azores
anticyclone moves northwards and the N NNE winds along the coast exert a southwards
surface stress that causes Ekman transport offshore (Blanton et al., 1987; Álvarez-Salgado et
al., 1993). Upwelling forces the Eastern North Atlantic Central Water (ENACW), either
subpolar or subtropical branches (Ríos et al., 1992), towards the coast during most of spring
and summer (Fraga, 1981; Tenore et al., 1995). The influence of the upwelling in this area is
generally restricted to a narrow band near the coast. In contrast, in winter the Iceland Low
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
strengthens and centers over the North Atlantic. As a result, downwelling-favourable southerly
winds prevail along the coast (Wooster et al., 1976; Fraga, 1981) and lead to the development
of the downwelling regime (Vitorino et al., 2002a,b) and a persistent poleward flow (Iberian
Poleward Current, IPC) along the continental slope (Frouin et al., 1990; Peliz et al., 2003).
Moreover, the Western Iberian Buoyant Plume (WIBP), a fresher surface water lens flowing
northwards, is also well developed during this season (Peliz et al., 2002) due to high river
runoff related to high rainfall.
These hydrodynamic and oceanographic features affect the primary production of this
region. Primary production in these upwelling areas is high due to the influx of nutrients in
spring and summer to the surface waters controlling phytoplankton activity, and especially that
of diatoms, which increases drastically. However, during winter, supply of nutrients may come
from the riverine runoff of the Atlantic Rivers draining hinterland. Water column studies on the
Portuguese shelf (Abrantes and Moita, 1999) indicate that when favourable upwelling
conditions prevail in summer, the diatoms, one of the main primary producers bloom. Diatom
assemblages have been used as qualitative indicators of primary production, changes in
paleo-upwelling intensity, as well as other environmental variables such as temperature and
salinity. In the studied area, different diatom assemblages are linked with the various surface
and subsurface water masses and freshwater inputs and hydrological fronts generated by the
upwelling processes (Abrantes and Moita, 1999).
Depositional systems of continental shelves are natural archives of data on changes in
past environmental conditions. A high number of paleoenvironmental tracers can be extracted
from their sediments. The continental margin west of Galicia is characterized by a relatively
narrow shelf. Sediment cover of the shelf is composed mostly by thin Pleistocene-Holocene
units (López-Jamar et al., 1992). Morphology and extent of the shelf sedimentary environment
is controlled by storm surges, supply of sediments, currents and a structural control
(Jouanneau et al., 2002; Dias et al., 2002a,b). The present sea-floor surface sediments are
dominantly silty clay with some areas of muddy sand sediments, and gravels at the ría mouths
near the coast (López-Jamar et al., 1992; Dias et al., 2002, Jouanneau et al., 2002). Large
loads of suspended fine sediment to the shelf are supplied by the main river system (Miño-
Douro), and other small rivers and streams along the coast, forming the Galicia Mud Patch.
Therefore, sedimentation on this margin is highly influenced by terrigenous inputs from the
river draining on the hinterland. The nature of sediments that accumulate in the Galician
continental shelf results from the combination of several main mechanisms as summarized: (i)
the rate of primary production in the surface waters and the subsequent accumulation of
biogenic material (organic, calcareous and siliceous) in the sediment; (ii) the diagenetic
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(dissolution) processes throughout the water column, at the sediment-water interface, and
within the sediment; (iii) and the supply of terrestrial material from the main river-systems
draining the area; (iv) the dispersal of the material due to the acting of current systems and
hydrographic conditions.
In this way, Galician shelf sediments contain geochemical, faunal and floral tracers of
terrestrial input, primary productivity, physico-chemical conditions of the surface waters and
terrestrial vegetation (Bernárdez et al., in press; González-Álvarez and Francés, 2005) which
show that this area is highly sensitive to climatic fluctuations. Little information about the
history of the marine paleoproductivity and paleoclimatology exists in this area, being some
studies restricted to the Rías Baixas and to the shelf (Diz et al., 2002; Desprat et al., 2003;
Álvarez et al., 2005; González-Álvarez et al., 2005; Martins et al., 2005; Martins et al.,
2006a,b; Martins et al., 2007; Muñoz-Sobrino et al., in press). However, the understanding of
upwelling variability and the ecological consequences during the late Holocene remains
limited.
In this paper we present a 4700-yr record of the climatic changes in the western
Galician continental shelf. This sediment record provides the opportunity for high-resolution
reconstruction of past environmental conditions in this temperate zone. We analyzed the core
sediment samples on the basis of a multivariable study based on sedimentological data, bulk
sediment composition, biogenic silica content, siliceous microorganisms (diatoms) and
concentration of several metals. Paleo-dataset was used as a basis for the reconstruction of
the history of marine environmental changes in the region.
In this way, the general objectives of this work are to detail the variations in siliceous
production and to assess the intensity of upwelling and productivity off Galician coast during
the last 5000 years. We also investigate how oceanographic and biological processes (e.g.,
upwelling, surface water dynamics, nutrients input, primary productivity) in the euphotic zone
control the production, export, and burial of some paleoenvironmental proxies, e.g., the
biosiliceous compounds and metals. Specifically we examine the production recording
potential of the diatom assemblages in the context of seasonal changes of the biological
productivity associated with the upwelling cycles. The results obtained are discussed in
relation to previously described climatic periods and from marine data in the study area. We
focus on the timing and the magnitude of the main production events during the Late Holocene
period and their relation to global climatic events and regional conditions.
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
2. FIELD AND LABORATORY PROCEDURES: DATA ACQUISITION AND METHODS
2.1. Core location and sampling
The material used in this work was obtained from a 260 cm-length gravity core SMP02-
3 (42°02.207’N, 9°02.363’W) retrieved from a water depth of 121 m in the core of the Galicia
Mud Patch offshore Galician coast during a cruise onboard R/V Mytilus in October 2002
(Figure VII.1).
Figure VII.1. Base map of the study area showing the core site (SMP02-3) and the geographical distribution of the sediments containing more than 50% of mud (modified from Dias et al., 2002a).
Core was stored at 4°C until it was longitudinally splitted in two equal parts (Ø 9 cm).
The core was visually described, logged, photographed, X-rayed and sub-sampled at constant
intervals. The working-halves of the gravity core were cut into 1-cm thick slices. Each slice
was then separated into subsamples and stored in plastic bags for different analyses. One half
was studied with regard to various sedimentological studies and the other half was used for
micropaleontological fossils identification and biosiliceous material counts, bulk component
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Chapter VII
analyses and metal determinations. Samples for siliceous microfossil analysis were collected
at the same depth intervals as sediment subsamples used for geochemical and other
micropaleontological counts (e.g., benthic and planktonic foraminifera).
2.2. Procedures and analyses
Chronological control
AMS 14C dating of foraminiferal tests (∼10 mg carbonate) recovered from several levels
of the core were performed at the Geochron Laboratories (Massachusetts, USA) and at the
AMS facility of the Aarhus University (Denmark) (Figure VII.2).
Ages are given in calibrated years before present (cal. yr BP) to facilitate comparison of
the present results with other records of different origins. The raw 14C ages were corrected
with a regional deviation from the global reservoir effect (apparent surface-water age) (Bard,
1988) according to the Marine Reservoir Correction Database, and, therefore reported as cal.
yr BP with the Calib 5.0.1 software (Stuiver and Reimer 1993 modified version 2002;
http://calib.qub.ac.uk/calib/) using the Marine04 calibration dataset for radiocarbon ages
younger than 26,000 14C years (Hughen et al., 2004). A calibrated age range within 2δ
confidence limits (95% probability) was obtained. The dates used in figures and discussion
represent the intercept of radiocarbon age with the calibration curve. No local-reservoir effect
was applied as shown by Abrantes et al. (2005).
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
Figure VII.2. Sedimentary logs of the core. Distribution of depositional facies are also shown. Dated levels throughout the core and the result of the calibration is shown on the right side
Bulk biogenic component analyses
Sediment composition was analyzed on bulk sediment and ground subsamples. One
portion of each 1-cm slice was used for the bulk geochemical analysis with a sampling
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resolution of 6 cm. CaCO3, organic carbon and nitrogen are standard parameters measured on
cores taken for marine studies. These tracers were measured on LECO carbon element
analyzers. Total carbon (TC) content and total nitrogen (TN) were measured with the CN-2000
analyzer using samples that had not been ashed prior to analysis. A portion of the sediment
was measured on a CC-100 analyzer for total inorganic carbon (TIC) content. Organic carbon
(TOC) was calculated as TC minus TIC. Calcium carbonate (CaCO3) concentration was
calculated multiplying the total inorganic carbon (TIC) content by the factor 8.33 supposing
that all the inorganic carbon is in the calcium carbonate chemical type.
The extraction of amorphous silica particles was made following a soda-hydrolysis
technique based on Mortlock and Froelich (1989). The resulting extract was determined for the
dissolved silicate concentration on the basis of molybdate-blue spectrophotometry at 812 nm
(Hansen and Grashoff, 1983) by means of a continuous flow analyser AutoAnalyser Technicon
II. Data are expressed as weight of Si (e.g., biogenic silicon) and opal content was calculated
as 2.14×Si. This conversion does not account for the bound-water content (2–15% depending
on the type and age of the material; Mortlock and Froelich, 1989) of the opal, which is difficult
to predict.
All the results are given in weight percent of dry, salt-free sediment. The proportion of
non-organic, non-opaline and non-calcareous constituents is regarded as the lithogenic,
siliciclastic or terrigenous sediment fraction and it is calculated as the residual from 100% of
the calcium carbonate, total nitrogen and organic carbon and opal sum.
Analytical procedures for metal determinations: Ba, Cu, Mn, Pb, Al, Fe
For chemical analyses of the metal contents, the dry bulk and homogenised samples
were subjected to total acid digestion. Approximately 40–50 mg of sediment was weighted
directly into Teflon® pressure vessels. The samples underwent an acid digestion using a
mixture of concentrated HNO3, and HF using 6 ml HNO3 (65%) and 2 ml HF (48%) in a
Milestone MLS 1200 Mega microwave oven following the EPA 3052 guideline (EPA, 1996) for
siliceous-type matrices. Handling and analysis of samples were carried out in a clean
laboratory (ISO class 7-8) and plastic labware employed for sampling, storage and sample
treatment was previously acid-cleaned (HNO3 10%) for at least 48 h and washed throughout
with Milli-Q water.
Fe, Al, Mn and Ba were determined using flame atomic absorption spectrometry (FAAS)
technique with a Varian 220FS apparatus. The concentrations of Cu and Pb were measured
by means of electrothermal atomic absorption spectrometry (GF-AAS) using a Varian 220
apparatus equipped with Zeeman background correction. The accuracy of the analytical
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
procedure was tested by repeated analyses of certified reference materials (CRMs), PACS-2
from the National Research Council of Canada (Table VII.1), excepting for Ba. In this case,
accuracy was determined analysing a control sample with a known concentration of Ba and
checking the reproducibility. RSD for Ba is 5.72%. The results obtained agree well with the
certified values. Relative Standard Deviations (RSDs) were typically lower than 10% excepting
Cu (14.8%) (Table VII.1).
Table VII.1. Comparison of the analytical results of the certified reference material PACS-2 (NRCC, Canada) with the measured data.
PACS-2 Al (mg g-1) Fe (mg g-1) Mn (mg g-1) Cu (µg g-1) Pb (µg g-1)
Measured value 67.6±3.9 41.6±2.0 0.48±0.04 318±47 186±11
Certified value 66.1±5.3 40.9±0.6 0.44±0.019 310±12 183±8
The excess/biogenic Ba concentration (Baexcess or Babio) was calculated following the
equation (1) (normative method). Ba/Al aluminosilicate ratio is required to assess the biogenic
barium, in particular in sediments with high detrital background (Reitz et al., 2004). This ratio
is presumed to be representative of the terrigenous input and serves as a correction factor for
substracting Ba associated to terrigenous inputs (Gingele and Dahmke, 1994). After detrital
barium is accounted for, the remaining one is attributed to biogenic barite and linked to the
productivity in surface waters (Gingele and Dahmke, 1994; Francois et al., 1995; Dymond and
Collier, 1996; Gingele et al., 1999).
