PALEOCEANOGRAPHY,VOL. 1, NO. 1, PAGES 43-66, MARCH 1986

LATE QUATERNARY PALEOCEANOGRAPHY OF THE TROPICAL ATLANTIC, 1: SPATIAL VARIABILITY OF ANNUAL MEAN SEA-SURFACE TEMPERATURES, 0-20,000 YEARS B.P.

Alan C. Mix

College of Oceanography, Oregon State University, Corvallis, OR 97331

William F. Ruddiman

Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964

Andrew Mcintyre

Department of Earth and Environmental Sciences,Queens College, City University, New York, NY

Abstract. At least two modesof glacial-interglacial INTRODUCTION climate changehave existedwithin the tropical Atlantic Ocean during the last 20,000 years. The first mode (defined The tropical oceansact as a solar collector and heat by cold glacial and warm interglacial conditions) occurred source for the earth's climate. Heat collected here is symmetricallynorth and south of the equator and advectedaway from the tropics in oceanic currents. Some dominatedthe easternboundary currentsand tropical of this heat is transferred to the atmospherein the middle upwelling areas. This pattern suggeststhat mode 1 is latitudes [Von der Haar and Oort, 1973] and is thus driven by a glacial modification of surface winds in both transported to the polar regions, where net radiative heat hemispheres.The secondmode of oceanicclimate change, loss occurs. defined by temperature extremescentered on the This paper examinesthermal variability of the tropical deglaciation, was hemispherically asymmetrical, with the Atlantic Ocean during the transition from the last northern tropical Atlantic relatively cold and the southern glaciation to the modern interglaciation between 20,000 tropical Atlantic relatively warm during deglaciation. A years B.P. and the present. Our approach is to study likely causefor this pattern of variation is a reduction of spatial variability of tropical Atlantic paleotemperatures the presentlynorthward cross-equatorialheat flux during estimatedfrom fossil foraminiferal assemblages.By deglaciation. No single mechanism accounts for all the determiningthe spatial effects of climate changes,we data. Potential contributorsto oceanicclimate changesare attempt to trace patterns of variability to the mechanisms linkage to high-latitude climates, modification of mon- governingtheir occurrence.Topics to be addressedinclude soonalwinds by ice sheetand/or insolation changes, (1) linkage betweennorthern and southernhemisphere atmosphericCO2 and greenhouseeffects, indirect effects oceanicclimate, (2) variations in Atlantic interhemispheric of glacial meltwater, and variations in thermohaline heat transport, and (3) relationshipsbetween low-latitude overturn of the oceans. and high-latitude climate changeson a glacial-interglacial time scale.

•Previouslyat Lamont-Doherty GeologicalObservatory, PHYSICAL OCEANOGRAPHY and Department of Geology, Columbia University, Palisades, New York. The modern seasurface temperatures of the tropical 2Alsoat Lamont-Doherty Geological Observatory, Atlantic Ocean are controlled by the winds. On an annual Palisades, New York. average, warmest temperatures (> 27øC) occur mostly in the westernequatorial Atlantic (Figure la), especiallyin Copyright 1986 the northern hemisphere[Levitus, 1982; Reynolds, 1982]. by the American GeophysicalUnion. This reflectsradiative heating of surfacewaters advected westward in the North and South Equatorial currents, the Paper number 5P0836. northern hemisphereposition of the Intertropical Conver- 0883-8305 / 86/005 P-0836510.00 genceZone (!TCZ), and northward cross-equatorialheat 4/4 Mix et al.' Tropical Atlantic Paleoceanography

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70W 50 30 10 10E Fig. 1. Modern mean annual seasurface temperature. Dotted linesmark estimatedmean annual positionsof the thermalequator. (a) Atlas values(plotted from data tablesof Levitus [1982]). (b) Averageof Augustand February temperatures,from the CLIMAP atlas values(see text) usedto simulatethe annual mean in the transferfunction calibration. (c) Foraminiferaltransfer function estimates of meanannual temperature; the coretop calibrationdata set (N. G. Kipp, personalcommunication, 1982). transport in the ocean [Hastenrath and Lamb, 1978; radiative heating of the large continental landmass in the Hastenrath, 1980]. northern hemisphere. Cool temperatures south of the Sea surface temperatures are relatively cool off equator in the Gulf of Guinea reflect a combination of northwest Africa ( < 21 øC) and in the easternAtlantic advection of cool eastern boundary current waters and south of the equator (<25øC) (Figure la). Off northwest Ekman divergencesouth of the equator responding to Africa this reflects southward advection of cool water in meridional winds [Cane, 1979; Philander, 1979] and of the Canaries Current and local "coastal" upwelling, which thermocline adjustmentsresponding to remote (western chills surfacewaters as much as 600 km offshore [Wooster Atlantic) wind forcing [Moore et al., 1978; McCreary et et al., 1976]. These effects are driven by northeasterly al., 1984]. trade winds that blow parallel to the coast [Newell et al., The asymmetrical pattern of cool sea surface tempera- 1972; Picaut et al., 1985]. tures south of the equator results in high radiative heat In the eastern equatorial Atlantic the winds are from the gain and low latent and sensibleheat loss in the South south [Newell et al., 1972; Picaut et al., 1985], driven by Equatorial Current [Hastenrath, 1980]. The mean annual Mix et al.' TropicalAtlantic Paleoceanography /45

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Fig. 1. (continued) position of the ITCZ in the northern hemisphereis tropical Atlantic sedimentsby B• et al. [1976], Gardner associated with a net flow of warm near-surface waters and Hays [1976], Prell et al. [1976] and Thiede [1977]. from the southern to the northern hemisphere in the Their resultsindicated that areas along the equator and off Guiana Current along South America. Return flow of cool northwest Africa were significantly cooler during the last water occursas North Atlantic Deep Water (NADW) flows glacial maximum than at present. They inferred that southward [Bryan, 1982; Stommel, 1980; Worthington, tropical Atlantic temperatures responded to equatorial 1976]. Through this cross-equatorialflow and thermohaline divergenceand upwelling of cool water driven by trade overturn, the Atlantic Ocean transports heat northward wind intensity. In their view, faster winds at the glacial acrossthe equator [Oort and Von der Haar, 1976; maximum reflected increasedhemispheric thermal gradi- Hastenrath, 1977]. Possible variations in these cross- ents causedby glaciation of the high-latitude continents. equatorial fluxes may have large consequencesfor climates This view of the oceansas a systemresponding elsewherethat are maintained by this redistribution of heat passivelyto external forcing by ice volume contrasts with between the hemispheres. Ruddiman and McIntyre's [1981] thermodynamic view of the Atlantic Ocean's amplifying role in the mechanics of PALEOCEANOGRAPHY glaciation. Ruddiman and Mcintyre hypothesized that a warm North Atlantic during ice growth provides the latent The Atlantic Ocean has played an extensiverole in the heat necessaryto fuel rapid glaciation, while a cold North development of paleoceanography. Early workers used the Atlantic, partially covered with sea ice, denies latent heat presenceor absenceof the tropical planktonic foraminifera (and moisture) during ice decay. G!oborotalia menardii to infer warm or cold climates in Still unansweredin Ruddiman and McIntyre's work is the tropical Atlantic [Schott, 1935; Ericson and Wollin, the question of what regulatesthe oceanic heat source that 1968]. Ruddiman[1971], however, showed that abundance acts as a glacial amplifier. One possibility they cite is that fluctuations of single speciessuch as G. menardii are not the ice sheets chill the North Atlantic with their melt optimal indicesof temperature. Emiliani [1955] used productsduring deglaciation. Chilling and fresheningof oxygen isotope analysesof planktonic foraminifera as a the high-latitude North Atlantic would reduce moisture paleothermometer. Shackleton [1967], however, demon- flux to the continentsby enhancingwater column strated that much of the oxygen isotope signal reflected stratification and seaice formation, thus reducing changingice volume on land rather than temperature. evaporation. This scenarioimplies that although the More rigorousapproaches to estimatingpaleotempera- oceansplay a role in glacial feedback, they are ultimately tures began with the work of Imbrie and Kipp [1971]. forced by glacial melt products and therefore are not Their "transfer function" approach related orthogonal independent of the ice sheets. representationsof modern foraminiferal faunas to modern Alternatively, the changing heat content of the North sea surface temperatures through regressionequations. By Atlantic may reflect variations in the oceanic heat flux applying theseequations to the downcore record of from the tropics. In this scenario the oceanscould be truly foraminiferal faunas, quantitative estimatesof paleo- active in the sensethat tropical climate changesnot forced temperatures were made. by the ice sheetsmay contribute to the mechanismsof The transfer function technique was first applied to glaciation. These two possibilitiesare addressedhere by Mix et al.: Tropical Atlantic Paleoceanography

