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data only lead to a 1.9°-2.5°C temperature decline [Ballentine and Hall, 1998]. A large temperature decline of -4°-5°C is supported by corralline St/Ca records ...
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

comparison toseasurface temperature estimates fromU37 ', oxygenisotopes,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) records from the EquatorialDivergence and the SouthEquatorialCurrentderivedfrom Mg/Ca ratiosin the plankticforaminiferGlobigerinoides sacculifer.The presentstudysuggests thatthemagnesium signature of G. sacculifer providesa seasonal SST estimatefrom the upper -50 m of the watercolumngenerated duringupwellingin australlow-latitudefall/winter.Common to both down-core records is a glacial-interglacial amplitudeof-3ø-3.5øCfor the last climaticchangesandlower Holoceneandglacial oxygenisotopestage2 temperatures compared withinterglacial stage5.5 andglacialstage6 temperatures, respectively. The comparison to publishedSST estimatesfromalkenones, oxygenisotopes,andforaminiferaltransferfunctionfrom the samecorematerialpinpointsdiscrepancies andconformities betweenmethods.

1. Introduction

1.1.

1.2. Foraminiferal as Paleotemperature

Mg/Ca

Ratios

Indicators

Paleo-Sea Surface Temperature Estimations

Although variousSSTu•7' approaches weredeveloped to reconstruct seasurfaceSSTu•'temperatures (SST),the glacial

Becauseof the incompatibilityof methodsand their inconsistentresults,the development of additionalSST

seems useful sinceclimatemodels largelyrelyon recordis especially debated. Ice corerecordsfrom equatorial proxies accurate estimations of the oceanpaleotemperature. The mountains[Thompson et al., 1995] and glacial snowline depression [Broecker andDenton,1989] suggestterrestrial interest in magnesium in bioticcalcite for reconstructing temperatures to be as muchas -5øC coolerthanduringthe paleo watertemperatures is continuously growing, although controlling its uptakeand Holocene. WhileStute etal. [1995]inferred a 5øCtemperaturethe viewson the processes drop for the Last GlacialMaximumfrom noble gas distribution,its susceptibilityto dissolution,and its remaincontroversial. measurements in groundwater, the revisionof the groundwater applicabilityfor paleoreconstructions dataonlyleadto a 1.9ø-2.5øC temperature decline[Ballentine Many studies couldnot revealany relationship between andHall, 1998]. A largetemperature declineof -4ø-5øCis magnesium andtemperature [e.g.,Krinsley,1960;Savinand that supported by corrallineSt/Ca records[Becket al., 1992; Douglas, 1973]. Delaneyet al. [1985] concluded environmental parameters mayaffectthe calcium Guilderson et al., 1994] and, recently,by temperatureadditional

reconstrutions fromplanktictropical•5•80records [Curryand substitution by magnesium. In contrast, otherinvestigators a relationshipbetweenwater temperature and Oppo,1997]. Theseresultscontradict the reduced glacial suggested temperaturedrop of only 2øC deduced from studieson marine magnesium fromtheinvestigation of coretopbiogenic calcite 1985;Izuka,1988].Theapplication for faunalassemblages [Climate:Long-Range Investigation, [e.g.,Puechmaille, Mapping, andPlanning (CLIMAP), 1981]andstableoxygen SSTreconstructions is recently growing, partlystimulated by isotopesin foraminifera[Broecker,1986]. Unsaturated culture experiments withplankticforaminifera showing a alkenonedataprovideevidence for a 2ø-4øC temperaturetemperature-related Mg/Cashellratio[Narnberg et al., 1996a, decline[Rostek et al., 1993;SikesandKeigwin,1994;Rosell- b]. Meld, 1998].

Cronblad andMalmgren [1981]werethe firstto report down-core magnesium variationsin plankticforaminiferal

Copyright2000 by the AmericanGeophysicalUnion.

tests,correlatingwith late Quaternary climaticoscillations.

Recently, Hastings et al. [1998]presented Mg/Ca-based SST Papernumber1999PA000370.

recordsfrom the equatorialAtlantic and the Caribbean

0883-8305/00/1999PA000370512.00

pointingout that the reliabilityof Mg/Carecords is not 124

125

NUERNBERGET AL.: Mg/Ca- DERIVED PALEO-SST

perturbedby dissolution. Mashiotta et al. [1999] used a Mg/Ca-basedSST recordto reconstructchangesin global ice volume.

In fact, calcite dissolution may significantly alter the magnesiumconcentrations within foraminiferaltests and thus may prevent the applicability of magnesiumas a tracer for water mass properties [Rosenthaland Boyle, 1993]. Brown and Elderfield [1996] recently estimated the effect of magnesium content on the solubility of low-magnesium biogenic calcite. They concludedthat the depth of the saturationhorizon for magnesiancalcitesis a function of their magnesiumcontent.Lohmann [1995] showedin additionthat G. sacculifermay startlosing shell masseven -2 km abovethe lysocline. Besides partial dissolution, other biases on the magnesiumsignal introducedby salinity and pH variations, amount of gametogenic calcite, and contaminant phases still needto be evaluatedbeforeforaminiferalMg/Ca ratios may be acceptedas a paleothermometer. The objective of this study is to pursue further the investigation on the potential of magnesium for paleothermometry. In continuation to Niirnberg et al. [1996a, b], who showed a well-constrained Mg/SST calibrationcurve for primary calciteof cultivatedG. sacculifer, this studycomparesmagnesium-derived SST recordsover the last -270 kyr to different paleo-SST proxies (foraminiferal transfer function, oxygen isotopes, and alkenone concentrations) on the same core material in order to resolve

discrepanciesand conformitiesbetween methods.