A normative approache on conservative elements like Al is commonly used to assess
the detrital barium background:
Baexcess=Batotal-Baterr =Batotal-(AlxBa/Aldet) (1)
Ba should be affected by early diagenesis (McManus et al., 1994; McManus et al.,
1998; Paytan et al., 1996; Schenau et al., 2001), but in this work we will designate the
biogenic barium as barium excess (Baexcess) since we consider that the diagenetic
remobilization is discarded (Moreno et al., 2004) and the terrigenous component was excluded
by normalization to Al (Schenau et al., 2001). Ba/Aldet ratio can be estimated independently
and is constant in time and source (Table VII.2c). Due to the impossibility of determining this
regional factor, we use several Ba/Aldet ratio recommended: a regional ratio determined for the
Atlantic ocean north of 30ºS (0.004, Gingele and Dahmke, 1994), two mean values of the
212
Chapter VII
crustal ratio (0.0075 and 0,0065, Wedepohl, 1991; Bowen, 1979) and the global average
crustal ratio (0.0037, Reitz et al., 2004). We also determined the Baterr using the relative
percentages of opal, carbonate, detritics and organic carbon and their association of the Baterr
(Hyun et al., 2002) taking into account that most of the barium is associated to the terrigenous
materials.
Siliceous microfossils preparation: Diatom counting and relative species percentages
The sediment samples for diatom identification were prepared by a standard method
devised by Abrantes (1988). Suspension volume and volume used to mount the glass slides
were known. A 500 µL of slurry (after mixing the solution for homogenization) across a 18×18
mm circular cover slips, placed in a circular Petri dish (Ø 47 mm) was pipetting, and they were
left to dry at room temperature. When evaporation is complete and cover slips are dry, they
were removed and fixed on onto permanently labelled smear slides using the high refractive
mounting medium Permount™ (Fisher Scientific).
Light microscopes (LEICA DMLB and Zeiss) with phase contrast optics and a
magnification up to 1000× was used for quantification and diatom identification. Several non-
overlapping transverses across the cover slips were examinated depending on the diatom
concentration. For each sample, at least 300 diatom valves were counted to ensure proper
assessment of diatom abundance and composition. Although 300 valves is generally regarded
as the optimum number needed for quantitative interpretations, the scarcity of preserved
diatoms within the sedimentary record in several levels of the core prevented this in all but a
few samples. However, in micropaleontological studies based on the interpretation of the
proportional distribution of the dominant taxa (>3%) counts of around 100 specimens are
sufficient (Fatela and Taborda, 2002). The counting procedure and definition of diatom counts
and total number estimates followed those of Schrader and Gersonde (1978). Specimens
representing more than one-half of the valve were counted as one and for pennate diatoms,
such as Thalassionema spp., each pole was counted as one-half specimen. Remaining
fragments were not counted as one specimen. Diatoms were identified to the lowest taxonomic
level possible, based principally on the keys of Hustedt (1930), Hustedt (1959), Hartley (1996)
Hasle and Syvertsen (1996) and Witkowski et al. (2000) (see taxonomic appendix). Each
individual was identified to the species level, otherwise they were assigned to a genus on the
basis of Round et al. (1990). Data concerning the ecology of the diatom taxa were compiled
from previous references in addition to several specific bibliographies, and, species were
gathered into freshwater, benthic and marine planktonic species.
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
Number of diatom fragments, silicoflagellates, sponge spicules, phytoliths,
crysophycean cysts and radiolaria per gram of sediment was also assessed, as well as
palynomorphs abundance (data in Bernárdez et al., in press). Amorphous silica particles were
also observed and analysed using a scanning electron microscope (SEM) at the Research
Support Service (CACTI) at the University of Vigo.
For evaluation of the state of preservation of diatom valves, the preservation index
defined by Abrantes (1988) was calculated.
3. PALEOENVIRONMENTAL PROXIES. PALEOCLIMATE AND PALEOPRODUCTIVITY APPROACHES
3.1. Chronostratigraphical features
Lithological characteristics of the core and the age model based on the 14C AMS
planktonic foraminiferal dates (Figure VII.2) are extensively described in Bernárdez et al. (in
press).
Sediments of the core are mainly composed by terrigenous and detrital material, quartz
and clay. Biogenic fraction represents a low proportion, consisting of calcareous remains of
gastropods, bivalves and foraminifers, and siliceous microfossils. Visual core description,
colour variation, and X-ray data do not show either significant evidences of sediment
disturbance, only some bioturbation at located levels.
The record spans from 4700 cal. yr BP to present, assuming that sample 0–1 cm
corresponds to 0 cal. yr BP. Sedimentation rate was quite variable over the age range studied,
ranging between 0.12 and 1.39 mm yr-1 (Figure VII.2), but it sharply increases during the last
1200 years (0–150 cm). This abrupt increase coincides with a change in sedimentary units.
3.2. Sediment composition
Bulk biogenic components
Sediments are predominately siliciclastic with the terrigenous fraction clearly dominating
(Bernárdez et al., in press). Concentrations of biogenic silica (BSi) reveal consistently high
values at the bottom of the core compared to those obtained in the upper muddy sequence.
Opal content falls in the range between 1.18 and 2.07 wt.%, accounting on mean for the 1.61
dry weight percent of the core sediment. Biogenic silica, as well as TOC constitute a small
source of biogenic components to the sediments, being CaCO3 the major component of the
marine derived biogenic sediments. TOC and BSi show a similar profile, displaying a strong
peak around 700 cal. yr BP (Table VII.2a).
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Chapter VII
Table VII.2a. Content of opal, TOC, calcium carbonate and detritics.
depth (cm) Age cal. yr BP TOC (%) CaCO3 (%) opal (%) detritics
(%) 0 19 1.72 3.92 1.43 92.94 6 214 1.54 3.58 1.38 93.50
12 259 1.70 3.75 1.33 93.23 18 303 1.61 3.79 1.37 93.24 24 347 1.49 3.87 1.49 93.15 30 391 1.52 3.62 1.43 93.43 36 436 1.62 3.29 1.51 93.58 42 480 1.70 3.00 1.44 93.86 48 524 1.62 3.00 1.58 93.80 54 569 1.64 3.21 1.42 93.73 60 613 1.76 2.78 1.48 93.98 66 657 1.86 2.58 1.40 94.16 72 701 2.03 3.28 1.64 93.05 78 746 1.95 3.87 1.85 92.33 84 790 2.13 3.29 1.76 92.83 90 834 1.65 4.58 1.76 92.01 96 878 1.66 4.78 1.67 91.89
102 921 1.58 5.33 1.78 91.31 108 964 1.72 5.21 1.78 91.30 114 1007 1.81 5.41 1.79 90.99 120 1051 1.72 5.33 1.58 91.38 126 1094 1.49 6.21 1.67 90.64 132 1137 1.37 5.83 1.59 91.22 138 1180 1.09 7.16 1.60 90.14 144 1223 1.19 7.30 1.56 89.95 150 1289 1.18 7.41 1.59 89.82 156 1464 1.08 5.66 1.54 91.73 162 1640 1.18 3.12 1.18 94.52 168 1816 1.11 2.25 1.29 95.35 174 1991 1.11 1.79 1.35 95.75 180 2167 1.18 2.25 1.36 95.21 186 2343 1.32 2.37 1.62 94.70 192 2518 1.35 2.62 1.45 94.58 204 3356 1.45 5.25 1.42 91.89 210 3511 1.37 7.25 1.75 89.63 216 3667 1.30 8.66 1.70 88.34 222 3823 1.27 10.08 1.79 86.86 228 3978 1.11 10.95 1.91 86.03 234 4134 1.09 12.50 1.92 84.50 240 4289 0.92 12.20 2.06 84.82 246 4445 1.05 10.70 1.95 86.31 252 4600 0.94 18.91 1.89 78.26 258 4756 0.81 14.41 2.07 82.71
Metals content
The major components of the core sediments are Si, which represents 227 mg g-1 as a
mean value (85–364mg g-1), and Al (33–119 mg g-1, averaging 71 mg g-1) (Bernárdez et al., in
press). Low Al values are found between 4700 and 4000 cal. yr BP. An increasing trend is
215
Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
recorded towards the top of the core peaking around 2000 cal. yr BP. Maximum values are
registered from 800 to 500 cal. yr BP.
Fe (ranging from 18 to 49 mg g-1) is also an important contributor to the sediment (Table
VII.2b). Good linear correlation between Fe and Al (Bernárdez et al., press) implies that Fe/Al
downcore variability is reduced, excepting for a small peak in the lower Unit 1 around 4200–
3900 cal. yr BP (Figure VII.3).
Table VII.2b. Concentrations of several metals analysed in the sediments of the core.
depth (cm) Age cal. yr BP
Cu (µg g-1)
Fe (mg g-1)
Mn (mg g-1)
Pb (µg g-1)
0 19 16.79 29.76 0.30 38.60 6 214 15.84 30.22 0.29 32.02
12 259 16.03 31.43 0.29 33.87 18 303 16.15 31.54 0.30 34.55 24 347 15.21 30.57 0.30 31.75 30 391 16.11 33.76 0.31 31.37 36 436 16.23 34.50 0.32 27.34 42 480 16.09 33.35 0.33 27.11 48 524 23.22 48.49 0.46 35.67 54 569 17.98 36.96 0.37 30.12 60 613 21.91 42.98 0.43 32.63 66 657 24.77 48.36 0.48 38.78 72 701 20.16 37.94 0.39 30.91 78 746 21.97 42.42 0.45 32.85 84 790 22.01 41.08 0.43 32.68 90 834 17.90 34.63 0.38 28.08 96 878 12.20 32.03 0.33 25.16
102 921 11.03 27.86 0.27 19.55 108 964 10.21 27.47 0.30 20.54 114 1007 11.57 28.21 0.28 21.65 120 1051 11.45 29.89 0.31 20.80 126 1094 10.12 25.49 0.25 18.65 132 1137 9.86 25.32 0.26 18.87 138 1180 8.30 23.96 0.25 18.08 144 1223 11.10 28.34 0.30 21.43 150 1289 11.52 29.09 0.30 29.23 156 1464 14.99 35.04 0.34 24.44 162 1640 21.25 39.81 0.37 25.25 168 1816 26.52 44.69 0.42 30.24 174 1991 25.95 45.19 0.43 32.09 180 2167 14.63 32.36 0.35 19.85 186 2343 15.07 36.28 0.35 22.63 192 2518 26.66 45.31 0.42 27.25 204 3356 18.48 37.81 0.37 22.90 210 3511 11.40 30.02 0.28 17.63 216 3667 10.20 28.39 0.31 17.31 222 3823 8.75 26.85 0.26 15.67 228 3978 5.81 23.41 0.17 8.71 234 4134 4.50 21.14 0.15 7.08 240 4289 4.51 19.72 0.15 7.34 246 4445 3.94 17.95 0.18 4.19 252 4600 3.06 17.85 0.09 1.99 258 4756 3.96 19.97 0.13 2.54
216
Chapter VII
Mn content shows high downcore variability (0.09–0.48 mg g-1). Higher amounts are
found at 790–524 cal. yr BP and around 2000 cal. yr BP (Table VII.2b). Mn/Al ratio displays
relatively stable values between 0.004 and 0.005, excepting at the bottom of the core where
ratios oscillate around 0.003. As observed for Fe/Al ratio, Mn/Al ratio shows a progressive
decrease from 1200 cal. yr BP up to 200 cal. yr BP (Figure VII.3).
Figure VII.3. Profiles of the chemical elements analysed in the core. Total concentration (solid circles) is shown on the left axis and metal/Al normalization (open squares) is shown on the right axis.
Pb is one of the minor components of the core sediments. This metal is detected in the
range of 2–39 µg g-1. Pb displays very low values in the lower part of the core, increasing
progressively up to 3300 cal. yr BP . From 1200 cal. yr BP to present, Pb content
progressively increases up to reach the maximum value in recent times (Table VII.2b). Pb/Al
ratio follows the same trend to that found for Pb content in the lower part of the core, and
around 3330 cal. yr BP values stabilize around 0.0003. From 400 cal. yr BP to present, ratio
progressively increases reaching the maximum value at present (Figure VII.3).
