record of el niño

8/4/2019 Record of El Niño

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Growth Increment and Stable Isotope Analysis of Marine

Bivalves: Implications for the Geoarchaeological- -'

Record of El Niño. - - - ~ - " ' - ~ ,

' - ~ ' V · < " u d , ~ Úl_é.', , F iJP(- I ' : , \ Au uIe oc,. ¡!

Harold B. Rollinst:. , ó l 'C ; 1 r e,,' "

Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260

f D a n i ~ I _ ~ . S a n ~ ± ) Departrrumt ofAnthropology, Cornell Uniuersity, lthaea, NY 14853

Uwe Brand

Department ofGeology, Brock Uniuersity, Sto Catharines, Ontario, Canada

Juditb C. Rollins

329 Bellwalt Dr., Bridgeville, PA 15017

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The 1982-1983 El Niño event afforded the opportunity to develop eri ter ia for the recognition ofancisnt EiNiños using mollusks from archaeological sites along coastal South America. A combination of growthincrement and stable isotope analyses indicated that elevated sea surface temperatures during large sealeEl Niños leave a record decodable from the growth patterns of selected bivalve shells. The intertidalvenerid Chione subrugosa displayed a pronounced break in the valve margin profile following the 1982

1983 évent bu t provided an inconsistent stable isotope pattern. The subtidal carditid Trachycardiumprocerum, on th e other hand, preserved a discernible and diagnostic growth interruption as weU as an

expected trend in stable isotope indicators of salinity and temperature change.We conclude that some of the major cultural ly disruptive El Niño events can be recognized in th e

geoarchaeological record by these techniques, especially if andllary information. such as faunal distribution pattems, are also considered. Perhaps the most serious constraint upon application ofthis a ~ r o a c h involves microstratigraphic resolution of shell midden deposits. Stratigraphic sampling ofmidden material should be accompanied, ifpossible, by sampling ofproximal natural strata. The chances ofdiscovery of

major El Niño perturbations in the geoarchaeological record of shell middens i s enhanced by the catastrophic nature of such events and by the indication that major El Niños have a high probability ofbeing

closely spaeed in time.

INTRODUCTION

The predominantly maritime-based economies of coastal Peru have existed for thousands of years (Moseley, 1975; Sandweiss et

al., 1983; Richardson, 1981). For at least the

last5000

yearsEl Niño

musthave been,

as it

is now, a major infl uence upon coastal occupation of that area (Rollins et al., 1986a). Therecognition of ancient large-scale El Niñoevents would certainly aid our understandingof the ecological vagaries that must have, inpart, affected the growth and development ofthese coastal economies.

The 1982 1983 El Niño event has affordedus the opportunity to develop criteria for the

recognition of ancient large-scale El Niño per

turbations, utilizing the widespread andabundant shell midden deposits along coastalPeru. This paper discusses the application ofgrowth increment and stable isotope analysesof bivalve shells as a tool for recognition ofancient El Niño events.

TBE 1982-1983 EL NIÑO

The 1982-1983 El Niño event was probably the most devastating Pacific Ocean climatic perturbation of this century (Arntz,1986). Historical records of El Niño eventsextend back to the 1700s and, using any available index, the 1982-1983 El Niño ranks as

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..

THE RECORD OF EL NIÑO

one of the worst (Cane, 1983; Quinn et aL,1978). The event was atypical in many respects. Thermal anomalies (as measured bySST-sea surface temperature) were extraordinarily high and the effects ofthe event did

not hi t coastal South America untillate summer, when local SST would normally be declining to its lowest value. Caviedes (1984)presented a detailed chronology of the 19821983 events that affected the coast of SouthAmerica. In June of 1982, the coast of centralChile received high rainfall with repeatedpassage of South Pacific winter depressions.By October, 1982, excessive rainfall occurredin coastal Ecuador and southern Colombiabut did not affect the region south of aboutlatitude 2 degrees south. However, September

and October were times of dramatic rise in

SST along the South American coast. The torrential rainfall moved southward during December, following the abnormal southern displacement of the intertropical convergencezone (lTCZ). Premature and excessive rainfallreached nor thern Peru by J anuary, causingextensive flooding and landsliding. Whilethere was rainfall in the desert of northwestPeru, drought struck the agricultural regionsof southern Peru (Rasmussen and Wallace,1983). Violent storms continued along the

coasts ofEcuador and norther Peru until midJune. Sorne degree of seasonal atmosphericnormality was restored by the end of June,

1983 (Caviedes, 1984). Although the ITCZhad passed beyond northern Peru, ai r temperatures were still higher than usual and theSST of coastal waters north of Callao, Peru

(about latitude 12 degrees south; Figure 1)

was as much as 8°C aboye normal. The Paita

coastal station (about latitude 5 degreessouth) showed signs of recovery of SST nor

mality by July, 1983 when the temperaturedecreased to 20°C (Barber and Chavez, 1983).