analyzing spatial patterns of climate change in the tropical Seasonalestimates would be particularly troublesome if Atlantic during the transition from glacial to interglacial the thermal equator moved. This would have the effect of conditions. reversingthe calendar seasonsthat are related to the warm season and cold season estimates and would result in METHODS erroneous seasonalpatterns of change near the equator. The discussionin this paper focuseson the estimatesof Time Scale mean annual temperature.'This does not mean that seasonalvariability of temperature is unimportant. A studyof spatial patterns of temperature change Seasonalcontrast estimateshave considerableimplications requires accurate correlations between records. To for interpretation of the mechanismsof climatic change. considerrelationships to possibleinsolation forcing and That topic is discussedin detail by Mix [1986] and A.C. allow comparison with events on land, a time scaleis Mix et al. (manuscriptinspreparation, 1986). needed.These requirements are met by oxygenisotope Comparison of the representation of mean annual tem- analyses(an ice volume proxy) of at leastone speciesof peratures based on the average of CLIMAP atlas values foraminiferaand radiocarbondating of ---1 O-gram samples for warm seasonand cold seasontemperatures (Figure 1b) of bulk calcium carbonate. A complete discussionof the with atlas values of modern mean annual sea surface methodsused to establishchronologies is given by Mix and temperature [Levitus, 1982] (Figure la) indicatesthat the Ruddiman [1985]. transfer function calibration temperaturesapproximate mean annual temperatures well. That is, seasonalbias in Transfer Functions this representationof the annual mean is negligible. Temperature estimatesbased on application of the transfer Temperature estimatesin this paper were generated functions to the core top calibration data set of from foraminiferal speciesabundance counts using the foraminiferal abundances(Figure 1c) generallysucceed in CLIMAP transfer function approach of Imbrie and Kipp representingoceanographic features. The location of the [1971], and the foraminiferal taxonomy of Kipp [1976] and thermal equator in the northern hemisphereis reproduced, B6 [1977]. The transfer functionsused here (FA-20) as are cool temperatures off northwest Africa and in the utilized six orthogonal factor assemblagesgenerated from South Equatorial Current. quantitativeabundances of 29 speciesof planktonic foraminifera. Calibration of the temperatureequations Spatial Variability was basedon foraminiferal faunas in 357 core top samples from the North and South Atlantic, regressedagainst atlas We utilize two methods for studying spatial variability valuesfor seasonalmaximum (August in the northern of the climate response.The first is simply to plot synoptic hemisphere,February in the southernhemisphere) and maps of mean annual temperature at 2000-year intervals minimum (Februaryin the northern hemisphere,August in (interpolatedlinearly from the data and agemodels) over the southernhemisphere) sea surface temperatures the last 20,000 years. The second,empirical orthogonal [Molfino et al., 1982;N.G. Kipp, personalcommunication, functions(EOF) analysis,reduces the temperature time 1982]. The atlas values adopted by CLIMAP are from a seriesto spatially coherent end-member patterns prepublicationversion of the Levitus [1982] atlas, and will (eigenvectors)that can be combinedwith different weights be referred to here as "CLIMAP atlas values". For the to reconstructthe original data. In this way, independent transfer function calibration of seasonaltemperatures, the (orthogonal) modes of climate variability can be examined boundarybetween the hemispheresis not the geographic separately.In addition, EOF analysishas the advantage equator but the mean annual thermal equator, which in the that only the spatially coherent signal is considered. Atlantic is in the northern hemisphere. Random noise and local uncorrelated signalsthat are The transfer functions used here were tested in South irrelevant to the large-scalepatterns are excluded from the Atlantic sedimentsby Molfino et al. [1982]. The standard analysis,which effectively enhancesthe signal-to-noise error of estimatefor the FA-20 equationsis _+1.2 øC. ratio in the data. Errors induced by counting statisticsalone, basedon Two different typesof EOF analysisexist: "Traditional" replicate sampleswithin our data set, are + 0.3øC. time domain EOF and "complex" frequencydomain Methods for writing and usingthese equations are EOF. Both are standardtechniques in meteorologyand discussedextensively by Imbrie and Kipp [1971]and Kipp physicaloceanography but have been usedrarely in [1976]. paleoceanographicstudies (two examplesare the studiesby Past studies(i.e., most CLIMAP and SPECMAP Imbrie [ 1980] and Lohmann and Carlson [ 1981]). In publications)have generallyestimated summer and winter traditional EOF analysis[Lorenz, 1956; Kutzbach, 1967; temperaturesseparately. For this study we expressthe Sullivan, 1980], principal componentsare generatedfrom estimatesas mean annualtemperature (the averageof the a correlation matrix. Although this is not a requirement, a summerand winter estimates)and annual temperature correlation coefficient is usedcommonly as a similarity range(the differenceof the summerand winter estimates). index becauseit effectively normalizes each seriesto unit This is done to eliminate assumptionsabout which varianceand thus preventsa few high-amplitudesignals calendar seasons match the thermal seasons around the from dominating the analysis.With this method, EOF equator, where the definitions of winter and summer are coefficients(that is, the weightingsof the orthogonal ambiguous for past climates. componentsto describeeach data series)are expressedin Mix et at.: Tropical Atlantic Paleoceanography 4 7

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;: 'RC9-49 •.'"'-"t?-I"" ' ' ' :l 10 I: I:':"'.."." '.'"",.',• RCl3-184V25•-60J ß ß --•-!.i-:::!.::'ii2"'•i!':,-. ßt •):..'. '. ..' ..'":.:':•. '-• ß I •v3o-36 }"->--'• '•/•:','.• •.'.'"'' - ' '..'.:.:_•VI5-168x •-ß ß /V30-41_--'RC24-01 J'." :1 [':-./_'ß '' .''" .'-•'.• [] v25-59 •-e =-'•R•'24-07 r-= 2:'.::q I•""'''•":...•.•:':.-'".•..'._•":XZ'x-, ß V30-40V"2•"-.__. •-'"' V•9-14•:'.i•] r.?i'..'. ' :.' .."' :!.':..'::%v•5-56 .c•.-•s= ß .... X:-:t V" ...' ''•':'::.'.:.::'L v22-•77[] RC24-27 •:t •os ..- .:..:: i .. :'i i" v22-•8 •2-•74 . •os 70W 50 30 10 10E Fig. 2. Map of core locations (seealso Table 1). standard deviation units. The number of principal frequency domain analysis. Second, on a glacial- componentsis limited in order to retain only those interglacial time scale(thousands of years), it is unlikely describinga significant fraction of the data. Choosing the that true propagating waves exist in the surface ocean number of componentsto retain is somewhat arbitrary. In analogousto those found at shorter periods, such as the practice, this dependson the quality of map patterns interannual E1Nino/Southern Oscillation events [e.g. produced by the EOF coefficients. Rasmussenand Carpenter, 1982]. Instead, apparent For our purposes it is desirable to map EOF coefficients propagating waves at glacial-interglacial periods would in temperature variation units rather than in standard probably reflect the spatial blending of geographically deviation units. Thus the EOF coefficients are multiplied separatedresponses at the same frequency but with by the original standard deviation of each data series. With different phase. In this case, isolating the different this transform, the normalized EOF coefficients have the responsesas different time domain EOF modes as we have same properties as regressioncoefficients, so that the done here is the most useful description of the original time seriesare representedby phenomenon.

n RESULTS Yi(t) = bi + • [ai,j ß Xj(t)] + Ri(t) (1) j--1 Time Series

where Yi(t) is the ith time series, bi is the mean of the ith Temperatures estimateswere generated from 685 series,Xj(t) are the n orthogonal functions (in this casethe analysesof foraminiferal speciesabundances in 28 cores time seriesof temperature responsemodes), and ai,j is the (locationsare given in Figure 2 and Table 1). Chronologic jth EOF coefficient for the ith time series. Ri(t) is a time control was provided by 82 radiocarbon dates and over series of uncorrelated residuals for each data series. If the 1000 oxygen isotope analyses. Data from 24 of the cores number of principal components retained (n) equals the are documentedby Mix and Ruddiman [1985]. Four of the number of original time series, then by definition, Ri(t)= 0 cores used here (A179-15, RC13-184, RC24-27, and for all series, and 100ø70of the variance is explained by the V22-222) were not consideredin that paper. Age models eigenvectors.The normalized coefficients ai,j have the for thosecores are documentedin Table 2. Oxygen isotope same units as the time series(in our case, oC) and represent data not tabulated by Mix and Ruddiman [1985] are the 1-a standard deviation of each mode in each series. included here in Table 3. When plotted on a map, the coefficients define spatial The foraminiferal data are new, with the exceptions of patterns of variation for each EOF mode. M-12392 [from Thiede, 1977], V25-56 [from Be et at., The other technique, complex EOF analysis, operates in 1976], A179-15 [from Kipp, 1976], and V22-174 [from the frequency domain [Wallace and Dickinson, 1972]. The J. Imbrie and N.G. Kipp, personal communication, 1984]. output of this technique is the amplitude and phase within New foraminiferal abundance data, factor loadings, a chosenfrequency band. Thus propagating waves can be seasonaland annual temperature estimates,and age models observedthrough the phase of a single EOF component. are tabulated in the work of Mix [1986]. The resulting We use time domain EOF analysis for two reasons. time seriesof mean annual temperature over the last First, our time series are too short in relation to the 20,000 years are plotted in Figure 3. dominant (orbital) periods of variability for meaningful A listing of the temperature time series, interpolated at 48 Mix et al.' Tropical Atlantic Paleoceanography