Cu levels in the core vary from 3 to 27 µg g-1, peaking around 1800–1990 cal. yr BP and
790–520 cal. yr BP. From 1200 to 800 cal. yr BP, very low and stable values are found (Table
VII.2b). Cu/Al distribution presents low variability. Only a small peak on the Cu/Al ratio is
recorded around 2000 cal. yr BP and also at the top of the core (Figure VII.3).
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
Table VII.2c. Al, total Ba and calculations of the Baexcess.
depth (cm)
Age cal. yr BP
Al (mg g-1)
Batotal (mg g-1) Baexcess
a Baexcessb Baexcess
c Baexcessd Baexcess
e
0 19 65.47 2.74 2.49 2.47 2.31 2.24 2.31 6 214 65.54 2.92 2.68 2.66 2.49 2.43 2.49
12 259 79.54 2.70 2.41 2.38 2.19 2.11 2.28 18 303 80.42 2.74 2.44 2.42 2.22 2.14 2.32 24 347 79.33 2.62 2.33 2.30 2.10 2.03 2.20 30 391 84.44 2.61 2.29 2.27 2.06 1.97 2.18 36 436 85.14 2.61 2.30 2.27 2.06 1.97 2.18 42 480 85.65 2.61 2.30 2.27 2.06 1.97 2.19 48 524 118.76 3.99 3.55 3.52 3.22 3.10 3.56 54 569 94.46 2.81 2.46 2.43 2.20 2.10 2.38 60 613 104.39 3.35 2.97 2.93 2.67 2.57 2.92 66 657 115.76 3.71 3.29 3.25 2.96 2.85 3.28 72 701 90.89 2.95 2.61 2.58 2.36 2.27 2.52 78 746 102.97 3.24 2.86 2.83 2.57 2.47 2.82 84 790 98.59 2.93 2.56 2.54 2.29 2.19 2.51 90 834 87.20 2.81 2.49 2.46 2.24 2.16 2.39 96 878 69.58 2.00 1.75 1.73 1.55 1.48 1.58
102 921 58.75 1.61 1.40 1.38 1.23 1.17 1.20 108 964 64.85 1.75 1.51 1.49 1.33 1.27 1.34 114 1007 66.36 1.54 1.29 1.27 1.11 1.04 1.12 120 1051 67.04 2.40 2.15 2.13 1.96 1.90 1.98 126 1094 55.85 2.05 1.85 1.83 1.69 1.64 1.64 132 1137 56.03 1.89 1.68 1.67 1.53 1.47 1.47 138 1180 49.60 1.62 1.43 1.42 1.29 1.24 1.20 144 1223 61.26 2.22 2.00 1.98 1.82 1.76 1.81 150 1289 61.15 2.18 1.95 1.93 1.78 1.72 1.77 156 1464 69.02 2.71 2.46 2.44 2.26 2.20 2.29 162 1640 78.91 2.79 2.50 2.48 2.28 2.20 2.36 168 1816 89.20 3.20 2.87 2.84 2.62 2.53 2.77 174 1991 91.00 2.94 2.60 2.57 2.35 2.26 2.50 180 2167 65.42 2.03 1.79 1.77 1.61 1.54 1.60 186 2343 67.99 2.36 2.10 2.08 1.91 1.85 1.92 192 2518 85.53 2.56 2.24 2.22 2.00 1.92 2.13 204 3356 77.38 2.26 1.97 1.95 1.75 1.68 1.84 210 3511 54.78 1.57 1.37 1.36 1.22 1.16 1.16 216 3667 61.61 1.39 1.16 1.15 0.99 0.93 0.99 222 3823 59.81 1.24 1.02 1.00 0.85 0.79 0.84 228 3978 37.70 0.34 0.20 0.19 0.09 0.06 -0.06 234 4134 34.30 0.69 0.56 0.55 0.47 0.43 0.30 240 4289 33.85 0.15 0.02 0.01 -0.07 -0.10 -0.24 246 4445 33.22 0.00 -0.12 -0.13 -0.22 -0.25 -0.40 252 4600 36.52 0.00 -0.14 -0.15 -0.24 -0.27 -0.36 258 4756 36.73 0.00 -0.14 -0.15 -0.24 -0.28 -0.38
a Reitz et al. (2004). new global average 0.0037 b Gingele and Dahmke (1994), 0.004 Atlantic ocean north 30º c Bowen (1979), 0.0065 d Wedepohl (1991), 0.0075 average crustal ratio e Hyun et al. (2002) where Baterr(mgg-1)=((30xCaCO3/100)+(120xopal/100)+(60xTOC/100)+(452xdetritics/100))/1000 using the percentages in dry weight.
218
Chapter VII
Barium in marine sediments occurs as biogenic barite or is associated with
aluminosilicates. The Ba/Al detrital ratio is the crucial factor in the Babio calculated from Batotal,
therefore we have calculated the amount of biogenic barium using different approaches.
Several discrepancies have been found between the methods and normative calculations
(Table VII.2c), but in order to get reliable values of this paleoproductivity proxy we have
decided to use the most recent global average crustal ratio (0.0037, Reitz et al., 2004). The
amount of biogenic barium in the sediment ranges from 3.6 (mg g-1) to almost nothing. Baexcess
exhibits a distinct profile, with two major peaks around 1800 cal. yr BP and 834–524 cal. yr
BP. In fact, values increase progressively from the bottom of the core towards the top. Total
Ba content follows the same trend, although values are slightly higher, between 0.12 and 0.44
mg g-1. Total Ba versus Al follows the same trend to that found for total barium from the bottom
of the core up to 1200 cal. yr BP. During the last 1200 years Ba/Al slightly increases, reaching
the highest values during last 200 years (Figure VII.3).
Diatoms
The frequency of the major species amounts to mean around 80% of the total species
percent throughout the core. The sequence of diatoms includes mainly species of the genera
Chaetoceros, Leptocylindrus, Paralia, and Thalassionema.
Chaetoceros resting spores (R.S.) (mostly C. affinis), are the main component of the
diatom assemblage, ranging from 20 up to 70% for the last 500 cal. yr BP. Their percentage
progressively increases from the bottom to the top of the core. C. cinctus R.S. has the same
profile as found for C. affinis R.S. but relative percentages are very low (0–3%). C. diadema
R.S. represents also a high contribution of the total Chaetoceros spores. Downcore pattern of
this species shows high abundances (~6%) around 1000–700 cal. yr BP (Figure VII.4).
Leptocylindrus danicus R.S. represents the 8–30% of the assemblage. Higher
percentages around 30% are found at 3300, 2200 and 1200 cal. yr BP. From this age to the
top of the record values become stable (around 18%) (Figure VII.4).
Paralia sulcata relative abundance varies from 5 to 43%, although a high amount of this
species is observed at around 1200–960 cal yr BP and around 400 cal. yr BP. The degree of
preservation of the assemblage calculated from the margin dissolution of this species follows
the same pattern to that found for P. sulcata. Higher abundances are found when preservation
is poor (Figure VII.4).
Thalassionema nitzschioides relative abundance varies between 2–6%. Higher values
(averaging 11.7%) are found from 3300 to 2200 cal. yr BP. A strong peak, with values up to
10% between 878–524 cal. yr BP, is also recorded (Figure VII.4).
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
4. DISCUSSION
Knowledge on the seasonal diatom succession and its record in the sediments of the
Galician shelf is limited and no studies on the composition of the diatom community in
sediment traps or water column have been carried out. There are only a few studies regarding
diatom water column assemblages of the Portuguese-Galician shelf (Bode and Varela, 1998;
Abrantes and Moita, 1999; Bode et al., 2002). Interpretation is mainly based on the ecological
characteristics of the species described in Abrantes (1988) and Bao et al. (1997).
From the study of the record archived in core SMP02-3, different sediment conditions
are inferred for the last 5000 years. Two periods barren of diatoms have been identified in the
record (4700–3300 and 1700–1200 cal. yr BP) intercalated between periods where diatoms
are present (Figure VII.4). Diatom barren sequences in the Galician shelf record may reflect
lower production, combined with more intense dissolution of diatom frustules. However, we
suggest that the low diatom abundances observed in some samples result from severe
dissolution, indicating a diagenetic control. Fe and Mn contents are indicative of early stages
of diagenesis (Froelich, 1979) especially in organic matter-rich environments. Normalization of
these chemical elements to Al (Hanson et al., 1993) reports the separation of metal
concentrations into natural (marine-derived and terrestrial-derived) and other fractions (e.g.
anthropogenic). In this way, variations in the concentration of Fe/Al and Mn/Al in our record
point to different reducing/oxidizing conditions within the sediment. Both periods barren of
diatoms (Figure VII.3) record the highest values of the Fe/Al and Mn/Al ratios, therefore they
are representative an oxic environment promoted by coarser sediments, which favours the
biogenic silica dissolution. Profile of relative abundance of P. sulcata (Figure VII.4) shows that
even heavily silicified forms disappear from the sedimentary record. This species has been
related to enhanced upwelling conditions (Abrantes and Sancetta, 1985; Abrantes, 1988;
Abrantes, 1991; Bárcena and Abrantes, 1998) and linked to vertically-mixed water column and
high surface water salinity (McQuoid and Nordberd, 2003 and references therein). In our
record, higher abundances of P. sulcata coincide with high values of the preservation index,
therefore, as reported by Bao et al. (1997) in the Galician shelf surface sediments, abundance
of this species is mostly related to preservation conditions.
220
Chapter VII
Figu
re V
II.4.
Com
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and
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ore
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Gre
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. Lith
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ts a
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MS
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ings
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221
Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
On the contrary, the anoxic-suboxic conditions which lead to the reduction of Mn and Fe
are indicated by low values of Fe/Al and Mn/Al. The remobilization of Fe and Mn and the
increase of the reduced species in solution occur when dissolved oxygen is consumed in pore-
waters after organic matter remineralization. In our record, during the last 1200 cal. yr BP
sediment grain size is finer and a high influx of organic matter occur (Table VII.2a), therefore,
anoxic conditions in the sediment are well established. In this case, most of the diatom
assemblages remain in this suboxic-anoxic environment and, therefore, they are preserved.
Similar geochemical patterns have been already observed in a nearby core (Martins et al.,
2006b; Martins et al., 2007), pointing to depressed levels of oxygen and early diagenetic
changes in muddy sediments between 2200–1200 cal. yr BP and 500–0 cal. yr BP. However,
imprecision in their age model impedes the correlation of both records.
Despite the preservation effects on the diatom assemblage, we consider that downcore
variations in diatom abundance are the result of paleochanges in primary production. Clear
trends are observed when looking at the individual species data.
The high input of allocthonous diatoms at 800–500 cal. yr BP to the shelf leads to the
maximum concentration of the diatom valves. A strong peak in the relative percentage of T.
nitzschioides and C. diadema occurs at this age. T. nitzschioides responses to a weakened
upwelling and/or high fluvial input discharge (Pokras and Molfino, 1986; van Iperen et al.,
1987; Abrantes, 1988; Bao et al., 1997). In this way, T. nitzschioides is present in significant
percentages where runoff influence is detected by the presence of freshwater, benthic species
and other riverine input tracers (Bernárdez et al., in press). The downcore profile of C.
diadema also follows the same pattern, reaching the highest abundances (~6%) at 1000–700
cal. yr BP (Figure VII.4). Although ecological preferences of C. diadema R.S. are not
extensively investigated, a few studies report the appearance of this species during spring
blooms (Tiselius and Kuylenstierna, 1996; McQuoid and Nordberg, 2006). Rebolledo et al.
(2005) also associate this species to high nutrient concentrations in the Chilean Fjords. In our
case, the clear relationship between the percentage of the freshwater assemblage and C.
diadema R.S. suggests high influx of river-derived nutrients to the shelf.
