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A contribution of diatom analysis to Lateglacial and Holocene environmental reconstructionsof Colombian lowland and montane ecosystems

Velez, M.A.

Link to publication

Citation for published version (APA):Velez, M. A. (2003). A contribution of diatom analysis to Lateglacial and Holocene environmental reconstructionsof Colombian lowland and montane ecosystems. Amsterdam: IBED, Universiteit van Amsterdam.

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Download date: 28 Jan 2020

Chapterr 5

Integratedd diatom-pollen based environmental reconstructionn of lake Las Margaritas,

Easternn Savannas of Colombia

Mariaa Isabel Vélez, Michael Wille, Henry Hooghiemstra and Sarah

Metcalfe e

Abstrac t t

AA multiproxy analysis including diatoms, pollen, element geochemistry, carbonn content analysis, and radiocarbon dates was carried out for sedimentt core Las Margaritas (3°23' N, 73°26' W) to reconstruct environmentall and climatic change. The record covers the last 11,150 cal BP.. Its location in the transitional area between the Amazonian rainforest andd the savannas of the eastern part of Colombia is strategic to document competitionn between both biomes induced by climate change. Dryy conditions, a landscape covered by savanna and an incipient lake still influencedd by the river system, characterised the period between 11,150 andd 9100 cal BP. More humid conditions prevailed between 9100 and 73300 cal BP, during which time the forest advanced over the savanna and aa swampy, isolated lake formed. From 7330 to 1870 cal BP, humid conditionss prevailed, except for two dry periods that caused a decrease in lakee level; these periods occurred between 5430-5200 and 2750-2500 cal BP.. A period of unstable climatic conditions, i.e. frequent alternations betweenn drier and wetter periods, causing alternation between the savanna andd the forest and oscillations of lake level, occurred between 4180 and 27500 cal BP. During dry conditions the lake level dropped, the water featuredd richer nutrient contents and higher conductivity, and swamps withh Arecaceae were formed. During thee humid periods, the lake had high waterr levels, the water was acidic, poor in nutrients and with low conductivity,, swamps with Mauritia surrounded the lake. Modern climatess and limnological conditions have prevailed since 1870 cal BP. Comparisonss with other environmental and climatic reconstructions from

114 4

thee Llanos Orientates and the Colombian Amazon allowed us to conclude thatt the humidity of this zone is highly dependent on moisture coming fromfrom the Amazon region.

1.. Introductio n

Thee late Quaternary environmental history of the tropical lowland ecosystemss in northern South America is relatively poorly known. Recentlyy a significant number of records of environmental change have becomee available through studies of lakes located in the savannas of the Llanoss Orientates. Many of these lakes were located along a west-to-east transectt from the foot of the Eastern Cordillera in the west to the Rio Orinocoo to the east, in the provinces of Meta and Vichada. The studied lakess were (from east to west): lakes Angel and Sardinas (Behling and Hooghiemstra,, 1998), lakes El Pinal and Carimagua (Behling and Hooghiemstra,, 1999; Berrio et al., 2000), lakes Chenevo and Mozambiquee (Berrio et al., 2002), lake Loma Linda (Behling and Hooghiemstra,, 2000) and lake Las Margaritas (Will e et al., 2003).

Sedimentt cores were analysed for pollen and loss on ignition (LOI) and chronologicall control was based on AMS radiocarbon dates. Lakes in closee proximity to the Cordillera contained up to 10 meters of sediment, facilitatingg reconstructions of high temporal resolution. Most of these lakess started sediment accumulation 12,000 to 8500 radiocarbon years beforee present (= 12,000 to 8500 BP). However, lake El Pinal shows a recordd that dates back to the Last Glacial Maximum (LGM).

AA paper by Berrio et al. (2002) attempts to synthesise the development of thee savanna vegetation. According to Berrio et al., from 22,000 to 12,850 call BP (18,000-10,700 BP) the savanna featured patchy gallery forest, non-permanentt lake conditions and maximum abundance of grasses, whichh reflect low precipitation and long dry seasons. From 12,850 to 11,1500 cal BP (10,700-9700 BP) gallery forests expanded, reflecting a moree humid climate. From 11,150 to c. 6300 cal BP (9700-5800 BP) grasslandss were at a maximum, indicating dry climatic conditions. From c.. 6300 to c. 3900 (5800-3810 BP) gallery forest increased. Around 4450 call BP (4000 BP) all records indicate changes in the savanna ecosystem;

115 5

inn particular , the palms Euterpe/Geonoma, Mauritia and Mauritiella becamee abundant which reflects a significant change in the general aspect off the savanna vegetation. The latter shows that taxa respond individualisticallyy and cause changes in floral composition across time. Ann overview of the history of the savanna vegetation north and south of thee Amazonian rainforest area was published by Behling and Hooghiemstra(2001). .

Lakee Loma Linda (Behling and Hooghiemstra, 2000) is the closest to the sitee examined in this article, and is also located today in the transitional zonee from savanna to rainforest. The pollen record of Loma Linda shows, withh a temporal resolution of about 125 yr, an alternation between forest andd open savanna vegetation since 8700 BP The pollen record of lake Las Margaritass has a temporal resolution varying from 20 to 50 yr (Will e et a l,, 2003).

Thee studies mentioned before were mainly based on pollen records, a proxyy that shows the competition between gallery forest and the open savannaa vegetation (regional vegetation change) and the competition betweenn aquatics from relatively deep water and shallow water habitats (locall vegetation change). However, the reconstruction of the developmentt of the local vegetation in these studies was based on a few pollenn taxa only, so a more detailed account of the environmental developmentt of the lake basins themselves was necessary. We therefore studiedd the diatom records of selected lakes, with a further objective of improvingg our understanding of changes in the abiotic conditions of these waterr bodies through time. In this paper we present the diatom record of coree Las Margaritas and the improved environmental reconstruction basedd on the integrated evidence from analyses of diatoms, pollen, lithology,, LOT, and geochemical data.

2.. Environmental setting

Lakee Las Margaritas (3° 23' N, 71° 26' W) is located in the province of Meta.,, about 75 km from the foot of the Eastern Cordillera and some 100 kmm to the southeast of the city of Villavicencio, at an elevation of 290 m. Thee lake is about 2 km long, 400 to 600 m wide, and approximately 1.5 m

116 6

deepp in the dry season. The lake originated in the late Pleistocene, and its elongatedd form suggests that it is an abandoned meander from an old drainagee system. Today, it is completely isolated from the main fluvial systemm and situated in a landscape with gently rolling hills in the northeasternn part of the catchment area of the Ariari River.