TABLE 1. Core Locations, Depths, and SedimentationRates

Mean Depth Sedimentation Core Latitude Longitude (m) Rate, cm/ky A179-15 24ø48'N 75ø56'W 3109 9.0 EN66-10G 6ø39'N 21ø54'W 3527 1.6 M-12392 25ø10'N 16ø51'W 2575 9.7 RC9-49 11ø11'N 58ø36'W 1851 4.4 RC13-184 3ø52'N 43ø18'W 3446 3.3 RC13-189 1ø52'N 30ø00'W 3233 3.5 RC24-01 0ø34'N 13ø21'W 3850 4.4 RC24-07 1ø21'S 11ø55'W 3899 7.3 RC24-16 5ø02'S 10ø12'W 3543 4.0 RC24-27 5ø25'S 0ø22'W 3718 4.2 V15-168 0ø12'N 39ø54'W 4219 6.9 V22-38 9ø33'S 34ø15'W 3797 1.9 V22-174 10ø04'S 12ø49'W 2630 3.3 V22-177 7ø45'S 14ø37'W 3290 3.5 V22-182 0ø33'S 17ø16'W 3776 4.4 V22-222 28ø56'N 43ø39'W 3197 3.3 V23-110 17ø38'N 45ø52'W 3746 1.6 V25-56 3ø33'S 35ø14'W 3512 5.0 V25-59 1ø22'N 33ø29'W 3824 3.6 V25-60 3ø17'N 34ø50'W 3749 2.9 V25-75 8ø35'N 53ø10'W 2743 6.3 V29-144 0ø12'S 6ø03'E 2685 4.6 V30-36 5ø21'N 27ø19'W 4245 1.9 V30-40 0ø12'S 23ø09'W 3706 3.8 V30-41K 0ø13'N 23ø04'W 3874 2.3 V30-49 18ø26'N 21ø05'W 3093 4.3 V30-51K 19ø52'N 19ø55'W 3409 3.0 V32-08 34ø47'N 32ø25'W 3252 4.0

2000-year intervals, is in Table 4. Temperature estimates In severalof the time series(e.g., RC9-49, V22-222, from sampleshaving communalities of lessthan 0.7 V23-110, V30-49, and V32-08; seeFigure 3) temperature (indicating the possibilityof foraminiferal faunas with no estimatesduring deglaciation(6000 to 14,000 yearsB.P.) exactmodern analogue within the calibration data set)are are as cold or colder than those at the glacial maximum. In enclosedin parentheses.These values are suspectand additionto thesepatterns, higher-frequency variability is shouldbe treated with caution. A completetabulation of presentin some cores. Significant late Holocene cooling is communalities is given by Mix [1986]. recordedin coresA179-15, V29-144, V30-49, and V30-51. Examination of the time seriesin Figure 3 reveals similaritiesand differencesbetween the cores.Many Map Patterns recordsdisplay glacial temperatures(14,000 to 20,000 yearsB.P.) colderthan full interglacialtemperatures (0 to Synoptic map patterns (Figure 4) illustrate the spatial 6000 years B.P.). Core V15-168, from the western developmentof tropical sea surface temperatures at equatorialAtlantic, recordssea surface temperatures 2000-year intervals over the last 20,000 years. Contours warmer in glacialtime than in interglacialtime. Although were drawn by computer after gridding the data using this is the only core with this pattern in our data set, it is cubic spline interpolation. Gridding induced some consistentwith the CLIMAP [1981] reconstruction, in smoothing to the data, within the 1.2øC standard error of which five coresfrom this area estimatedglacial the temperature estimates. Final contours were drawn by temperatures warmer than at present for one or both hand where the data coveragewas not sufficient to allow seasons.Some of the valuesolder than 14,000 yearsB.P. legitimate spline interpolation between data points. In in coreV15-168, however,have relativelylow communali- Figure 4, machine-drawn contours are solid, and hand- ties (0.6 to 0.8), reflecting faunal assemblageswithout a drawn contours are dashed, indicating uncertainty of exact modernanalogue. While this doesnot necessarilymean position. that the temperature estimatesare erroneous,it does In the core top map of sitesused for downcore study suggestcaution in their interpretation. (Figure 4a), the thermal equator (> 26øC) is located Mix et al.' TropicalAtlantic Paleoceanography /49

TABLE 2. Age Models Other Than Those Documented by Mix and Ruddiman[1985]

Depth Age Core cm years Error Comment A179-15 0.0-1.0 700 + 200 14C, bulk carbonate* 49.0-50.5 4,200 + 200 14C, bulk carbonate* 94.0-99.0 7,600 + 130 '4C, bulk carbonate* 110.0-113.0 10,700 + 480 '4C, coarse fraction* 129.0-132.0 14,700 + 500 •4C, coarse fraction* 215 24,000 -- r5'80 Event 3.0

RC13-184 0 2,000 -- core top 8 6,000 -- (5•80 Event 1.1 38 14,000 -- (5•O Event 2.02 78 24,000 • (5'•O Event 3.0

RC24-27 0 1,500 • correlation of 24 6,000 -- %CaCO3 data to 54 14,000 -- cores RC24-01 100 24,000 • and RC24-07.

V22-222 0 1,500 • core top 15 6,000 -- (5•O Event 1.1 40 14,000 -- (5•O Event 2.02 80 24,000 -- (5'8180 Event 3.0 *from Ericson et al. [1961]. between 2 ø and 8øN. Cool temperatures off northwest During the period of deglaciation of the continents Africa ( < 20øC) and in the eastern equatorial Atlantic (14,000-6000 years B.P.), the thermal pattern changed (< 25øC) reflect the divergence,upwelling, and advection considerably. By 14,000-years ago (Figure 4e), tempera- of cold waters to these areas. Comparison of this core top tures in the South Equatorial Current had warmed by map (Figure 4a) with that of the transfer function 1ø-3øC, to 23ø-24øC. Northwest African and subtropical calibration (Figure 1c) indicatesthat the spatial coverage North Atlantic sites(e.g., M-12392, V22-222, V30-49, of the 28 coresused here is generally sufficient to define V30-51K, and V32-08) reachedtheir coolesttemperatures. oceanographicfeatures. Exceptions to this, where core This observation is consistent with data farther north, coverageis not sufficient, are the southeastcorner of the which indicate that temperatures in the northern subtropi- map, where the confluence of cool Benguela Current cal Atlantic reached a minimum during deglaciation waters with the South Equatorial Current is not resolved, [Ruddiman and Mcintyre, 1981]. and the northern edgeof the map (north of 30øN), where By 12,000 years B.P. (Figure 4f), the Gulf of Guinea contours in the Subtropical Gyre of the North Atlantic are may have warmed to above 25øC (warmer than its core top poorly defined. value). This estimate, however, is based on just one core. Prior to the maximum of the last ice age, 20,000-years Low communalitiesat the site of V29-144 suggestthat ago (Figure 4b), the mean annual thermal equator was in these estimates be treated with caution. Conditions the northern hemisphere, as it is today. The tropical without a modern analogue may have been presentin the Atlantic was significantly cooler than at present, with Gulf of Guinea at this time. Along the equator, cool temperatures of lessthan 22øC in the eastern equatorial temperatures(< 23øC) indicate that upwelling still region and lessthan 18øC (possiblydown to 12øC, see occurred at this time, but the confluence of cool equatorial discussion)off northwest Africa. A strong front at 15ø to waters with those of the Benguela Current may have been 20øN separatedthe cold (< 18øC) Canaries Current and weaker than at present. Relatively cool temperatures northwest African waters from the warmer subtropical and persistedoff northwest Africa, and in the North Atlantic tropical waters. Subtropical Gyre. The pattern was similar at 18,000 (Figure 4c) and 16,000 The thermal maxima in both hemispheresmake a years B.P. (Figure 4d). At thesetimes of maximum global continuous thermal equator 14,000 to 12,000 years ago glaciation, temperature estimatesalong the equator and in difficult to define (Figures4e and 4f). Relatively warm the South Equatorial Current were cooler than those to the (> 24øC) water extendedfarther eastin the South Atlantic southand east, suggestingsignificant equatorial divergence. than in the North Atlantic at these times, opposite the Throughout the period from 20,000 to 16,000 years B.P., glacial maximum pattern. The thermal equator may have the thermal equator was clearly located in the northern beensouth of the geographicequator, at least in the central hemisphere, between 0 ø and 10øN. and easternAtlantic. More data should be sought to test 50 Mix et al.: Tropical Atlantic Paleoceanography