It has been assumed that upwelling is the main mechanism for enhancing primary
production off Galicia. However, influence of river runoff during winter has not been properly
assessed. Some attempts for estimating the primary production on the shelf have been carried
out by Teira et al. (2001) and Joint et al. (2002), but they failed in the estimations during the
winter months, since few experiments were performed. Prego and Bao (1997) indicated that
upwelling is the main forcing controlling the silicate concentrations in the Galician continental
waters during summer, but they also pointed to the possible influence on productivity of
222
Chapter VII
silicate derived from freshwater inputs during winter. deCastro et al. (2006) also suggested
that high freshwater discharge fertilizing the area can induce a phytoplankton bloom in the
Galician shelf. This is in apparent contradiction to Bode et al. (2002), who have suggested that
relatively low numbers of diatoms in Galician waters occur under high runoff.
Baexcess is commonly used as a tool for the reconstruction of paleoproductivity although
some authors have discussed its validity (e.g., Anderson and Winckler 2005). In core SMP02-
3, high correlation between Ba and Al in bulk sediment reveals that the barium comes from
several different sources and some of the barium is associated with terrigenous
aluminosilicate materials. The non-terrigenous barium follows the same profile to that found for
other paleoproductivity indicators, such as TOC and opal (Table VII.2a, Figure VII.3).
Maximum values of Baexcess (Figure VII.4) coincide with strong humid periods reported by
Bernárdez et al. (in press): around 2000 cal. yr BP during the Roman Warm Period (RWP) and
at 800–500 cal. yr BP. During the RWP we can not evaluate the primary production and the
oceanographic conditions since it is one of the periods barren of diatoms. However, we can
conclude that at 800–500 cal. yr BP primary production is enhanced.
Downcore values of total Cu are within the range reported for the recent sediments
deposited in the western Galician shelf (Araújo et al., 2002; Corredeira et al., 2005). Two
strong peaks are observed around 2000 cal. yr BP and at 800–500 cal. yr BP paralleling the
pattern found for total Ba (Figure VII.3). These Cu peaks are related to strong input of detrital
materials from continental soils and weathered rocks, since when normalising to Al values
Cu/Al profile becomes stable, except for maximum values found at 2000 cal. yr BP. Cu is an
essential micronutrient for phytoplankton being part of the biogeochemical cycles (Sunda et
al., 1981). A strong biological uptake for Cu is observed in the Galician shelf waters (Santos-
Echeandía et al., 2005) since Cu concentrations in the suspended particulate matter are in the
range of dissolved values. Peak in Cu/Al around 2000 cal. yr BP and in recent sediments could
be linked to more biological activity and diatom productivity in surface waters that can raise its
content in the sediments.
Chaetoceros R.S. are heavily silicified and characteristic of turbulent waters rich in
nutrients of upwelling areas (Schuette and Schrader, 1981; Abrantes and Sancetta, 1985;
Abrantes, 1988; Pitcher, 1990; Abrantes and Moita, 1999; Abrantes et al., 2002; Romero and
Hebbeln, 2003). Under present-day conditions in the western Iberian shelf, Chaetoceros spp.
proliferate in spring and summer when upwelling is well developed (Abrantes and Moita,
1999). As one of the main contributor to diatom community in our record (Figure VII.4), it is
suggested that the presence of these spores is linked to more frequent upwelling events.
There are important amounts of different spore morphotypes of the diatom genus
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
Chaetoceros, mostly related to high nutrient input. One of these high nutrient input indicators
is C. cinctus. It is absent from the record, but appears during the last 500 cal. yr BP, reflecting
the upwelling intensification. C. affinis presents the same pattern, a species also related to the
coastal upwelling phenomena off Chile (Romero and Hebbeln, 2003).
Ecologically, L. danicus blooming is related to nutrient depletion, upwelling relaxation
and the breakage of summer stratification after the mixing period (Varela et al., 2001; Bárcena
et al., 2001; Bárcena et al., 2004). A strong decrease in the abundance of L. danicus is
recorded at recent times, when high nutrients and enhanced productivity by upwelling
intensification are recorded. Baexcess also exhibits high values (~2.5 mg g-1) during the last 500
cal. yr BP (Figure VII.4). However, these values do not reach those found during the humid
period registered at 800–500 cal. yr BP. Therefore we can conclude that primary productivity
during this period is triggered by river-derived nutrients, which could be supplied in higher
amounts that those derived from upwelled waters.
The intensification of the upwelling in the Galician-Portuguese margin during
approximately the last 1000 cal. yr BP is a well known fact (Soares, 1993; Diz et al., 2002;
González-Álvarez et al., 2005; Lebreiro et al., 2006; Martins et al., 2006a; Soares and Dias,
2006; Martins et al., 2007; Muñoz-Sobrino et al., in press). Our record confirms this
interpretation, however high temporal resolution in the upper part of our core allows us to
enclose the enhancement of the upwelling regime to the last 500 years. This upwelling
intensification is observed under a well-described NAO negative regime (Bernárdez et al., in
press). This apparent contradiction is explained by the seasonality of the oceanographic
processes acting in the region. We could define a predominance of a NAO negative-like phase
with SW winds and winter storm conditions during the last 1200 cal. yr BP, but this fact does
not implies the ceasing of the upwelling regime during summer. This climatic situation could be
blurred by prevailing winter conditions, but during the last 500 cal. yr BP, N NNE winds and,
therefore, the upwelling regime, would predominate in a seasonal scale. Combining both
climatic situations, the final result is the increase in productivity, but the diatom assemblages
permit to discriminate between both processes.
Other studies show increased upwelling in recent years in multiple areas around the
world and, in particular, in the Canarian-Iberian margin (Bakun, 1990; Anderson et al., 2002;
Goes et al., 2005; Santos et al., 2005; McGregor et al., 2007) most of them attributed to 20th
century global warming and atmospheric CO2 rise. Temporal resolution of our record does not
permit to discriminate enhanced upwelling on a decadal scale, but shows similar patterns to
those found by earlier authors on a century-decadal range. This fact has also important
implications on diatom productivity since, although dependent on a complex balance of several
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factors, it is controlled by upwelling. These results imply an increasing in phytoplankton
biomass and the response of the ecosystem and the consequent biological, climatological and
socioeconomical impacts. Upwelling may continue to strengthen with global warming
(McGregor et al., 2007), even in our study area, provoking ecosystem and diatom composition
changes.
On the other hand, it is important to point out that in the recent years a strong increase
in the Pb concentrations occurred (Figure VII.3). These values are within the range point out
by Araújo et al. (2002) for surface sediments in the muddy patch. When Pb is normalized with
Al, in order to eliminate the natural lithogenic influence, the variability of Pb/Al ratios is lower,
but still shows a significant rise in recent times. One of the primary sources of lead into the
aquatic domain is the atmospheric deposition (Clark, 2001). However, in the Galician shelf the
input by rivers flowing at the shelf could also be an important source. Enrichment factors and
isotope ratios seem to indicate that atmospheric Pb pollution started around 3500–3000 years
BP related to human activities (Martínez-Cortizas et al., 2002a), but in the NW Iberian
Peninsula atmospheric Pb pollution is only apparent since 2500 years BP (Martínez-Cortizas
et al., 2002b). A maximum in concentration was found during the Roman colonization and also
for the last 300 years due to industrial development (Martínez-Cortizas et al., 2002b). A small
peak in the concentration of this element during Roman Warm Period is recorded in our core.
Moreover, the increase in the Pb/Al ratios occurs approximately during the last 400 cal. yr BP
(Figure VII.3). This result confirms the age model of the core, but we are not able to detect the
decreased Pb concentrations for the using of unleaded gasoline also observed in other paleo-
records in Galician marine sediments and peat bogs (Cobelo-García and Prego, 2003;
Martínez-Cortizas et al., 1997). Conversely, the Pb enrichment in the sediments of the core,
which provides a high temporal resolution during the recent years, is in good agreement with
the Pb enrichment due to industrial and naval development (Martínez-Cortizas et al., 1997;
Martínez-Cortizas et al., 2002b).
5. CONCLUDING REMARKS
A multidisciplinary study covering sedimentology (texture, organic matter content),
geochemistry (major, minor and trace elements), palaeoecolgy (diatoms, biosilica compounds)
and isotopic dating (14C) has been carried out on the gravity core SMP02-3 retrieved from the
Galician continental shelf (NW Iberian Peninsula). Paleoproductivity changes in the NW
Iberian Peninsula have been inferred from microflora and geochemical distributions of this
core. The study of microfossil assemblages and texture, composition and geochemistry of
sediments has been proved to yield relevant information on Holocene primary production
changes in the NW Iberian Peninsula.
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Paleoproductivity changes and upwelling variability in the Galicia Mud Patch during the last 5000 years: geochemical and microfloral evidences
Disappearance of diatoms during some periods of the core has been related to
oxygenation conditions and early diagenetic processes occurring during burial. Despite the
presumably large influence of biogenic silica preservation conditions in periods 4700–3300
and 1800–1200 cal. yr BP, relative abundances of diatom species are still reliable tracers of
oceanographic and indeed climatic conditions. Based on diatom valves abundance and
dominance, the assemblage zones present in the core can be divided in sub-environments that
have evolved through time. The sediments are characterized by a dominance of diatoms
representative of high nutrient environments (e.g., spores of Chaetoceros spp.), accompanied
by coastal planktonic, benthic and freshwater species.
The geochemical characterization of the sediment core indicates that sediments
deposited at the Galicia Mud Patch come from three different sources: lithogenic,
anthropogenic and biogenic. Peaks in metal concentrations are mainly related to detritic inputs
from the river plumes, especially at 800–500 cal. yr BP.
We have pointed that the sediment features (biogenic compounds and chemical
features) are triggered by climatic and oceanographic conditions in the Galician area, as well
as, by anthropic activities. The sharp rise in the Pb and Pb/Al concentrations occurred during
the last 400 cal. yr BP is linked to Pb enrichment after the industrialization.
With respect to the timing of coupled marine and terrestrial palaeoenvironmental
processes and their effect in the primary production, the record presented here shows that:
(1) In general, high productivity due to nutrient enrichment is recorded during the last
1200 cal. yr BP due to both upwelling intensification and river runoff.
(2) The major wetter event occurring at 800–500 cal. yr BP, provokes a high diatom
production, being the river plumes the main source of nutrients to the coastal waters off
Galicia.
(3) This study indicates that Chaetoceros R.S. and some morphotypes are useful
tracers of paleoupwelling conditions on the NW Iberian margin. The other proxies considered
show consistent patterns indicating high productivity in the Galician shelf during the last 500
years. This fact is due to the intensification of the upwelling phenomena related to prevailing N
NNE winds. Their expansion is registered for the last 500 cal. yr BP from the onset of the
humid event
In brief, marine productivity in this area is triggered by two processes that increase the
nutrient availability for phytoplankton growth: nutrient input by river discharge and/or coastal
upwelling.
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Chapter VII
Acknowledgements
We would like to RV Mytilus crew and the technicians and scientists who have participated in the ría cruises and sampling. We are grateful to Clemente Trujillo, Paula Ferro and Jesús Roncero for laboratory technical support with sample preparation for diatom and metals analysis at the Institute of Marine Research (IIM, Vigo) and Micropaleontology Laboratory of the University of Salamanca. We also thank Dr. Antonio Cobelo-García for the valuable reviews and suggestions. Funding for this study was received from the MECD and Xunta de Galicia under the projects METRIA-REN2003-04106-C03, REN2003-09394, PGIDIT05PXIB31201PR PGIDT04PXIC31204PN, EVK2-CT-2000-00060 and PGIDT00MAR30103PR. Xunta de Galicia (Secretaría Xeral de Investigación e Desenvolvemento) and Ministerio de Educación, Cultura y Deporte (Secretaría de Estado de Educación y Universidades) financed P. Bernárdez with a grant.