Ecological disruption following the onset ofthe 1 9 8 2 ~ 1983 El Niño was dramatic bu t not

at all unexpected. Modification of the normalupwelling of cold nutrient-r ich water adjacentto coastal South America inevitably affectspopulations of phytoplankton, fish, and seabirds. The response of larger marine pelagicorganisms to SST anomalies can be very

rapid, ostensibly reflecting a behavorial adaptation triggered by temperature change,when there is actually a complex linkageamong nutrient supply, light intensity, temperature, productivity, and food (Barber and

Chavez, 1983). The stresses created byreduced food availability are apparentIy amplified up the food web, leading to decreasedgrowth rates and reproductive failure mostnoticeable at higher trophic levels. These catastrophic ecological disturbances are oftenunappreciated over short spans oftime due tothe spatial displacement ofmany species fol-

lowing the onset ofEI Niño. Local "catches" ofsorne fish and shellfish may actually increasedue to movement by currents ofdisplaced populations into shallower, more accessible

coastal regions or due to the temporary introduction of thermally anomalous "exotic" species. Arntz (1986) noted, for example, that the

1982-1983 El Niño caused rapid growth andhigh den sities of scallops and sorne other invertebrate species. On the other hand, íntertídal and shallow subtídal mussel banks and

kelp forests were destroyed by the evento Human inhabitants of these coastal areas whoare dependent upon gathering of nearshoreinvertebrates for food were hardest hit by the1982-1983 catastrophe. This most likely has

been true in this region for every large-scaleEl Niño event over thousands of years.

The economically important bivalve speciesof coastal Peru are, for the most part, sessilebenthic and thus incapable of short-term migration with changes of water currents and

temperature. Over the long term, however,such species exhibit ecological resilience dueto their"high reproductive recruitment potential. Although scattered anecdotal information is available concerning the effects of El

Niño upon shellfish harvests, Httle researcheffort has been directed toward understanding physiological and morphological responses of various shellfish species to the ecological stresses ofEI Niño. Sessile bivalve specíes are sensitive indicators of changingcoastal condi tions and record, on a dailyandlor subdaily basis, a chronology of environmental perturbations. This record is manifested in the pattern of shell growth incre

182 VOL. 2, NO. 3




TALARA

VIRU °oCHIMBOT E

HUAYNUNA OOCASMA

o 100 200

"' ILES

PERU

BRAZIL

Figure l. Map ofPeru showing cities mentioned in texto

ments, types of shell microstructure, andprofile of the value anterior margino

METHODS

The bivalve specimens utilized in this studywere harvested alive or purchased from ven

dors at several sites along coastal Peru, during a series of collecting trips in 1984. CoBecting stations ranged from Tumbes, in extremenorthwestern Peru, tri the Paracas península,about latitude 15 degrees south (Rollins et al.,

1986b; Figure 1).Following preliminary screening of several

harvested bivalve species, 35 specimensofthe

intertidal Chione subrugosa and 15 specimens of the subtitdal Trachycardium pro-

GEOARCHAEOLOGY:ANINTERNATIONALJOURNAL

cerum were selected for detailed growth increment analysis. Description of the techniquesof bivalve growth increment analysis are

readily available from the geoarchaeologicalliteraturé and will only be briefly describedhere (Deith, 1983; Koike, 1980; Rollins et al.,in press). Individual valves were cut with a

Felker rock saw along the axis of maximumgrowth (normal to surface growth lines).Fragile valves were embedded in Epon 815epoxy resin prior to cutting (Kennish et al.,1980). Cut valves were ground and polishedusing a Buehler Isomet lapidary machine andthen etched in 5% HCl for approximately oneminute. Etching time varied according tospecies and condition of valves. The etchedvalves were carefully washed, air-dried, im

183




mersed in acetone and quickly placed uponpieces of sheet acetate (0.1 mm thick). Afterdrying, the acetate peel replicas weremounted between glass microscope sUdes. Ac-etate peel replicas were studied and photo

graphed using a MPV-2 microscope and automatic camera. Growth increment counts weremade upon assembled photo mosaics.