2. Study Area, Material, and Methods

dissolution, which should be critically consideredwhen performingmagnesiumanalyses. Recently, culture experiments with living G. sacculifer revealeda significantmagnesiumincreasein the foraminiferal testsfor a 10øCincreasein watertemperature[Niirnberget al., 1996a,b]. It could also be shownthat gametogeniccalcite has a significantlyhigher magnesiumconcentrationthan primary calcite, althoughsecretedat the sametemperature.A sac-like chamber, which may be built during the foraminifer's final stageof development[Bd et al., 1983; Hemlebenet al., 1987], however, doesnot show enhancedmagnesiumconcentrations comparedto a normalfinal chamber[Niirnberget al., 1996a]. 2.2.

Core Samples

Mg/Ca studieswere performedon two sedimentcores from the tropical Atlantic, for which major stratigraphical, sedimentological,and geochemicalinformation was available. SedimentcoresGeoB 1105-3/4 from the Equatorial Divergence and GeoB

1112-3/4

located

-400

km

south

in

the

low

productiveSouthEquatorialCurrent(SEC) were recoveredfrom the Guinea Basin from 3225

(1ø39.9'S,

12ø25.7'W)

and

3122m water depth (5ø46.2'S, 10ø44.7'W), respectively (Figure 1). Stable isotope geochemistry, foraminiferal assemblageanalysesand related SST reconstructions, fluxes of sedimentarybiogenic components, grain size analyses, and organiccarbonfluxes were previously publishedby Meinecke [ 1992], Bickert and Wefer [1996], Schneideret al. [ 1996], and Wefer et al. [1996]. A record of alkenone SST estimates is availableonly for core GeoB 1105-3/4 [Schneideret al., 1996] because the content

of alkenones

was insufficient

for the

propercalculation of U3• in coreGeoB1112. 2.1.

Foraminiferal

Species Selected for

Analyses

G. sacculifer inhabits shallow euphotic waters in the tropical and subtropical oceans [Shackleton and Vincent, 1978; Hernleben and Spindiet, 1983] because of the photosyntheticrequirementsof their dinoflagellatesymbionts [Leeand Anderson, 1991]. Inferred from both abundancesand isotopic temperatures,the depth habitat lies at-25-75 m [Fairbankset al. , 1980; Erez and Honjo, 1981; Fairbanks et al., 1982; Hemleben et al., 1989] with prevailing habitat temperaturesof 20ø-30øC.For the equatorialAtlantic a depth habitatof-0-50 m is proposedby Erniliani [1954] andHecht [1971]. From plankton tow studies close to our area of investigation, Ravelo et al. [1990] concludethat highest abundances of G. sacculiferoccurin -0-40 m. Salinities from 24 to 47 are tolerated[Hemlebenet al., 1989]. Reproductionis linkedto the lunar cycle [Bijma and Hernleben,1994]. In order to reproduce,G. sacculifermovesfar below the euphoticzone. During gametogenesis a secondary layer of calcite, called gametogenic calcite, is secretedin 80-100 m [Bijma and Hemleben,1994]. Accordingto modelresults,nearlya third of the populationeven addsgametogeniccalcite within the main thermoclineat 300-800 m waterdepth[Duplessyet al., 1981]. Growth rate and frequencyof gametogenesisare a function of feeding and illumination [Bd, 1980; Spindler et al., 1984]. G. sacculifer commonly feeds on copepods[Spindler et al.. 1984] and prefers a low-nutrient oligotrophic environment [Bijma and Hernleben,1994]. Accordingto the ranking of Berger [1970], G. sacculifer is fairly susceptible to

For both cores, stratigraphicinformation is available from the PANGAEA Paleoclimate Data Center (Alfred-WegenerInstitute,Bremerhaven).The age modelsare basedon graphic

correlation of Cibicidoides wuellerstorfi /5180records to the /5180standard recordof Martinson et al. [1987].We choose the benthic record for stratigraphic correlation to minimize sea surfacetemperatureeffectsthat may occurbetweenupwelling (Equatorial Divergence) and non upwelling regions (South Equatorial Current) and may include a time lag between planktic isotope signals. For detailed stratigraphic information,seeBickert and Wefer [1996] and Schneideret al. [1996]. The depth-age relationships for both cores demonstratenearly 2 times higher overall sedimentationrates (4.16 cm/kyr) in core GeoB 1105 comparedwith core GeoB 1112 (2.54 cm/kyr) due to the enhanced biogenic sedimentation underthe equatorialupwellingareaat site GeoB 1105 [Bickert and Wefer, 1996]. In contrast

to the western

South Atlantic

basins

and the

Cape Basin,the deep-seaareasthat are mainly filled by Lower CircumpolarDeepWater (LCDW) tindersaturated with respectto carbonateconcentration,the deepestpartsof the GuineaBasin are dominatedby slightly supersaturated North Atlantic Deep Water (NADW) [Broecker and Peng, 1982]. This causesthe lysoclineof calcite to stay much deepercomparedto the other South Atlantic basins [Biscaye et al., 1976; Thunnel, 1982].