TABLE 3. Previously unpublished 6180 data TABLE 3. (continued)

Depth, cm b• 80 Depth, cm b• 80 A179-15 G. ruber, white, 250-355 tan V22-222 G. ruber, white, 250-355/am (continued) 10.0 -2.16 42.0 0.20 20.0 -2.09 44.0 0.14 30.0 -2.01 46.0 0.35 40.0 -2.02 48.0 0.24 50.0 -2.24 50.0 0.15 60.0 -1.98 60.0 -0.14 70.0 -2.06 70.0 0.16 80.0 -1.95 80.0 -0.11 90.0 -1.83 90.0 -0.05 100.0 -1.60 110.0 -1.20 120.0 -0.52 130.0 -0.55 140.0 -0.20 this possibility. It is not clear from the present data 150.0 -0.34 whether a southernposition of the thermal equator 160.0 -0.16 persistedthroughout the year, or if large seasonal 170.0 -0.59 variations in its position occurredduring deglaciation. It is 180.0 -0.52 also unknown whether or not the atmospheric Intertropi- 190.0 -0.56 cal ConvergenceZone was closely associatedwith the 200.0 -0.55 oceanicthermal equator, as it is today. 210.0 -0.63 As at 14,000 and 12,000 years B.P., the position of the 220.0 -0.92 thermal equator 10,000 years ago (Figure 4g), was ambiguous. The Gulf of Guinea warmed to above 26øC RC 13-184 G. sacculifer, 355-4 15/am (again, basedon a no-analoguefauna in core V29-144). 0.0 -1.43 The equatorial divergencewas weaker than at 12,000 years 5.0 -1.52 B.P. Temperature estimatesremained very cool (< 15øC) 10.0 -0.80 off northwest Africa. 15.0 -0.65 By 8000 years ago, near the end of deglaciation (Figure 20.0 -0.38 4h), developmentof warm (> 26øC) temperaturesin the 25.0 -0.25 western Atlantic between 2 ø and 8øN indicates that an 35.0 -0.41 unambiguous thermal equator was again beginning to 40.0 0.03 form in the north. Warm water (> 25øC) persistedsouth of 45.0 -0.02 the equator and in the Gulf of Guinea. Cooler 50.0 -0.09 temperatures(< 24øC) along the equator at 10øW indicate 55.0 0.07 that equatorial divergencewas presentbut weak. 65.0 0.13 Persistenceof relatively cool water ( < 16øC) off northwest 70.0 -0.04 Africa indicates that advection of cool water in the 75.0 0.02 Canaries Current and regional upwelling continued to be 80.0 -0.32 strongerthan at present but weaker than at the glacial 85.0 -0.26 maximum. 90.0 -0.28 By middle Holocene time, 6000 years ago (Figure 4i), the thermal equator was clearly in the northern hemisphere, V22-222 G. ruber, white, 250-355/am just north of the geographic equator. Temperatures 0.0 -0.87 peaked at 26ø-27øC in the Gulf of Guinea, indicating that 10.0 -1.27 the thermal equator may have remained close to the 20.0 -0.59 equator in the eastern Atlantic. Equatorial divergencewas 22.0 -0.26 minimal at this time. 24.0 -0.17 Since the middle Holocene, temperature estimates from 26.0 -0.22 core V29-144 in the Gulf of Guinea have cooled by about 28.0 -0.14 2øC, while the waters off northwest Africa have continued 30.0 -0.07 to warm (4000 years B.P., Figure 4j; 2000 years B.P., 32.0 -0.40 Figure 4k). The position of the thermal equator in the 34.0 -0.40 northern hemisphere became better defined as the eastern 36.0 -0.29 limb swung northward and the linkage between the South 40.0 0.29 Equatorial Current and the southern hemisphere Benguela Mix et al.' Tropical Atlantic Paleoceanography

o. •. • o

• o

o

.

I

O O O O O O O O O O O O AGE(ky BP) AGE(ky BP) AGE(ky BP) AGE(ky BP) •. 3. •sfimateso• meanannual s•a su•ace tempem[u•e,in dc•ees •dsius, plotted•e•sus a•c.

(easternboundary) Current returned. By 2000 years B.P. data set (Figure 5). For comparison, Figure 5a is a (Figure 4k) the thermal pattern approached that of today compositeb•80 (ice volume proxy) record created from the (Figure 4a). same cores [Mix and Ruddiman, 1985]. The dominant EOF mode 1 (Figure 5b) explains65;% of Empirical Orthogonal Functions the't•olalvariance. The apparentwavelength of about 20,000-year is consistentwith a longer time seriesfrom The above description of the synoptic temperature maps tropicalAtlantic coreV30•40 that is dominatedby the showsthat the tropical Atlantic has experienceda 23,000-year precessioncycle (A. Mcintyre et al., complicatedclimate history that may be simplified through unpublisheddata, 1986).Positive EOF coefficientsfor this EOF analysis. This analysis isolates two modes of climate mode reflect cold temperaturesduring glacial time and variability that account for 77% of the variance within the warm temperaturesduring interglacial time. 52 Mix et al.: Tropical Atlantic Paleoceanography

TABLE 4. Mean Annual Sea Surface Temperature Estimates AGE, 1000 years B.P. Core TOP 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0

A179-15 25.1 25.1 25.3 25.5 25.2 24.9 24.6 24.5 24.4 24.5 24.3 EN66-10 26.2 26.2 25.7 25.7 24.5 23.4 23.3 23.3 23.0 23.8 24.7 M-12392 19.8 19.6 19.8 18.3 14.6 14.2 15.5 11.4 12.9 13.4 11.6 RC9-49 26.1 26.1 26.7 26.1 25.6 25.9 25.7 25.4 25.8 25.6 25.3 RC13-184 26.5 26.5 26.7 27.2 25.7 26.0 26.1 26.1 26.3 26.0 25.5 RC13-189 26.2 26.3 26.4 26.1 26.1 25.3 24.6 24.3 24.6 23.7 24.3 RC24-01 24.5 24.6 24.9 24.5 23.8 22.5 22.8 22.7 22.0 21.7 (22.0) RC24-07 24.9 25.1 24.6 24.8 23.4 23.2 23.3 (23.8) (23.8) 20.3 21.1 RC24-16 25.2 25.2 25.1 24.7 25.0 24.9 24.3 23.2 22.0 21.9 23.1 RC24-27 24.9 23.4 24.7 25.6 24.9 23.0 23.4 23.2 23.7 22.7 21.2 V15-168 26.1 26.1 26.2 26.1 25.5 26.0 26.8 27.2 (27.6) (27.6) (27.4) V22-38 25.3 25.7 25.5 25.5 25.8 25.8 25.6 25.4 25.5 25.6 25.4 V22-174 25.5 25.5 25.7 25.5 25.0 25.1 25.1 24.1 23.0 22.6 22.0 V22-177 25.4 25.4 24.8 24.8 25.3 24.2 23.3 22.1 22.5 22.0 21.9 V22-182 25.1 25.1 24.9 24.4 24.1 23.2 22.7 22.1 (22.1) (22.5) 22.6 V22-222 23.4 23.7 24.0 21.9 22.3 21.0 22.0 20.7 21.4 22.3 21.7 V23-110 25.3 25.2 25.2 25.5 25.1 25.1 24.9 25.0 25.2 25.4 25.8 V25-56 25.9 25.9 26.2 26.0 26.0 25.6 25.3 26.1 25.6 25.5 25.6 V25-59 26.4 26.3 26.4 26.3 25.7 25.5 25.1 25.2 25.0 25.8 25.4 V25-60 26.5 26.4 26.1 26.2 26.0 25.3 25.5 25.6 25.6 25.7 25.3 V25-75 26.2 26.4 26.2 26.3 26.0 25.9 26.2 25.7 26.6 25.8 25.5 V29-144 24.6 (25.4) (26.5) (27.0) (26.2) (26.4) 25.5 (23.2) (22.8) 22.3 22.0 V30-36 27.4 27.2 27.3 27.0 27.3 26.7 26.1 25.7 26.5 26.3 26.4 V30-40 25.4 25.6 25.3 25.1 24.1 23.6 22.6 (22.3) (22.2) 21.1 23.5 V30-41K 24.7 25.5 25.3 25.6 25.1 24.2 22.4 22.2 (22.3) (19.9) (21.5) V30-49 21.2 21.5 22.5 21.9 20.8 19.4 19.1 17.2 20.8 (20.4) (21.6) V30-51K 19.2 19.8 20.0 18.0 18.0 17.5 15.0 14.9 15.4 15.8 15.5 V32-08 21.2 21.2 20.8 20.6 20.2 20.0 20.7 19.9 20.1 20.3 20.3 Values in parentheseshave communalities of lessthan 0.7, and should be treated with caution.