Stable isotope analysis of bivalve shells isalso widely used in geoarchaeological studies,mainly as a technique for ascer taining seasonofharvesting (Shackleton, 1983; Bailey et al.,1983; Jones et al., 1984; Deíth, 1986; reviewedby Rollins et al., in press). Killingley and

Berger (1979) were able to detect upwellingevents by stable isotope analysis of California

mussel shells. We know of no other study,however, which has attempted to use stableisotopes ofmolluscan shells as indicators ofEINiño sea surface temperature perturbations.Prior to stable isotope analysis all bivalveshells were initiálly cleaned of adhering organic and inorganic material . Then the shellswere cut along the axisof maximum growth inthe manner described aboye and subjected tofurther cleaning by immersion in an ultra-

sonic bath of de-ionized water (to minimizecontamination) for 15 minutes. Additional

cleaning consisted of brief immersion in 15%HCI, rinsing with de-ionized water, and subsequent air-drying (Brand and Veizer, 1980).Shell samples were taken (by drilling and

sawing) at specific intervals along the axis ofmaximum growth in order to traverse the

growth before, during, and after the 1982-

1983 El Niño event (as indicated by examination ofvalve margin pro file and growth increment pattern). These samples were powderedand dissolved in 5% HCI, in order to minimizethe leaching of elements from material that

was not removed during the cleaning process.All samples were analyzed for Ca, Mg, Sr,

Na, Mn, Fe, Al, Ni, Cu, and Zn usíng an automated and computerized Varían 1475 atomícabsorption spectrophotometer (see Brand and

Veizer, 1980; Brand and Hinsperger, 1986 forcomplete details of sample preparation and

analysis). Trace element compositions werecalculated on a 100% carbonate free basis tominimize trends due to fluctuations in

organic matter content ofthe shells. Accuracyof chemical values was determined by analyzing N.B.S. standard rocks (633,634,636) and

precision of data was calculated using duplicate analyses.

The shell samples were also analyzed foroxygen and carbon stable isotopes. Powderswere reacted with 100% phosphoric acid at

50°C for 10 minutes, and evolved gas wasanalyzed on a V.G. 903 Micromass spectrometer. The isotope ratios are expressed in 8 relative to PDB and corrected for oxygen-17.

Chemical data were evaluated using the

statistical packages (e.g., SPSS) on a Burrough's B7900 computer, and with the Stat-

view program on an Apple Macintosh computero Graphs were computer-generated.

A total of 14 samples from three specimensofChione subrugosa and 19 samples from fourspecimens of Trachycardium procerum wereanalyzed for Ca, Mg, Sr, Mn, Na, Fe, Zn, Cu,and Ni. AIso, a representative suite of15 sampIes was analyzed for 8180 and 813C.

RESULTS

Growtb Increment Analysis

Chione subrugosa and Trachycardium pro-cerum display different patterns of growth.The former has a reflected mantle margin, as

do other venerid bivalves, and easily countedarcuate growth increments are deposited(Figure 2). Trachycardium procerum possesses a nonreflected mantle margin and

growth increments intersect the shell margin

at high angles (Figure 3). In Chione subru-gosa daily growth increments were prominentwhile semi-daily growth increments weremost distinct in Trachycardium procerum.

Detailed analysis of the daily and semi-dailygrowth increments of bivalve shells has

proven to be a useful indicator of short and

long term environmental stress, intensity of

seasonal zonation, and habitat relative toonshore/offshore depth gradients (Richardsonet al., 1980; 1981; Barker, 1964; Jones, 1983;Rhoads and Lutz, 1980; Hall et al., 1974; Evans, 1975; Clark, 1975; Kennish and Olsson,1975; Thompson, 1975; Pannella and Mac-

VOL 2, NO. 384




E

a.......-oO.1mmA

--...0.1 mmeFigure 2. A-C. Chione subrugosa (photomicrographs of acetate peels). A. Specimen#

Ptl-10, collected 4-16-84, Tumbes, Peru. Note distinct daily lines and subtle,díscontinuous sub-daily lines. Crossed-Iamellar microstructure (X) is in typical unstressed pre El-Niño orientation. Growth margin of the shelI is to the left. B. Samespecimen. Area immediately below El Niño break. Note transgressive crossedlameIlar microstructure (X) invading upper composite prismatic layer. C. Specimen #FPr-1, collected from shell midden, Pampas las Salinas, Peru. N arrow break in valveprofile is typical ofa storm break, not a major El Niño disturbance. Recovery was rapidfollowing the storm break. Growth margin of the shell is to the right. D-E.Trachycardium procerum. Arrows indicate onset of El Niño break. D. Specimen #TPl-6, collected 4-18-84, ChicIayo, Peru. E. Specimen # 2TP2-1O, collected 12-1-84,Trujillo, Peru .