Delta CO32-valuesfrom the nearestGeochemical Ocean SectionsStudy(GEOSECS) site 109 [Bainbridgeet al., 1981 ], indicating the difference between the in situ carbonate ion concentrationand the pressure-corrected saturation carbonate

NUERNBERGET AL.' Mg/Ca- DERIVED PALEO-SST

.40 ø

.20ø

126

0o

10 ø

10 ø

o

o





.5 o -10 ø

-10 ø

-15 ø

-15 ø

10 ø

10 ø

GD

o

o



0ø GeoBl105

.5 o

.5 o GeoB1112

AG

-10 ø

.10 ø AC

-15 ø

-15 ø

.40ø

.20ø

0o

Figure1. Areaof investigation schematically SSTu•,' indicating thehorizontal distribution patternof currents for thetropical surfacelayer at -0-100 m water depthfor (a) northernspringand (b) northernfall accordingto Strammaand Schott[1999]. Core locationsare indicatedby large dots.NEC, North Equatorial Current; NECC, North Equatorial Countercurrent;GD, Guinea Dome; GC, Guinea Current; SEC, SouthEquatorial Current with the northern (NSEC), equatorial (ESEC), central (CSEC), and southernbranches(SSEC); EUC, EquatorialUndercurrent;SEUC, SouthEquatorialUndercurrent;SECC, South EquatorialCountercurrent; NBC, NorthBrazil Current;GCUC, Gabbon-Congo Undercurrent;AG, Angola Gyre; AD, Angola Dome; AC, Angola Current.The hatchedareaindicatespotentialupwellingin the studyarea.

ion concentration (with respectto calcite),point towardwater

cleaning step and, again, distilled deionized water rinses

depthsof •-4800 m for the calcite saturationhorizon. Even

followed,alternated with ultrasonical cleaning.Subsequently, samples weretreatedwith a hot (80øC)oxidizingNaOH/H202 solution(30 ml 0.1 N NaOH (analyticalgrade);10 pL 30%

duringthe entirelate Pleistocene interval,the lysoclineis not expectedto haverisenabove•-3800m waterdepth[Bickertand Wefer,1996], suggestingthat the coresare well aboveany reasonablecalcitesaturationhorizon. The magnesiancalcite

H20: (supraput))for 30 min. Every 10 min, ultrasonical

cleaningsteps(2 min) wereapplied.Afterward, threestepsof

horizoncalculated for Mg/Ca = 10 mmol/mol (the upper distilleddeionizedwaterrinsing andultrasonicaltreatmentas above followed. The clean foraminiferal fragmentswere could have been as much as 150 m above the calcite saturation broughtinto new vials previouslycleanedwith 1 N HCI horizon. (subboileddestilled)and subsequently dissolvedby 0.1 N HNO3 (subboileddestilled)duringultrasonictreatment.The limit of whatis appropriatefor planktic foraminiferalcalcite)

2.3.

Preparation

and Analytical

Approaches

Specimens of G. sacculifer were selected from the 250-500pm size fraction. The large size ensures that specimenscan be easily identified and that a relatively small number of specimens is sufficient for the geochemical investigation. Specimens visibly contaminated by ferromanganese oxideswere discarded.Magnesiumand calcium analyses were carried out by inductively coupled plasma optical emissionspectrometry(ICP-OES). Approximately 0.5-1.2 mg sample material usually consisting of •-30 specimens of G. sacculifer were gently crushedbetweenglassplatesin orderto openthe chambersand were subsequentlyplaced into vials. In order to remove contaminantphasesthe material was rinsed three times with distilled deionized water with ultrasonical cleaning (2 min) steps after each rinse. One methanol (subboiled destilled)

samplesolutionwas then diluted with distilled deionizedwater up to 3 mL.

In order to assessthe efficiency and necessity of our cleaning procedure we subjected sieved foraminifera

(Globigerinabulloidesand Orbulinauniversa,250-500 pm) from core top samples off Portugal (39ø04'N, 10ø40'W;

1605m waterdepth)to sequentially morerigorouscleaning. Variousforaminiferalcleaningprotocolsdescribed by Boyle [1981], Boyle and Keigwin [1985], Brown [1996] and

Hastings et al. [1996]wereadopted and/ormodified.Cleaning step 1 comprisesthree rinses with destilled deionizedwater

alternated with sonication(2 min). Cleaningstep2 subjects the foraminiferato threerinseswith destilleddeionizedwater,

one rinse with methanol,and two subsequent rinses with destilleddeionizedwater,eachstep alternatedby sonication (2 min). This step significantly reducedthe magnesium

127

NUERNBERG ETAL.:Mg/Ca-DERIVED PALEO-SST

contents of G. bulloidesandO. universain comparison to step 1. Further cleaning (step 3 as describedabove) with hot alkalineperoxidefor 30 min with alternatingsonicationafter each10 min did not changethe magnesiumcontentwithin the measurement error nor did step 4, which applies a solution of NHnC1insteadof hot alkaline peroxide.Step 5 is similar to step3, but hasno methanolrinse.Finally,the mostextensive

cleaning,step6, comprises,besidessonicationandrinsing

glacial SST• and wouldreducethe glacial-interglacial SST•otop• amplitude by maximum of 1øC. 3. Results and Discussion

3.1. Mg/Ca Signal in Planktic Foraminifera From the Equatorial Divergence and the South Equatorial Current

with distilled dionized water and methanol, the treatment with

NHnCLsolution andhot alkalineperoxide.All cleaningsteps after step 2 did not changethe Mg/Ca ratiossignificantly. However,sincewe expectstrongercontamination in the downcoresamplescomparedto coretops,we systematicallyapplied cleaning step 3.