In map pattern (Figure 6a), mode 1 is strong in the DISCUSSION eastern portions of both the northern and southern hemispheres.Its highestamplitude occursin the northern Accuracy of the Time Scales hemisphere off northwest Africa, with a coefficient of 2.7 in core M-12392 (Table 5). Relatively high amplitudesalso A danger in the use of EOF analysis is that errors in the occur near the equator eastof 25øW (EOF coefficientsof time scalesfor individual corescould produce artificial 1.0 to 1.7) and in the South Equatorial Current (EOF phase shifts between time series. In our data, erroneous coefficientsof 1.0 to 1.3). Smaller EOF coefficientsin the phaseshifts would produce a secondresponse mode, which westerntropical Atlantic indicate diminishedamplitudes would appear as a "spike" centeredon the deglacial of mode 1 away from the sensitiveupwelling and eastern transition, similar in appearance to our mode 2. Two tests boundary current areas. demonstrate that time scale errors are not the source of our EOF mode 2 (Figure 5c) explains 12070of the total mode 2. variance. Positivecoefficients indicate cool temperatures First, it is unlikely that time scaleerrors would have a during deglaciation(14,000 to 6,000 yearsB.P.), and systematicmap pattern. If mode 2 were produced solely by relativelywarm conditionsduring the last glacial time scaleerrors, a random map pattern of strengthwould maximum (> 14,000 yearsB.P.), and at present.In map be expected. Figure 6b illustrates clearly that this is not the pattern(Figure 6b), mode2 revealshemispheric asymmetry. case. It could be argued, however, that regional variations Its valuesare generallypositive in the northern hemisphere in radiocarbon dates could causespatially coherent time and negativein the southernhemisphere. This reflects scaleerrors. The map pattern alone supportsbut doesnot relatively cool conditions in the North Atlantic and warm prove the reality of the correlations. conditionsin the SouthAtlantic during deglaciation. Second,the reliability of the chronologieswas tested by Eastward extensionof positive (northern hemisphere) performing EOF analysison oxygenisotope data from the valuesoccurs along the equator betweennegative same cores (excepting RC24-27, which has not yet been (southernhemisphere) values to both the north and south. analyzedfor •i180). All •i180data usedin this analysisare Mix et al.: TropicalAtlantic Paleoceanography 5 3 tabulated in the work of Mix and Ruddiman [ 1985] or in within the euphotic zone, speciesthat prefer very cold Table 3 of this paper. As theseb•80 data are dominated by conditionsproliferate below warm surfacewaters [Fairbanks the effectsof changingice volume, little spatial variability and Wiebe, 1980]. If tropical upwelling were significantly should be detected if the chronologiesare correct. This is greater in the past than within the calibration data set, the the case.For b'80, the first (glacial-interglacial) EOF mode transfer function may estimate erroneous temperatures. accountsfor 94ø7oof the pooled variance. A secondEOF Proliferation of cold water forms due to a rise in the mode was detected in the •80 data (as a noisy deglacial thermocline could yield estimates that are too cold. spike), but it accountsfor lessthan 2ø7oof the variance. Calibration of the equations to modern sea surface This is not distinguishablefrom noise. Further, the map temperatures that are warmer than temperatures actually pattern of this •80 mode 2 appearsto be random. This controlling faunal distributions, however, may desensitize result is striking, consideringthat the time scaleswere the equationsto the presenceof cooler faunas. Although basedlargely on radiocarbon data and not on correlation the magnitudeof changerecorded by a transfer function of oxygenisotope records [Mix and Ruddiman, 1985]. could be wrong, the senseof change should be correct, and Thus the existenceof two responsemodes of sea surface useful interpretations can still be made. Excepting the few temperature variation is not an artifact of miscorrelation. samplesnoted in Table 4, communalities are high enough The two modesrepresent spatial variability of surface- in our data set (greater than 0.7, typically 0.9) to suggest ocean climate changeson a glacial-interglacial scale. that the transfer functions are operating within their range of calibration. Accuracy of the Sea Surface Temperature Estimates Mode 1

The transfer function technique assumesthat foraminif- The similarity of the mode 1 temperature responseto the eral faunas are related to an unspecifiedcombination of oxygenisotope record of ice volume (Figure 5a and 5b) oceanic parameters that is linearly related to sea surface suggeststhat tropical Atlantic temperatures respond temperature [Imbrie and Kipp, 1971]. This linear (directly or indirectly) to the presenceof ice on the relationshipmust remain constantthrough time for the continents. We can exclude the possibility that variations paleotemperatureestimates to be correct. In other words, in the strength of mode 1 are artifacts of dissolution or the transfer function equations estimate conditions well bioturbation effects, becausethere is no clear relationship when interpolating within the range of their calibration between mode 1 coefficients and water depth (Figure 7a) data set but are suspectif they are forced (by no-analogue and/or sedimentation rate (Figure 7b). faunas) to extrapolate too far outside of their calibration The strong concentration of mode 1 in the eastern data set. boundary currents and upwelling regions (Figure 6a) Rind and Peteet [1985] suggestthat transfer function implies that it reflects glacial-interglacial changesin temperature estimatesfor the last glacial maximum surface winds. With greater wind intensity, more cold [CLIMAP, 1976; 1981] are too warm in the tropics to be water was advected to the tropics in the northern consistentwith data from the continents. The oxygen hemisphereCanaries and the southern hemisphere isotopedata, however, may argue that some of our glacial Benguela Currents. Enhanced upwelling of cold water may maximum temperature estimatesare too cold. The have occurred near the equator and along the continental temperature variations we have estimated are not margin of Africa. A likely explanation of cool tempera- detectablein •'80 data from planktonic foraminifera in the tures or shallow thermocline in these areas is enhanced samesamples (see discussion by Mix and Ruddiman divergenceand upwelling induced by stronger trade winds [1985]). This may reflect seasonaland/or depth habitat (also suggestedby Gardner and Hays [1976] and Prell et al. shifts of foraminifera to maintain preferred temperatures, [1976]). or it may mean that the transfer functions overestimate The relative warmth at the equator off South America in glacial-interglacialtemperature changes. glacial time, although partially without a modern For example, temperature estimatesof lessthan 20øC analogue,could also be consistentwith strongertrade off northwest Africa for the last glacial maximum are winds. In the modern ocean, increased wind stressalong within the range of the calibration data set for subtropical the equator during southernhemisphere winter enhances and subpolar waters but outside the range of modern advection in the South Equatorial Current and thus lifts calibration for tropical upwelling systems.Glacial the thermocline in the east, yielding cool temperatures in maximum temperature estimatesas low as 12øC in this the Gulf of Guinea. Water "piles up" in the western area (core M-12392) are below the temperatures of equatorial Atlantic, however, depressingthe thermocline subthermocline source waters that upwell off northwest in the west and yielding warm temperatures off South Africa today [Wooster et al., 1978]. If the estimated 12øC America [Merle, 1980]. This pattern from the seasonal seasurface temperatures are correct, they imply either a cyclemay also apply to the longer time scalesstudied here. sourcefor upwelled waters deeper (and therefore cooler) Strongerglacial trade winds could have enhancedthe than at present or considerable cooling of glacial "hinging" effect of the equatorial thermocline, forcing subthermocline waters. cooler temperatures in the east and warmer temperatures In tropical upwelling areas, where the thermocline lies in the west. 54 Mix et at.: Tropical Atlantic Paleoceanography

a 70W 50 30 10 10E 30N;"K...... • ß • '•...... • •'k'%...... "•'O [] ß '-'1' ..... "• os ...... - ",:4':'.'"."':•.":":"!".'".'q.UE"?'"''.....:.... "" . ' ...'.:...''":"-•30 N ,, [] , ,::•".'.' . : • •25 ,,• m:'.'...'.:: .''A.ß NNUAL MEAN: - [. •--" '•, • -•_•'5/i•';: •-EMA- PER SURFA TURE! ACE'" --'2 6 [] :.i'::.: ::.:.,'-.-'..' .' ".'- ...... i ...... ":."-';':':"::"''" "L: :,::...' = [] _-- . ..'.:: [] []

10S 10S ...... ß'-.;'. •' , , ,• f-.:t 70W 50 30 10 10E

b 70W 50 30 10 10E

30N • ,... i ?' I,,7.I / . )/:'-'.''. 30 N x•,•'•o,,.,,.,• ,,, o,• )'• il•'•'.•x•...'-.\-'•o •• /.•:'..::.• !;!."!' ' .. 'i,[] •m'?.•..'" ' [-;- [ ^-s u, ^c [ '"" • ".".':'..:-i:. •,..': ' . ' - I 0 = ...... '..... '-i";-'.'" :.'.'.: .'..".'.' ß I 0 ?.6 ß -"'m...... i• / '':.".':.•' ;.::'::"'"....::::':' • ..o:.:' ...' ß1• [] [] '?."'......

