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PALEOCEANOGRAPHY, VOL. 6, NO. 1, PAGES 1-20, FEBRUARY 1991 BENTHIC FORAMINIF RAL also IN THE OCEAN'S TEMPERATURE-SALINITY-DENSITY FIELD: CONSTRAINTS ON ICE AGE THERMOHALINE CIRCULATION RainerZahnl Departmentof Oceanography,University of British Columbia, Vancouver, Canada Alan C. Mix Collegeof Oceanography, OregonStateUniversity, Corvallis Abstract.Benthic15180 datafrom95 coresitesare used to infer possible temperature-salinity(T-S) fields of the Atlantic and Pacific oceans at the Last Glacial Maximum (LGM). A constraint of stable densitystratificationyieldslogically consistentscenarios for both T and S. The solutions are not unique but are useful as a thinking tool. Using (AT-3øC) than overlying deep waters,in conflict with otherdata, suggestingice agedeepwater much colder than at present. It is also possiblethat the observed15cgradientsare an artifact of laboratory intercalibration.If Atlanticdeepandbottomwater values were similar to deep Pacific values, this wouldbe consistent with thehypothesis of a stronger GEOSECS data, we solve for the modem relation- southern shipbetween 1518Owate r (15w) andsalinity inthedeep deep-oceanventilation at the LGM. Taking the observedgradientsat face value, however,a solution couldbe thattheLGM bw-Sslopein deepandbottom waters was higher than at present, conceivably becauseof a strongercontributionof saltto the deep oceanvia more intenseseaice freezing. This would allow PacificdeepwatersandAtlantic bottomwaters to have a common source,again in the Antarctic. Both wouldbe moredensethanArianticdeepwaters, even thoughthe deepwaterswere much colderthan at present. To better constrain these inferences sea:15w(SMOW) = 1.529 * S - 53.18. As a starting point, we assumethat the slope of this equation applies toLGMconditions andpredict 1518Ocalcit e(•c) gradientsin equilibriumwith probableT-S fieldsof LGM deepandbottomwaters. Benthicforaminiferal 15180 datafromthedeep Pacific (2-4kmdepth), and the bottom Atlantic (> 4 km depth), are 0.1-0.2%o lower than from the deepAtlantic (2-4 km depth) at the LGM. If the modem15w-S slopeapplies,Atlantic deep and bottom waters were more dense than Pacific deepwaters. This assumptionwould imply bottom waters both fresher (AS >0.5) and colder •Now at GEOMAR, Kiel, Federal Republic of Germany. Copyright 1991 by theAmericanGeophysical Union. Papernumber90PA01882. 0883- 8305/91/90PA-01882510.00 ocean versus North Atlantic source for drawn fromthespatial distribution ofbenthic 15180, we mustreducescatterin the 15180datawith more high-qualitymeasurements in high sedimentation rate cores.This is especiallytrue at bottom water sites. Also, we must intercalibratemassspectrometers at different isotope laboratories more accurately, to insureour isotopedataarecompatible. INTRODUCTION Rhythmic variations offoraminifera115•80 indeepsea sediments,first detected by Emiliani [1955], ZahnandMix: BenthicForaminiferal 15180 primarily reflect changes inglobal ice volume Shackleton, 1967]. Intercore differences in the variationsare commonlyattributedto spatialvariations of deep water temperature [Chappell and Shackleton, 1986; Labeyrie et al., 1987]. Yet we know from work on the modernocean [e.g., Craig andGordon, 1965]thattheb•80 composition of differentwatermasses(bw)in the oceanis quitevariable. Thus changesin water massare anothervariable that must be considered.Broecker [ 1986, 1989] discusses possiblevariationsin the distributionof in the glacial oceans. He emphasizeswater mass changesin surface waters relative to mean deep the same vessel. Memory between samples,less than 1% of the isotopeoffset between samples,is undetectablein normalmarinesamples.Calibration to the international Pee Dee Belemnite (PDB) scale was done primarily through the U.S. National Institute of Standardsand Technology carbonate stan,,dard "NBS-20" and secondarily through "NBS19. Long-term reproducibility (+1(5)for15180 and 15•3C over1year(1989)is0.09and0.04%0, respectively, for a local calcitestandard(n=229), and0.04 and0.03%0,respectively,for NBS-20 (n=71). The15•80 dataused herearefromthetwowidely usedbenthicgeneraUvigerinaandCibicidoides.We waters,whichwouldaffectb•80 in planktonic corrected all15•80 datatotheUvigerina scale, which foraminifera. isthought tobeclose to15•80 equilibrium withsea water[Shackleton, 1974].Thecorrection for15180 Here we take the next stepand explorethe useof spatial distributions ofbenthic foraminiferal b•80as data from Cibicidoides to Uvigerina is +0.64%0 a tracerfor both temperatureand salinityin the deep sea. We start by evaluatingthe equilibrium frac- tionation of 15ISOcalcite (15c) in theocean's temperature-salinity-density field. Next we derivebw-Srelationshipsfor the modernocean. To interpretthe ice agedatain termsof T and S, we fin:stapplythe slope of the moderndeep-seabw-Srelationship. We later relax this restriction and consider how the bw-S patternmay have changedin the past. Finally, we infer the range of possibleice age distributionsof temperature,salinity,anddensityin thedeepsea. A uniquesolutionto two variables(T and S) cannotbe found from measurementsof one variable (bc). In spiteof this, the constraintof densitystratification gives insight into possibleT and S distributionsin the deepocean,which hasimplicationsfor the mode of deep and bottomwater formationin the ice age ocean. [Shackletonand Opdyke, 1973; Shackleton,1974]. The error introducedhere may be as large as 0.2%0 for individual samples[Mix and Fairbanks,1985]. On average,though,the adjustmentappearsto be quitestablein boththeAtlanticandPacific. We haveusedcoretopdatato examinethemodem distribution pattern ofbenthic b•80.Coretops with anomalously lowb•3Cwererejected. Wesuspect theseare contaminatedwith glacial material. The LGM is deftnedasthemostrecentforaminiferal