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 185




Figure 3. Trachycardium procerum (Speciman 1/: TP3-l), collected 4-20-84,Chimbote, Peru, A-B. Acetate peel, negative print (scale 1 mm). Note broadshallow El Niño break in valve profile and increase in crossed-Iamellar microstructure at the valve margino Anterior tip is to the right in all of the photographs.Semi-daily growth incrementa are shallowly inclined below the El Niño break. Arrows indicate 307 days (614 increments). C. Photomicrograph (scale = 0.1 mm).Enlargement ofarea C in Figure 2A. Note massÍ'Ve crossed-lameIlar microstructureand nearly horizontal growth increments. D. Photomicrograph (scale == 0.1 mm) of

area D in Figure 2A. Cross-lameIlar microstructure stil l present, but growth increments have resumed normal inclination to the valve margino

Clintock, 1968; Ekaratne and Crisp, 1982). colored bands, the result of calcium carbonateThere is general agreement that bivalve deposition when val ves are open and the forshells are gauges of a hierarchy oflunar and mation of a thin dark line, presumably due tosolar rhythms, as well as extrinsic environ anaerobic dissolution of shell material, whenmental events. An individual growth incre valves are c10sed (Lutz and Rhoads, 1977).ment consists of a couplet of light and dark Intertidal bivalves (e.g., Chione subrugosa)

186 VOL 2, NO. 3



therefore have incremental patterns that reflect both lunar tidal (exposure and immersion) and solar day rhythms where valve cIo-

sure accompanies exposure and increasedinput ofheat Oight). Sorne controversy exists

over the temporal significance of singlegrowth increments in bivalve shells. Richardson et al. (1980, 1981) have'made a strong case

for tidal exposure and temperature controlling the expression of growth increments.Prominent growth lines are the result oftidal

exposures coinciding with elevated temperatures, as when low tide occurs durir.g daylighthours. Alternating strong and weak lines oc-

cur when one low tide is in early morning and

the other is in mid-afternoon (Richardson et

al., 1981). We suspectthatsuchfactorslead toa pronounced difference in strength of growthlines so that bivalves which are totally exposed at low tide will appear to form one lineper day whenever one low tide coincides withdaylight hours. Counting of growth lines is

difficult and the tendency is to observe and

record only the more prominent lines. Presumably, growth line counts on high íntertidal bivalves will tend to emphasize dailyperíodicities whereas counts on lower intertidal and subtidal bivalves stress semi-daily

intervals. This may explain why daily linesha ve been commonly recognized in veneridssuch -as Mercenaria mercenaria, Meretrixlusoría, and Chione spp., which inhabit intertidal mud flats or lagoons (Koike, 1973;Pannella and MacClintock, 1968). The subtidal carditids, in contrast, tend to displaynearly equal development of semi-daily lines,corresponding to each low tide. Our analysesof Trachycardium procerum and Chione sub-rugosa support this interpretation.

As noted by many workers (e.g., Thompson,

1975), shell increments may also be groupedinto alternating close and wide-spaced bundles that correlate with semi-monthly neapand spring tides. The tidal patterns are generally less obvious in subtidal bivalves (e.g.,Trachycardium procerum). In more temperate regions annual patterns of increment deposition reflecting seasonality may also bediscernible.

Specific environmental or biological


stresses may also result in pronounced growthbreaks in bivalve shells. Such stresses may

accompany storms, temperature shocks (hotor cold), or spawníng. Breaks due to hot and

cold shock may be quite similar, exhibiting

sudden bunching of growth increments at theonset, with slow recovery. Storm breaks, however, typically display rapid onset and recovery, and may result in the depositional incorporation of sediment grains at the base of anarrow notch in the shell margino Excellentdiscussions of the types of growth patterns

and breaks are provided by Kennish and

Olsson (1975) and Cunliffe (1974). Kennishand Olsson (1975) determined that the majorcontrolling factors affecting the growth ofthe

venerid clam Mercenaria mercenaria weretemperature, tides, substrate type, water

depth, and age ofthe individual. Environmenta l stress also may be indicated by irregularities in the shell microstructure and in the

profile of the valve margino Kennish and

Olsson (1975) noted that thermal shockbreaks in venerids may cause a transgressionof massive crossed lamellar microstructureinto the outer prismatic or composite prismatic microstructure. Wide notches may oc-

cur on the valve margins as a result of the

temporary withdrawal of the mantle duringprolonged stress. They are quite dist inct fromthe narrow storm-induced notches mentionedaboye and may result from long-term clima icperturbations such as El Niño events. Under

such conditions the ventral margin of the

valve may be blunted with a precipitous slope(Rollins et aL, 1986b).