Analyseswererun on an ICP-OES(ISA Jobin Yion-Spex Instruments S.A. GmbH)with polychromator applyingyttrium asaninternalstandard. We selected elementlines for analyses

The down-coreMg/Ca recordsof both Guinea Basin cores

GeoB 1105 and 1112 coverthe last --270,000 years(oxygen isotopestages1-8), thus comprisingthree glacial-interglacial changes.In both cores, Mg/Ca ratios rangebetween2.6 and

4.4 mmol/molat most(TablesA1 andTableA2•). The longterm variability in Mg/Ca ratios by far exceedsthe standard error of the measurements and thus has to be considered as real.

concentration range. The relative standarddeviation is < 1%

Although the short-term scatter of the Mg/Ca ratios is also significantfrom the analyticalpoint of view, its paleothermal relevanceis questioned.It is likely that these small-scale variations are dueto the intraspecificinhomogeneityof the magnesiumincorporation.Narnberg [1995] alreadystatedthat the large rangeof magnesiumconcentrationswithin a single

for magnesiumandcalcite.The Mg/Ca reproducibilityof 15 replicatesamplesof G. sacculiferis good(? = 0.98, mean

chamberwalls, may lead to quantitativedifferencesbetween

which appeared most intensive and undisturbed (Ca:317.93nm; Mg: 279.55nm; and Y: 371.03 nm).

Elementdetectionwas performedwith photomultipliers,the hightension of which was adapted to each element

standarddeviation is 0.04 mmol/mol, and maximum standard deviation is 0.11 mmol/mol). The error in terms of SST is maximum of + 0.4øC.

Theaccuracy of ourcalciumandmagnesium analysescanbe evaluated withliteraturedata.Hastingset al. [ 1998] published inductivelycoupledplasmamassspectrometry (ICP-MS) and atomicabsorption spectrometry (AAS) derivedMg/Ca ratiosof G. sacculifer selectedfrom an equatorialAtlantic sediment core, the glacial-interglacialrange of which lies between--2.9 and 3.9 mmol/mol, thus being consistentwith our ICP-OES-

foraminiferaltest, which may exceeda factorof 3 along

differenttestsof one sample.ReplicateICP-OESanalyses varying on averageby --0.05 mmol/mol contribute to the scatterof the magnesiumdata down-core.In order to exclude the short-term scatter of the data from further discussion we

applied a simple seven-point smoothing procedure(least squares smoothing filter) [Paillardet al., 1996]onthe original Mg/Ca records(Figures2 and 3). The comparisonof Mg/Ca ratios in both coresreveal that

theMg/Caratiosin theEquatorial Divergence (GeoB1105) are lower comparedto the SouthEquatorialCurrent(core GeoB

based data (--2.9-4.0 retool/tool).

1112). This observation coincides with the observation of

2.4.

generallyheavierstableoxygenisotopevaluesin coreGeoB 1105 [Meinecke, 1992] (Figures2 and 3). The down-core variationsof both Mg/Ca ratiosandoxygenisotopesmatch

SST From Oxygen Isotopes

For core GEOB 1112 we calculated SST from the available

•5•80recordof G. sacculifer by applyingthe ErezandLuz [1983] temperatureequation:

T = 17.0- 4.52(•80•- •80,,) + 0.03(•80•- •O,,) 2,

(1)

fairly well, thereby clearly reflectingglacial-interglacial changes. HighestMg/Ca ratiosoccurduringinterglacials,and lowest values occur during glacials with pronounced concentration gradientsat stageboundaries8/7, 6/5, and 2/1. Spectral analysis [Paillard et al., 1996] reveals that the

where•180•istheoxygen isotopiccomposition of thecalcite Mg/Ca variability in core GeoB 1112 is dominatedby (permillePeedee belemnite(PDB)and•5•80•v is the oxygen Milankovitch frequencies with periodscloseto 100, 41, and isotopic composition of seawater (permille PDB; the

23 kyr. The lengthof time seriesof--270,000years,in fact, is

conversion from•5•80•v SMOWto •5•80•v PDBis performed too short to definitely resolve for variability in the according to Hut [1987]. The •5•80•v is assessed from the eccentricity band(100-kyrcycle).Thecross-spectral analysis salinityversus •5•80•v relationship established by Wanget al. [Blackmanand Tukey,1958]of theMg/Caandthe•5•SO records [1995] for the low-latitude Atlantic. Assuming that G. sacculiferinhabitsthe upper-100 m of the watercolumn, we considered salinitiesof 35.5-36.1 at 100-0 m waterdepth

derivedfrom G. sacculiferfrom the samesamplesindicates that the two time seriescorrelate(R =-0.54) with Mg/Ca

leading the•5lSOsignal.Thecomparison of phaseanglesof

[Levitus andBoyer,1994] leadingto a rangeof •5180•v values. Mg/Caand•5lsOreveals thatchanges in Mg/Caleadchanges in Glacial-interglacial variationsof seawater •5180•v, which are •80 by 12.0+2.1kyrat the 100kyr orbitalperiod, by drivenby ice volumechanges aretakenfromthe mean•5•80•v 5.5+1.0kyr at the41 kyr orbitalperiod,andby 2.6+0.7 kyr recordof Vogelsang[1990]. Vogelsang[1990] proposesa

meanocean•5180•v increase by 1.1%oduringthe LastGlacial Maximum,which is in accordance with pore water •5•O measurements in sedimentcores from the equatorialAtlantic

•Supporting Tables AI andA2areavailable electronically atWorld Data

Center-A forPaleoclimatology, NOAA/NGDC, 325Broadway, Boulder, [Schraget al., 1996]. Assuming a largermeanocean•5•sO•vColorado (e-mail: paleo•mail.ngdc.noaa. gov; URL: http://

increaseof 1.2-1.3%o[Fairbanks, 1989] would resultin warmer

www.ngdc.noaa. gov/paleo).

NUERNBERGET AL.: Mg/Ca- DERIVEDPALEO-SST

GeoB 1112 South 5o46.:-s •0o4•.7-w, smm water depth Equatorial Current (SEC) Mg/Ca SST (øC) (mrnol/mol) 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

20

22

24

128

SST (øC) 26

28

16 18 20 22 24 26 28

ß

...