b•80 maximumbelow the last glacial-interglacialtransition. This may differ slightly from published chronologies, but given the potentialage errorsin someof the datausedhere,we preferthe objective criterion ofusing the15•80 maximum. FORAMINIFERAL 15•80AND PALEOTEMPER AT[ IRE EQUATIONS METHODS AND DATA BASE Glacial-interglacialfluctuationsof foraminiferal Wecompile benthic 15180 datafrom58northeast I5•80 combine the of changing temperature •8 signals Ariantic and 39 Pacific core sites for the modern and are listed in Table 1. Someof the isotopedata used here are new. Thesewere measuredat OregonState University, on a Finnigan MAT 251 mass spectrometer.This facility is equippedwith microvolume inlet and an AutoprepSystemsautomatedcarbonate device. Sample preparation follows the standard techniques[Shackletonand Opdyke, 1973], except and changing150water(15w).TO solvefor temperature,we mustchooseamongavailableisotopicequilibrium equations. Figure la showsempirical 15c equilibriumpredictions,defined by sevendifferent equations(see review by Mix [1987]). The equationspredictshiftsin 15c of 0.21-0.27%oper 1øC temperaturechange(Table 2). At temperaturesabove 10øC,all but the earliestequationsyield nearlythe same isotopevalues. However, at typical abyssal temperatures, i.e., below5øC,theequationsdiverge. To test the palcotemperatureequations,we com- that reactions occur at 90øC. parecoretop15180 datawithdepth profiles ofpre- the Last Glacial Maximum (LGM, approximately 18,000 years ago) (Table 1). Most of the data used here were available from the literature. References It is a "common acid bath" system,in which up to 40 reactionsoccur in dictedequilibrium15c (Figure lb). The equilibrium ZahnandMix: BenthicForaminiferal•80 o •5o oO 0 ,-• t.• oo oo .q- 0 i i i i •2uo i i i oo (•,1 i i ZahnandMix: BenthicForaminiferal b•80 oo o• • o •uOUOuuu o uu•u•uuuu ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! OuuuuuOOUO• uuuu ZahnandMix: BenthicForaminiferal •0 oou UUU ••U uuoo UU UU•U•UU ou •2ouuuuo U U U•UUUUU ZahnandMix: BenthicForaminiferal •5180 •5'80,•z( %oPDB) 1 2 3 4 0 18 2 P_•T.•.OTEMPERATURE EOUATIONS AFTER: 14 a) MCCREA [1950] 12 b) EREZANDLUZ [1983] 10 c) O'NEmETAL. [ 1969] 8 d) SHACKLETON [1974] 6 e) EeST•INETAL. [1953] 4 2 0 C•dO [1965] 0 g) HORmE • -2-2 -1 0 1 2 3 4 ++ 4• O•^ [1972] 5 5 a 1: •te•[ 6 /5•SOeee • minusõ•Owater A B Fig.1. (a)Empirical relationships between •518Ocalcite (•) andwater temperature. (b)Vertical profries of•5c, using temperature and•S18Owater profiles from GEOSECS stations 29and115(data from(Sstlund etal.[1987a]). Crosses arebenthic •5180 values (Uvigerina scale) fromNortheast Arianticcoretops.Bestfit is withequation b. profilesfornortheast Atlanticcoresitesarecomputed from vertical temperature and •Swsectionsof GEOSECSstations29 and 115 [Ostlundet al., 1987a]. The bwvalueswere transformedfrom the StandardMean Ocean Water (SMOW) scaleto the PDB carbonatescaleby subtracting 0.27%0fromthe data tabulatedon the SMOW scale[Hut, 1987]. Offsetsbetweenpredicted valuesandcoretopdata (Figurelb) pointeitherto errorin calibratingthe equations at lowtemperatures or to isotopic disequilibriumin Uvigerina.The bestfit to thecoretopdata below2 km depthis with theequationof Erez and Luz [1983]. This equationwas calibratedbetween 14øC and 30øC, with cultured planktonic foraminifera. We will usethisequationto estimate equilibrium•5c.Our conclusions wouldnot change had we chosenoneof the otherequations,however. The fit of the Erez and Luz equationto core top does not indicate failure of the Erez and Luz [ 1983] equationbutrathera watermasseffect. For the Pacific, we use T and •Sw data from GEOSECS stations 306, 322, and 345 [Ostlund et .o al., 1987a]. Here, the Erez and Luz equationpredicts •5clower than some of the core-topdata (see Figure 5 below). We cannotprovewhy this mismatchoccurs.We speculate, however,thatbiological mixingof foraminiferalshellsfrom late glacial coresectionsto the coretopsis more severeat some of the Pacific core sites we have used. In the Pacific, foraminiferal shells are most common in carbonate- rich glacialsediments andlesscommonin coretops wherecarbonatecontentsare stronglyreduced.The opposite is u'ueof manyAtlanticsites.Thusthereis a higherchancefor Pacificcoretopsto be contami- nated withhigh•5•80 glacial material duetobioturbation. Birchfield [ 1987] reacheda similarconclusion. benthic •5180 values atwaterdepths < 2 kmispoor. Predictions aresystematically lessthanthemeasured values(Figurelb). At thesedepths,mostof thecore sites used here are near the Strait of Gibraltar and are influenced by180-enriched Mediterranean outflow waters. The hydrographicprofilesof GEOSECS stations29 and 115, which we use to computethe vertical•5cprofilein Figure1, arefar fromGibraltar. They arenot stronglyinfluenced by Mediterranean waters. Thusthe mismatchof coretop dataandthe predictedwatercolumnvaluesabove2 km depth TABLE 2. SlopeandTemperature Predictions Using Different Paleotemperature Equations Equation Slope T øCa McCrea [1950] 0.21 -0.75 Epsteinet al. [1953] Craig[1965] 0.26 0.27 3.17 3.79 O'Neil et al. [1969] 0.25 2.80 Horibe andOba [ 1972] 0.27 3.81 Shackleton[1974] 0.25 2.90 Erez andLuz [1983] 0.23 1.52 a calculatedfor 15w of +0.27%o(SMOW) and 15c of +3.5%o (PDB). ZahnandMix: BenthicForaminiferal 8]80 WATER MASS PATI'E•S OF 8wAND 8cIN (Abw/AS = -1.5 in Figure 2) thus yielding a "fictitious" zero-salinity intercept of 8180,much THE DEEP SEA lowerthanthe8180valueof thetruefreshwater dilu- Thedistributions of b]80in seawater (bw)and ent. Water salinity(S) arebothcontrolledmostlyby evaporation and precipitation [Dansgaard, 1964; Craig and masses from the southern ocean and the North Atlanticaretheprincipalsources of ventilation in the modem deep ocean. This was probablyalso true (at varying mixing