Sitee Las Margaritas is located in an area where savanna vegetation is transitionall to rainforest, and it features a relatively undisturbed sedimentaryy record. Because of these characteristics, core Las Margaritas presentss a sensitive record of climatic and environmental change. The firstt study of the core Las Margaritas was carried out by Will e et al. (2003).. Making use of 14C wiggle-match dating (WMD) and detailed pollenn analyses these authors presented the vegetation and climate reconstructionn for site Las Margaritas. They stated that the development fromfrom open savanna to forest during the mid-Holocene is synchronous with aa decreasing caloric seasonality and a southward migration of the Intertropicall Convergence Zone (ITCZ), suggesting that large scale climaticc and vegetational change in Colombian savannas was precession forced. .

2.12.1 Vegetation

Thee region features mostly open savannas interrupted by gallery forests withh trees from 10 to 25 m tall concentrated in depressions and low lying areass that form the drainage system. In undisturbed conditions, continuouss cover of wet evergreen rainforest starts between 30 and 80 km too the south from the site (Fig. 1). Due to the well-drained and nutrient-poorr soils, and more recently to human activity, conditions are drier, trees aree shorter and more slender. Areas with stagnant water in the proximity off the lake are partly covered with stands oïMauritia palm swamp forest. Inn various places open grassland reaches the lake shore. Aquatics, mainly Cyperaceae,, Ludwigia, and Sagittaria, grow in a narrow zone around the lakee (see Will e et al. 2003 for a more detailed description of the regional vegetation). .

2.22.2 Climate

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Thee climate of the Llanos Orientates is seasonal. Approximately 90% of thee precipitation falls during the rainy season. The length of the dry season,, which varies geographically, is longest in Puerto Carrerio, near thee Venezuelan border, where it lasts from December to May. The dry seasonn there is more intense than at the site of Laguna Las Margaritas, wheree it only lasts some 3 to 4 months. The seasonal cjimate is caused by thee annual migration of the intertropical convergence zone (ITCZ). When thee ITCZ moves north from June to August, the trade winds, bearing moisturee from the Amazon and the tropical Atlantic, can reach the area; thiss explains the long and unimodal pattern of precipitation. Total annual precipitationn varies from about 1500 mm in Puerto Carreno to about 2500 mmm near lake Las Margaritas (Fig. 1). At the transition to rainforest, near Sann José del Guaviare, annual precipitation is 3000 mm and the dry seasonn lasts up to 2 months. The savannas near the foot of the Andes receivee annually >2000 mm of precipitation. The mean annual temperaturee at she Las Margaritas is around 26°C.

3.. Methods

Thee 10-m-Iong sediment core was drilled with a modified Livingstone corerr in the centre of the lake, using a floating platform. Compact sandy sedimentss below 10 m prevented further sediment recovery. Sediments weree collected in 1 -m sections in aluminium tubes and transported to Amsterdam.. The core was stored in the laboratory in cold (4°C) and dark conditions. .

Forr element analysis and LOI, samples were taken every 5 cm between 1000 and 500 cm core depth, at the same depths as the samples for diatom andd pollen analyses. Element analysis was carried out using atomic emissionn spectrometry, Pekin Elmer 5000, except P and Ti that were measuredd with the colorimeter Cecil C1Q10. Samples for LOI were burnedd at 800°C. The lithological description of the core is taken from Will ee et al. (2003). LOI was used as an indication of the Carbon content inn the sample (Hakanson and Janesson, 1983) and elements were used as indicatorss of erosion from the littoral and/or allochthonous input.

Timee control of the sediment core was based on 9 bulk samples of ca. 0.5 cmm collected at depths where significant changes in the pollen record

118 8

and/orr in the sediment column occurred. The AMS 14C ages of the bulk sampless were calibrated with the Cal25 programme (Van der Plicht, 1993).. For more details, see Table 2 in Will e et al. (2003).

ForFor diatom analysis, samples were taken at 5 cm intervals along the core fromfrom the same levels as the pollen samples. The procedure for the preparationn of the fossil diatoms was the following: about 0.3 mg dry sedimentt of each sample were immersed in 30 ml H202 (30%) for 24 hourss at room temperature, after which a few drops of potassium permanganatee were added. After the reaction stopped, 10 ml of HC1 were added.. Samples were washed with distilled water. Permanent slides were preparedd with naphrax. A total of 124 slides were examined at lOOx magnification,, and a minimum of 400 valves were counted on each one. Forr diatom identification and ecology the following literature was used: Patrickk and Reimer (1966), Krammer and Lange-Bertalot (1986, 1991, 1997),, Lange-Bertalot (2000), Moro and Ftirstenberger (1997), Gaiser and Johansenn (2000), Torgan and Biancamano (1988) and Evans (1958).

AA survey to collect modern diatoms analogous to the fossil material and too examine the chemical characteristics of the lake was carried out in Januaryy 2000. The pH values were measured in the field with pH indicatorr paper. A few samples were collected using glass bottles and takingg the sample directly from the water and/or the sediment. Water sampless were transported in cool containers to the laboratory of the Universidadd de Antioquia in Medellin (Colombia) for chemical analyses. Too separate the diatoms we added 30 ml of H2O2 (30%) to the water and lett it decant for 48 hours. Macrophyte roots were scraped and the residues mountedd on slides. For sediment samples we applied the same technique ass for the fossil samples.

Onee unidentified fossil species, Aulacoseira species 1, was also present in thee modern diatom collection, which facilitated its ecological interpretation.. Some other species, such as Eunotia transfuga, Eunotia rabenhorstianarabenhorstiana and Aulacoseira pseudoamericana, were present in both thee fossil and the modern samples, a fortunate situation given littl e has beenn published about their ecology.

119 9

Forr pollen analysis samples were collected following standard procedures describedd in Will e et ah (2003).

4.. Results

4.14.1 Chronology

Detailss of the radiocarbon dating were published in Will e et al. (2003) andd presented in this study in Table 1. The age for the periods presented inn this study were calculated through linear interpolation between radiocarbonn dates (BP) and between calibrated years (cal BP), as we providee the two ages for each zone. Fig. 2 shows the depth vs. age graph off the bulk samples.