105 'lOS

70W 50 30 10 10E Fig. 4. Synopticmaps of estimatedsea surface temperature, in degreesCelsius, interpolated from downcore time series.Dotted lines mark estimatedannual positionsof the thermal equator. (a) Core tops, (b) 20,000 yearsB.P., (c) 18,000 yearsB.P., (d) 16,000 yearsB.P., (e) 14,000 yearsB.P., (f) 12,000 yearsB.P., (g) 10,000 years B.P., (h) 8000 years B.P., (i) 6000 years B.P., (j) 4000 years B.P., (k) 2000 years B.P.

Glacial trade windsalso may have changeddirections mostly meridional to mostly zonal over the eastern relative to the modern winds. At present, winds over the equatorialAtlantic could producesubstantial equatorial easternequatorial Atlantic are from the south [Newell et cooling[Cane, 1979;Philander and Pacanowski,1981]. al., 1972;Picaut et al., 1985]. The relative lack of easterly This interpretation of modified glacial trade winds is winds in this area precludesa significant contribution of consistentwith proxy data from eolian sediments. Grain Ekman divergenceto cool temperaturesalong the equator sizesof silt blown from the Sahara are greater in glacial [Cane, 1979; Philander and Pacanowski, 1981]. The time than in interglacial time, indicating more vigorous location of an isolatedband of coolesttemperatures winds [Parkin, 1974; Sarnthein et at., 1981] and/or source (< 22øC) at the glacial maximum 18,000 yearsago (Figure area aridity [Sarnthein, 1978; Pokras and Mix, 1985]. The 4c) and the high amplitude of mode 1 (Figure 6a) along the eolian data, however, are most sensitive to middle equator may point to equatorial Ekman divergence,driven tropospheric winds, not surface winds, so a direct by increasedzonal wind stress,as a mechanism to cool the comparison of the two data setsmay not be appropriate. It seasurface. This mechanismneed not require an increase is not yet clear from the eolian data whether or not wind in total wind stress.A changein wind direction from velocity increasedin the southern hemisphere. Our Mix et al.' TropicalAtlantic Paleoceanography 55

C 70w 50 30 10 10E

30N [' • %.• x , ' ,'--:, ,,.,',.,, / , ,•..:•":'. '.'.'."-.' . ß '. ''..'.:;. ..:•o,,,. •.m \\\ ! I •1I []",::"'"' .'.::.'..ß' :'

lO ' :'.:'.::-'.'.:'- ' .: i.' .. ' : •o [] ...... ".}.".: '.L :...'.'.''-'..' ': ...... _-.=• •...i ...... •..-•.!...::! ?.."f...- ..._. _ -.•_ [] '.::::'J

108 10S

70W ,50 3,0 10 10E d 7ow ,50 3,0 lO lOE :"F-'...... g...... r"'"l"g 3ON[- •,,,. '"'7%'•"•-- •""'\ • ; • '"""fl ):i'"_"""'' ' :''.".•30N [:..... e½._ '•. %. \ \ •\ ,,,,'t %• ;'.'..•/..'-.'." ,'.''' ' ' -: •'[] 25 x\\ %I[]":':'"'"::'.i' ' : [• ' _ • -'""'-•••"•'i.:,': 1'T.E M P E R A T U R E i ,• '.".':".'.:.::.i;;.'.'i. ßi ,o •...... [] • ' o-..,.: 27" [] - -- X ""- ß . , :.-:.'3:"..'L...ß.-" [] i• [] <••:.'.:: '":'.. -_'! lOS"'"' ß. ..v..:..'..,. [] m ':!11lOS

70W 50 30 10 10E

e 70W 50 30 10 10E

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m ".:.':'..:-::. :...': ' ß ' ß I 0 m ".:•::-'..':.: .'.,.:.,'..".'.' 10 26 ....'• ...... '•...... '::.i.::::.::'i::": .....:!:,'.' ' :...... [] '[] ß . 23•....•-:•:: ...... [] im [] [] [] ....?o ooOø'o..o ß'•'.' ' 14 BP,000 ' ' ':.':.':..:),'•.... •,, ..... 10S

70W 50 30 10 10E Fig. 4. (continued) 56 Mix et al.: Tropical Atlantic Paleoceanography

f 70 w 50 30 10 10E

30N ';'•'."'.' .' ' ' '".':"::30N

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, ß 'ANNUAL MEAN X• •• IIIi'.:".'.'"S.'.?'." EA- SURFACE' ', +EMp.ERATURE I 10 ....•'•__•, • ••.'-•:'..•-;.-.:... : '. '.. : 10 :.....,...:•,::...... ,•..•' '"•' •••i.'-.'-...:....:•.:-::.-'•::...:...... I'"?"'?'-=A...... ,•..'::. ': ;:.:'.'. ß ß ß :.L;"x• ß J -L•.• / ....,t•"'-•.:'.":i.; :•i-:.i•'.'- ."•:•:•/.-:.':'.:."-.-..•,---? LL -, ...... •;" •,•.•:• ß . - 000 BP ' "..'-.' ...... •':" ' 8 ' '. '..' '-":"':"."...... 25 [] ...-"' • '.:10S 10S ...... : ...... ?.. • •

70W 50 30 10 10E Fig. 4. (continued) Mix et al.' Tropical Atlantic Paleoceanography 5 7

k

30N

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Fig. 4. (continued) 58 Mix et al.: Tropical Atlantic Paleoceanography

b180 MODE I MODE 2 2.0 0.0 -1.5 1.5 -2.0 2.0

C

o10 o

• 80 Fig. 5. EOF modes of sea surface temperature change. (a) Composite •80 record (ice volume proxy) from the same cores [Mix and Ruddiman, 1985], with interglaciation from 0 to 6000 years B.P., deglaciation from 6000 to 14,000years B.P., and glacialmaximum before 14,000years B.P. (b) Sea surfacetemperature mode 1, which has cold glacial and warm interglacial conditions, accountsfor 65 ø70of the total variance. (c) Sea surface temperature mode 2, which has temperature extremescentered on the deglaciation, accounts for 12ø70 of the total variance.

inference of hemispheric symmetry of trade wind changes et al. [1982]. Seasonal contrast estimates do, however, is based on the presenceof cool sea surface temperature point to the possibility that other mechanismsshould be estimatesin both hemispheres. considered. But what causedthe glacial change in tropical winds in Climate changesin the northern hemisphere could have both hemispheres?Trade wind velocities are generally influenced wind directions over the South Equatorial consideredto be a function of the thermal gradient within Current and thus affected sea surface temperatures in the the hemisphereof the wind system [e.g., Krauss, 1977; southern hemispheretropics. The present southerly winds Nicholson and Flohn, 1981]. Model results show that over the eastern equatorial Atlantic south of the equator albedo and altitude effects of the northern-hemisphereice reflect pressuregradients induced by radiative heating of sheetssignificantly increasedthe velocity of the northern the large African landmass north of the Equator [e.g., trades only [Hansen et al., 1984; Manabe and Broccoli, Nicholson and Flohn, 1981]. The presenceof northern 1984]. hemisphereice cover in the high latitudes may have In the southern hemisphere, changesin the volume of reducedthis meridional monsoonal effect [Manabe and the Antarctic ice sheetswere relatively small [Stuiver et al., Hahn, 1977; Kutzbach and Guetter, 1984] and thus 1981]. Antarctic sea ice, however, may have doubled its modified wind directions over the easternequatorial area and reduced its summer meltback [Hays, 1978]. This Atlantic. By this mechanism, surface winds over the Gulf could have resulted in steeperhemispheric thermal of Guinea may have become more zonal at the glacial gradients and thus stronger southern trade winds over a maximum, thus contributing to local cooling at the longer period of the year. It is not clear at present how equator via Ekman divergence. It is not clear, however if sensitive the southern trades are to sea ice boundaries in this northern hemisphere monsoonal effect could reach as the Antarctic. It seemspossible, however, that glacial sea far as 10øS,as is required by the map pattern in Figure 6a. ice increasein the southern oceanscould provide a Further, monsoonal climate effects on land were more a mechanism for enhancing southern hemispheretempera- deglacial/icegrowth phenomenonthan a glacial/interglacial ture gradients, as is needed to explain the map pattern for phenomenon [Rossignol-Strick, 1983; Street-Perrott and mode 1. Harrison, 1984; Pokras and Mix, 1985]. This hypothesisof linkage to the high southern latitudes, If linkage to Antarctic sea ice variations is needed to however, is not supported by estimatesof seasonal explain the cooler glacial maximum sea surface tempera- contrast [Mix, 1986; and A.C. Mix et al., manuscript in tures in the South Equatorial Current, what caused the preparation, 1986]. Foraminiferal faunas estimate higher linkage between the northern hemisphere ice sheetsand seasonalcontrast in the eastern equatorial Atlantic at the southern hemisphere sea ice? Two possible mechanisms, last glacial maximum (7ø-9øC) than at present (2ø-5øC), suggestedby Broecker [1984], are (1) changing CO2 opposite the pattern inferred for Antarctic sea ice [Hays, content of the atmosphere from about 200 ppm in glacial 1978]. These estimatesdo not eliminate linkage to the high time to 300 ppm in interglacial time [Berner et al., 1978] latitudes as a mechanism to cool the tropical South and (2) reduced deep ocean heat and salt flux to the Atlantic on an annual average during glaciation. The southern oceans during glacial maxima. Antarctic reconstructionshave been questioned by Burckle Manabe and Wetheraid [1980] suggeste4that CO2 Mix et al ßTropical Atlantic Paleoceanography 59