ratios) in the glacial ocean eachregion8wis linearlyrelatedto watermasssalin[Curry and Lohmann, 1985; Curry et al., 1988; ity (Figure2). The slopeof the 8w-S relationship, Duplessy et al., 1988; Keigwin, 1987; Oppo and however, varies between 0.1 (for tropical surface Fairbanks,1987]. Mixing betweentheseend-memwaters) and 0.6 (for high-latitude surfacewaters) ber water massesis most easily seenin the Atlantic [Craig and Gordon, 1965]. This reflects greater Oceanwhich representsthe geographicconnection Rayleighdistillationof precipitation in highlatitudes, between the deep and bottomwater sourceareasin which preferentially removes •80fromhigh-latitude fresh waters. Without additional mechanisms such the high-latitude North Atlantic and the southern ocean. Using GEOSECS 8w and salinity data asfreezingof seawater, thezero-salinity intercept in Gordon, 1965; Joussaume et al., 1984]. Within the8w-Splotgivesthe8•80 valueof themean [0stlundet al., 1987a],we evaluate8w-Srelation- regionalfreshwater diluentof seawater. shipsfor theAtlanticmodemdeepandbottomwaters (Figure 3). Becauseof their close similarity, we combinethe 8w-Sequationsfrom Figure 3 into one singledeep-oceanequation: I 1 8w(SMOW)=l.529*S-53.18 (r2=0.95, n=110) (1) . it, J • [SURFACE / 7 Again, note the relatively negativezero-salinity intercept, below -50%0 (SMOW). In contrast,the average 8•80composition ofprecipitation andfiver NORTHATLANTIC_DEEP WATER runoff in the high-latitudeNorth Atlantic is about- 0 /WEDDELL /l[ PACIF[•_.AN_._.D,,,I ,ND.I_.•_,•_OCF_• ' 21%o(SMOW) [CraigandGordon,1965;0stlund ] -11 33 DEEP WATER ANTARCTIC . 0.4 BOTTOM WATER , 34 . , 35 . , 36 . i 37 . 0.3 38 SALINITY Fig.2. Regression linesandslopes(s)for 8wand 0.1 salinity. Line A, NorthAtlanticsurfacewaters.Line B, tropicalsurface waters[afterCraigandGordon, 1965]. Line C, Mediterranean Waters[Stahland Rinow,1973].Dashed lineisfordeepandbottom watersamples in theAtlantic(seealsoFigure3). 0.0 -0.1 This is not true in polar regions,where salinityis influencedby freezing of sea ice. Freezingrejects 34.6 34.7 34.8 34.9 35.0 35.1 SALINITY A: $•,= 1.45* S- 50.55(r2 = 0.95) Thus southern ocean waters havea relativelywide rangeof salinities(33.5-34.5) even thoughthey havevirtually the same8w(about0.2%0 versus SMOW) (Figure 2) [Craig and 2-4kin (A) >4kin (B) -0.3 salt,butcauses almost no8•80fractionation [Craig and Gordon, 1965]. [] A -0.2 B: $w= 1.68* s - 58.36 (r2 = 0.96) Gordon, 1965; Weiss et al., 1979; Jacobs et al., Fig.3. Relationships between 8]SOwater andsalinity 1985]. For deep water, mixing of water masses derived from sea ice freezing with other water massesresultsin high slopesof the8w-Srelationship in Atlanticdeepwaters(opensquares,line A) and bottomwaters(opentriangles,line B). Data from .. Ostlund et al. [ 1987a]. ZahnandMix: BenthicForaminiferal •80 andHut. 1984;Osttundet at., 1987b]. The esti- mated •f80ofprecipitation andmeltwaters inthe SouthernOcean rangesfrom about -13%oto -50%0 (SMOW) [Weiss et al., 1979; Jacobset at., 1985]. At its most negativeextreme,this approachesequation (1)'s interceptvalue of-53.18%o. The contribu- tionof180-depleted Antarctic icesheet meltwaters to •80 waterbelow4 km depth(Figure4a). This patternis similarin the westernandeasternbasinsof the North Atlantic. The isotopicoffsetbetweendeep andbottomwatersis slightlylessin the east,because theMid-AtlanticRidgeformsa topographic barrierto AABW [Metcalf et al., 1964]. dituent[Craig andGordon,1965]. We will usethe slopeof equation(1) as a starting pointto relatesalinityto f•win the glacialocean. It is importantto notethe limits in this assumption, however. Using equation (1) for the past oceans assumes thattherole of freezingandsaltrejection(as Using publisheddata setsof •w and temperature [Ostlundet al., 1987a] and applyingthe paleotemperatureequationof Erez and Luz [1983], we esti18 mate fi Ocalcite (•5c)valuesfor the deepAtlantic and Pacific oceans (Figure 4b and Table 3). In the Atlantic, the decreaseof fiwfrom NADW to AABW goesalongwith a decreasein temperature.The net result is similar tic values for Atlantic deep and bottom water masses. In the Pacific, a slight (-•0.1%o)increasein predictedfieoccursin bottom waters,due mostlyto lower temperatures. Consistentwith equilibriumpredictions,below-•2 a fractional contributionto water masses)is the same kmwaterdepth, theAtlantic coretopbenthic fi180 as at present. We relax this constraintlater in the paper, to explore the possibilityof varying slopeof the deep-seaf•w-Srelationship. valuesreach a roughly constantlevel of 3.1-3.3%o (Figure 5 andTables 1 and 3). At the LGM, benthic Antarctic Bottom Water, however, is less than 0.1% of the total [Weiss et al., 1979]. Thus the low inter- ceptof•180in equation (1)mustreflect mixing of North Atlantic Deep Water with a freeze-enhanced Antarctic end-member, rather than a true freshwater In addition, the role of intermediate waters of different •w is neglected. For example, Craig and Gordon[ 1965] notethatmodemPacificdeepwaters have slightly lower salinitiesthan expectedfrom simplemixing betweenNADW and AABW (Figure 2). They suggestthat this offset may be explained by an admixture of lower salinity intermediate waters. Birchfield [1987] disagreed. Arguing that Craig and Gordon'sAntarctic end-memberwas too oo fi180is higher everywhere. Thisreflects thecombined effects of increasedglobal ice volume and decreased watertemperature(Figure6 andTable4). The meanAtlanticincreaseof 1.7%ois largerthanthe mean Pacific increaseof 1.5%•. In Figure 6, hypotheticalverticalticprofilesfor the LGM assumethat modemwater masspatternsapply. We accountfor icevolume byshifting themodem equilibrium fi•80 profiles by +1.3%o,consistentwith recent data on sea level [Fairbanks, 1989]. Deviations of the data lowin •180,heinferred thatonlytwoend-members from