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Sea Surface Temperature Calculations in the Equatorial East Atlantic From PALEOCEANOGRAPHY, VOL. 15, NO. 1,PAGES 124-134, FEBRUARY 2000 Paleo-seasurface temperature calculationsin the equatorial east Atlantic from Mg/Ca ratios in planktic foraminifera- A comparisonto seasurface temperature estimates from U37 ', oxygen isotopes,and foraminiferal transfer function D. Niirnberg and A. Miiller GEOMAR ResearchCenter, Kiel, Germany R. R. Schneider FachbereichGeowissenschaften, University of Bremen, Bremen, Germany Abstract. We presenttwo -270 kyr paleo-seasurface temperature (SST) recordsfrom the EquatorialDivergence and the SouthEquatorial Current derived from Mg/Ca ratiosin the plankticforaminifer Globigerinoides sacculifer. The presentstudy suggests that the magnesiumsignature of G. sacculiferprovides a seasonalSST estimatefrom the upper -50 m of the watercolumn generated during upwelling in australlow-latitude fall/winter. Common to both down-core recordsis a glacial-interglacialamplitude of-3ø-3.5øC for the last climaticchanges and lower Holoceneand glacial oxygenisotope stage 2 temperaturescompared with interglacial stage 5.5 andglacial stage 6 temperatures,respectively. The comparisonto publishedSST estimatesfrom alkenones,oxygen isotopes, and foraminiferal transfer function from the samecore material pinpoints discrepancies and conformities between methods. 1. Introduction 1.2. Foraminiferal Mg/Ca Ratios as Paleotemperature Indicators 1.1. Paleo-Sea Surface Temperature Estimations Althoughvarious SSTu•7' approaches were developed to Becauseof the incompatibilityof methodsand their reconstructsea surface SSTu•' temperatures (SST), the glacial inconsistentresults, the developmentof additionalSST recordis especiallydebated. Ice corerecordsfrom equatorial proxiesseems useful since climate models largely rely on mountains[Thompson et al., 1995] and glacial snowline accurateestimations of the oceanpaleotemperature. The depression[Broecker and Denton, 1989] suggestterrestrial interestin magnesiumin biotic calcite for reconstructing temperaturesto be as muchas -5øC coolerthan during the paleowater temperatures is continuously growing, although Holocene.While Stute et al. [1995]inferred a 5øC temperature the viewson the processescontrolling its uptakeand drop for the Last GlacialMaximum from noble gas distribution,its susceptibilityto dissolution,and its measurementsin groundwater, the revisionof the groundwater applicabilityfor paleoreconstructions remain controversial. dataonly leadto a 1.9ø-2.5øCtemperature decline [Ballentine Many studiescould not revealany relationshipbetween andHall, 1998]. A large temperaturedecline of -4ø-5øCis magnesiumand temperature [e.g., Krinsley, 1960; Savinand supportedby corrallineSt/Ca records[Beck et al., 1992; Douglas, 1973]. Delaney et al. [1985] concludedthat Guildersonet al., 1994] and, recently,by temperatureadditional environmental parameters may affect the calcium reconstrutionsfrom planktic tropical •5•80 records [Curry and substitutionby magnesium. In contrast, other investigators Oppo,1997]. Theseresults contradict the reducedglacial suggesteda relationshipbetween water temperatureand temperaturedrop of only 2øC deducedfrom studieson marine magnesiumfrom the investigation of core top biogenic calcite faunalassemblages [Climate: Long-Range Investigation, [e.g.,Puechmaille, 1985; Izuka, 1988]. The application for Mapping,and Planning (CLIMAP), 1981] and stable oxygen SSTreconstructions is recently growing, partly stimulated by isotopes in foraminifera[Broecker, 1986]. Unsaturated cultureexperiments with planktic foraminifera showing a alkenonedata provide evidence for a 2ø-4øCtemperature temperature-related Mg/Ca shell ratio [Narnberg et al., 1996a, decline[Rostek et al., 1993;Sikes and Keigwin, 1994; Rosell- b]. Meld, 1998]. Cronbladand Malmgren [1981] were the firstto report down-coremagnesium variations in plankticforaminiferal Copyright2000 by the AmericanGeophysical Union. tests,correlating with late Quaternaryclimatic oscillations. Recently,Hastings et al. [1998]presented Mg/Ca-based SST Papernumber 1999PA000370. recordsfrom the equatorialAtlantic and the Caribbean 0883-8305/00/1999PA000370512.00 pointingout that the reliabilityof Mg/Ca recordsis not 124 125 NUERNBERGET AL.: Mg/Ca- DERIVED PALEO-SST perturbedby dissolution. Mashiotta et al. [1999] used a dissolution, which should be critically consideredwhen Mg/Ca-basedSST recordto reconstructchanges in global ice performing magnesiumanalyses. volume. Recently, culture experiments with living G. sacculifer In fact, calcite dissolution may significantly alter the revealeda significantmagnesium increase in the foraminiferal magnesiumconcentrations within foraminiferaltests and thus testsfor a 10øCincrease in watertemperature [Niirnberg et al., may prevent the