4.24.2 Stratigraphy

Thee stratigraphy of the sediment core was taken from Will e et al, (2003). Thee bottom 10 cm interval is composed of pure sand (Fig. 3). From 990 too 932 cm there is mainly sand but with a higher content of clay and plant remains.. The interval 932-626 cm is composed of light clay with plant remains.. From 626 to 385 cm peat is the main component, with a sand layerr at 439-437 cm. From 385 to 208 cm peat continues to be the main componentt but there is a generalised increase in clay, particularly at 362-3522 cm. The interval 208-48 cm, is composed of peat, with less clay contentt and many plant remains. The top 48 cm are composed of fine detrkall mud.

4.34.3 Element geochemistry

Informationn on element geochemistry and loss-on-ignition (LOI) is only availablee for the interval 100-500 cm (Fig. 3). This is due to a lack of sedimentt between 0-100 cm, and to the limited diatom valve preservation betweenn 650 and 1000 cm. Element geochemistry refers to the total contentt of Si02> A1203, CaO, FeQ, Na20, K20, Fe2Oj, MnO, Mn02, MgO,, P205, Ti02 and LOI in a sample. The content is given in percentages.. The inorganic fraction is composed of Ti02, MgO, A1203, FeO,, Fe203, MnO, Mn02, CaO, Na20, K 20, Si02. LOI represents the organicc fraction. Si02 is the most abundant mineral with an average of c.

120 0

35%% (max. 58% at 240-255 cm; min. 10% at 265 cm) followed by A1203

c.. 20% (average). LOl has an average of c. 30% (max. 50% at 145 cm; min.. 10% at 245-250 cm). CaO has a peak at 440 cm. At 205 cm all mineralss increase except Si02 that records its minimum value. Between 130-1355 cm Na20, K20, MgO Ti02 have a peak. P2Os increases at 105 cm.. We interpreted the peaks in the inorganic fraction as an increase of thee input material from the margins of the lake.

4.44.4 Fossil diatoms

Fossill diatoms valves were well preserved only in the upper 560 cm of thee core; preservation in the rest of the core was poor. In the samples correspondingg to the intervals between 395-390 cm; 255-240 cm; 170-1655 cm; 155-140 cm and 15-0 cm we could not count 400 valves, the minimumm required number; these samples were therefore excluded from thee analysis. Sixty species were identified in total and the main results are graphedd in a diatom percentage diagram (Fig. 4). Two species, species-1 andd Aulacoseira species-1 remain unidentified (Plate 1). It is unclear whetherr species-1 belongs to the genus Cyclotella or Aulacoseira. The distributionn of species by genera is as follows: 23 species belong to Eunotia,Eunotia, 8 species belong to Pinnularia and 6 species belong to Aulacoseira',Aulacoseira', the remaining species are distributed among the genera Surirella,Surirella, Actinella, Stauroneis, Gomphonema, Cymbella, Sellaphora, Luticula,Luticula, Diadesmis, Navicula and Frustulia. For the ecological reconstructionn we used published ecological data and information from thee modern diatom assemblages collected in the modern lake Las Margaritas.. The ecological ranges for the important diatoms are presented inn Table 2.

121 1

Figuree IA . Map showing main vegetation zones in the Eastern part of Colombiaa and the location of sites mentioned in the text. IB: Detailed mapp showing the location of lake Las Margaritas. 1C: Monthly precipitationn values of the period 1987-2001 of the study area after Institutoo de Estudios Ambientales (IDEAM), Bogota, Colombia.

122 2

UtCNo. .

10040 0

10041 1

10042 2

10043 3

4956 6

5477 7

4957 7

5478 8

4958 8

Depth h (cm) )

~Ï6 6

51 1

81 1

204 4

300 0

500 0

592 2

800 0

1000 0

«CBP P

8541366 ~

4 4

1902139 9

6 6

3789136 6

5396149 9

5998141 1

8760160 0

9760160 0

513C C

_ . _ _ „ . .

-27.9 9

-27.7 7

-29.9 9

-27.4 4

-25.8 8

-23.1 1

-22.8 8

-22.4 4

Sample e type e

bulk k

bulk k

bulk k

bulk k

bulk k

bulk k

bulk k

bulk k

bulk k

CalBP. .

907-861 1 831-811 1 795-687 7 1867-1845 5 1835-1707 7 1641-1637 7 1925-1907 7 1903-1731 1 3355-3205 5 3193-3163 3193-3163 3149-3143 3 3123-3119 9 3089-3083 3 4345-4337 7 4287-4271 1 4261-4085 5 4029-4013 3 4005-3999 9 6289-6169 9 6149-6109 9 6099-6091 1 6075-6055 5 6051-5999 9 6939-6925 5 6909-6729 9 6695-6691 1 10145-10139 9 10111-10081 1 10065-10061 1 10031-10021 1 10009-9997 7 9919-9553 3 11257-11243 3 11237-11087 7 11079-11075 5 10939-10887 7 10881-10871 1

Appr.. date afterr WMO (calBP) )

760 0

1775 5

1850 0

3250 0

4150 0

6200 0

6830 0

9775 5

Tablee 1. Chronological control of the core Las Margaritas (taken from Will ee et al., 2003).

123 3

E E o o

£ £ o. . o o D D

O O

100 0

200 0

300 0

400 0

500 0

600 0

700 0

800 0

900 0

1000 0

10000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Radiocarbonn years (BP)

Figuree 2. Depth vs. age graph of bulk and macrofossil radiocarbon ages off core Las Margaritas (after Wille et al., 2003).

4.54.5 Modern diatoms

Modernn diatoms were collected in January 2001 from a variety of sites in thee lake. From the centre of the lake we collected surface sediments and a waterr sample; from the littoral we collected sediments, roots and a water sample.. Qnly the sediments from both locations, the littoral roots and the littorall water samples contained diatoms. Modern assemblages are presentedd in Table 3. The species Aulacoseira species-1, A. pseudoamericana,pseudoamericana, A. cf. italica, Eunotia aff transfuga and E. rabenhorstianarabenhorstiana were present in the modern as well as in the fossil samples.. We used the modern ecological ranges of the diatom species as a guidee to reconstruct paleoenvkonmetal conditions. The modern littoral diatomm assemblages consisted mainly of Eunotia minor, Fragilaria javanicajavanica and Pinnularia braunii; we interpreted the presence of such a diatomm spectrum in the fossil assemblages between 365 and 360 cm as indicativee of a littoral environment during the period corresponding to thatt core interval. Samples between 300 and 280 cm were dominated by

124 4

AulacoseiraAulacoseira species-1 and E, aff. transfuga; today this assemblage is foundd in the surface sediments, therefore we intepret that similar conditionss with respect to water depth and water chemistry might have prevailedd during the period this interval represents. In the uppermost 83 cmm the assemblage is composed mainly of Aulacoseira granulata v. angustissima,angustissima, A, species-1, and in a smaller proportion, of .4. herzogii and A.A. pseudoamericana. This diatom assemblage is similar to that found in thee plankton from lake El Pinal, located some 240 km northeast of lake Lass Margaritas (pH, 4.52; Conductivity 7.7. uS/cm and 1 m water depth). Thereforee we concluded that the period represented by the uppermost 83 cmm was characterised by open water. Modern chemical characteristics of lakee Las Margaritas are presented in Table 4.