30' ß

60'

90'

Isotopic

temp.

120-

150. 180

210-

240-

;ST ß dev.

std. dev.

270

1

0

SSTMg/Ca

SSTMg/Ca

-1

--*--- fi180 (%0PDB) C.sacculifer

Figure2. (left) Down-core Mg/Carecordof coreGeoB1112-3/4for thelast-270,000yearsin comparison to the stable oxygen isotope curve(G. sacculifer) [Weferet al., 1996].Replicate Mg/Caanalyses are indicated by squares. Thesmoothed recordisincluded asa thickline.(middle)SSTtemperature curvefromoxygen isotopes in comparison to thesmoothed SSTMedC a

record. Thebroad band ofSSTIsotop egives reference todifferent salinities applied forcalculating 15•8Ow. (right) Calculated $STMg/Ca of coreGeoB1112versus agein comparison to $$T derived fromforaminiferal transfer functions [Weferetal., 1996].

GeoB 1105 1ø39'9'S 12ø25'7'W'3325 mwater depth Equatorial Divergence

Mg]Ca 2.6

2.8

(mmol/mol) 3.0 3.2 3.4

SST (øC) 3.6

23

24

25

26

SST (øC) 27

28

15 17

19 21 23 25 27

..

30'

6O 90

•,..,• 120 < •50. 180' 210

240

Mg/Ca

$td.dev.

o

270

2.0 1.5 1.0 0.5 0.0 '0.5

SSTMv/C:•

---*-- •80 (%0PDB)a. ruber Figure 3. (left) Down-core Mg/Ca record of core GeoB 1105-3/4 for the last -270,000 years in comparisonto the stable oxygenisotopecurve(G. ruber) [Weferet al., 1996].ReplicateMg/Ca analysesareindicatedby squares.The smoothedrecord

isincluded asa thickline.(middle)SSTtemperature curvefromalkenones in comparison tothesmoothed SSTMg/C a record. (right)Calculated$$T

of coreGeoB1105versusage in comparison to $$T derivedfrom foraminiferaltransferfunctions

[Wefer etal.,1996].MgtCa

129

NUERNBERGET AL.: Mg/Ca- DERIVED PALEO-SST

at the 23 kyr orbital period. The coherencyof the two time seriesis 0.87 at 1/!00 kyr, 0.82 at 1/41 kyr, and0.78 at 1/23 kyr. All thesecoherencies exceedthe 95% confidence interval for coherency(= 0.62). The correlationcoefficientof the two time seriesimprovesto R =-0.71, whenassuminga time lag of 5500 years(at the 41 kyr orbital period)betweenboth the

equally sampled andsmoothed Mg/Caand• 180records. In coreGeoB 1112 the stage8/7 boundary(TerminationIII) exhibits a pronouncedMg/Ca increase from -3.2 to -3.9 mmol/mol. At stage 6/5 boundary (Termination II), Mg/Ca ratiosincreasefrom 3.1 to up to 3.9 mmol/mol, andat stage1/2 boundary(TerminationI)they increasefrom 3.0 to 3.8 mmol/mol. The overall glacial-interglacial amplitudeof the smoothedMg/Ca recordsis -0.7-0.8 mmol/mol. For core GeoB 1105 we observea Mg/Ca ratio increasefrom

-2.7 to 3.2 mmol/mol at stage boundary8/7, from 2.8 to 3.5 mmol/mol at stage boundary 6/5, and from 2.6 to 3.4 mmol/mol at stageboundary2/1, thus showinga glacialinterglacial amplitude of 0.5-0.8 mmol/mol, which is comparableto the valuesin coreGeoB 1112. 3.2. SST Information Ratios in Comparison

From Foraminiferal Mg/Ca to Recent Conditions

It is of considerableimportance, for the paleo-SST reconstruction from down-core Mg/Ca records (hereafter

indicated asSSTMg/Ca), whichMg/SSTrelationship is applied. Up to now, only a few species-specific Mg/SST calibration curvesfrom cultivatingexperimentsexist. For primary calcite of G. sacculifer,Niirnberget al. [1996a, b] suggesteda strict relationship between the foraminiferal Mg/Ca ratios and temperature.They chosean exponentialmodelto fit their data, althoughstudieson the partition coefficient of magnesiumin magnesiancalcite overgrowthsare still ambivalent, showing both linear and exponentialrelationshipsbetweenmagnesium and temperature [Chilingar, 1962; Kinsman and Holland, 1969; Katz, 1973; Fiichtbauer and Hardie, 1976; Mucci, 1987;

Oomori et al., 1987]. Since the foraminiferal Mg/Ca ratios are orders of magnitude lower compared to inorganically precipitated calcite, it was suggestedthat the foraminifera are capable of physiologically controlling the magnesium concentrationin their shells [Niirnberg et al., 1996a]. Such physiological control is apparently the dominant driving force, althoughtemperatureseemsto affect theseprocessesthe way that a direct magnesium-temperaturerelationship is pretended. The slight deviation of the relationship applied here from the one originally proposedby Niirnberget al. [1996b] stems from the correction made to adjust the microprobe measurements of Niirnberg et al. [1996b] to the ICP-OES data of this study:

Table 1. CoreTop SSTEstimates andGlacial-Interglacial SST Amplitudes FromVariousProxies SST

SST

GeoB1105, øC

GeoBl112, øC

Levitus [1994]

26.2-20.4

26.5-23.2

May-June

Levitus [1994]

24.8-19.9

25.1-22.8

May-Aug.

Method

Period

Levitus [1994]

26.2-22.9

25.7-23.3

Sept.-April

Levitus [1994]

26.2-22.3

25.5-23.3

annual

Mg/Ca•moo• U•'37

24.1 25,7

26.1 n.d.