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SUMMARY AND CONCLUSIONS
SUMMARY AND CONCLUSIONS
El análisis de la sílice biogénica y las asociaciones de diatomeas en sedimentos
recientes de las rías de Vigo, Pontevedra y Ferrol y en un registro Holoceno de la plataforma
continental gallega, junto con las características sedimentarias y biogeoquímicas de dichos
sedimentos dan lugar a las siguientes conclusiones generales:
Conclusión general I: Las variaciones espaciales del contenido en ópalo en los
sedimentos superficiales y subsuperficiales de la ría de Vigo están causadas
fundamentalmente por el flujo de sílice biogénica hacia el fondo.
Conclusión general II: Los patrones de producción de diatomeas en la columna de
agua, así como su posterior sedimentación y registro en el sedimento superficial de las rías de
Pontevedra y Ferrol han permitido:
1. Caracterizar la variación estacional y las condiciones oceanográficas bajo las
cuales se desarrollan y proliferan cada una de las especies de diatomeas.
2. Determinar los procesos que afectan a la preservación de las mismas en el
sedimento y su fiabilidad como trazadores climáticos e hidrográficos.
3. Sentar las bases para la interpretación paleoecológica de los ambientes de
ría basada en estos organismos y su aplicación como marcadores
paleohidrológicos y paleoceanográficos.
Conclusión general III: Se ha elaborado una reconstrucción paleoclimática y
paleoceanográfica de las condiciones reinantes en la plataforma continental gallega durante
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aproximadamente los últimos 5000 años basada en el análisis multidisciplinar de diferentes
proxies. Se ha puesto especial énfasis en relacionar estas variaciones climáticas y
oceanográficas y su impacto en la productividad primaria con otros eventos de escala regional
y local descritos por diversos autores.
Las conclusiones derivadas de los objetivos específicos que se plantearon al empezar
el estudio se desglosan en los siguientes apartados.
El registro de ópalo en el sedimento superficial y subsuperficial de la ría de Vigo
Se ha llevado a cabo satisfactoriamente la técnica de determinación de ópalo en
muestras de sedimento superficial, lo que garantiza una mayor calidad de los resultados. La
desviación estándar del método de digestión alcalina empleado es ±0.2.
Dada la elevada resolución espacial del muestreo superficial llevado a cabo en la ría de
Vigo los resultados obtenidos permiten la cuantificación precisa de las variaciones en el
contenido en ópalo en dicha ría, incluso en aquellas zonas en las que no existían datos
previos de este parámetro, como es el caso de la ensenada de San Simón. Es precisamente
en esa zona donde se localizan los mayores valores de sílice biogénica, que son debidos a la
importante presencia y crecimiento de diatomeas con forma de vida bentónica. Además, esta
elevada concentración también se puede relacionar con al aporte de diatomeas de agua dulce
a través del río Oitabén-Verdugo y con la alta concentración de sus frústulos en los pellets
fecales procedentes de los residuos del cultivo de mejillón en bateas instaladas en la
ensenada.
La margen norte de la zona interna de la ría de Vigo se caracteriza por porcentajes
elevados de sílice biogénica, mientras que la concentración disminuye hacia la boca de la ría,
de modo que los valores más bajos se localizan en los canales de entrada a la misma. Por
otro lado, el giro antihorario que describe el agua en las zonas internas, así como la tendencia
a que el agua dulce se concentre por la orilla norte pueden ser las causas de los valores altos
de ópalo detectados en la zona. La concentración de bateas en el margen norte de la ría
también contribuye a un aumento de sílice biogénica en los fondos adyacentes.
Todos estos procesos modelan el contenido de sílice biogénica que se halla en los
sedimentos superficiales, si bien esta distribución está principalmente controlada por la
producción primaria en la columna de agua, que aumenta hacia las zonas internas de la ría,
donde se registran los valores mayores.
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El contenido en ópalo de los sedimentos superficiales de la ría de Vigo y la producción
biosilícea de la columna de agua están correlacionados linealmente. La ecuación obtenida
relaciona pues el medio pelágico y el bentónico. La variación espacial del flujo de sílice
biogénica hacia el fondo a escala anual presenta la misma variación que la distribución de
ópalo en el sedimento superficial. De este modo, se ha podido validar la utilización del ópalo
como trazador de paleoproductividad en ambientes de ría y estuarios y en zonas litorales con
alta producción primaria y, lo que es más importante, cuantificar ese flujo de Si hacia el fondo.
Sin embargo, tal y como se ha puesto de manifiesto anteriormente, en las zonas más internas,
donde existe un aporte alóctono de material biosilíceo, esta máxima debe ser tomada con
cautela, y se hace indispensable un estudio sistemático de las diatomeas que aparecen.
Asimismo, se pone de manifiesto un efecto de dilución derivado de la presencia de
partículas de sedimento más gruesas y de los elevados porcentajes de carbonatos y materia
orgánica. Es por ello que los valores más bajos de ópalo que se localizan en la zona externa
de la ría se expliquen tanto por la menor productividad primaria superficial en estas áreas
como por la textura del sedimento, predominantemente de tamaño grueso. Con el fin de
eliminar este efecto de dilución se han llevado a cabo análisis de ópalo sólo en la fracción <63
µm. La elevada correlación entre los valores medidos en la muestra total y en la fracción fina
indica que para determinar adecuadamente las variaciones espaciales en la producción
biosilícea en la ría no es necesario realizar análisis por fracciones granulométricas.
Sin embargo, cuando se ha seguido el mismo procedimiento en es testigo CGPL00-1,
obtenido en la plataforma continental, se llega a la conclusión de la conveniencia de medir la
concentración en ópalo en la fracción fina. El testigo CGPL00-1 presenta dos partes que se
diferencian claramente por sus características granulométricas. La basal es
fundamentalmente arenosa, mientras que la superior es predominantemente fangosa. Los
datos permiten afirmar que el análisis de ópalo en la fracción fangosa (<63 µm) expresa de
forma más fidedigna los cambios en la paleoproductividad de la columna de agua que el
análisis en muestra total. Ello se debe a que es posible estandarizar los resultados y además
eliminar las interferencias debidas a la presencia de partículas relativamente gruesas que
enmascaran el registro de las diatomeas.
Respecto a los perfiles de distribución de ópalo en el sedimento subsuperficial se han
observado patrones muy diferentes en función del área de la ría que consideremos. La
distribución vertical de ópalo es la respuesta a diversos procesos: los cambios en el flujo y
acumulación de material biosilíceo y, por tanto de la producción primaria en la columna de
agua; los procesos postdeposicionales y el aporte de frústulos de diatomeas en los pellets
fecales producidos por los cultivos de mejillón. Los perfiles de contenido en ópalo típicos del
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Summary and conclusions
eje fangoso de la ría se caracterizan por un ligero descenso del porcentaje de ópalo en los
centímetros subsuperficiales. Por el contrario, en la zona interna se registran concentraciones
de ópalo muy elevados a profundidades de hasta 11-15 cm, y que se asocian al desarrollo de
un ambiente reductor que posibilita un aumento del potencial de preservación de la sílice
biogénica.
Producción primaria, flujos biosilíceos al sedimento y registro del material biosilíceo en el sedimento superficial de las rías de Pontevedra y Ferrol
Ría de Pontevedra
El estudio de la variación estacional de la comunidad de diatomeas en la columna de
agua pone de manifiesto que:
1. El género Chaetoceros se asocia al desarrollo de condiciones de producción altas
causadas por procesos de upwelling ligados a la dominancia de los vientos del Norte.
2. Thalassiosira spp. aparece bajo condiciones de aporte continuo de nutrientes,
mientras que las especies del género Rhizosolenia responden a las eclosiones invernales y
proliferan cuando el contenido en nutrientes de las aguas superficiales es bajo.
3. S. costatum se desarrolla cuando existe un elevado aporte de nutrientes a las zonas
más internas de la ría procedente del río Lérez. También los aportes fluviales, especialmente
durante el invierno, determinan la abundancia de T. nitzschioides, si bien esta especie tolera
condiciones oceanográficas variables.
4. L. danicus prolifera tras el afloramiento cuando el contenido en nutrientes es más
bajo y existe estratificación de la columna de agua al final del verano. La especie P. sulcata y
las diatomeas de carácter bentónico sólo aparecen en la columna de agua cuando la
descarga fluvial es elevada y se produce la resuspensión del sedimento superficial. El grupo
de diatomeas de agua dulce se detecta en las estaciones más internas, y su presencia
constituye un buen trazador de la intensidad de la pluma de agua dulce del río Lérez.
La comparación entre los taxones recogidos en la columna de agua, en las trampas de
sedimento y en el sedimento superficial pone de relieve que en la interfase sedimento-agua se
produce una severa disolución, que afecta más a las especies con frústulos pobremente
silicificados. Así pues, S. costatum se disuelve progresivamente a medida que va cayendo a
través de la columna de agua. Este mismo patrón se observa para las especies del género
Thalassiosira, a excepción de la estación localizada en la zona más interna de la ría. Por su
parte, Rhizosolenia spp. sufre procesos de disolución una vez que llega al sedimento, de tal
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manera que desaparece del mismo completamente. Por el contrario, T. nitzschioides muestra
un patrón inverso, de tal manera que su abundancia relativa es mayor en las trampas y en el
sedimento superficial que en las muestras de agua. Las especies de Chaetoceros se
preservan en las trampas en forma de esporas en elevada concentración. Este hecho también
se obseva para L. danicus, el cual se preserva muy bien en el sedimento superficial,
especialmente en la ría media. Las especies bentónicas y de agua dulce se localizan en los
sedimentos de las zonas más internas. P. sulcata aparece en el sedimento a lo largo de toda
la ría debido a que es una especie robusta, e incluso cuando las condiciones de preservación
no son las óptimas.
De todo el material biosilíceo encontrado en el sedimento superficial se concluye que
las diatomeas son los organismos que aparecen con mayores abundancias, especialmente en
la parte más interna de la ría. La elevada representación de diatomeas de agua dulce,
crisófitas y fitolitos en la margen norte e interna de la ría caracterizan la extensión geográfica
de la pluma de agua dulce del río Lérez. Incluso, la entrada por la boca sur de la ría de agua
relativamente dulce procedente de fuertes descargas del río Miño queda reflejada en el
sedimento superficial mediante estos marcadores. Este hecho tiene importantes implicaciones
en la reconstrucción de los aportes fluviales en el pasado, y subsecuentemente de la
intensidad de las precipitaciones sobre el continente. El análisis de las diatomeas de agua
dulce, crisófitas y fitolitos en registros de ría interna permitirían monitorizar el desplazamiento
de la zona estuarina a lo largo del tiempo.
El análisis conjunto de los marcadores geoquímicos y micropaleontológicos usando el
análisis de componentes principales muestra el contraste que existe entre las zonas de la ría
influenciadas por el upwelling y aquellas de la zona interna dominadas por procesos
estuáricos.
En definitiva, se ha observado una buena correlación entre las propiedades del
sedimento superficial y los patrones oceanográficos actuales. Así, el estudio muestra que la
distribución de diatomeas y restos biosilíceos se relaciona con las variables ambientales que
controlan la hidrografía y la producción primaria, como son la descarga fluvial y el aporte de
nutrientes por el upwelling.
Ría de Ferrol
Del estudio de la comunidad de diatomeas en la columna de agua y de las
asociaciones registradas en los sedimentos superficiales de la ría de Ferrol se derivan estas
conclusiones:
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Summary and conclusions
Los patrones de producción de diatomeas en la columna de agua difieren ligeramente
de los encontrados en la ría de Pontevedra. En primer lugar, la abundancia de diatomeas en
la ría de Ferrol a lo largo del año es considerablemente mayor, exceptuando el invierno. Se
trata de una ría semicerrada que recibe un aporte continuo de nutrientes a través del río
Grande de Xubia que desemboca en la cabecera de la ría. Aunque la descarga fluvial sea
más baja de la que reciben otras rías, referida el volumen total de agua contenido en la ría es
parecida a la de Pontevedra. Además, el estrecho canal que da paso a la parte media de la
ría y la alta influencia mareal que se observa favorece la continua renovación de aguas y, por
tanto, el aporte de nutrientes, lo que da lugar a una producción continua.