Species s

AulacoseiraAulacoseira granulata

AulacoseiraAulacoseira herzogii

AulacoseiraAulacoseira cf. Hfdka

Aulacoseira Aulacoseira pseudoamericana pseudoamericana

AulacoseiraAulacoseira species 1

CymbeBaCymbeBa affspicula

Ecolog y y

hh and brackish water -Wei!! mixed water, moderately eutrophic,, moderately alkaline -Inn surface sediments from the modemm lake Turbid,, standing and shadow waters s -Inn littoral sediments from the modemm lake inn very shallow environments pronee to dissecarjon

Surfacee sediments from lakes in northernn Michigan. In a small bog lakee surrounded by a floating mat -Inn the plankton and surface sedimentss from lake El Pinal

nn surface sediments and less commonn in the littoral from the modemm lake Stagntantt water, pH: 4.3-6.5 -abundantt in pH: 7 and conductivityy 5013piS/cm

Referenc e e

-Torgann and Biancamano 1988 8 -Salaetal.. 1999. -Personall observation

-Jewsonetal.. (1993) -Personall observation

-Vetoo etal. (in prep.)

Cambumm and Kingston (1986) )

-Personall observation

-Hustedtt (1937) -Personall observation

Indicato rr of

Shallower,, eutrophic andd well mixed waters

Shallowerr water level. Stagnantt water

Veryy shallow waters pronee to dissecation Veryy shallow water conditions s

-Shalloww waters (85-1500 cm). -pH:: 4,52- 5,3 -Conductivity:: 18(jS/cm

Stagnantt waters with higherr conductivity

Tablee 2. Ecological characteristics of the modern diatom analogous. The ecologicall characteristics were taken from published material and personall observations in the modern lake Las Margaritas.

125 5

Diadesmis Diadesmis coafervacea coafervacea

EunotiaEunotia bilunarisv. mucophila mucophila EunotiaEunotia óidyma eteganiub eteganiub

EunotiaEunotia camelus

EunotiaEunotia fonnica

EunotiaEunotia minor

EunotiaEunotia rebenhorstiana

EunotiaEunotia transfuga-

LuticulaLuticula mutica

NaviculaNavicula gallica laevissime laevissime

Pinnularia Pinnularia miscrosteuron miscrosteuron

PinnulariaPinnularia viridis

-Aerophi ll or shallo w water , prefer s sof tt war m water . -Fromm the Amazo n region : pH: 4,6-66 and low conductivitie s (5-84u.S/cm )) and warm water (29-*.

) )

-acidd lakes and bogs as epiphyti c

-Oiigohalobe ,, ackktphi t -Inn plankto n and surfac e sediment ss fro m Lagun a grand e -- In the littora l of moder n lake

Circu mm neutra l Loww conductivit y Planktoni c c Highh O Ï concentratio n -Inn stagnan t or slow runnin g water s s -Haloph-lndiff-Sapro . . pH:: acid-lndiff-Circumn . -Eutrophi cc water s Acidi cc water wit h loww minera l conten t Althoug h theyy tolerat e highe r Ca concentratio nn that other Eunotia spp , , -Inn the plankto n from t|ie littora l in thee moder n lake -Fromm Amaur i creek (Amazone) , pH:: 4$ Conductivity : % S / c n v Temperatur ee 29 C -Inn surfac e sediment s in mode m lake e

-Acidobiontic-Dystrophi cc water s -Inn surfac e sediment s fro m moder nn lake

Oftenn as aerophi l

Terrestria ll on wet moss

-Widee rang e of pH but prefer s slightl yy acidi c and oligotroph y water r -Optimu mm depth : 18 cm; pH: 4,6 Intermiten tt water

-Patric kk and Reimer (1966) -Salaa et at. (2000)

-Patric kk and Reimer (1966)

-Moraa and Förstenberge r (1997). . -Dee oliveir a et at. 1992 -Persona ll observatio n -Moroo and Fürstenberger , (1997). .

-Patric kk and, Reimer (1966) -Moroo and Furstemberge r (1997). .

-Patric kk and Reimer (1966J -AHesetal .. (1991) -Persona ll observatio n

-Salaa etal . (2002) -Persona ll observatio n

-Patric kk and Reimer (1966) -Persona ll observatio n

Patric kk and. Reimer (1966)'

Kramme rr and Lange -Bertalot(1997 ) ) -Patric kk and Reimer (1966) -- Gaiser and Johanse n (2000) )

Evanss (1958)

Noo permanen t water

Stagnan tt water s wit h highe rr conductivit y Acid ,, oligotroph y water .. High water level . . Plankto nn ic-epiphyti c Acidd water . High waterr level

Stagnan tt water , slightl y y eutrophicated . . Benthi c c

Shallowe rr water level .. Increas e in Ca concetratio n n

Acidi cc waters.Wit h loww conductivity . Highh water level

Acidi cc water s wit h loww conductivity . Highh water level Benthic/epiphvti c c Noo permanen t water

Noo permanen t water

Shallowe rr water level s s

Intermitten tt water

Tablee 2. continued.

on on E E

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Figuree 3. Diagram showing the stratigraphy, element geochemistry,, LOI and radiocarbon ages of the core Las Margaritas s

127 7

HH LJ

P! !

Figuree 4. Diatom percentage diagram of core Las Margaritas showingg the records of the main taxa.