Conspicuous El Niño-induced growthbreaks were present in all the specimens ofChione subrugosa collected in 1984 fromTumbes (Figure 4). The growth breaks dis

played rapid onset and slower, often erratic,recovery. Most specimens did not achieve tota l recovery and the ventral shell margins are

strongly blunted. In contrast, the ventral

margins of unstressed specimens of Chionesubrugosa are much thinner, as indicated bycomparison of these specimens with a collection of 194 Chione subrugosa valves from an

archaeological site at Valdivia, Ecuador.Sixty-one percent of the Ecuadorian shells

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 187




\

•\

: : r = ~F

Figure 4. Chione subrugosa. Hachure marks indícate position ofEI Niño breaks.A-B. Specimen # PTl-8, Tumbes, Peru, collected 4-16-84. C-D. Specimen #

Vald-2, Valdivia midden, Cut J, coastal Ecuador. E-F. Specimen # PTl-17,

Tumbes. Peru, collected 4-16-84. G-H. Specimen # PTl-19, Tumbes, Peru, collected 4-16-84. I-J . Specimen # PT1-10, Tumbes , Peru, collected 4-16-84. K-L.

Specimen # 2PTl-6, Tumbes, Peru, coUected 11-25-84. M-N. Specimen #

2PTl-ll, Tumbes, Peru, collected 11-25-84.

had smooth profiles, with no stress breaks

(Figure 4C, D). When breaks were observedthey were not the major interruptions described above (Rollins et al., 1986b).

Moreover, the Ecuadorian shells that had

stress breaks near the ventral margins of theshells showed rapid recovery and a thin ventral margin was reestablished after the

growth interruption in the val ve profile. TheEl Niño-stressed specimens did not exhibit

such recovery.Growth increment counts taken on the

photomosaics of acetate peel replicas ofChione subrugosa extended from the ventraltips to the beginning of the major growth interruption. Increment counts ranged from

VOL 2, NO. 388




300-375 (average of 328) for specimens col-

lected alive on April 16, 1984. The incrementcounts on another specimen of Chione sub-rugosa killed on November 25, 1984 totaled531 (Table 1). Counting backward from the

ventral margin and assuming one incrementaddition per day places the time of onset ofmajor stress on Chione subrugosa in late May,1983. This is coincident with the maximumSST anomalies measured at the Peruvianports of Paita and Chicama when water temperatures reached 10°C aboye normal. Suchincrement counts are probably underestimates because ofthe likely anaerobic dissolution or nondeposition of increments duringintervals of maximum stress. Our resultsclosely agree with the growth increment

counts of Pallant (in press), who used growthbreaks and the absence of sub-daily lines toinfer stressful habitats of Chione subrugosain a study of environmental changes in CostaRica. Our data support Pallant's interpretation as we noted that sub-daily lines weredeposited only prior to the onset ofthe El NiñoSST anomalies. Stress resulted in closelyspaced lines of about equal strength.

Specimens of Chíone subrugosa exhibitedslow and erratic recovery from the El Niñoinduced stress (Figure 5). The growth ínter

ruptions recorded by notches in the valveprofiles, unlike storm breaks, sometimesspanned several months and presumably indicate extended periods of partial mantle retractíon. The trauma of the El Niño-inducedstress is further indicated by a lack of correspondence, among individuals, in the amountof post-break shell deposition. In fact, manyspecímens displayed as much post-stress

growth directed inward as there was horizontally along the radial growth gradient. This isindicated by the scatter and overlap of the

meaSUTements plotted in Figure 6. The erratic pattern of growth recovery in this spe

cies substantiates the interpretation of astress-induced break in contrast to sorne typeof ontogenetically programmed slow-down ofshell growth.

Increment analysis of Trachycardium pro-cerum corroborates the results of the Chionesubrugosa counts. Comparable counts fromthe ventral tip to the initiation of the growthbreak were about double those in Chionesubrugosa (Table 1), and denoted major stress

commencing on April 30,1983 and June 20,1983 for the two specimens examined. These

dates are well within the interval of elevatedSST.

El Niño-induced stress profiles are less accentuated on Trachycardium procerum than

on Chione subrugosa and may be subtlebunching of growth lines or broad shallowdepressions (Figure 3). Recovery of Trachy-cardium procerum following El Niño stress

involved resumption of relatively normalgrowth and the addition of several centimeters of shell material. Only a few specimensdisplayed pronounced inward growth but inthese cases the slopes ofthe valve profiles are

more gentle, presumably due to the nonreflected mantle margin in this species. During

major stress the angle of inclination of the

growth increments to the valve surface is onlyabout 10 degrees. Normally (i.e., under

nonstressful conditíons) the inclination angle

is about 35 degrees.Trachycardium procerum showed trans-

Table l. Counts of Growth Increments-Ventral Margin to Onsetof El Niño Break.