TF,o,d

23.5

25.4

TF•m

26.1

27.3

(5•0•.

n.d.

24.7

8•Om•.

n.d.

25.5

SST Amplitude,GeoB1105, øC Method

SST Amplitude,GeoB1112, øC

MIS 2 / MIS 1

MIS 6 / MIS 5

MIS 8 / MIS 7

MIS 2 / MIS 1

MIS 6 / MIS 5

MIS 8 / MIS 7

3,4

2.9

2.9

1.9

n.d.

n.d.

n.d.

Mg/Ca•moo• U•'37

3.5

2.9 3.1

2.9 3.8

TF,•d

7.2

6.1

6.5

8.0

7.9

6.3

TF,,•m

5.2

3.9

3.7

4.9

5,1

2.7

•5•gO•.

5.1

7,7

6,5

•5•gOm•.

5,1

7,7

6,5

(top) SSTestimates arefor coresGeoB1105andGeoB 1112in comparison to modemLevitusandBoyer (1994) watertemperatures. The Levitus andBoyer (1994) temperatures of theupper50 m of thewatercolumnaregivenfor thebeginning equatorial upwelling(May to June),theupwelling period(May to August),theremainingyear(September to April), andtheentireyear.(bottom)Glacial-interglacial SSTamplitudes of variousSST proxiesare comparedfor TerminationI, (MIS 2/1), II, (MIS 6/5), andIII, (MIS 8/7), differentiatedfor bothcores.Here n.d.is no data.

NUERNBERGET AL.: Mg/Ca- DERIVEDPALEO-SST

T = (logMg/Ca- log0.491)/0.033 R2= 0.92.

(2)

The uppermostHolocene samplesin cores GeoB 1112 and

GeoB 1105 reveal a-2øC offset in SSTM•ca with lower temperaturesat core GeoB 1105 (Table 1), which generally persistsover the entire time period investigated.Today, only during equatorialupwelling from May to August/beginning September,SST in the EquatorialDivergenceare considerably cooler than in the South Equatorial Current at 0-50 m water depth [Levitusand Boyer, 1994] (Figure 4). Unfortunately, information about abundancemaxima of the oligotrophic G. sacculifer during that time is sparse. B. Donner (personalcommunication,1998) gathereda longterm abundancerecord for planktic foraminifera from the equatorial East Atlantic revealing an abundance peak of G. sacculiferduring the beginningupwellingfrom australlowlatitude late fall/early winter. During summer, instead, no G. sacculifer were observed in the eastern South Atlantic

July/ August

T (øC) 13

0 i

15

.

17

i

.

i

May/ June

19

_

i

130

[Oberhiinsliet al., 1992]. Ufkes et al. [1998] report high abundancesduring austral low-latitude spring (October to November),when a deep mixed layer has establishedin the eastern

South Atlantic.

Taken together, the-2øC differencebetween the two core sitesandthe presumedabundance maximumduringaustrallow-

latitudefall/wintersuggest that at siteGeoB1105 the SSTMg/C a signalreflectsthe thermalsituationwithin the upper-50 m of the watercolumnduringupwelling(May to August),while site GeoB 1112 is not affected by upwelled waters. The latest

holocene SSTMg•c aof -26 ø+0.4øC and24ø+0.4øC at core sites GeoB 1112 and GeoB 1105, respectively, fairly well reflectthe upwellingsituation[LevitusandBoyer, 1994]. The

errorin SST•eca calculations to lowervalues,whichmayraise from the presenceof varying portions of gametogeniccalcite enrichedin magnesium[Niirnberget al., 1996b], may provide enoughamountof scopeto even explain the rapid SST drop, which takes place during the ongoing upwelling in July and lastsuntil August[Levitusand Boyer, 1994] (Figure4). From Septemberon when upwelling ceases, SST slowly increasesagain in both areas by -1-3øC, and surfacemixed layer temperaturesin the Equatorial Divergence never fall below the ones observedwithin the South Equatorial Current (Figure 4). We thereforeconcludefor the area of investigation

that the SSTuvca signal reflects the austral low-latitude

30

fall/winter upwelling situationwithin the uppermost-50 m of the water column (the assumedhabitat of pregametogenic G. sacculifer). Whether salinity changesperturbthe magnesiumsignal is still a matter of debate. From live culturing of OrbMina

60 .

I I

90

universa, Lea et aL [1999] infer an increase of 4 _+3%

Upwelling II

•120

MaytoAugustI

I(a)

I

150 i

......... 13 0

I

15 ß

I

SouthEquatorialCurrent(SEC) EquatorialDivergence 17

ß

I

19 ß

I

21 .

I

23 ß

I

25 ß

I

27 .

........•..,:.•:•,....-• ......•:?:....... .............................. •........• ......... .,................... .•:..,•............... •..•

ao '•

60



90

oo-øø

I

3.3.

•120

September toApril

in

Mg/Ca per salinityunit at 22øC.For G. sacculifer,Narnberget al. [1996a] pointed out that pronounced salinity changes (greaterthan -10) apparently dominate over the temperature effect with respectto the magnesiumuptake, possibly caused by an enhanced metabolic activity at high salinity levels. Small-scale salinity differences below 3, instead, are not resolvedby the Mg/Ca ratios [Niirnberget al., 1996a]. Since, at the study sites, annual salinity variations of-35.1-36.1 [Levitus and Boyer, 1994] within the upper 100 m water column (which is the assumeddepth habitat of G. sacculifer) give rise to Mg/Ca ratio variations of-0.25 mmol/mol at most, which is close to the analytical error, we do not expect salinity to have any pronouncedeffect on the foraminifera/ Mg/Ca ratio.