128 8

Specie s s

ActineifaActineifa pararobusta AnomoeoneisAnomoeoneis brachysira AulacoseiraAulacoseira aff pseudoamericana AA cf. agasizii AA granulata A.A. herzogii cfcf Achanthes minutissima CymbellaCymbella aequalis CC silesiaca DiadesmisDiadesmis confervacea EunotiaEunotia bkjentula ££ camelus ££ dydima ££ fermiformis ££ fiexuosa EE aff. gtaoalis ££ gladalis E.E. cf. faba ££ cf rabenhorstiana EE minor ££ naeglii EE pectinalis undulata ££ subacuartoides ££ transfuga ££ trigbba FragilariaFragilaria javanica FrustuliaFrustulia rhomboides GomphonemaGomphonema archaevibrio G.G. cf. acustiusculum GG gratile LuticulaLuticula muOca NavhulaNavhula cryptocephala N.N. minima N.N. subtilissima N.N. radhsa PinnulariaPinnularia cf. subgfaba P.P. cf. laucensis P.P. g«b6a P.. hemiptera P.P. ignobilis P.P. obscura P.P. rostratissima P.P. subcapitata SellaphoraSellaphora pupula StauroneisStauroneis anceps StenopterobiaStenopterobia delkatissima SurveliaSurvelia linearis

Littora l l (epiphytic ) ) + +

+ +

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+ + + +

Littoral : : (planktonic ) )

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

+ +

_ _

Surfac e e sedimen t t

+ +

+ +

~ ~ + +

— —

+ + + +

+ +

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+ +

+ +

Tablee 3. Composition of the modern diatom flora collected in January 20011 from the littoral and surface sediments of lake Las Margaritas (+ meansmeans presence; - means absence).

129 9

Paramete rr ____ Value Depthh {m)~ t T , pHH (field) 5.3 PHH 4.6 Conductivityy (|AS) 18.4 Totall Alkalinity (mg CaC03/l) 8 mgofS04/!! 1.4 mgg of Cl/I 2.36 mgofCa/ ll 0.140 mgg of Mg/I 0.204 mgg of Na/I <0.023

j T £ ^ K / [ __ JM

Tablee 4. Chemical water charateristics of the modern savanna lake Las Margaritas. .

4.64.6 Cluster and Detrended Correspondence Analyses

Clusterr analysis was applied to identify zones in the diatom and pollen diagrams.. In addition to the standard CON1SS cluster analysis, we applied DCAA analysis for the diatom samples (Fig. 5). Five DCA-based diatom zoness were identified. This zonation supports the CONISS-based zonation. .

4.74.7 Diatom zones

Diatomm zone D-l (565-403 cm; 30 samples): the lower and middle parts off this zone are dominated by Eunotia didyma elegantula (20%), E. formicaformica (10%), E. off. transfuga (5%) and Aulacoseira species 1 (5%). E. cff rabenhorstiana, E. minor and K bihmaris mucophila are also present Thee upper part of the zone is dominated by Cymbella aff. spicula (40% at 450-4400 cm and at 420-405 cm) and in lower percentages by C microcephalamicrocephala (450-405 cm). Aulacoseira granulata angusiissima, A. herzogiiherzogii and A. cf. italica are more abundant between 500 and 405 cm. Althoughh with low percentages the aerial/subarial diatoms Pinnularia microstauron,microstauron, Luticula mutica and P. viridis are more abundant between 485-4500 cm. At 435 cm Navicula gallica laevissima, L mutica and DiadesmisDiadesmis confervacea show a maximum. Lithologically this zone is

130 0

characterisedd by peat. LOI values and CaO content reach maximum valuess at 445-440 cm.

Diatomm zone D-2 (403-303 cm; 18 samples): this zone is dominated by EunotiaEunotia formica (20%), E. didyma elegantula (10%), E. trigibba (10%) E.E. aff. transfuga (8%) and in a smaller proportion by E. minor (5%) and E.E. bilunaris mucophila (5%). E. camelus and Frustulia rhomboides are common.. The bottom of this zone occurs in the peat unit and the mid-upperr part occurs in the peat unit that contains more clay. LOI has low values.. Between 365-360 cm there is a marked increase in Fragilaria javanica,javanica, E. minor, Pirmularia braunii which resembles today's littoral assemblage. .

Diatomm zone D-3 (303-160 cm; 22 samples): there is a clear dominance of EunotiaEunotia aff. transfuga (30-45%), and E. cf. rabenhorstiana (380-250 cm: 60%;; 250-160 cm: 20%). K formica reaches high percentages but does so intermittentlyy (295-285 cm: 35% ; 280-275 cm: 1%; 270-265 cm: 30%; 255-- 235 cm: 1%; 205-195 cm: 40%). Aulacoseira species-1 dominates thee bottom (20%) where Luticula mutica (a peak at 260 cm of 8%) and DiadesmisDiadesmis confervacea are common. The bottom assemblage dominated byy A. species 1 and E. aff. transfuga is similar to the one found in today's surfacee sediments and occurs in the peat unit with higher content of clay, loww LOI and a peak in most minerals at 265-270 cm (except Si02). The mid-upperr part (from 200 to 160 cm) of this zone (200-160 cm) occurs in aa peat unit that contains plant remains and increasing values of LOI and increasingg values of Eunotia fermiformis, E. bilunaris mucophila and E. minor. minor.

Diatomm zone D-4 (160-83 cm; 13 samples): this zone is dominated by AulacoseiraAulacoseira species \ (20%), A. herzogii (10%), Eunotia minor (10%), E. bilunarisbilunaris mucophila (8%), Cymbella aff. spicula (5%). Aulacoseira cf pseudoamericanapseudoamericana and Pirmularia microstauron are present. This zone occurss in the peat unit that contains many plant remains. LOI values are increasingg (max. 50% at 145 cm). K20 has a peak at 130-135 cm.

Diatomm zone D-5 (83-0 cm; 13 samples): this zone is dominated by AulacoseiraAulacoseira granulata v. angustissima (40%) and to a lesser extent by A.

131 1

speciesspecies 1 (15%). Compared to the zone below, there is a marked reductionn of E. minor, E. hihmaris mucophila, C. aff. spicula. Pinnularia microstauron,microstauron, A. herzogii and A. cf pseudoamericana are present. This zonee occurs in the peat unit with plant remains at the bottom and in the finee detritus mud at the top. This assemblage is very similar to the planktonicc assemblage found today in the lake EI Pinal, composed of A. gramdalagramdala (24%), A. pseudoamericana (16%), A. herzogii (10%), A. speciesspecies I (5%) E.minor (3%) and E. camelus (4%).

4.84.8 Pollen zones

AA summary of the most important pollen taxa is presented in Fig. 6. Pollenn zones were based on the CONISS analysis of the pollen taxa that reflectt the regional vegetation. Since the diatom zones reflect local conditionss in the water body of lake Las Margaritas, diatom and pollen zonationss are not necessarily similar. We expect that the pollen zonation off aquatics (of local importance) is more in accordance with the diatom zonation.. A description of the pollen zones was given in Will e et al (2003). .