Chione subrugosaDate of # of # of Onset of El Niño

Specimen # Locality Death Increments Days Break-Date

2PI'1-11 Tumbes 11/25/84 531 531 6/16183PI'1-19 Tumbes 4/16/84 325 325 5/28/83PI'1-1O Tumbes 4/16/84 300 300 6/22/83PI'1-17 Tumbes 4/16/84 375 375 4/8/83

Trachycardium procerum

TPl-3

TP3-1Chiclayo 4/18/84Chimbote 4/20/84

715614

358307

4/27/836/19/83

GEOARCHAEOLOGY; AN INTERNATIONAL JOURNAL 189




c.

B

DFigure 5. Chione subrugosa. Specimen # PTl-19, collected 4-16-84, Tumbes, Peru.A. Acetate peel, negative print (scale = 1.0 mm). Arrow indicates onset ofEI Niñobreak at 325 days (325 increments) prior to death. Note extreme bunching of growthincrements from the initial break in valve profile to the anterior tip ofthe valve. B-D.Photomicrographs (scale = 0.1 mm). Enlargements of the stress profile shown inFigure 4A.

gressive development of crossed lamellar support to the recent studies ofCerastoderm:;

shell microstructure as far as the shell margin edule, another carditid clam (Richardson et

in association with the El Niño break. In al., 1980, 1981). In that species, tidallevel wasChione subrugosa the inner crossed lamellar determined to be a major factor influencinglayer also extended part way into the outer the rate of growth and more rapid growthcomposite prismatic layer concomÍtant with correlated with longer periods of immersion.the El Niño break, but never carne close to the

shell margin (Figure 2).Stable Isotope Analysis

The nearly equal strength of semi-dailygrowth increments in Trachycardium pro- For the most part, the elemental analysis ofcerum follows the pattern expected for low the shells of Chione sub rugosa and Trachy-

intertidal and subtidal bivalves and lends cardium procerum resulted in anomalous or

190 VOl. 2, NO. 3



l.5

14

13

12

11

10 •9

E E x

8

x

> 1 • \ NOV 25, '8 4 •


x

x

••

6

5\ y= 0476X + 347

x •4

X

•

x

r = 04

•• •

X

• NOVEMBER 25, 1984 (N= 15)

X X APRIL 16, 1984 ( N-20)le X X

216, '8 4 XX

XX X X

O30 31 32 33 34 35 36 31 38 39 40 41 42 43

TH (mm)

Figure 6. Two collections ofChione subrugosa. Graph shows lack of correlation between amount ofpost-ElNiño shell growth (as measured by DVM-horizontal distance from El Niño break to growing margin oftheshell) and total shell height (measured from umbo to growing margin of the sheJl). The El Niño-inducedshock resul ted in long-term growth interruption. DVM does not record th e amount of inward-directed, or

vertical, component of growth, however.

insignificant chemical differences betweenthe pre- and post-El Niño events. However,SrlNa and stable isotope trends in the shellcomposition ofTrachycardium procerum wereindicative of changing oceanographic conditions during the El Niño evento In contrast,

Chione subrugosa exhibited a statistically insignificant SrlNa and stable isotope pattern

(compare Figures 7-12 and Table ID.

Upwelling currents are generally enrichedin the light carbon isotope beca use the totaldissolved inorganic carbon contains moreC-12 than C-13. Therefore, the incursion of

warm surface waters ofthe equatorial countercurrent into the colder waters ofthe Peruvian

Province should result in heavier values of

GEOARCHAEOLOGY:ANINTERNATIONALJOURNAL

013C in the carbonate shell material precipitated during the incursion (Killingley andBerger, 1979). At the same time, the warmer

waters should result in lighter 8180 values in

molluscan shell carbonate.The anomalous values of both 813C and

818

0 exhibited by specimens of Chionesubrugosa (Figures 8, 9) are consistent withthe habitat ofthis species. Lagoonal mud flatsare subject to frequent episodic localized temperature spikes and we anticipate that the

shell record of such an intertidal specieswould exhibit a virtually indecipherable pattern of temperature sensitive stable isotoperatios.

In contrast, the elemental and isotopic com

191



0. 3


1\oZ"..V'l 0.2

oe PTI-17

t 6 . PT'-8

EN O.PTI-19

Relative Growth Increment

Figure 7. Sr/Na-growth increment distribution diagram for three specimens of Chione subrugosa. Solidsymbols represent pre-EI Niño shell growth. EN is theonset of the El Niño-induced shock, as indicated bygrowth abnormality along the valve margin profile.Open symbols represent shell carbonate deposition during and after the El Niño shock.

Table 11. Unpaired student t-Test of Sr/Na (wt) inChione subrugosa and Trachycardium procerumgrowth increment samples.