Mg/Ca

Paleo-SST

Information

From

Foraminiferal

Ratios

150 •'/ September to April i(b)

The paleo-SSTu•ca recordsof both coresresembleeach other,although theSSTu•ca estimates of coreGeoB1105 are

Figure 4. Watertemperature datafor the EquatorialDivergence(thin dashed lines) and the South Equatorial Current (thick solid lines) averagedfor (a) the upwellingseason(May to August)and (b) the remainingyear [Levitusand Boyer,1994].For the upwellingseasonwe differentiatedbetweenthe beginning(May to June) and the remaining upwelling(Julyto August).The shaded areamarksthepresumed habitat of pre-gametogenic G. sacculifer(upper50 m of the water column). Time and depth at which the EquatorialDivergenceis significantly coolerthanthe SouthEquatorialCurrentand at which the Mg/Ca signal

is presumably beinggeneratedare markedby the hatchedarea. The arrow pointsto considerable surfacewater coolingduringongoing upwellingin australlow-latitudefall/winter.

generally lower in comparisonto core GeoB 1112. It is

apparent thatbothSSTMg•c arecords are-1ø-2øChigherthanthe correspondingpaleosummerSST• recordsof Wefer et al. [1996] derivedfrom planktic foraminifera/ transfer functions (Figures2 and3). The overallrangeof thedown-core SSTu,•c ais smaller (3.6 ø and 4.0øC in cores GeoB 1112 and GeoB 1 105, respectively)than the maximumrangeobservedfrom transfer functions, which even exceeds 8.0øC. In fact, the glacial-

interglacialSSTMg/C a amplitudes at stage8/7, 6/5, and2/1 boundaries are-3øC

for each of the boundaries (Table 1),

131

NUERNBERGET AL.: Mg/Ca- DERIVEDPALEO-SST

I8'

28

Entire dataset 27



26

[• 25 24

23

a) 22

GeoB

27

1112

SSTMg/Ca r•

26

o

• 25

t$3 24 23

GeoB

1105

SSTMg/ca

b)

22

0

30

60

90

120

150

180

210

240

270

Age (kyr) Figure5. (a) Comparison of SSTMyca andSSTu•;fromcoreGeoB1105and(b) comparison of SSTMg/C a for bothcores investigated. Thehatched areamarksthe timeperiodfor whicha goodcorrelation betweenSSTMg/c a and SSTu•'exists (R = 0.78). Marine oxygenisotopestagesareindicated.

whereasthe SSTvFamplitudesare larger and often exceed5øC.

SSTMg/C a andwarmSSTvF. In pre-Holocene sections, SSTisotop e

The magnitudeof the glacial-interglacial SSTMg/c a gradient reconstructionssignificantly deviate in amplitude from the corresponds to that of Hastings et al. [1998], who describeda warmSSTTr data,although theSSTi.•otop• amplitude betterfits to temperaturedrop of-2.5øC during glacial stage 2 in the warmSSTTrthanto SSTMyca. The temporalamplitude of the equatorial Atlanticbasedon SSTMg/c a.Studies of VanCarnpoet SSTM• ais considerably smallerthanfor SSTisotop e(Table1). It al. [1990], who pointedout that African sea level temperatures needs to be pointed outthatin contrast to SSTM• athe accuracy were 3ø-4øClower during the Last Glacial Maximum, further of SST•.•otop• suffersconsiderably fromassumptions regarding supportthe magnesium-derived SST reconstructions. theiceeffect,therecent,andthepaleo-5•SO Wvalues.Sincethe Both the smaller amplitude and the generally higher magnesium signalin G. sacculiferdefinitelyleadsthe oxygen temperature estimates mayresultfromthe factthat the SSTMr./caisotopesignalanda considerable portionof the 5•sOsignal signal reflects seasonal temperatureconditions from the reflectsglobalice volumechanges,we concludethat changes

uppermost watercolumn,whereasthe SST,r integratessummer in $ST precedechangesin globalice volumeat this site. $$T estimates obtained from alkenone concentrations temperatures from the surface waters down to the subthermocline when considering the entire planktic (SSTu•,') establishedfor core GeoB l105 [Schneideret al., foraminiferal faunal spectrum [Meinecke, 1992]. Also, 1996]exceedthe SSTM•aby -lø-2øC, while for mostof the foraminiferal abundancesin this specific area might be related time spancovered,the temporalpatternof both temperature to vertical water column hydrographyand nutrient distribution rather than to sea surface temperature alone [Ravelo and Fairbanks, 1992; Sikes and Keigwin, 1994]. Holocene SST estimates derivedfrom oxygen isotope data

of G. sacculifer (SSTisotope) from coreGEOB1112 rangefrom -23ø-25.5øC (Figure 2) and thus are considerably lower than

reconstructionsmatch well (Figure 5). The overall correlation

betweenboth the smoothedSSTM•/c a time seriesand the smoothedSSTu•; time seriesis R = 0.49. The correlationfor samplesyoungerthan 90,000 years,for which a good visual correlationexists,improvesto R = 0.78. The overall glacialinterglacialamplitudesat Terminations I andII rangefrom 2.9ø

NUE•ERG

ET AL.: Mg/Ca- DERIVED PALEO-SST

to 3.5øCin both the SSTMg/c a and SSTu}'records,while at Termination III the amplitude of the SSTMgra is only -2.9øC compared withthe3.8øCamplitude of theSSTug/c arecord(Table 1). Major discrepanciescan be observedin oxygen isotope stage 7 and basal stage 6. While the SSTu•i record clearly

indicateswarmingof surfacewaters,the SSTuvca stayslow. Also, in substage5.3 the SSTug'recordindicatesa significant warming, which is not reflectedby the magnesiumsignal.