theselinesreflectchangesnot directlyrelatedto are needed. The general point that intermediate storage of •80-depleted iceonland,i.e.,duetolocal waters could have had different •w-S patternsthan modem deep water, however,remains. We cannot yet quantifytheimportanceof intermediatewatersin the glacialocean,but it appearsthat they were better ventilated than now [Boyle, 1988; Oppo and Fairbanks, 1987; Zahn et al., 1987; Kallel et al., changesin temperatureor f•wof the watermass. Most of the measured benthic tic data from the LGM are significantlyheavier than would be predicted by a 1.3%oice volume effect (Figure 6). In the Atlantic, 15cvalues below 4 km approachthose predictedand are on average-0.2%0 lower than the deepwatertic mean between2 and 4 km. A cross section (Figure 7) shows that the deep water tic 1988]. Thus it is probablethat the deepwater•w-S relationshipwas different in the glacial ocean. We will illustratethe effectsof varyingf•w-Srelationships maximum on the reconstruction the Pacific, the data do not reach true bottom water of T and S from foraminiferal fi•80inthefinalsection ofthisstudy. is most obvious north of about 10øN. In depths.The latitudinalcrosssectionhintsat a deepwater maximum of benthic tic to the north of 40øN DISTRIBUTION OF BENTHIC FORAMINIFERAL b•80:MODERNAND LGM In the modemAtlantic,North AtlanticDeepWater (NADW)formsa fi180maximum between 2 and3 km depth with •w values >+0.2%0 (SMOW). Antarctic Bottom Water (AABW) is evident in low (Figure 7). At present,however, we cannot con- clude thatanystatistically significant fi•80gradients existbetweenglacialPacificdeepandbottomwaters. The Atlantic offsetbetweendeepandbottomwaters is statisticallysignificantat the 99% level (t testfor differencebetweenmeans)if thereareno systematic measurementerrors between depth intervals. The ZahnandMix: BenthicForaminifera115 •80 LATITUDE 0ø 10ø 20ø 30ø 40ø 50ø 60øN A 0o 10o 20ø 30ø 400 50ø 60ON B Fig. 4. Modemdistributions of (a) •, and(b) t•½ in theNorthAtlantic(datafromOstlundet al. [1987a];equation of ErezandLuz [1983]).NADW andAABW canbeseenin hwbutnotin hc. Thedecrease of f• is offsetby lowertemperatures atdepth. offsetbetweenAtlanticandPacificdeepwaters,with Pacific valueslower than Atlantic valuesby 0.10.2%0at the LGM, are also significantat the 99% level. Beforewe interpretthe isotopedatain terms of water masschanges,however,we mustevaluate potentialgeological andanalyticalproblems thatmay limit our conclusions. INTEGRITY OF BENTHIC•5180: BIOTURBATION AND MASS SPECTROMETER CALIBRATION A possibleexplanationfor intercoredifferencesof stratigraphicrecordsis to assumesmoothingof signalsby bioturbation[Bergerand Heath, 1968; TABLE3. Hydrography andOxygenIsotope Signalof ModemWaterMasses Depth Level, km 2-4 >4 2-4 >4 InSireTemperature, øC Salinity 15w, %0SMOW NorthAtlantic: (Geosecs Stations29, 115) 2.9 34.92 0.26 2.4 34.89 0.21 15c, %0PDBa 3.17 3.25 CentralPacific: (Geosecs Stations 306,322, 345) 1.7 1.1 34.67 34.70 0.00 -0.01 a calculated usingtheseawater-•equilibrium equation of ErezandLuz [1983]. 3.19 3.32 ZahnandMix: BenthicForaminiferal •j180 1o IMODERNI NE-ATLANTIC PACIFIC •80 (%.PDB) 6 180 (%oPDB) 1.5 2.5 3.5 1.5 0 2.5 i ß 3.5 i . i 0 1 • •' 1 © 2 []CAMBRIDGE • 2 OGIF S. YVETTE + KIEL ß LDGO 3 • 3 $OSU •x WHOI 4 4 6 6 Fig.5. Modem depth distributions ofbenthic foraminifera16180 (PDB scale, normalized to Uvigerina).Symbols identifylaboratories. Solidcurvesarepredicted equilibrium 6c(equation of ErezandLuz [1983]). Valuesfor temperature and6warefromGEOSECSstations 29 and115in theAtlanticandstations 306,322and345in thePacific[•stlundet al., 1987a]. ILASTGLACIALMAXIMUMi NE-ATLANTIC PACIFIC •5'80 (%oPDB) •5'SO (%oPDB) 3 4 5 3 4 i i 5 . i 0 [21 2 N• 000 [] CAMBRIDGE O GIF S. YVETTE + KIEL ß LDGO $ OSU o 4 a WHOI 5 6 Fig.6. Depth distributions ofbenthic foraminiferal •j180 (PDB scale, normalized toUvigerina) at theLastGlacialMaximum.Symbols identifylaboratories. Solidcurvesarethesameasin Figure 5 except increased by 1.3%o to account foricevolumechange. ZahnandMix: BenthicForaminiferal •5180 TABLE 4. Benthic•5180(%0PDB) at Northeast AtlanticandPacificCoreSites M o d ½r n 518 0 a st WaterDepth,km _Last GlacialMaximum A•j180 5180a St n LGM-Rec n (cm1000yr4) Northeast Atlantic 2-4 >4 Whole-basin mean c 3.21 (b) 0.1 15 4.99 0.2 35 1.78 3.22 3.22 0.1 0.1 5 20 4.82 4.97 0.1 0.2 5 40 1.60 1.75 2-4 > 4 Whole-basin mean c 3.41 3.14 3.39 0.1 0.1 10 1 11 0.2 0.3 0.2 29 2 31 1.46 1.50 1.47 5.8 1.9 Pacific 4.88 4.64 4.86 5.2 st,lo standarddeviation;n, numberof coresitesper depthhorizon. a Uvigerina scaleof •5•80. bWithout valueof 2.89%0 fromcoreEND066-10GGC (seeTable1). c Weightedmean. CLIMAP Project Members, 1984]. Bioturbation worksmostefficientlywheresedimentation ratesare kyr-1 (Tables1 and4). Thuswemighthypothesize low. from differentialsmoothingasa functionof sedimen- Sedimentation thatthescatter inour•180profiles maycome inpart rates at the core sites from > 2 tation rate. km water depthusedherevary between1 and 8 cm ILASTGLACIALMAXIMUM I S LATITUDE 0 10 20 30 N 40 50 60 0 4.83 [] CAMBRIDGE o GIF s. YVETTE + KIEL -10 0 10 20 30 40 50 60 0 6 ß LDGO ß OSU t, WHOI 6 !.'