5.. Multi-proxy-based paleoenvironmental reconstruction

Wee first present an interpretation of the diatom record of the upper 630 cmm of core Las Margaritas in combination with the evidence taken from changess in the lithological column, element geochemistry and from aquaticc plants. This combination allowed the reconstruction of the local developmentt of the lake (Fig. 7). We also make use of information from thee pollen record published by Will e et al. (2003) that reflect the local vegetationn (the pollen record starts at the bottom of the core at 1000 cm). Thiss section is followed by a comparison of the local environmental historyy with the record of regional environmental change based on the analysiss pollen taxa of regional significance. Table 5 shows the depth intervall of the diatom zones and the pollen zones and the estimates ages off all periods.

132 2

1500 200

Depthh (cm)

Figuree 5. Detrended Correspondence Analysis (DCA) for the diatom sampless of core Las Margaritas showing axis 1 vs. depth.

Zone e Coree interva l (cm) no. sample s perio d (14C BP)/ periodtca l BP) diatom-11 565-403 diatom-22 403-303 diatom-33 303-160 diatom-44 160-83 diatom-55 83-0 averagee temporal resolution

pollenn LM-1 990-752.5 pollenn LM-2 752.5-627.5 pollenn LM-3 627.5-451.5 pollenn LM-4 451.5-352.5 pollenn LM-5 352.5-137.5 pollenn LM-6 137.5-0

30 0 18 8 22 2 13 3

6645/96== I

40 0 25 5 37 7 41 1 42 2 299 (214 total)

5820-4620 0 4620-3810 0 3810-2640 0 2640-1920 0 133 (96 total)

399 cal years

11,190-8130 0 8130-6470 0 6470-4630 0 4630-3870 0 3870-2490 0 2490-250 0

6645-5200 0 5200-4180 0 4180-2750 0 2750-1870 0

1920-2500 1870-300

11,150-9100 0 9100-7330 0 7330-5430 0 5430-4270 0 4270-2500 0 2500-300 0

averagee temporal resolution: 11,150 / 214 = 52 cal years

Tablee 5. Radiocarbon and calibrated ages for diatom and pollen zones of coree Las Margaritas.

133 3

:-L ii u i i

co o CD D CD D

CD D

lFtJ*PlFtJ*P <& »* <&

Figuree 6b

VV O O O O O D O O Ö Ö i—— c( n * to to r-. « ai o

Figuree 6A. Pollen percentage diagram of the main pollen taxa of core Las Margaritass (modified after Will e et al., 2003). 6B: Summary pollen diagram. .

134 4

5.11 Local environmental reconstruction

Period-1:: 6645-5200 cal BP (5820- 4620 BP; including zones D-l, LM3, LM4 ) )

Thee abundance of benthic diatoms, rather than planktonic taxa, at the beginningg of this period is indicative of permanent water with relatively shalloww water conditions. The presence of the shallow water aquatics Sagittaria/EchinodorusSagittaria/Echinodorus and Ludwigia also support this interpretation. Swampp forest with Mauritia palms occurred around the lake. The abundancee of Eunotia diatom species indicate that waters were acidic withh low conductivity and low nutrient concentration. Betweenn c. 5700 and 5230 cal BP (4990-4630 BP: 450 to 405 cm core depth)) there was a change from benthic to epiphytic diatoms (Cymbella) andd an increase of aerophilous diatoms adapted to intermittent waters. Thiss is indicative of greater proximit y of the littora l to the present-day centrall part of the lake where the core was raised. Therefore, the lake levell must have lowered, the water of the lake had higher conductivity, a circum-neutrall pH and was mesotrophic. Shallower conditions are also supportedd by the decrease of aquatics and by the increasing area of swampyy palm forest. The sandy horizon between 439 and 437 cm, and the increasee in LOI and CaO, might indicate that exposed littora l areas becamee eroded under low water level conditions.

Period-2:: 5200-4180 cal BP (4620-3810 BP; including zones D-2, LM4, LM5) )

Thee abundance of benthic, epiphytic and planktonic diatoms indicate a permanentt water body with shallow water. With respect to the final part off the previous period, there is an increase in water level, also supported byy the increase of the palm Mauritia, that indicates the expansion of the swampp area. The increase in the aquatic plant Cyperaceae, the increase in clayy content and decrease in LOI , indicate also a proper lacustrine sedimentatingg environment. The dominance of Eunotia diatom species are indicativee of acidic waters, with low conductivity and low nutrient concentration. . Period-3:: 4180-2750 cal BP (3810-2640 BP; including zones D-3, LM5)

135 5

Thee abundance of benthic diatoms at the beginning of this period indicatess that open water conditions prevailed and the lake must have had aa similar size, depth and chemical conditions to today (Table 4). Open waterss are also suggested by the low abundance of Cyperaceae, and an increasedd clay content of the sediments. Towards the latter part of this periodd the decrease and eventual increase in LOI, the increase in most elements,, in E. formica, in aerophilous diatoms, and Eunotia species tolerantt to more concentrated conditions of Ca, and the periods with absencee of diatoms, are indicative of lowering water levels and, probably, erosionn of exposed littoral surface. Lower water levels and littoral conditionss are also supported by the presence of cyperaceous reed swamp, thee increased of palm swamp {Arecaceae reticulata), and shallow water aquaticss (Ludwigia).

Period-4:: 2750-1870 cal BP (2640-1920 BP; including zones D-4, LM5 LM6) )

Thee dominance of benthic and epiphytic diatoms are indicative of low waterr levels and acidic water. Based on the presence of Cymbeila aff. spiculaspicula and Aulacoseira herzogii conductivity and turbidity may have increased,, compared to the previous period. Low lake levels and meso-eutrophicc water conditions are also supported by the increased content of organicc matter (higher LOI values) and P205, starting process of peat formation,, the decrease in cyperaceous reed swamp, and the palm Arecaceae.. The low diatom counting at beginning of the record may be duee to corrosion of the valves by their exposure to the atmosphere.

Period-- 5: 1870-300 cal BP (1920-250 BP; including zones D-5, LM6)

Thiss period started with a change in the diatom assemblages from epiphyticc to planktonic and benthic species which indicates increasing lakee levels. Also the increase of cyperaceous reed swamp, and Mauritia-dominatedd swamp forest, and the increased clay fraction of the sediments supportt higher water levels. Water had a low conductivity and low pH. It seemss the lake reached at this time its modern physical and limnological conditions. .