La comunidad de diatomeas presentes en la columna de agua también es similar a la
encontrada en la ría de Pontevedra, así como su evolución estacional. Sin embargo, ambas
difieren en la abundancia absoluta y relativa de cada especie. La discrepancia más importante
es la aparición en esta ría de un elevado porcentaje de N. longissima durante la campaña de
invierno, de tal manera que se comporta como indicadora de las condiciones de mezcla de la
columna de agua. Esta especie también presenta abundancias elevadas en septiembre, y
junto con L. danicus y S. costatum, es característica de este periodo. A pesar de ello, N.
longissima y S. costatum no quedan preservadas en cantidades importantes en el sedimento,
así como los taxones Rhizosolenia spp. y Thalassiosira spp. Por el contrario, L. danicus
prolifera en mayores abundancias en la estación más exterior de la ría, quedando también
bien representada en los sedimentos superficiales de la zona externa en forma de esporas, al
igual que sucede en la ría de Pontevedra. Esta especie es indicadora de las condiciones de
estratificación de la columna de agua y de la entrada de aguas oceánicas.
Otra de las diferencias importantes entre las rías de Ferrol y Pontevedra es la escasa
entrada a la primera del agua aflorada Eastern North Atlantic Central Water (ENACW) a través
del canal de entrada a la ría, lo que se traduce en una elevada producción de individuos del
género Chaetoceros en la parte externa, quedando preservadas además en el sedimento en
forma de esporas. T. nitzschioides prolifera en mayores abundancias en la columna de agua
durante condiciones de alta producción en verano en la parte media de la ría. Sin embargo,
aparece en mayores porcentajes en el sedimento superficial en el canal y la zona externa, un
patrón que también se detecta en la ría de Pontevedra.
Es común a ambas rías la aparición de diatomeas bentónicas y de P. sulcata en la
columna de agua durante la campaña de invierno, incluso en las zonas externas, lo que refleja
el transporte desde zonas internas y la resuspensión del sedimento en condiciones altamente
energéticas. Esa especie caracteriza la comunidad de diatomeas de los sedimentos
superficiales de la zona media. No obstante, una peculiaridad de la ría de Ferrol estriba en el
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elevado porcentaje de diatomeas bentónicas en la zona interna a profundidades menores de 5
m. Teniendo en cuenta la morfología estrecha de esta ría y el aporte fluvial relativamente
escaso que recibe, la asociación de diatomeas de agua dulce está restringida a la zona de
cabecera. Por consiguiente, el grupo de diatomeas bentónico pueden ser utilizado como
trazador de la posición del nivel del mar y de la línea de costa en el pasado, al menos en rías
con características semejantes a las de Ferrol.
Interpretación de los cambios en la productividad y la influencia terrestre en el registro Holoceno de la plataforma continental
La reconstrucción climática que se ha realizado en el testigo SMP02-3, recogido en la
plataforma continental gallega a una profundidad de 121 m, muestra evidencias de sucesivas
variaciones ambientales a lo largo del Holoceno reciente. Su estudio ha permitido identificar
diversos periodos que muestran diferentes grados de intensidad de la dinámica de upwelling,
de la influencia fluvial y del impacto humano.
Los marcadores biogeoquímicos indican que los sedimentos depositados en esta zona
provienen de tres fuentes principales: litogénicas, antropogénicas y biogénicas. En este
sentido, los datos geoquímicos y litoestratigráficos permiten identificar periodos alternativos
de mayor o menor influencia continental. Así, entre 3300 y 1700 cal. yr BP y durante los
últimos 1200 años los procesos que gobiernan el dominio emergido se hacen sentir también
en la plataforma, en contraposición a los periodos 4700–3300 cal. yr y 1700–1200 cal. yr BP,
durante los cuales tales procesos no afectaron a la plataforma media. Estos periodos de
condiciones netamente marinas se identifican por la presencia de sedimentos arenosos, con
valores bajos de Fe, Al, LSi, C/N y otros indicadores terrígenos, así como valores altos de la
relación Ca/Al y la falta de registro de diatomeas. Por el contrario, los estadios en los que se
detectan aportes desde las zonas emergidas se caracterizan por valores elevados de Fe, Al, y
C/N, así como por porcentajes relativamente elevados de diatomeas bentónicas y de agua
dulce.
La desaparición del registro biosilíceo en los períodos 4700–3300 and 1800–1200 cal.
yr BP puede estar relacionada con las condiciones redox en sedimento y procesos
diagenéticos que ocurren durante el enterramiento. En concreto, en estos niveles se infiere
que las condiciones son relativamente óxicas, lo que provoca la disolución del material
biosilíceo. Sin embargo, durante los últimos 1200 años el aumento en el contenido de carbono
orgánico y la aparición de materiales más finos, da lugar a condiciones reductoras que
favorecen la preservación de diatomeas. En estas zonas donde el material biosilíceo se
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Summary and conclusions
preserva, se observa una dominancia de diatomeas representativas de ambientes ricos con
nutrientes, y un elevado número de especies bentónicas y de agua dulce transportadas.
Los mecanismos climáticos que se invocan para explicar estos cambios en la firma
terrígena de los sedimentos de la plataforma son el establecimiento de periodos sostenidos de
NAO positiva o negativa. Durante fases con preponderancia de NAO positiva prevalecieron las
condiciones de aridez relativa sobre el continente y vientos dominantes del N-NE. Por el
contrario, durante las fases mantenidas de NAO negativa, el incremento de las precipitaciones
sobre las cuencas de drenaje de los ríos Miño y Duero y el consecuente incremento de la
descarga fluvial dejaron su huella en los sedimentos de la plataforma media. En definitiva, el
incremento de los trazadores de procedencia continental y litogénicos se asocian a un
aumento de las precipitaciones en momentos de NAO negativa dominante.
Es importante destacar que uno de esos periodos de elevado aporte continental a la
plataforma se registra entre 2000 y 1800 cal. yr BP, momento que coincide con el
establecimiento de condiciones más calurosas durante el Periodo Cálido Romano. La
degradación de los bosques y por lo tanto el aumento de la erosión de los suelos, unidos a
condiciones más lluviosas y cálidas reforzaron aún más la señal terrígena en los sedimentos
de la plataforma. Alguna de las causas consideradas, como la deforestación, poseen una
componente antropogénica bien documentada. Por otro lado, el registro del contenido de Pb y
la relación Pb/Al presenta valores muy altos durante los últimos 400 años, lo cual es
consistente con los datos de incremento de este parámetro en otros registros terrestres y
marinos. Este hecho indica que las actividades industriales y antropogénicas en tierra también
se pueden identificar en este registro marino.
El mayor evento lluvioso que se registra a lo largo del testigo se localiza a 800–500 cal.
yr BP. El proceso climático que dio lugar al registro de este evento puede estar relacionado
con un incremento de la actividad solar coincidiendo con el denominado Máximo Solar, del
que también existen evidencias de temperaturas más cálidas.
La cronología de todos estos eventos responde a cambios en los sistemas de presión
atmosférica que afectan a la región. Así, las condiciones variables o predominantes de NAO
positiva o negativa controlan los patrones de lluvias y descarga fluvial o el desarrollo del
upwelling. Dentro de la incertidumbre que se deriva de la propia realización del modelo de
edad del testigo, todos los eventos registrados y las interpretaciones realizadas se relacionan
adecuadamente con los que describen otros autores en estas áreas, así como con eventos
climáticos de escala global.
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Chapter VIII
En lo que se refiere a las variaciones en la productividad, en general se detecta un
aumento durante los últimos 1200 cal. yr BP. Concretamente, hay dos procesos principales
que controlan el aporte de nutrientes y por tanto la producción biosilícea. Durante el periodo
800–500 cal. yr BP, el elevado aporte de material de origen fluvial provocado por las plumas
de los ríos Miño y Duero, y por tanto de nutrientes, estimula la producción fitoplanctónica.
Durante los últimos 500 años se observa además la intensificación del upwelling costero
durante la estación de verano, quedando registrado por un aumento en el porcentaje relativo
de esporas de Chaetoceros. Esta intensificación del upwelling es consistente con los registros
actuales de escala decadal que lo relacionan con el incremento de CO2 en la atmósfera. Así,
este resultado implica que en el futuro podría producirse un aumento de la biomasa
fitoplanctónica como respuesta del ecosistema a estos cambios y el consiguiente impacto
socioeconómico.
247
[Chapter IX]
SELF-CRITICISM AND PERSPECTIVES
SELF-CRITICISM AND PERSPECTIVES
Ningún trabajo de investigación es perfecto y siempre hay determinados aspectos u
objetivos que quedan escasamente resueltos, o bien interpretaciones que podrían pulirse.
Seguramente hay aspectos de esta Tesis Doctoral que son mejorables y no quería renunciar a
la oportunidad de ponerlos de manifiesto y, en la medida de lo posible, justificarlo.
En primer lugar, aunque se ha llevado a cabo un estudio muy detallado de la
acumulación de ópalo en la ría de Vigo, no se ha realizado el análisis cuantitativo y exhaustivo
de las asociaciones de diatomeas presentes en el registro sedimentario reciente, puesto que
ya existen trabajos publicados al respecto, ni tampoco de la composición de ese material
biosilíceo (silicoflagelados, esponjas, fitolitos, quistes de crisofíceas, etc.). Sólo se apunta a la
existencia en elevadas cantidades de diatomeas de carácter bentónico y de agua de dulce en
las partes más internas de la ría. A pesar de ser una ría muy estudiada y de la que existe un
conocimiento bastante bueno de los aspectos biogeoquímicos, hidrográficos y sedimentarios,
es necesaria una investigación integral de las condiciones de sedimentación del material
biosilíceo a una escala estacional, con una alta resolución de muestreo y su comparación con
el registro actual. Además, todo ello debería estar combinado con un amplio estudio de las
condiciones hidrográficas y geoquímicas. De esta manera se podrían parametrizar
adecuadamente todas las variables y procesos que interactúan y disponer para el futuro de
una buena base de datos susceptible de ser utilizada para inferir condiciones de
paleoproductividad.
Respecto al estudio de las asociaciones de diatomeas en la columna de agua, en las
trampas de sedimentos y en el sedimento superficial en la ría de Pontevedra, es necesario un
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Self-criticism and perspectives
análisis más completo del ciclo anual de producción y sedimentación y su relación con las
condiciones hidrográficas del medio. La falta de muestreos en la columna de agua y en
trampas a lo largo de todo el ciclo estacional, principalmente durante el final de verano y en
otoño debido a un problema logístico, impidió un registro completo de esa variabilidad. Sin
embargo, este muestreo integral de todos los compartimentos permitió realizar la primera
caracterización del transporte de diatomeas hacia el sedimento en las rías gallegas.
Con relación al estudio de las asociaciones de diatomeas en la columna de agua en la
ría de Ferrol, cabe decir que la resolución temporal del muestreo, aunque limitada, sí es
representativa y caracteriza de forma importante los principales periodos oceanográficos y los
procesos hidrográficos y de producción que afectan a esta ría. Tal y como se ha apuntado en
el caso del estudio en la ría de Pontevedra, sería necesario un análisis más detallado del ciclo
anual de producción de diatomeas. Además, puesto que es una ría tan dinámica, resultaría de
una gran importancia conocer el proceso de sedimentación del material orgánico y también
detrítico, el cual no ha sido llevado a cabo.
Por otro lado, y aunque se ha puesto de manifiesto el aporte y registro de organismos
biosilíceos por parte de los principales cursos fluviales en su cabecera, no se ha efectuado
una estimación de ese flujo de diatomeas de agua dulce a ambas rías. Este proceso podría
ser abordado simplemente muestreando la zona fluvial y la desembocadura.