Sr/Na (wt) X

Allochem N EL t-value

C. subrugosa 0.234 0.248 0.756 ,,;;; 0.375

T. 0.413 0.271 -3.796 ,,;;; 0.005

N '= pre-El Niño growth increment samples; EL '= postand El Niño increment shell samples.

positions of Trachycardium procerum appear

to reflect the physiochemical conditions of the

ambient seawater. The Sr/Na ratios in the

shell incrcment samples decrease after the El

Niño event (Figure 10). The pre- and post-El

Niño values are significantly different at the

99.5% confidence level (TabIe II). Similar

trends are also depicted by the 813C compositions of Trachycardium procerum, and the

heavier 813e values correlate with the elevated SST along coastal Peru (Figure 11).

Concurrently, the 8180 values are lighter in

the post-El Niño precipitated shell carbonate(Figure 12), presumabIy an indication of the

warmer counter current water.

+ 0.5

0.0I

ti-0.5

-. .ID

-1.0 /oa.; ¿

o

u

<.O

-1. 5 III

-2.0 II

-2.5I

6tEN


Figure 8. o13C-growth increment distribution diagram ofC hione subrugosa. Explanations and symbols asin Figure 7.

Water salinities and temperatures can becalculated with:

Salinity (±1.0, ppt) = -5.037 In A + 28.627,

where A is the Sr/Na (wt.) ofindividual aragonite samples (Brand, 1984) and with:

Toe = 19.0 -3.52(8A 8w)

+ 0.14(8A

- 8w

)2,

where the 8A is the 160/180 ratio of the molluscan aragonite and 8w is the 160p80 ratio

of the ambient seawater (Grossman and Ku,1981). Application of these equations to the

Sr/Na values of Trachycardium procerum(Figure 10) resulted in calculation of an aver

age pre-El Niño water salinity of about 33 pp t

and a post-El Niño average value of 35 ppt.Similarly, the calculated water temperature

VOL 2, NO. 392



1.4 . . - - - - - - - - - - - . . . ,

l?1.6 \

\ \-1.8

ro \ Q...

<> \ <> -2.0

o 0--..\ !?ID

-2.2 rr l

-2.4

f N


Figure 9. &lSO-growth increment distribution di agram ofChoine subrugosa.Explanations and symbols as

in Figure 1.

increased from a pre·EI Niño average of 17°Cto 22°C after the incursion of the warm equa

torial counter current.

GEOARCHAEOLOGICAL IMPLICA'l'IONS

We believe that the results of this study

have the potential for establishing a baselinefor recognition of ancient El Niño events, utilizing the abundant and widespread shellmidden material along coastal South America. For the most part, species that are har-

vested today along coastal South America are

available from middens and associated natu-

ra l deposits that reach back through thousands of years of maritime culture, althoughnot without occasional widespread changes in

species distribution patterns (Rollins et al,

1986a).Recognition of large scale El Niño events

from analysi s of shell midden material willhave to proceed with caution, however. Wesuspect that a detailed chronology of El Niño

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL


will never be attained by these techniques,but we are optimistic that sorne of the majorculturally disruptive El Niño events can be

. recognized in the geoarchaeological record.As such, we urge that attention be paid to

the following assumptions and constraints:1. Clearly, detailed growth increment and

stable isotope analyses of bivalve shells are

limited by time and cost constraints. The typeof growth increment analysis employed in

this study utilizing numerous photomosaics ofacetate peel replicas is labor intensive andany study must necessarily be restricted to arelatively small number ofbivalve shells. Bythe same token, geochemical analysis of bivalve shells is constrained primarily by costand the large number of samples required per

shell.Techniques for more rapid processing of

molluscan shell material will be essential.The results of our study suggest that the profile of the valve margin will provide a quickand relatively easy method for recognition of

El Niño-induced stress among large collections of bivalve sheIls. The El Niño "signatures" recognized in Chione subrugosa andTrachycardium procerum involved uniquepatterns of growth disruption that could be

c1early separated from shorter-term environ

mental stress, such as isolated storm events.The use of the valve profile technique is enhanced by shell growth in tropical and subtropical latitudes where strong seasonalgrowth disruptions are absent.

We suspect that the role of detailed growthincrement and stable isotope analyses will berestricted to occasional checks of randomlyselected shells from midden material (and as·sociated natural deposits) and confirmation ofsuspected El Niño-induced patterns.