5.5. 2 Regional environmental reconstruction

136 6

Fig.. 7 shows the periods identified in the diatom-based and the pollen-basedd records. Some boudaries are coeval, others not. In the following sectionn we integrate the evidence of all available proxies for all discrete periods. .

Periodd 1: 11,150-9100 cal BP (11,190-8130 BP; zxme LM-1). At the beginningg of this period the drainage system still provided coarse sands to thee site reflecting relatively high energy. Towards the end of this interval thee lake gradually became isolated from the river system. The presence of aquaticc plants is indicative of low water levels. Open grass savanna dominatedd the landscape. According to Will e et al. (2003), during this periodd the dry season was longer and precipitation values were lower comparedd to thee present-day.

Periodd 2: 9100-7330 cal BP (8130-6470 BP; zone LM-2). During this periodd the extension of gallery forest increased and in the basin, isolated fromfrom the drainage system, a shallow lake and swampy area developed. Accordingg to Will e et al. (2003), at this time the difference between wet andd dry seasons became less extreme.

Periodd 3: 7330-6645 cal BP (6470-5820 BP; bottom zone LM-3). During thiss period, a proper lake conditions developed in the small basin. Aquaticss became more abundant and in the centre of the lake peat started too accumulate. On a regional scale open savanna, savanna bush, and galleryy forest in the lower parts along the drainage system were in competitionn and show marked oscillations. Climate variability on century timee scales occurred.

Periodd 4: 6645-5430 cal BP (5820-4630 BP; local environmental period 1).. During this period the lake had open and permanent waters and there wass a proper lacustrine depositional environment. The regional pollen signall showed a continuous change in the balance between forest and savanna.. Gradually forest expanded suggesting that the transitional area betweenn forest and savanna reached closer to site Las Margaritas and humidd conditions must have prevailed. An increase in climatic humidity wass also recorded at the nearby site Loma after 6000 BP (Behling and Hooghiemstra,, 2000), and also after 5800 BP in the northern part of the

137 7

Llanoss Orientales (Berrio et al., 2002). This humid period coincided with aa continuing decrease in seasonality.

Periodd 5: 5430-5200 cal BP (4630-4620 BP; local environmental period 1).. During this period the basin conained littl e water, with higher conductivity,, than during the previous interval, circumneutral pH, and loww water levels prevailed. Sandy sediments and increased element values aree indicative of exposed littoral areas and erosion. Possibly the lake reachedd its minimum level in the entire record. Trees characteristic of forestt disturbance (Cecropia) became more abundant suggesting the lake basinn experienced dynamic conditions during this short period. Open savannaa was mainly restricted to the hill tops.

Periodd 6: 5200-4180 cal BP (4620 to 3810 BP, local environmental periodd 2). The lake level increased, compared to the previous period, and openn waters remained during this interval. Waters were oligotrophic with loww conductivity. Aquatics became rare around the lake while a palm swampp expanded. Gallery forest and savanna woods expanded in the surroundingss suggesting that climatic conditions became more humid duringg this period.

Periodd 7: 4180-2750 cal BP ( 3810 to 2640 BP; local environmental periodd 3). During this period the lake level changed frequently. On a regionall scale a frequent alternation between forest and savanna suggest thatt centennial scale change in precipitation values were significant duringg the interval.

Periodd 8; 2750 to 2500 cai BP (2640-2490 BP; local environmental periodd 4). During this period the lake level dropped and the water had higherr conductivity and turbidity, if compared to the previous period. Aquaticc plants became rare. Periods dominated by savanna alternated withh periods dominated by forest. We conclude that dry conditions prevailedd during this period.

Periodd 9: 2500-1870 cal BP (2490-1920 BP; local environmental period 4).. Low water levels prevailed during this period, but they were higher if comparedd to the previous period. During this period forest is still the dominantt vegetation in the surroundings, however, savanna vegetation is

138 8

expanding.. We infer that dry conditions similar to the previous period weree present in Las Margaritas. Drier climatic conditions between 2700 andd 1900 BP are supported by coeval drier conditions in the Caqueta are off Colombia Amazonas (Van der Hammen et al, 1992) and the northern partt of the Llanos Orientales (Behling et al., 1999).

Periodd 10: 1870-300 cal BP (1920 to 250 BP local environmental period 5).. Open water conditions, probably similar to today's, were present duringg this time. Savanna vegetation was the main vegetation in the surroundingg area. It is possible that the replacement of the forest by savannaa was caused by human impact rather than as a result of drier climaticc conditions. However, we do infer that modern climatic conditionss were established during this time.

6.. Discussion

Ourr climate reconstruction shows dry climatic conditions between 11,200 andd 8130 BP and a change towards wetter climates after 8130 BP. This climaticc reconstruction agrees with that derived from biome reconstructionss for Colombia (Marchant et al., 2002), that indicate dry andd cold conditions for several places in Colombia during the Lateglacial, betweenn 0 and 0 BP, and a change towards warmer andd wetter climates by 0 BP. These climatic conditions were also presentt in the highlands from the Eastern Cordillera where the record fromfrom lake Füquene (Fig. 1) shows dry and cold conditions from c. 19.000 too c. 8700 BP and humid and warm climates from c. 8700 to 5500 BP. In sitee Las Margaritas we arrived at a more detailed climatic reconstruction, particularlyy after 5820 BP (6645 cal BP). Our reconstruction indicates humidd climates from 5820

139 9

# # f * --

..K, , ^ ^

^ ^

4? 4? <?&<*? <?&<*? yy y

854+36 6 1000 0

1843+34 4 1902+39 9

3043+36 6 2000 0

T l ö ö ö & & <?u<r r fif fif ^?> i i ^ ^

vT T iff iff

3000 0 3789+36 6

4000 0

5000 0

5396+49 9

5998+41 1 6000 0

7000 0

8000 0

8760+60 0 9000 0

9760+60 0 10,000 0

:—~~ =.|5

11,000 0

Presentt day conditions acidic,, oligotrophic, loww conductiv ity

MauritiaMauritia palm swamp andd Cyperaceae

Loww lake level Concentratedd and

turbidd water Decreasee lake level

Raree aquatics

Transitionall stage Oscillatingg take level

Shalloww open acidic watei oligotrophicc with loww conductivity

Raree aquatics Arecaceaee swamp

Shalloww open acidic watei pligouophtcc with lowconductrvrty y

II Mauritia palm swamp TT "Decrease in lake level

concentratedd water jj Arecaceae palm swamp

A.A. jShallqwopen acidic watei LJJ i oligotrophic with foww conductivity with,, Sagyttaz/a andd luawtgia

LL Matjtïtia palm swamp_ increasee in aquatics

permanent t loww lake level

Lakee became isolated fromm the regional drainage

system. .