Respecto a la reconstrucción paleoclimática llevada a cabo en el archivo sedimentario
de la plataforma continental, hay que poner de manifiesto la falta de registro de los
indicadores biosilíceos en diversos niveles, lo que impide una interpretación precisa de los
procesos climático-oceanográficos que afectan a la región durante la segunda mitad del
Holoceno. Aún así, el objetivo principal ha sido cubierto satisfactoriamente, combinando el
registro de diatomeas y sílice biogénica con el uso de diversos marcadores o proxies de otra
índole. En este sentido, y debido a la falta de registro y con el fin de precisar los diversos
periodos climático-oceanográficos inferidos sería necesario un aumento de las dataciones de 14C. Por otro lado, este registro sedimentario todavía puede aportar mucha más información,
puesto que también se están llevando a cabo estudios sobre las asociaciones de
foraminíferos planctónicos y bentónicos y su composición geoquímica, análisis isotópicos y
estimaciones de temperatura superficial de la columna de agua. Además, existen otros
testigos de sedimento en esta escala temporal que también pueden ser utilizados mediante el
uso de estos trazadores para corroborar las conclusiones derivadas de este trabajo.
Sin duda el inicio, desarrollo y conclusión de una Tesis Doctoral es un proceso de
aprendizaje continuo que no acaba cuando es defendida, sino que te acompaña a lo largo de
toda la carrera investigadora. En este sentido todo este trabajo ha permitido establecer un
252
Chapter IX
orden de prioridades y necesidades inmediatas de estudio. La Tesis Doctoral es el inicio de
esa carrera de investigación, y es a partir de este momento cuando el planteamiento de
nuevas hipótesis de trabajo y problemas científicos que se traducen en la redacción de
proyectos de investigación es la prioridad. Es por ello que no quería dejar pasar la
oportunidad de plantear algunas inquietudes, ideas futuras y necesidades científicas con
respecto a la línea de investigación en la que está enmarcada esta Tesis.
1. Ampliar a otras rías el estudio de la concentración y distribución de ópalo biogénico
y de las asociaciones de diatomeas en el sedimento. En concreto, el análisis del ciclo anual,
flujos y registro de las asociaciones de diatomeas en el sedimento superficial en cada una de
las Rías Altas también puede ser una buena continuación de este trabajo, debido a sus
características fisiográficas, oceanográficas y productivas claramente diferenciadas de las de
las Rías Baixas.
2. Por otro lado, y en continuación con la línea de trabajo propuesta anteriormente,
sería asimismo muy interesante ampliar la escala espacial de este estudio, de tal manera que
incluyera, por ejemplo, el análisis de muestras de sedimento de todo el Margen Atlántico
Ibérico, el Golfo de Cádiz y el Golfo de Vizcaya, tanto en las zonas más litorales como en la
plataforma. En este sentido, ya existe una importante base de datos de diatomeas recientes
en toda la costa portuguesa y plataforma y rías gallegas, que debe ser ampliada y que pueda
ser susceptible de ser utilizada como base de datos regional combinándola con datos
oceanográficos para la elaboración de ecuaciones de transferencia.
3. También es oportuno un estudio más amplio de la producción biológica y su
registro en el sedimento a escala anual por medio de trampas de sedimentación en la
plataforma continental, e incluso en zonas más externas, como el talud continental.
4. El desarrollo e implementación de una estación fija océano-metereológica que
contemple el análisis de variables oceanográficas y la consiguiente respuesta biológica en
series temporales sería de vital importancia para su variabilidad a una escala supraanual. Así,
un estudio multidisciplinar que contemple la evaluación de diversos parámetros, por ejemplo
geofísicos, geoquímicos, metereológicos, etc. permitiría la integración de resultados para un
futuro estudio paleoclimático. Se han llevado a cabo diversos intentos de control de diversos
parámetros a lo largo de varios años (principios de los 90 hasta 2003) exclusivamente en la
ría de Vigo y en la bahía de A Coruña por parte del Instituto Español de Oceanografía
(programa de radiales) y el Instituto de Investigaciones Marinas de Vigo (IIM-CSIC). Sin
embargo, no existe una estación fija o de control para ninguna de las rías gallegas en la cual
observar, en nuestro caso, la abundancia o flujo de diatomeas al fondo y las variaciones de
las mismas a escala decenal. Esto resulta de crucial importancia para determinar la influencia
253
Self-criticism and perspectives
sobre el medio biológico y geológico de diversos procesos climático-oceanográficos que
varían en ese rango temporal. Es necesario identificarlos para diferenciarlos de aquellos
derivados de las actividades antrópicas que actúan en esa misma escala de tiempo. Este
esfuerzo de identificación de los procesos que actúan a la escala temporal a la que lo hace la
actividad humana sobre el ambiente es clave para entender la futura evolución climática.
5. Continuando con la anterior línea de investigación, sería muy interesante un
estudio de alta resolución de registros relativamente recientes (200 años aproximadamente)
para correlacionar con las condiciones actuales y con datos históricos e instrumentales de
diversos parámetros (temperatura, precipitación, etc.). Los estudios de registros de escala
centenaria y decenaria a alta resolución son muy escasos en esta zona y, por tanto, la
identificación de los procesos climáticos que actúan a esa escala. La correlación y calibración
de los marcadores paleoclimáticos con datos históricos a estas escalas de tiempo son de
especial importancia para el estudio del cambio climático reciente y la elaboración de modelos
de predicción y evolución del clima.
6. Otro aspecto importante de esta investigación es la aplicación del uso de los
organismos biosilíceos a registros costeros, por ejemplo, en playas-barrera-lagoon o lagunas
litorales de esta zona, que puedan abarcar por ejemplo la transgresión holocena. La
caracterización y evolución de los ambientes sedimentarios en zonas litorales puede ser
llevada a cabo con estos organismos. De hecho, pequeñas variaciones en el nivel del mar y la
consecuente respuesta de esos ambientes sedimentarios implica un desplazamiento y/o
aparición de diversas especies de diatomeas en función de las características del ambiente.
La estratigrafía usando diatomeas puede ser utilizada como indicadora de los factores que
afectan a la evolución costera en el pasado.
7. Se ha puesto de manifiesto que tanto en las rías como en la plataforma continental
gallega los sedimentos poseen una importante contribución de material procedente de la
descarga fluvial. Así, se opina y propone un análisis desde diversos aspectos (biológico,
geoquímico, etc.) de las partículas (orgánicas y no orgánicas) que son transportadas por
carga de fondo y suspensión por los ríos que desembocan a cada ría y plataforma. Una
caracterización a escala anual de todo el proceso de aporte y su variabilidad relacionada con
las características climáticas de la zona, cálculo de flujos, permitiría abordar íntegramente el
análisis de estas zonas costeras altamente influenciadas por procesos que se producen en el
continente y que dependen de él.
8. Por otro lado, en esta zona altamente influenciada por la descarga fluvial sería muy
interesante realizar también un análisis del origen y proveniencia de la materia orgánica que
queda registrada en el sedimento, mediante análisis de 15N y 13C. Asimismo, también sería
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Chapter IX
necesario un análisis de estos parámetros en la materia particulada que aportan los diversos
cursos fluviales.
9. En lo que respecta a estudios paleoclimáticos y paleoceanográficos, el extremo
noroccidental de la plataforma continental gallega está relativamente poco estudiado. En
concreto, la mayoría de trabajos se centran en el Holoceno reciente, y existen pocos registros
que abarquen el Ultimo Máximo Glacial y la variación glacial-interglacial. Actualmente existe
una colaboración con investigadores de la Universidad de Bremen, Max Planck Institute y
RCOM que incluye estudios geofísicos, sedimentológicos, geoquímicos y
micropaleontológicos en la plataforma gallega frente de las Rías Baixas y en el talud
continental, que nos va a permitir abordar este tema.
El desarrollo de todas estas futuras líneas de investigación depende sin duda alguna
de su posible financiación a través de proyectos de investigación, contratos I+D+I o estudios
con empresas.
255
TAXONOMIC APPENDIX
Taxonomic appendix
259
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Benthic group Achnanthes spp. Bory 1822 X X X Achnanthes brevipes Agardh 1824 X Amphora spp. (Ehrenberg) Kützing 1844 X X X X X Auliscus sculptus (Smith) Ralfs in Pritchard 1864
X X X
Campylodiscus spp. Ehrenberg ex Kützing 1844
X X
Campyloneis grevillei (Smith) Grunow 1867 X X Catenula adhaerens (Mereschkowsky) Mereschkowsky 1903
X
Cerataulus radiatus (Roper) Ross 1986 X Cerataulus turgidus Ehrenberg 1843 X X X Cocconeis spp. Ehrenberg 1837 X X X X X Cocconeis californica Grunow 1881 X X Cocconeis costata Gregory 1855 X X Cocconeis dirupta Gregory 1857 X X Cocconeis disculoides Hustedt 1955 X Cocconeis disculus (Schumann) Cleve 1895 X X Cocconeis distans Gregory 1855 X X Cocconeis pediculus Ehrenberg 1838 X X Cocconeis peltoides Hustedt 1939 X Cocconeis placentula Ehrenberg 1838 X X X Cocconeis pseudomarginata Gregory 1857 X X Cocconeis scutellum Ehrenberg 1838 X X X X X Delphineis spp. Andrews 1977 X X Delphineis surirella (Ehrenberg) Andrews 1981 X Dimeregramma marinum (Gregory) Ralfs in Pritchard 1861
X
Dimeregramma minor (Gregory) Ralfs in Pritchard 1861
X X
Diploneis spp. (Ehrenberg) Cleve 1894 X X X X Diploneis bombus (Ehrenberg) Ehrenberg ex Cleve 1894
X X X X
Diploneis cabro (Ehrenberg) Cleve 1894 X Diploneis didyma (Ehrenberg) Cleve 1894 X X X Diploneis interrupta (Kützing) Cleve 1894 X X Diploneis notabilis (Greville) Cleve 1894 X Diploneis papula (A. Schmidt) Cleve 1894 X X Diploneis smithii (Brébisson ex Smith) Cleve 1894
X X X X
Eunotogramma marinum (Smith) Peragallo ex vanLand 1978
X
Grammatophora angulosa Ehrenberg 1840 X X Grammatophora spp. Ehrenberg 1840 X X X Grammatophora marina (Lyngbye) Kützing 1844
X X X X X X
Grammatophora oceanica (Ehrenberg) Grunow 1881
X X
Grammatophora serpentina (Ralfs) Ehrenberg 1844
X X X
Taxonomic appendix
260
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Benthic group Gyrosigma spp. Hassall 1845 X X X Gyrosigma fasciola (Ehrenberg, 1839) Griffith and Henfrey, 1856
X
Hantzschia spp. Grunow 1877 X X Licmophora spp. Agardh 1827 X X Licmophora flabellata (Carmichael) Agardh 1830
X