."2• 4.755.0o/o0 (PDB) "'::• ..... >5.0 o/o0 (PDB) Fig.7. Meridional cross sections ofbenthic foraminiferal •5•80 attheLastGlacial Maximum (PDB scale,normalizedto Uvigerina). Symbolsidentifylaboratories.Relativelylow valuesarefoundat depthsbelow 3.5-4.0 km. ZahnandMix: Benthic Foraminifera115180 12 from six isotopelaboratories (Table 1). To checkfor consistency betweenlaboratories, we averagedLGM 15180 valuesfrom the northeastAtlanticandPacific between2 and 4 km depth. In this interval thereis A numericalbioturbationmodel (similar to that of Penget al. [ 1979])predictsthatsmoothing becomes important at sedimentation ratesbelow4 cmkyr-1 (Figure 8). We assumea 10 cm mixed layer (probablyan overestimate), with diffusivityof 60 no15180 trendwithwaterdepth, andfortheAtlantic, cm2 kyr-1, andconstant downcore abundances of data are available from all six laboratories. benthicforaminifera. The modelpredictsthat core Atlantic, four of the laboratoriesgive mean values within one standard deviation (+ 0.17%o)of the mean. At the extremes,two laboratorymeansdiffer by nearly 0.4%0from each other (Table 5). The Pacificdeepwatermeanvalue (Table4) is dominated tops are affected more than glacial maximum samples. Upward mixing ofhigh-15180 glacial shells ismoreeffective thandownward mixing oflow4180 Holocene shells. This is becausedownwardmixing is limited to one mixing depth. There is no clear correlationbetweenthe model predictionsand the For the b[8 data from only twolaboratories (Table 5).The 15 O gradientbetweenthe oceans,if calculatedon with increasingsedimentationrates (Figure 8). Changes in theabundance of benthicforaminifera in glacialandinterglacial sediments wouldmodifythe the basisof datafrom thesetwo laboratories,is only 0.05%0, i.e., at the border of analytical detection. Thus, we cannot exclude the possibility that the observedintraoceanicandinteroceanicgradientsare, results shown here. Based on the available data, at least to some extent, an artifact of calibration off- however,we concludethat biomrbationis unlikely to setsbetween the laboratories. This does not prove thattrue calibrationoffsetsexist. Our comparisonis basedon relativelyfew samples,and it is not based on analysesof aliquotsof the samecarbonatesample core data. The scatter of the data does not decrease bethesolecauseof scatter in the15180 data. Anotherpotentialbiasmay resultfrom improper intercalibration of massspectrometers. We usedata ß i - i - i - i - ß - i . w - l.• [ LGM m/nu• MODERN I 0.0 0 •.• 12 2 ß i 4 - i 6 - i 8 10 12 14 16 - [] CAMBRIDGE o Gig s. YVETTE 5.0 o + E 4.5[ •o ¸ 4.0[ 35 0 • 4.5 12 4.0 .... 2 ß KIEL ALDGO ß OSU AWHOI LAST GLACIAL MAXIMUM 4 6 8 10 8 10 12 14 ............... •O 3.0 '• + • MODERN 2.5 0 2 4 6 SEDIMENTATION 12 14 16 RATES ( cm 1000yr 4) Fig. 8.Modem and Last Glacial Maximum benthic foraminiferal 15180 and glacial-interglacial 1518 O shifts atdepths > 2kmcompared tosedimentation rates. Solid curves represent numerically predicted values if bioturbated layer is10cmthick, withdiffusivity of60cm 2ky-1. ZahnandMix: BenthicForaminiferal •j180 13 Table5. MeanBenthic •j180attheLGM FromCoreSitesBetween 2 and4 kmWaterDepth Northeast Laboratory a Atlantic •518 Ob st Cambridge 4.95 0.22 6 Gif sur Yvette Kiel • OSU WHOI Mean 5.17 5.08 4.81 4.96 4.85 4.99 0.01 0.10 0.07 0.18 0.16 0.18 3 12 3 7 5 36 Pacific n Ac •5180 b st n A c -0.04 4.85 0.18 16 -0.03 4.73 4.97 4.85 4.88 0.27 0.11 0.29 0.18 2 9 2 29 +0.18 +0.11 -0.18 -0.03 -0.14 . -0.15 . +0.09 -0.03 st, 1 o standarddeviation;n, numberof datapoints. a For explanationof laboratorycodesseeTable 1. b Uvigerina scaleof •5••0 (%.PDB). c Deviation from mean. measuredat differentlaboratories.A more thorough intercalibration between the laboratories is needed before corrections to the data can be made. It appearsthat at least some of the variability observedin Figures 5, 6, and 7 comes from true differencesin the core samplesanalyzed. Given the noise level in individual measurements, our discus- sionwill emphasizeaveragevalueswithindeepwater (2-4 km) and bottomwater (> 4 km) core sites(Table 4). Because of questionsabout intercalibration, however, the difference in mean •5cbetween the glacialAtlantic andPacificdeepwatersandAtlantic deep and bottomwatersis questionable.We will interpretthe •5½ patternsconsideringtwo options:that themeasured gradients arereal,or thatthereis no gradient. (4) the global salt budgetmustbalance;and (5) any verticalprofile musthave eitherconstantor increasing densitywith increasingwaterdepth. The first and second points are physical constraints. Note that the freezingconstraintappliesto sourcewaters at the sea surface. By the time these waters mix into the deep-sea, ambient heat fluxes require that they are well abovethe freezingpoint [Mix and Pisias, 1988]. The third point is an assumption. It is not foolproof, as warm salty bottomwatersare conceivable,but it appearsto be reasonablefor the LGM. The fourthandfifth points arephysicalconstraints whichcannotbe violated,but they do not yield unique solutions. Becausewe do not have full coverageof the glacial oceans,one could always argue that the salt budget balances somewhere FORAMINIFERAL •5180IN THE OCEAN'S TEMPERATURE-SALINITY-DENSITY FIELD For the densityconstraint,stablewater massstratification may come from a wide rangeof T-S combinations. The major purposeof points4 and 5 is to eliminate unrealistic Foraminifera115•80 monitorsthecombinedvariationsof watertemperature andbw. Unfortunately,bw is not uniquelylinked to salinity. Thus, measure- outside our field of measurements. scenarios that would create an unstable water column,or make it impossibleto balancethe saltbudget. Several water masspatternsare possible. Each ments of benthic •5180 ontheirowncannot uniquely would be consistent with the vertical and horizontal reconstructbothtemperatureandsalinityin thepast. distributions of benthic•5•80at theLGM. We show The•5•80 gradients inbenthic foraminifera, however, theseon traditionaltemperature-salinity(T-S) diacan narrow the field of acceptablesolutions,if we includethe followingrequirements andassumptions: (1) temperatures of the sourcewatersmustnot fall belowthe freezingpointof seawater;(2) in situtemperaturesare usedto calculate•5c(not potentialtemperatures as there are no pressure effects on foraminiferal•5c);(3) the deepestwater massesform at high latitudesand have the lowest temperatures; grams. Also plotted are •5cequilibrium isolines on the diagrams.To do thiswe initially