applicability of magnesiumas a tracer for 1996a,b]. It could also be shownthat gametogeniccalcite has water mass properties [Rosenthaland Boyle, 1993]. Brown a significantlyhigher magnesiumconcentration than primary and Elderfield [1996] recently estimated the effect of calcite, althoughsecreted at the sametemperature. A sac-like magnesium content on the solubility of low-magnesium chamber, which may be built during the foraminifer's final biogenic calcite. They concludedthat the depth of the stageof development[Bd et al., 1983; Hemlebenet al., 1987], saturationhorizon for magnesiancalcites is a function of their however, does not show enhancedmagnesium concentrations magnesiumcontent. Lohmann [1995] showedin addition that comparedto a normal final chamber[Niirnberg et al., 1996a]. G. sacculifermay start losing shell masseven -2 km abovethe lysocline. Besides partial dissolution, other biases on the 2.2. Core Samples magnesiumsignal introducedby salinity and pH variations, Mg/Ca studieswere performedon two sedimentcores from amount of gametogenic calcite, and contaminant phases still the tropical Atlantic, for which major stratigraphical, needto be evaluatedbefore foraminiferalMg/Ca ratios may be sedimentological,and geochemicalinformation was available. acceptedas a paleothermometer. Sedimentcores GeoB 1105-3/4 from the Equatorial Divergence The objective of this study is to pursue further the and GeoB 1112-3/4 located -400 km south in the low investigation on the potential of magnesium for productiveSouth Equatorial Current (SEC) were recoveredfrom paleothermometry. In continuation to Niirnberg et al. the Guinea Basin from 3225 (1ø39.9'S, 12ø25.7'W) and [1996a, b], who showed a well-constrained Mg/SST 3122m water depth (5ø46.2'S, 10ø44.7'W), respectively calibrationcurve for primary calcite of cultivatedG. sacculifer, (Figure 1). Stable isotope geochemistry, foraminiferal this study comparesmagnesium-derived SST recordsover the assemblageanalyses and related SST reconstructions,fluxes of last -270 kyr to different paleo-SST proxies (foraminiferal sedimentarybiogenic components, grain size analyses, and transfer function, oxygen isotopes, and alkenone organiccarbon fluxes were previously publishedby Meinecke concentrations) on the same core material in order to resolve [ 1992], Bickert and Wefer [1996], Schneideret al. [ 1996], and discrepanciesand conformitiesbetween methods. Wefer et al. [1996]. A record of alkenone SST estimates is availableonly for core GeoB 1105-3/4 [Schneideret al., 1996] 2. Study Area, Material, and Methods because the content of alkenones was insufficient for the propercalculation of U3• in coreGeoB 1112. 2.1. Foraminiferal Species Selected for Analyses For both cores, stratigraphicinformation is available from the PANGAEA Paleoclimate Data Center (Alfred-Wegener- G. sacculifer inhabits shallow euphotic waters in the Institute,Bremerhaven). The age modelsare basedon graphic tropical and subtropical oceans [Shackleton and Vincent, correlationof Cibicidoideswuellerstorfi /5180 records to the 1978; Hernleben and Spindiet, 1983] because of the /5180standard record of Martinsonet al. [1987].We choosethe photosyntheticrequirements of their dinoflagellatesymbionts benthic record for stratigraphic correlation to minimize sea [Leeand Anderson, 1991]. Inferred from both abundancesand surfacetemperature effects that may occur betweenupwelling isotopic temperatures,the depth habitat lies at-25-75 m (Equatorial Divergence) and non upwelling regions (South [Fairbankset al. , 1980; Erez and Honjo, 1981; Fairbanks et Equatorial Current) and may include a time lag between al., 1982; Hemleben et al., 1989] with prevailing habitat planktic isotope signals. For detailed stratigraphic temperaturesof 20ø-30øC.For the equatorialAtlantic a depth information,see Bickert and Wefer [1996] and Schneideret al. habitatof-0-50 m is proposedby Erniliani [1954] and Hecht [1996]. The depth-age relationships for both cores [1971]. From plankton tow studies close to our area of demonstratenearly 2 times higher overall sedimentationrates investigation, Ravelo et al. [1990] concludethat highest (4.16 cm/kyr) in core GeoB 1105 comparedwith core GeoB abundancesof G. sacculiferoccur in -0-40 m. Salinities from 1112 (2.54 cm/kyr) due to the enhanced biogenic 24 to 47 are tolerated[Hemleben et al., 1989]. Reproductionis sedimentationunder the equatorialupwelling areaat site GeoB linked to the lunar cycle [Bijma and Hernleben,1994]. In order 1105 [Bickert and Wefer, 1996]. to reproduce,G. sacculifermoves far below the euphoticzone. In contrast to the western South Atlantic basins and the During gametogenesis a secondary layer of calcite, called Cape Basin, the deep-seaareas that are mainly filled by Lower gametogenic calcite, is secretedin 80-100 m [Bijma and CircumpolarDeep
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