For coreGeoB 1112 the SST•r•c a estimatesfall into the range of the SSTu}' published for core GeoB 1105, being slightly higher (maximum of 0.5øC) duringpeak interglacials

(Figure5). Theglacial-interglacial SST•r•c aamplitude of-3øC

132

functionsare furtherexpressedby the fact that bothchemically derived temperature reconstructions reveal (1) higher interglacial stage 5e temperaturesin comparisonwith the Holocene and (2) colder glacial stage 2 conditions in comparison with glacial stage 6. Similar conditions are reportedfrom SSTu[' reconstructionsof the northernNorth Atlantic [Villanueva et al., 1998] and the Indian Ocean [Rostek et al., 1993]. The temperaturerecordsderivedfrom transfer functions, instead, reveal warmer Holocene and stage 2 temperatures comparedto stages5e and 6 conditions. 4.

Conclusions

is comparableto the SSTu•( amplitudeof core GeoB 1 105. Similar to core GeoB 1105, discrepanciescan be observed duringlate stage7 andbasal stage6, wherethe SSTu,•;record

Since the disparity between terrestrial and marine SST estimatesis most obvious in tropical regions, this study definitelyshowsseasurface warming,whilethe SSTMgra stay attemptsto contributenew aspectsby applying Mg/Ca ratios in the planktic foraminifer G. sacculifer as a tool for cool. Also, substage5.1, which exhibits warm SSTu,•(,is not estimating SST in the past to two sedimentcoresfrom the reflected in SSTMg/c a. Both the absolutetemperatureoffset andthe discrepancies easternequatorialAtlantic covering the last -270 kyr. The between the down-core records of SSTu•d andSST•, in the down-coreMg/Ca variationswerecalibratedusingthe speciesspecific, exponential Mg/SST relationship establishedfor EquatorialDivergence are most presumablyexplained by the primary calcite of G. sacculifer[Niirnberget al., 1996a]. SST time of signal generationand the different depth habitats of the U 37•' signalproducingcoccolithophorids on the onehand estimationsrevealan accuracyof approximately+0.4øC. and foraminifera on the other hand. Conte et al. [1993], Sikes and Keigwin [1994], Herbert et al. [1998], and Miiller et al.

Holocene

SST estimates amount to -26øC

for the South

EquatorialCurrentand---24øCfor the EquatorialDivergence suggestingthat the chemicalsignaturemusthave generated [1998] consistentlyconcluded that the sedimentaryU 37•' ratio essentially reflects annual mean mixed layer temperatures duringupwellingin australlow-latitudefall/winter in -0-50 m (-0-10 m waterdepth).In fact, the U •' temperature calculated water depth. Within the South EquatorialCurrent, pre37 Holocene SST vary between ---24øC and 27.5øC at most, for the uppermost sample in core GeoB l105 of 25.7øC whereasthe coolerEquatorialDivergenceexhibitsSST of only matchesthe annualmeantemperatureof the uppermost---10 m 22ø-25.5øC for the last 270,000 years. Common to both of the water column of 25.6øC [Levitus and Boyer, 1994]. Seasonal variations of 24ø-27.5øC observed in this area are recordsis a glacial-interglacialamplitudeof -3-3.5øC for the apparently averaged out.TheSST•g/c , of-24øCatthesamesite, last climatic changes, thus being consistent t o instead, reflects the austral low-latitudefall/winter signal paleotemperature estimates derived from unsaturated beneaththe mixedlayer downto ---50m waterdepth [Levitus alkenones.Furthermore,both recordsimply lower Holocene and Boyer, 1994]. andglacial oxygenisotopestage2 temperatures compared to interglacial stage 5.5 and glacial stage 6 temperatures, The prevailing temperatureoffset of-lø-2øC through time betweenEquatorialDivergence and South EquatorialCurrent respectively. (Figure 5) points to continuous upwelling within the Thecomparison of SSTMg/C ato otherSSTproxiesunderlines the applicability of foraminiferal Mg/Ca ratios for EquatorialDivergence,comparableto the recentsituation. The increasingtemperature offsetobservedin stage7, stage6, and paleothermalreconstructionsand thus may reinforce the discussion on SSTreconstructions. Ourstudyemphasizes the substage 5.3 maybe interpreted asthethermaldecouplingof a warm thin surfacelayer from cooler water massesbeneath and

need for cautionwhen applying single paleotemperature

thusa strongeror longerperiodof stratification.In contrast,a decreasing temperature offsetmainly observedduringthe cold eventsof glacial and interglacial stageswouldproposeless strong seasonaldifferencesin the upper -50 m of the water

techniquesto reconstructclimate dynamicsand stressesthe importanceof a multiproxyapproach.

column.

Thecorrespondance of the SSTMyca andSSTu,•' records and their disparity to temperaturerecordsderived from transfer

Acknowledgements. For intensivediscussion and improvement of the manuscriptwe thank Jelle Bijma, Ralf Tiedemann, and Christine C. NiJmberg. Forcriticalreviewswe aregratefulto E. Rohling andtwo anonymous reviewers.Thisstudywas fundedby the German ScienceFoundation(DFG) grantNu60/4-1/2.

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A. Mailer and D. Narnberg,GEOMAR Research Center,Wischhofstrasse 1-3,D-24148

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[email protected]) R. R. Schneider,FachbereichGeowissen

schafien, University of Bremen, Klagenfurther Strasse, D-28334 Bremen, Germany. (rschneider•uni-bremen.de) (ReceivedJanuary28, 1999; revised October 12, 1999; acceptedOctober 19, 1999.)