Lakee formation

Riverr influence

3000 cal BP (250 BP)

J? J? Presentt day

climate e

Lowestt values of seasonality y

-18700 cal (1920 BP)

Dryy conditions

Drierr conditions Unstablee climate Moree seasonal?

Alternationn forest/savanna Decreasee in the difference

betweenn wet and dry seasons s

Forestt expansion Humidd conditions

Dryy candtions?or increase tempe-raturee and in evaporation rates?

Forest/Savannaa competition increasee in Uauriia, Moracwee

t-tcóyomium,t-tcóyomium, decease t ^^ Poeceae

Increasee in humidity

Dynamicc competition forest/savanna Transit)) onai zone forest/savanna

evenn closer to the site Climatee variability oncentury scale

Galleryy forest moree abundant.

Transitionall zone forest/savanna a

closerr to the lake

Lesss extreme difference betweenn wet and dry

25000 cal {2490 BP) 27500 cal (2640 BP)

41800 cal (3810 BP)

52000 cal (4620 BP)

54300 cal (4630 BP)

66455 cat (5820 BP)

73300 cal (6470 BP)

Landscapee dominated byy savanna

Dryy conditions: longer dryy season -

lesss precipitation

91000 cal (8130 BP)

J11,1500 cal (11,190 BP)

Figuree 7. Summary of local and regional environmental change in the areaa of lake Las Margaritas.

140 0

too 4630 BP (6645-5430 cal BP) and from 4560 to 3800 BP (5200-4160 call BP). After c. 3000 BP indications of the onset of drier climates start to appearr and by 2640 BP (2750 cal BP) dry and possibly warmer conditions weree established until 1920 (1870 cal BP). According to Marchant (op. cit),, at , present day conditions were established for many placess in Colombia. In the record from Fuquene, present day conditions weree reached after c. 3600 BP, and in Las Margaritas after 1920 BP.

Dryy conditions in Las Margaritas between 11,150 and c. 9000 cal BP are explainedd by the northern position of the ITCZ during the early Holocene (Martinn et al., 1997; Haugh et al., 2000) that caused not only decreased precipitationn but also increase in the length of the dry season and blocked thee northern trade winds coming from the Atlantic into this area. Increasingg precipitation and the prevalence of humid conditions after c. 73000 cal BP (c. 6500 BP) is explained by the southward migration of the ITCZZ that brought humidity to the Amazon region. By comparing our climatee record with the one published by Van der Hammen et al. (1992) inn which several dry periods were recorded in a humid region and with thee shifts of the ITCZ during the Holocene, we see that the influence of thee Amazon over Las Margaritas is not only manifested by moisture but alsoo by dryness which suggests that the climatic influence from the Amazonn over site Las Margaritas is greater than that from the more centrall parts of the Llanos Orientales. More palaeoclimatic studies in the southernn part of the Amazon would allow us to better differentiate betweenn the climatic influence on the north-eastern and south-eastern Llanos. .

Inn general terms the results from diatom and pollen analyses converged, exceptt for the regional period between 5430-5200 cal BP where regional pollenn indicate humid climates while local proxies indicate a drop in lake levell sugesting drier climates. This contradiction could be explained by increasee in temperature, increasing evaporation rates, rather than by a decreasee in precipitation.

Thee pollen signal of the aquatics and palms, indicative of lake level changes,, supported adequately the diatom-based lake level

141 1

EunotiaEunotia aff. transfuga x1000

EunotiaEunotia cf. rabenhorstiana x1000 AulacoseiraAulacoseira species 1

** 1 '

ËwnoffaforflwexIQQQ Q

Platee 1. Digital images of common fossil diatom species from sediments off lake Las Margaritas.

reconstruction.. The similarity between the curves of Arecaceae (palms) andd the Cymbella aff. spicula (an indicator low lake level due to its

142 2

preferencee for stagnant water of high conductivity) suggests that when the lakee level decreases, and the littoral has less water cover, other palms differentt to Mauritia, appear. On the contrary, when the lake level rises andd the littoral is covered by permanent water, Mauritia palm is more abundant;; similarly in the Caqueta region, Mauritia appears in those areas subjectt to frequent inundations caused by rises in the river (Urrego, 1996). . Throughh this study we could compare pollen and diatoms as proxies for environmentall reconstruction. Pollen grains proved to be better preserved thann diatoms. They are preserved in coarser sand grains and resist high-energyy transport while diatoms are more vulnerable. In peaty conditions, diatomss are also more vulnerable to dissolution while pollen grains are moree resistant. The diatom record, on the other hand, gave a good insight intoo water level due to their high sensitivity to changes in depth (habitat) andd chemistry. For example, the periods of decrease in lake level like thosee occurred between 5430-5200 cal BP and 2750-2500 cal BP were nott deteced by the pollen record. It is clear that combining these proxies cann add to our broader understanding of climate change in Colombia.

7.. Acknowledgements

Diatomm research was funded by WOTRO-DGIS grant WB 75-368 to H. Hooghiemstraa / MI . Vélez. Pollen analysis was funded by NWO-ALW grantt 750.197.08 to H. Hooghiemstra / M. Wille. The core was collected inn the frame of a NWO-GOA funded post-doctoral project (grant 750.195.100 to H. Hooghiemstra / H. Behling). We are much indebted to Pedroo Botero, Carlos Botero and Hermann Behling for support in the field.. We thank the successive directors of the TROPENBOS-Colombia officee in Bogota, J. Saldarriaga and C. Rodriguez, for logistic support duringg our field work. We thank Nico de Wilde (Amsterdam) for the sedimentt analysis and Annemarie Philip (Amsterdam) for the preparation off pollen and diatom samples. Silvia Sala assisted the identifications of somee of the diatoms.

8.. References

143 3

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Jewson,, D., Khondker, M., Rahman, M. and Lowry, S., 1993: Auxosporulationn of the freshwater diatom Aulacoseira herzogii in Lakee Banani, Bangladesh. Diatom Research 8, 403-418.

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