a 70w 50 30 10 10E

.5 1.0-==1 30N ß ß 30N • . .

..

ß .

'ANNUAL MEAN

.. MODE 1

ß

ß ...... 10 ß . .. 10

ß

ß .

ß

ß

ß

10S 10S .5

70W 50 30 10 10E

70W 50 30 10 10E

30N

ß , ß , ß '. ßANNUAL MEAN ,.. ', '. MODE 2 ß

ß 10 ß .

,

ß

10S ' '.'..'." 10S

ß

70W 50 30 10 10E

Fig. 6. Map patterns of the EOF coefficientsßValues mapped indicate the standard deviation of variation for each mode at a site. (a) Mode 1 (cold glacial and warm interglacial temperatures)is strongestin the eastern boundary and tropical upwelling areasof both hemispheres.(b) Mode 2 is oppositein sign in the northern and southernhemispheres. Positive values indicate relatively cold conditions,and negativevalues indicate relatively warm conditions during deglaciation.

effects on climate would be strongestin the northern 1ø-2øC/a times EOF range of 3a; Table 5, Figure 5b). hemisphere, but significant in the Antarctic as well, Thus if the model studies are correct and if our becauseof polar amplification related to changing sea ice temperature estimates are correct, other factors may extent. Bryan et al. [1982] calculated that a quadrupling of contribute to the glacial cooling of the tropics. present atmospheric CO2 would increase average tropical The casefor changing deep water flux from the northern sea-surfacetemperatures by 1ø-2ø(2. A more recent study hemisphereto the southern hemisphere calls for reduced by Schlesingeret al. [1985] gives a similar value, and North Atlantic Deep Water (NADW) flux during glacial suggeststhat a doubling of present atmospheric CO2 would time. If NADW did not form in glacial time, its present induce sea-surfacetemperature changesof lessthan 2ø(2 in heat and salt flux to the southern oceans [Gordon, 1975] the tropical Atlantic. would have been removed. Weyl [1968] speculatedthat For the ice-agecase of reducedpCO2 (--- 200 ppm) Manabe removal of these fluxes would enhance water column and Bryan [1985] suggestmean tropical sea surface stability, increasesea ice extent, and thus strengthen temperature changesof < 2øC. Typical ranges for our southernhemisphere thermal gradients. Martinson [personal mode 1, however, are 3ø-6ø(2 (1-t• EOF coefficients of communication, 1985], however, suggestsexactly the 60 Mix et al.: Tropical Atlantic Paleoceanography

TABLE 5. EOF Coefficients

Mean EOF Coefficient Temperature Variance Core Mode 1 Mode 2 øC Explained, ø7o A179-15 0.4 -0.1 24.9 88 EN66-10 1.0 0.5 24.4 88 M-12392 2.7 0.3 15.1 88 RC9-49 0.4 0.2 25.7 64 RC13-184 0.3 0.0 26.3 56 RC13-189 0.9 -0.1 25.1 93 RC24-01 1.1 0.0 23.1 89 RC24-07 1.0 -0.5 23.5 50 RC24-16 1.0 -0.5 23.9 87 RC24-27 1.1 -0.5 23.7 77 V15-168 -0.6 0.3 26.6 73 V22-38 0.0 -0.1 25.6 40 V22-174 1.0 -0.6 24.4 93 V22-177 1.3 -0.3 23.7 95 V22-182 1.0 0.1 23.4 95 V22-222 0.7 0.5 22.1 78 V23-110 0.0 0.2 25.2 63 V25-56 0.2 0.0 25.8 45 V25o59 0.4 0.2 25.7 88 V25-60 0.3 0.1 25.8 82 V25-75 0.2 0.0 26.1 28 V29-144 1.6 -0.7 24.6 88 V30-36 0.5 0.2 26.7 81 V30-40 1.3 0.0 23.6 89 V30-41 1.7 -0.6 23.5 95 V30-49 1.0 0.8 20.6 80 V30-51 1.7 0.2 17.0 89 V32-08 0.3 0.1 20.3 70

oppositeeffect, that removal of the salt flux would flux uncertain, the sign of the effect is also unknown. In diminish Antarctic stability, and thus inhibit sea ice any case,chemical and isotopic data from benthic formation. foraminifera have shown that the North Atlantic Rigorous testsof deep water hypotheseshave not yet continuedto export deepwater (perhapswith some been made with ocean-climate models. Thus not only is the reduction of rate) during the glacial maximum [Boyle and magnitudeof the climatic effect of changingdeep water Keigwin, 1982; Mix and Fairbanks, 1985].

3.0 L 73.0 b o

0 0 oo Cbo 0 1.0 o o 0 o o 0 •0 0 0

o 0 -1.0 ; ß ß ß ß ß ß i I -1.0 1500 3000 4500 1.0 4.0 7.0 10.0 WATER DEPTH (meters) SED. RATE (cm/ky) Fig. 7. Comparisonof mean annual temperaturemode 1 coefficientsto (a) water depth and (b) mean sedi- mentation rate (over the period 0-24,000 yearsB.P.). Lack of any significantrelationship precludes sig- nificant contributions of carbonate dissolution and/or bioturbation to mode 1. Mix et al.: Tropical Atlantic Paleoceanography 6 !

1.0 1.0 a 0 b o o o o o o o oø o o • 0.0 o o.o o o o

o o o

-1 .o -1.0 i - 1500 3000 4500 1.0 4.0 7.0 lO.O WATER DEPTH (meters) SED. RATE (cm/ky) Fig. 8. Comparison of mean annual temperature mode 2 coefficients to (a) water depth and (b) mean sedi- mentation rate (over the period 0-24,000 yearsB.P.). Lack of any significantrelationship precludes signifi- cant contributions of carbonate dissolution and/or bioturbation to mode 2.