Mastogloia spp. (Thwaites) Smith 1856 X X Mastogloia pseudoexigua Cholnoky 1956 X Navicula spp. Bory 1822 X X X X Navicula cancellata Donkin 1872 X Navicula digitoradiata (Gregory) Ralfs in Pritchard 1861
X X X
Navicula distans (Smith) Ralfs in Pritchard 1861
X X X
Navicula forcipata Greville 1859 X X Navicula palpebralis (de Brébisson) Smith 1853 X X X Navicula peregrina (Ehrenberg) Kützing 1844 X Navicula transitrans (Grunow) Cleve 1883 X X Nitzschia panduriformis Gregory 1857 X Opephora spp. Petit 1888 X X X Opephora schulzii (Brockmann 1950) Simonsen 1962
X X
Petroneis monilifera (Gregory) Stickle & Mann 1990
X X
Plagiogramma pulchellum Greville 1859 X Plagiogramma staurophorum (Gregory) Heiberg 1863
X X
Pleurosigma acutum Norman ex Ralfs in Pritchard 1861
X X
Pleurosigma angulatum (Quekett) Smith 1853 X Pleurosigma elongatum Smith 1852 X Pleurosigma spp. Smith 1852 X X X X X X Podosira stelligera (Bailey) Mann 1907 X X X X X Psammodiscus nitidus (Gregory) Round and Mann 1990
X X X
Rhabdonema minutum Kützing 1844 X X Rhabdonema adriaticum Kützing 1844 X Striatella unipunctata (Lyngbye 1819) Agardh 1832
X
Surirella spp. Turpin 1828 X Surirella fastuosa (Ehrenberg) Kützing 1844 X X X X Toxarium spp. Bailey 1854 X X X Trachyneis aspera (Ehrenberg) Cleve 1894 X X X X X Trachyneis spp. Cleve 1894 X X X Tryblionella Smith (1853) X X X Tryblionella coarctata (Grunow) Mann in Round et al., 1990
X X
Tryblionella punctata Smith 1853 X X X
Taxonomic appendix
261
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Freshwater group Achnanthes lanceolata (de Brébisson in Kützing) Grunow in Cleve et Grunow 1880
X X X
Achnanthes minutissima Kützing 1933 X X Aulacoseria spp. Thwaites 1848 X X X Aulacoseria granulata (Ehrenberg) Ralfs in Pritchard 1861
X X X
Aulacoseria islandica (O. Müller) Simonsen 1979
X X X
Ctenophora pulchella (Ralfs ex Kützing) Williams et Round 1986
X X
Cyclotella spp. (Kützing) de Brébisson 1838 X X X Cyclotella litoralis Lange and Syvertsen 1989 X X Cyclotella menenghiniana Kützing 1844 X X X Cyclotella ocelata Pantoscek 1912 X X Cymbella spp. Agardh 1830 X X X Diatoma spp. Bory 1824 X X X Diatoma hyemalis (Roth) Heiberg 1863 X X X Diatoma vulgare Bory 1824 X X Epithemia spp. Kützing 1844 X X Epithemia adnata (Kützing) Rabenhorst 1853 X Epithemia argus (Ehrenberg) Kützing 1844 X Eunotia spp. Ehrenberg 1837 X X X Eunotia arcus Ehrenberg 1837 X Eunotia pectinalis (O.F. Müller) Rabenhorst, 1864
X X X
Eunotia pectinalis var. minor (Kützing) Rabenhorst 1864
X
Eunotia triodon Ehrenberg 1837 X X Fragilaria crotonensis Kitton 1869 X X Fragilaria vaucheriae (Kützing) Petersen 1938 X X Fragilariforma spp. Williams and Round 1988 X X Fragillaria spp. Lyngbye 1819 X X X X Gomphocymbela spp. Muller 1905 X X Gomphonema spp. Ehrenberg 1832 X X X Gomphonema acuminatum Ehrenberg 1832 X X Gomphonema truncatum Ehrenberg 1832 X X Hannaea arcus (Ehrenberg) Patrick 1966 X X X Hantzschia amphioxys (Ehrenberg) Grunow in Cleve and Grunow 1880
X X
Luticola spp. (Kützing) Mann 1990 X X Melosira moniliformis (Müller) Agardh 1824 X Meridion circulare (Greville) Agardh 1831 X X Pinnularia spp. Ehrenberg 1843 X X X Pinnularia acoricola Hustedt 1934 X X Pinnularia borealis Ehrenberg 1843 X X Sellaphora spp. Mereschkowsky 1902 X X X Sellaphora pupula (Kützing) Mereschkowsky 1902
X X
Stauroneis spp. Ehrenberg 1843 X X
Taxonomic appendix
262
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Freshwater group Staurosirella spp. Williams and Round 1988 X X X Staurosirella pinnata (Ehrenberg) Williams and Round 1987
X X X
Stephanodiscus spp. Ehrenberg 1845 X X X Surirella angusta Kutzing 1844 X Surirella biseriata Brébisson 1836 X Synedra spp. Ehrenberg 1830 X X X Synedra closterioides Grunow 1881 X Synedra ulna (Nietzsche) Ehrenberg 1838 X X X X Synedra undulata (Bailey) Gregory 1861 X Tabellaria spp. Ehrenberg 1840 X X X X Tabellaria flocculosa (Roth) Kützing 1844 X X X
Taxonomic appendix
263
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Other species Actinocyclus curvatulus Janisch 1878 X X Actinocyclus octonarius Ehrenberg 1838 X X Actinoptychus senarius (Ehrenberg) Ehrenberg 1843
X X X X
Actinoptychus splendens (Shadbolt) Ralfs ex Pritchard 1861
X X
Anaulus spp. Ehrenberg 1844 X X Anaulus balticus Simonsen 1959 X Anaulus minutus Grunow in van Heurck 1881 X Asterionella formosa Hassall 1850 X Asterionellopsis glacialis (Castracane) Round in Round et al., 1990
X X X
Asteromphalus flabellatus (Brébisson) Greville 1859
X X
Azpeitia nodulifera (Schmidt) Fryxell and Sims 1986
X
Azpeitia tabularis (Grunow) Fryxell and Sims 1986
X
Bacteriastrum spp. Shadbolt 1854 X X Bacteriastrum delicatulum Cleve 1897 X Bacteriastrum hyalinum Lauder 1864 X X X Biddulphia alternans (J.W. Bailey) Van Heurck 1885
X X X
Bidulphia pulchella Gray 1831 X Biddulphia obtusa (Kützing) Ralfs in Pritchard 1861
X X
Caloneis spp. Cleve 1894 X Cerataulina pelagica (Cleve) Hendey 1937 X X X Chaetoceros spp. Ehrenberg 1844 X X X X Chaetoceros spp. R.S. Ehrenberg 1844 X X X X X X Chaetoceros affinis Lauder 1864 X X X X X X Chaetoceros atlanticus Cleve 1873 X Chaetoceros brevis Schütt 1895 X X Chaetoceros cinctus Gran 1897 X X X Chaetoceros compressus Lauder 1864 X X X Chaetoceros constrictus Gran 1897 X Chaetoceros convolutus Castracane 1886 X Chaetoceros curvisetus Cleve 1889 X X X Chaetoceros danicus Cleve 1889 X X Chaetoceros debilis Cleve 1894 X X X Chaetoceros decipiens Cleve 1873 X X X Chaetoceros densus Cleve 1901 X X X Chaetoceros diadema (Ehrenberg) Gran 1897 X X X Chaetoceros didymus Ehrenberg 1845 X X X X Chaetoceros neogracilis Schütt 1895 X Chaetoceros laciniosus Schütt 1895 X Chaetoceros lauderi Ralfs in Lauder 1864 X Chaetoceros lorenzianus Grunow 1863 X X X X X Chaetoceros perpusillus Cleve 1896 X
Taxonomic appendix
264
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Other species Chaetoceros pseudocurvisetus Mangin1910 X Chaetoceros radicans Schütt 1895 X Chaetoceros seychelarum Karsten 1907 X X Chaetoceros simplex Ostenfeld 19014 X Chaetoceros socialis Lauder 1864 X X X Chaetoceros subsecundus (Grunow in Van Heurck) Hustedt 1930
X
Corethron criophilum Castracane 1886 X Coscinodiscus spp. (Ehrenberg) Hasle and Sims 1986
X X X X X
Coscinodiscus argus Ehrenberg 1838 X X X Coscinodiscus concinnus Smith 1856 X Coscinodiscus decrescens Grunow in Schmidt 1878
X X X
Coscinodiscus oculus iridis Ehrenberg 1840 X Coscinodiscus radiatus Ehrenberg 1841 X X X X Cylindrotheca closterium (Ehrenberg) Reiman and Lewin 1964
X
Detonula pumila (Castracane) Schütt 1896 X X X Ditylum brightwellii (West) Grunow in Van Heurck 1883
X X
Eucampia zodiacus Ehrenberg 1840 X X X Fragilariopsis doliolus (Wallich) Medlin and Sims 1993
X X
Guinardia delicatula (Cleve) Hasle 1996 X X X Guinardia flaccida (Castracane) Peragallo 1892 X X X Guinardia striata (Stoltherfoth) Hasle1996 X X X Hemiaulus hauckii Grunow in Van Heurck,1882 X Hemidiscus cuneiformis Wall 1860 X X X Huttoniella reichardtii Karsten 1928 X Lauderia annulata Cleve 1873 X Lauderia borealis Gran 1900 X Leptocylindrus danicus Cleve 1889 X X X Leptocylindrus danicus R.S. Cleve 1889 X X X Leptocylindrus mediterraneus (H. Peragallo, 1888) Hasle 1975
X
Leptocylindrus minimus Gran 1915 X X X Lioloma pacificum (Cupp) Hasle X Navicula spp. Bory 1822 X Nitzschia spp. Hassall 1845 X X X X Nitzschia bicapitata Cleve 1901 X X X Nitzschia longissima (Brébisson in Kützing) Ralfs in Pritchard 1861
X X X
Odontella mobiliensis (Bailey) Grunow 1884 X X X X X X Paralia sulcata (Ehrenberg) Cleve 1873 X X X X X X Planktoniella sol (Wallich) Schütt 1892 X X Pseudonitzschia spp. Peragallo in Peragallo 1900
X X X
Taxonomic appendix
265
Ría de Pontevedra Ría de Ferrol SMP03-2
Water colum
Sediment traps
Sediment Water
column Sediment Sediment
Other species Pseudonitzschia pungens (Grunow ex Cleve) Hasle 1993
X X
Pseudonitzschia subpacifica (Hasle) Hasle 1993
X
Pseudonitzschia delicatissima (Cleve) Heiden in Heiden and Kolbe 1928
X X X
Rhizosolenia spp. Brightwell 1858 X X X X Rhizosolenia alata Brightwell 1858 X X Rhizosolenia bergonii Perigallo 1892 X Rhizosolenia fragilissima Bergon 1903 X X Rhizosolenia hebetata (Bail) Gran 1904 X X Rhizosolenia robusta Norman 1861 X Rhizosolenia setigera Brightwell 1858 X X X Rhizosolenia shrubsolei Cleve 1881 X X Rhizosolenia styliformis Brightwel 1858 X Roperia tesselata (Roper) Grunow ex Pelletan 1883
X X
Skeletonema costatum (Greville) Cleve 1878 X X X X X X Stephanopyxis turris (Greville and Arnott) Ralfs ex Pritchard 1861
X X
Thalassionema nitzschioides (Grunow) Mereschkowsky 1902
X X X X X X
Thalassionema frauenfeldii (Grunow) Hallegraeff 1986
X X
Thalassiosira spp. (Cleve) Hasle 1973 X X X X X Thalassiosira angulata (Gregory) Hasle 1978 X X X Thalassiosira anguste-lineata (Schmidt) Fryxell et Hasle 1977
X X X X
Thalassiosira fallax Meunier 1910 X Thalassiosira ferelineata Hasle and Fyxnell 1977
X
Thalassiosira levanderi Van Goor 1924 X X X Thalassiosira nordenskiöldii Cleve 1873 X Thalassiosira oestrupii (Ostenfeld) Hasle 1972 X X Thalassiosira rotula Meunier 1910 X X X Thalassiothrix longissima Cleve and Grunow 1880
X
Triceratium spp. Ehrenberg 1839 X Tryblionella navicularis (Brébisson ex Kützing) Ralfs in Pritchard 1861
X
Esta Tesis se centra en el estudio del ópalo biogénico, organismos biosilíceos y marcadores biogeoquímicos en diferentes compartimentos del medio marino. Estos trazadores oceanográficos se han analizado en la columna agua, trampas de sedimentos y sedimento superficial de las rías de Vigo, Pontevedra y Ferrol, y en el registro sedimentario Holoceno de la plataforma continental gallega. El objetivo de esta Tesis es relacionar las condiciones de productividad primaria con los patrones oceanográficos y/o climatológicos actuales, así como evaluar el flujo de material biosilíceo a través de la columna de agua y su acumulación en los sedimentos. Se ha realizado una reconstrucción paleoceanográfica y paleoclimática precisa del registro sedimentario Holoceno teniendo este conocimiento de los procesos que afectan a los marcadores estudiados.
This Thesis deals with the biogenic opal, biosiliceous organisms and biogeochemical markers in the marine environment. These tracers were analysed in the water column, sediment traps and superficial sediment of the ría de Vigo, Pontevedra and Ferrol, as well as in the Holocene sediment record of the Galician continental shelf. The aim of this work is to correlate the primary production conditions with the recent oceanographical and climatological patterns in the area. In addition, the vertical fluxes of the biosiliceous material and its accumulation in recent sediments has been evaluated. On the basis of diatoms, biosiliceous compounds and biogeochemical tracers a paleoceanographic and paleoclimatic reconstruction of the Holocene sediment record has been done taking into account the processes affecting these markers.