convertsalinity to •Swusingequation(1) and usethe seawater-calcite equilibrium equation of Erez and Luz [1983]. Densityis computedfrom the internationalequation of stateof seawater[UNESCO, 1981] andexpressed in conventionalsigma(o) units. We calculatedensity at the 4 dbarlevel (04, approximately4 km water ZahnandMix: Benthic Foraminiferal $•80 14 depth) to eliminatepressureeffectson seawater densityat depth. Note thatthediagramaxesareAT andAS, rather than T and S. We avoid absolute T-S estimates, becausethesedependon knowingthe ice volume effect well. In eachdiagram,T, S, 15c, and o4 are givenin comparison to a reference watermass.We wishto emphasize the spatialvariationsin the signals, rather than the absoluteglacial-interglacial changes.The differencecalculations arerelatively insensitive to small errors in the choice of a reference water mass. We use a hypotheticalmeanAtlantic deepwaterasthereference.ThemodemT-S values of this reference water mass are T=2.5øC S=34.9. M0DERN ,•, • o and For the glacial maximum, we set the deepwater reference temperature to 0øC[Duplessy et al., 1980;ChappellandShackleton,1986;Labeyrie et al., 1987], and salinityto 35.9 to accommodate lower glacialsealevels. PositiveAT, AS, Abc,or Ao4 numbersindicatehigherT, S, bc,or o4 values with respectto thereferencewatermass. TheAtlantic-Pacific Gradient ofBenthic i•180 at Depths2-4 Ion Pacificdeepwaterstodayarecolder(AT = -1.2), lesssaline(AS = -0.2) andslightlymoredense(Ao4 = +0.05) than deep watersof the North Atlantic (Figure9a andTable3). At thesametime,benthic 15•80 obtained fromcoretopsamples is essentially LAST GLACIAL MAXIMUM LAST GLACIAL option 1 optionMAXIMUM 2 +1 I ATLANTtC= .... +1 o I t , :--..-•.--•/. ß ,+1 Co ..•5"' ß' ["'PACIFIC.•:."'-/f.." t ' .-' PACIFIC.-'" ... ' '-' A' '"''"".'• 9."/..." B'1 -1t" -'" .?""-" 5'"' • t.."•"P" V• ..,•.5". i" ...,., .1117 '11' Ct •! - 0.2 0 +0.2 - 0.2 • +0.2 - 0.2 0 +0.2 A SALINITY EQUILIBRIUM FRACTIONATION LINES FOR•180calcite (ASA•18Ocalcit ½) ........ DELTADENSITY ISOLINES ((h) Fig.9. Temperature, salinity, andwatermass density (Ao4,dotted lines)anomalies of deep watersrelativeto NADW reference.Solidlinesshowequilibrium bcanomalies (asA&). Positive numbers forallproperties indicate higher values thantheAtlantic deepwaterreference. (a) Modem:NorthAtlanticdeepwater(solidsquare) andNorthPacificdeepwater(solidtriangle). ThePacificis colder(AT=-1.2),lesssaline(AS=-0.2),andslightlymoredense(Ao4=+0.05)than theAtlantic.The•cvaluesaresimilarin bothoceans (Abc=0).(b)LastGlacialMaximum,option 1:Benthic foraminiferal •80 inthedeep NorthPacific islowerthaninthedeepNoahAtlantic (A•c=-0.2).Opentriangle shows anomalies if ASstayed atitsmodem value(AT=-0.4,Ao4= 0.09). Opencircleshows anomalies if salinity wasthesamein AtlanticandPacificdeepwaters (AT=+0.8,Ao4=-0.13).Bothscenarios predict thatAtlanticdeepwaters weremoredense than Pacific deep waters attheLGM.(c)Last Glacial Maximum, option 2: Benthic foraminiferal •80 inthedeepNorthPacific isthesame asinthedeepNorthAtlantic (Abc=0). Solidpentagon shows anomalies if ASwasthesameastoday(AT=-0.4,Ao4=-0.03).Dashedarrowshowsshiftof Pacificwatersif theirsalinity wasthesameasin theAtlantic(openpentagon). In thiscasethe Atlantic andPacificproperty fieldswereidentical (AT=O, Ao4---0). Scenarios forLGM assume •salinityslopesameastoday. ZahnandMix: BenthicForaminiferal •j180 the samein both oceans(3.2 to 3.3%0;Tables 3 and 4). This is illustratedin Figure 9a by the •5cequilibrium fractionationline, which passesthroughboth the Pacific and Atlantic water masses. At theLGM,the•5•80excursion relativetomod- 15 bothoceanshad identicaltemperatures anddensities (openpentagonin Figure9c). Other scenariosare also possible. If inferred Pacific waterswere falling alongthe A•5cisolineof0.29'00relative to the Atlantic reference water mass, a em is largestin benthicforaminiferafrom the North Atlantic (averageof 1.8%o). This exceedschanges observedat Pacific core sitesby about0.2%0(Table 4; seealsoDuplessyet al. [1980], Duplessy[1982], and Shackletonet al. [1983]), yielding an interoceanicA•5cvalue of-0.2%0 at the LGM. This gradient may be questionabledue to inaccurateintercalibrationof massspectrometers (seeabove). Figure 9b showsthe line of acceptablesolutions for T-S of Pacific deep watersfor a 15c value 0.2%0 solutionwith deep Pacific densitiesas great as or greaterthanAtlantic densitieswould not be possible without changingthe slopeof the •5w-Srelationship as it would requiredeepPacific temperaturesbelow freezing. The inferenceof higher-densitywatersin the Atlantic at the LGM would suggesta separate source of cold and/or salty deep waters to the Atlantic. Thisproblemdisappears if the observed•5c gradient between both oceanswere an artifact of mass spectrometerintercalibration.If Pacific and lower than the Atlantic. Atlantic •5cvalues were similar, this would be more If we maintain the modem salinity contrastbetweenoceans(AS = -0.2), then the Pacific-Atlantictemperatureoffset decreasedto AT =-0.4 (opentrianglein Figure 9b). This is less than half of the modern interoceanictemperature difference. If this scenariois correct, it reversed the densitycontrastbetweenthe deepPacific and deep Atlantic, with Ate4= -0.09. The deepAtlanticwould have been denser that the Pacific, because of the extra glacial cooling of the Atlantic coupledto its highersalinity. If howeverPacific•Jcvalueswerethe same as Atlantic 15c values (Abc= 0), then Pacific waterwouldlie alongthe same•5cisolineasAtlantic waters. If the modem salinityand temperaturecon- consistent with recentinferencesfrom carbonisotope andcadmiumdataof strongersouthernoceanbottom water sources, and weaker NADW sources at the LGM [Boyle and Keigwin, 1982; Broecker,1986; Oppo and Fairbanks, 1987], and perhapsreplacement of NADW with a North Atlantic intermediate