2.Our study demonstrated a strong habitat andJor biological constraint upon the use

of geochemical analysis of bivalve shells forrecognition ofEI Niño-induced stress. Wecannot expect that intertidal species will respond,in terms of stable isotope ratios, similarly tosubtidal species. The results of our study suggest that subtidal species will be more reliablegeochemical prospects, but even intertidal

species can be used if detailed growth incre

193



THE R E C O ~ D OF EL NIÑO

0 . 6 ~ - - - - - - - - - - - - - - - - - - - - - - - - - - ~

0.5

0.4

-:

oz 0.3

"..Vl

0.2

/I/

/¿

o. TP3 -1

c.. 2TP2 -10

Q. TPI -6

o. TPI -9

f EN

Relafive Growfh Incremenf

Figure 10. Sr/Na-growth increment distribution diagram

for four specimens of Trachycardium procerum. Explanationand symbols as in Figure 7.

tI . 2. . . .------------- . . . ,

-- tl.Oenao...

o

o::!?

u +0.8,..,

<.O

tO.6 tEN

Relaflve Growth Increment

Figure 11. &13C-growth increment distribution diagram of Trachycardium procerum. Explanations and

symbols as in Figure 7.

ment analysis is performed. We emphasizethe desirability of establishing a careful set ofEl Niño indicators for every species used. Thisclearly cannot always be tied into an opportunity for direct observation (as was the casewith the 1982-1983 event) but should alwaysinvolve careful and detailed integration Cif

growth increment andstable isotope tech

niques.3. Ancillary information regarding sudden

and dramatic changes in taxonomic distribution should be considered in any attempt torecognize ancient El Niño events. Our observations during the 1982-1983 El Niño suggest that such changes will most likely beselective. Large clams (e.g • Mesodesma don-acium) and rock-dwelling gastropods (e.g.,Fissurella spp.) might be preferentially affected. and an abrupt decline in abundance of

VOl. 2, NO. 3194



00 ....-------...------,

-0.2

-0.4

ID

oo..

-0.6,,('•o

O

tD

- 0.8

1.0

t1.2EN


Figure 12. &lBQ-growth increment distribution díagram of Trachycardium procerum. Explanations andsymbols as in Figure 7.

these species in a particular stratum might be

significant. Sudden increases in thepercentage of small mussels (especially Semimytilus

algosus and Brachiodontes purpuratus) and

the large gastropod Thais choco lata may signal the presence ofa large scale El Niño eventoThese species appeared to be minimally affected during the 1982-1983 evento The

small beach clam, Donax obesulus, recoveredquickly following the 1982-1983 El Niño and

its sudden abundance in a stratigraphic sequence might also be indicative. Dramatic increases in certain motile species (e.g., the scal

lop Argopecten purpuratus) should al so benoted.

This approach must be carefully applied, as

many taxic changes in shell middens may

indicate changes in resource exploitation unrelated to El Niño events (e.g., shifts in harvesting technologies or cultural preferences,alterations in coastallandscapes , etc.). .

4. Perhaps the most serious potential constraint upon application of these techniques

GEOARCHAEOlOGY: AN INTERNATIONAl JOURNAl


involves the nature ofthe midden record. Certainly, the search for ancient El Niño eventswill be most successful under conditions conducive to microstratigraphic sampling ofrelatively undisturbed midden material. Shellmiddens, however, are often culturally dísrupted (although the midden material associated with seasonally occupied base camps or

shellfish harvesting sites along coastal Peru

may be less culturally disturbed than shellrefuse heaps associated with more continuously occupied sites). Stratigraphic samplingof midden material should be supplemented,wherever possible, with sampling ofproximalnatural strata. Many ofthe sites along coastalSouth America provide this opportunity (seeRollins et al., 1986a), but even in these cases

care must be taken to avoid seriously timeaveraged faunal assemblages. Ideal conditions for detection of ancient El Niño eventsinvolved catastrophic burial; fortunately thismay commonly accompany an El Niño perturbation (e.g., beach ridge deposits associatedwith El Niño flooding events as discussed bySandweiss, 1986). The temporal "window"available to be recorded is admittedly small,dependent upon the duration of the El Niñoevent and the longevities of the bivalve species. Adequate conditions do exist, however,

in the fossil record, as documented by Clark(1987) during growth line analysis of aPliocene scallop assemblage. The chances ofrecovering a large scale El Niño event in the

ancient record are enhanced by the fact that

major El Niños have a high probability ofbeing closely spaced in time. Quinn et al.(1978) assessed the probability of strong El

Niño events recurring within 7-8 years at 82percent.

We gratefully acknowledge the research support of aNational Geographíc Society grant and a S e ~ í o r Facultygrant from the Latín American Studies program at the

University of Pittsburgh. We profited from discussionswith Mr. Thomas DeVries of Oregon State Universityand we are thankful for the draft ing assistance of Mr.Frank Benacquista.

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197GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

record of el niño

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