water source [Boyle, 1988]. However, we will return to this questionfollowing discussionof isotopicvariabilitywithin the North Atlanticand show that if Pacific •5cvalues were to be 0.2%0 lower than Atlantic values,a reasonablechangein the •5w-Srelationshipcanalsogive an acceptable solution. trast between both oceans were maintained, the TheVertical Distribution ofBenthic i5180 densityof PacificandAtlanticwaterswouldbe more similarthantoday(Ate4= +0.03; closedpentagonin Figure 9b), becauseof the steeper slopesof the isopycnalsunderlow, glacialtemperatures. This is not the only acceptablescenario,however. in the North Atlantic Stablewatermassstratification betweendeep(2-4 km) and bottom (>4 km) waters in the modem The measuredPacific 15c valueswould requiredeep Atlanticis dueprimarilyto a decrease of temperature with depth. As shown in Figure 10a, AT = -0.3 (bottom-deepAtlantic temperatures).This temperature decreasewould increase•Jc,but it is offsetby a slightdecreasein salinitywith depth(AS = -0.03). The net effect is a small density increase(Ate4 = water in the Pacific to be warmer than in the Atlantic +0.03)withdepthandbenthic •180essentially the (AT = +0.8, opencirclein Figure9b). Note thatthe increasein deepPacificsalinityrelativeto the mean oceanwould require lower salinitieselsewherefor the salt budgetto balance. As in the previouscase, deep Pacific water would be lessdensethan deep North Atlantic water. Here, the interoceanicdensity contrastwouldbe evenlarger(Ate4=-0.13) thanthat predictedin the first scenario.If, however,Abc= 0 and AS = 0, then AT = 0 and A(•4 = 0. That is, if therewasno 15c gradientbetweenbothoceans,andif salinitywasthe samefor PacificandAtlanticwaters, same at deep and bottom water sites in today's Atlantic(Figure10aandTable 3). In this first scenario, we assumed that the intero- ceanicsalinitygradientwasthe sameastoday. What if we assumeinsteadthatLGM salinitywasthe same in the Atlantic and Pacific Oceans (i.e., AS = 0)? At theLGM,Atlantic benthic 15•80 isincreased by 1.6-1.89'oo relativeto themodem. The verticalprof'fie in thenortheast Atlanticindicates a slight151•O decreaseof about0.2%0from deep to bottomwater sites (Figures 6 and 7, and Table 4). As noted above,this gradientcan be questioned,due to the low numberof analysesin the bottomwatersand possible problems withinterlaboratory calibration of themassspectrometers. Giventhisuncertainty, we ZahnandMix: BenthicForaminiferal 8180 l0 LAST GLACIAL MAXIMUM option 1 MODERN +1 LAST GLACIAL MAXIMUM option 2 I'''•AN¾I'C ./ .'! " ..... ATLAN+),_ '• BOTToMIC I3 ½. - 0.2 0 + 0.2 ....... DE•.WA• 1 WA ' _.-' '"'" t -0.2 .] .t •...' ...... / o'.:.ATLANTI•"q I.. -... -1 C........ .........ß ....... ....... 1 XTLANTf/:: /Lff'" / 0 1 ß .•.o....•.WAT .E• +0.2•0 0 + 0.2 A SALINITY EQummanm FRACnOSAZ•OS Lmms FOR 815OCalci• (ASA815 Ocaici•) ...... D•.LTADENSITYISOL•ES ( o4 ) Fig. 10. SameasFigure9, but for Atlanticdeepandbottomwaters. (a) Modem: North Atlantic deepwater(solidsquare)andbottomwater(solidtriangle).The bottomwatersarecolder(AT= -0.3), lesssaline(AS=-0.03) andslightlymoredense(Ac•4=+0.03)thandeepwaters. The 8cvaluesare similarat bothdepths(ASc=0).(b) LastGlacialMaximum,option1: Assumes8w-Sslope sameastoday,andno differencebetween8cin deepandbottomwaters. If the deep-bottom water salinitygradientwas the sameas at present(opentriangle),the verticaldensitygradientwould be lower (Ac•4= +0.01), dueto the lowerreferencetemperature.To maintainverticalstabilityequal to today,T and S gradientsmustincrease(opencircle,AT=-0.9, AS=-0.12, Ac•4=+0.03). (c) Last Glacial Maximum, option2: Assumesbottomwaterbenthic8cis 0.2%0lessthanin deepwater. If 8w-Sslopewasthe sameastoday,neutralstabilityrequiresbottomwatersto be colderandless salinethanoverlyingdeepwaters(AT=-2.9, AS=-0.56). Stablestratification wouldrequireeven largergradients.Alternatively,a steeper8w-Sslope(bolddashedlines)wouldallow for morereasonablehydrographicpropertiesof bottomwater (opencircle, AT=-0.9, AS=-I.2, Ac•4=+0.03), while maintaininglower 8cin bottomwater. In thiscase,PacificdeepwaterscouldhaveT-S propertiescloseto Atlanticbottomwaters. discussthe implicationsof two possiblescenarios: first one with no 8c gradientbetweenAtlantic deep today. In contrast, the well-documented benthic and bottom waters, and secondone with the 0.2%08c LGM [Curry and Lohmann, 1985; Curry et al., 1988] is generallythoughtto reflectlessactivemixing (hencegreaterdensitycontrast) betweendeepand gradientasmeasured. In the first scenario(Figure 10b), we assumethat bcis the sameat the deepandbottomwatersites. If so, the T-S gradientsbetween deep and bottom waters in the LGM North Atlantic could have been essentiallythe sameastoday(opentrianglein Figure 10b). In this scenariothe densitygradientbetween deepandbottomwaterswouldhavebeenlowerthan today (Ac•4= +0.01). This resultsfrom changesin the slopeof isopycnalsunderthe glacialconditions of lowertemperatures. This lower-density stratification, if correct,would presumablyallow for more verticalmixingbetweendeepandbottomwatersthan 813Cdecrease fromdeeptobottom watersitesatthe bottom waters in this area. To havea verticaldensitycontrast atLGM equalto today's(stillassuming no•c offsetbetweendeepand bottom water), bottom water must have been both freshet(AS = -0.12) and colder (AT = -0.8) than the deep water (open circle in Figure 10b). This is possible,givenreconstructions of strongerinfluence of southernoceanwaterin the deepglacialAtlantic [e.g. Boyle and Keigwin 1982, 1985/1986; Oppo and_ Fairbanks,1988]. It doesrequire,however,that thiswater masswas muchcolderthanat present,at ZahnandMix: BenthicForaminiferal •5•80 least-0.8øC if our assumedtemperatureof 0øC for the NorthAtlanticdeepwatertemperature at LGM is correct.This may be an acceptable scenario. In the second scenario, we take the measured benthicforaminiferal •5•80gradient of -0.2%0 betweendeepand bottomwater sitesat face value. This would requirea thermohalinestructureof the deepAtlanticoceanquitedifferentthanat present.If the •5w-Sslope was 1.5, as it is now, to maintain neutral buoyancy bottom waters must have been much colder (AT = -2.9) and fresher (AS = -0.57) than the overlying deep waters (open triangle in Figure 10c). Even larger temperatureand salinity differences would be needed to achieve the stable stratification neexled topreserve abenthic •5•80 gradient. This scenario,however,would either put bottom waters below the freezing point (which is impossible)or requiredeepwatersto approachmodem temperatures, thusrequiringan ice volumeeffect on•5•80 ofnearly 1.7%o (which violates thesealevel constraintof Fairbanks[1989]). This appearsto be an unacceptablesolution. Either the Atlantic deepbottomwater • gradientat LGM or the assumptions goinginto theT-S-o4-•cplot arewrong. A solutioncouldbe foundif the slopeof the •5w-S relationship was greater at LGM than at present. This would be possibleif the contributionof salt from seaice freezingto the deepoceanwatermasses was more important at the LGM. Source water temperaturesin today'ssouthernoceanare close to the freezing point and could not have been much colder at the LGM. A plausible way to maintain higher bottom water densitiesover the cold glacial deepwaterswould be to increasethe southernocean salt contentrelative to its •5w.More freezingwould inject more salt into the source waters, and thus increasetheir density, without changingthe end- member •Sw. Thisrelatively salty, •80-depleted bottom water, after mixing with deep waters, would increasethe slopeof the deep ocean•5w-Srelationship. If this slopedoubled,to a glacialvalue of 3.0 (dashedlines in Figure 10c), water mass stability couldoccurwith reasonableT-S propertiesin bottom waters(AT = -0.9, AS = -0.15; opencircle in Figure 10c). A steeper•5w-Srelationshipwould also solve a problemwith thepossibleAtlantic-Pacificdeepwater •5cgradientdiscussedaboveand illustratedin Figure 9. Using the modem •5w-Sslope, we inferred that Atlantic deep and bottom waterswere much more densethanPacificdeepwatersat the LGM (if the •5c offset between both oceans is real). This would 17 require different sourcesof Pacific deep water and Atlantic bottom water. If the •Sw-Sslope was 3.0 instead of 1.5, this problem goes away. Bottom watersin the Atlantic couldhavehad T-S properties similarto the deepPacific. Thesewaterswouldhave beenslightlyfresherandcolderthanthe diminished NADW sources,but more dense consistentwith their positionas bottom water in the Atlantic. A likely source for both water masses would be the Antarctic. SUMMARY AND CONCLUSIONS Craig [1965, p. 173] stated,"It is obviousthat a detailedknowledgeof Pleistocenevariationsin isotopic compositionof the oceanis very importantfor the understanding of the causes of glaciation". Broecker[1986, p. 133] repeatedthis statementand added that"unfortunately aknowledge of•80distribution in the glacial oceanis currentlybeyondour grasp". We pick up the task here, and attemptto constrain thedistribution of •5•80withinthedeep sea. A completepictureremainsbeyondour grasp, but with new datawe have madesomeprogress. We do not arrive at a uniquesolutionto the distri- bution of•5•80 intheglacial ocean. Instead, wehave usedhypotheticaldistributionsof •Swater (•Sw)in the ocean'stemperatureand salinityfields as a thinking tool to examinethe logicalconsequences of possible circulationschemesin termsof densitydistributions. Finding true relationshipsbetween•5wand S in the pastwill be very difficult. It dependson ratesand patternsof evaporation,precipitation,runoff, the ratio of seaice freezingto deepwaterformationrates, and the mixing patternsof water masseswithin the deep sea. For simplicity, we have not considered intermediate water masses. Some of these water massesat presenthave slightlydifferent•5w-Srelationshipsthandeepwaters(Figure2), sovariations in theirisotopebudgetscouldaddcomplexity. We findthatglacialmaximum •5180 values of benthicforaminifers(•5c)fromdeepAtlanticsites(2-4 km depth) are on averagehigher than thosefrom Atlanticbottomwatersites(>4 km depth)or from the deepPacific. This finding, thoughconsistentwith other studies,may in part reflectproblemsof interlaboratory calibration. It can, however, be interpretedin termsof reasonable watermasscharacteristics. Consideringthe constraintaddedby densitystratification, it appearsthat changesin the slopeof the •5w-Srelationshipare critical to making senseof ice age •5cpatterns. If we assumethat the measured•5c ZahnandMix: BenthicForaminiferal •5180 18 gradientsin the glacialoceanare real andif themodem relationshipof 15wand salinityin the deep sea appliesto thelastglacialmaximum,Atlanticdeepand bottom waters must have been more dense than those in the Pacific. This densitydistributionwould seem to requirea majorsourceof highdensityglacialdeep water in the Atlantic and a separatesourceof deep water somewhere in the Pacific. This is inconsistent with circulationpatternsinferredfromcarbonisotope distributions.If we assumethat ice age 15c was the same at Atlantic and Pacific core sitesor if the •5w-S slopewas higher than it is now, this conflict goes away. In both cases, Pacific deep waters and Atlantic bottom waters could have had a common source in the southern ocean. A weak northern sourceof glacialAtlanticdeepwaters(or mixingwith intermediatewaters) is permitted. We tentatively conclude that sea ice formation contributed more salt to the glacial deep oceanthan today. If true, this would change the salinity distribution within the ocean[BroeckerandPeng, 1987], whichhasimplicationsfor paleo-CO2and other geochemicalbudgets. 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