In summary, the map pattern for glacial-interglacial sea glacial meltwater and icebergs.Third, it may be an indirect surfacetemperature changes(mode 1) requires inter- responseto insolation, mediatedby continentalmonsoon hemisphericlinkage of climate. Our favored explanation effects. Fourth, mode 2 may reflect changesin the of this tropical Atlantic pattern is linkage of the trade presentlynorthward cross-equatorialheat flux in the winds to hemisphericthermal gradients. Although Atlantic Ocean. changing atmospheric CO2 is a likely mechanism for Like mode 2, the 23,000-year precessionalcycle of linkage of high-latitude climates, present models [e.g. insolation shifts 180ø in phase acrossthe equator for Bryan et al., 1982; Schlesingeret a1.,1985; Manabe and equivalentseasons [Berger, 1978]. Hemisphericinsolation Bryan, 1985] suggestthat CO2 changesalone cannot extremesoccurred 11,000 years ago. Thus insolation might account for the magnitude of sea surface temperature seemto be a likely candidate to explain the phase and changeswe estimate for the tropics. Possibleexplanations hemisphericasymmetry of mode 2. of the model-data magnitude mismatch are (1) that present The high thermal inertia of the tropical oceans, modelsunderestimate the sensitivityof the tropical however, arguesagainst a direct responseof the tropical sea surface temperatures to atmospheric CO• and related oceansto insolation. For the 23,000-year precessioncycle climatic feedbacks, (2) that other mechanisms,such as an increase of insolation in one seasonwas largely modification of equator-crossingmonsoonal winds by compensatedby a decreasein the opposite season. northern hemisphere ice cover, contribute to interhemi- Consideringthe thermal inertia effect, Kutzbach and sphericlinkage within the tropics, and (3) that transfer Otto-Bleisner [1982] estimated the direct effects of functions overestimatetropical temperature changes, insolation change on the ocean as lessthan 0.01 øC on an perhaps becauseforaminiferal faunas senselarge thermo- annual average. Further, the northern hemispherereceived cline effects rather than small seasurface temperature slightlymore insolation 11,000 years ago on an annual changes. averagethan the southern hemisphere, becausethe earth was at perihelion during northern hemispheresummer. Mode 2 This was the opposite of the senserequired to explain mode 2. Therefore, the mode 2 oceanic temperature The secondEOF mode of temperature variability is responsewas not a direct responseto local or hemispheric defined by relatively cold temperatures in the northern insolation variations. The origin of mode 2 must lie within tropical Atlantic and relatively warm temperatures in the the climate system. southern tropical Atlantic during deglaciation, between The secondhypothesis, melt-product chilling of North 14,000 and 6,000 years B.P. (Figure 5c). Typical ranges of Atlantic surfacewaters, could possiblyexplain tempera- this effect in each hemisphereare 2ø-3øC (1-a EOF ture patternslocally within the North Atlantic [Ruddiman coefficients of 0.5ø-0.8øC/a times EOF range of 3.7 •; and Mcintyre, 1981], but alone it cannot account for the Table 5, Figure 5c). As was discussedabove, mode 2 is not hemisphericasymmetry of mode 2. If equatorward an artifact of errors in chronology. In addition, the lack of advection of iceberg-chilled waters were the whole story, any clear relationships between this mode and water depth one might expectthe signalto diminish toward the equator (Figure 8a) or sedimentation rate (Figure 8b) eliminates and disappearin the southern hemisphere. The phase carbonate dissolution or bioturbation as significant inversionat the equator and the relatively strong deglacial contributors to mode 2. warmth in the southern hemispheretropics would not be We consider four possiblemechanisms to explain expected. mode 2. First, it could be a direct responseto orbitally Further, the energy requirementsto cool much of the modulated changesin seasonalinsolation. Second, it may subtropicalAtlantic precludethe melt product hypothesis reflect chilling of the North Atlantic during deglaciation by as a significantcontributor to mode 2. Even if the entire 62 Mix et al.: Tropical Atlantic Paleoceanography

excessPleistocene ice volume of 60 million km 3 [Denton 10-50 W m-2), and approachesmodern net radiation heat and Hughes, 1981] were delivered to the North Atlantic as fluxes (typically 50-150 W m -2) for the subtropicalNorth icebergs(clearly an overestimate) in a period of 5000 years, Atlantic [Bunker and Worthington, 1976]. the heat of fusion (H0 would extract only 2 W m -2 from the For comparison, in the Bryan et al. [1982] ocean- surface of the North Atlantic (Q = Hr x Ice Volume / atmospherecoupled GCM study, quadrupling atmospheric Time / Area = 334 J cm -3 X 60x 102' cm 3 / 1.5 x 10 '• s / CO2 (roughly equivalent to an 8 W m -2increase in heat flux 5 x 10'3 m 2 = 2 Wm-2). This effect would be offset by the - 2 to the ocean), increasedtropical and subtropical sea W m -2increase in annual average insolation for the surface temperature by 1ø-2øC. A simpler energy balance northern hemisphere [Berger, 1979]. Thus, although model by Harvey and Schneider [1985], which changed the icebergchilling may contribute locally to deglacial solar constant by 8 W m -2, yielded tropical sea surface temperature reduction in the subpolar North Atlantic, it temperature changesof about 3øC. These results indicate cannot explain the entire mode 2 thermal pattern. that heat extraction on the order of of 30 W m -2 (for An indirect responseof the oceansto low-latitude cessationof cross-equatorial heat transport) could yield insolation may be possible. Kutzbach and Otto-Bleisner thermal changeseasily large enough to explain the 2ø-3øC [1982] hypothesizedthat high seasonalcontrast in northern range of mode 2 in our data. hemisphere insolation 9000 years ago enhanced the Ultimately, this pattern may be forced in part by ice monsoonal circulation over Eurasia and Africa. In volume change.The presentnorthward heat transport has atmosphericgeneral circulation model (GCM) runs by been attributed to thermohaline overturn as warm surface Kutzbach and Otto-Bleisner [1982] and Kutzbach and water flows north and cooler deep water flows south Guetter [1984], decreaseof pressureover Africa in summer [Bryan, 1982; Worthington, 1976]. Fresheningof North was accompanied by a slight increase over the ocean. The Atlantic surface waters during deglaciation may have enhancedpressure gradients would presumably result in suppressedthis thermohaline overturn by decreasingthe stronger summer winds in the northern hemisphere. J.E. density of the North Atlantic Deep Water source areas Kutzbach [personal communication, 1984] suggestedto us [Worthington, 1968]. Evidence for this occurrenceduring that this could "spin up" gyre flow and thus cool the many deglacialevents has been found in benthic oceansthrough enhanced advection and upwelling. foraminiferal bl3C data by Berger [1985] and Mix [1986]. Although the opposite effect should occur in the winter, At present,however, it appearsthat glacial-interglacial nonlinear sensitivity of the summer could yield a net changesin deep water circulation are more prominent in change on the annual average. In the southern hemisphere the geologicrecord than eventsoccuring during climatic the phasing would be reversed, in keeping with mode 2. transitions. As noted above under mode 1, the effects of Although the annual temperature effects on land were changingdeep water circulation on climate are largely small (< 0.5øC) in these model runs, the effects on the untested in climate models. Thus, although changing ocean are unknown. Both models used modern sea-surface cross-equatorialheat transport is a likely candidate temperatures as input. Sea-surface temperatures were not for explaining mode 2, the exact mechanismsinvolved explicitly modeled. require further study. A possibleargument against the monsoon model for Given the recent emphasison multiple stepsto explaining mode 2 is that the thermal effects of enhanced deglaciation[Duplessy et al., 1981; Berger et al., 1985; Mix winds should be strongestin the gyre margin upwelling and Ruddiman, 1985], it is tempting to speculatethat the areas. The map pattern for mode 2 (Figure 6b) suggests two peaks in mode 2 (Figure 5c) at - 11,000 and • 14,000 (on the basisof limited data) that the strongesteffect of yearsB.P. representseparate pulses of meltwater flux to the this mode is not in the gyre margins but in central subpolar North Atlantic. This is not certain, however, subtropical waters, at least in the northern hemisphere. becausethese peaks do not date exactly the same as b' 80 Further data should be obtained to substantiate this stepsin the samecores [Mix and Ruddiman, 1985j. The pattern. If it is confirmed, the monsoonal responseto apparent structureswithin mode 2 may be within the noise insolation is unlikely to account for mode 2. Note, level of our data. however, that this argument is speculative. The effects of In summary, the hemispheric asymmetry of mode 2 monsoon intensity (which are largely seasonal)on annual reperesentsa deglacial transfer of heat from the northern mean sea surface temperatures have not yet been tested in to the southern hemispheretropics, relative to the modern coupled ocean-climate models. We can not yet exclude pattern. The direct chilling of seawater by glacial melt monsoonal effects as contributors to mode 2. products is unlikely to explain the temperature patterns The fourth possibleexplanation of mode 2 is that outsideof the high-latitude North Atlantic. Indirect effects changing cross-equatorial heat transport within the ocean of meltwater on Atlantic heat transport, perhaps through produced opposite effects in the northern and southern modulation of the thermohaline overturn, may contribute hemispheres.During deglaciation, chilling of the subtropi- to the mode 2 deglacial temperature anomalies that we cal North Atlantic and warming of the subtropical South have observed. Monsoonal effects on oceanic circulation Atlantic would occur if the northward advection of heat during deglaciation are also possible, but at present are (presently 1.4 x 10" W [Hastenrath, 1980]) were reduced untested. or eliminated. A complete cessationof this heat transport, Whatever the origin of mode 2, it implies deglacial when averaged over the entire North Atlantic, would transfers of heat within the ocean that may be significant effectively extract 30 W m -2 from surface waters. This for global climatic linkage. Ruddiman and Mcintyre value is similar to modern sensibleheat fluxes (typically [1981] speculatedthat temperature variations in the North Mix et al.: Tropical Atlantic Paleoceanography 63

Atlantic play a role in the mechanismsof glaciation. If we Karlin, M. Raymo, M. Feliciano, and F. Steininger. are correct that changesin interhemisphericheat transport Funding from National ScienceFoundation grants in part control the temperature of the North Atlantic, then OCE80-18177 and OCE83-15237 is greatly appreciated. thesevariations in heat transport may be involved in the Lamont-Doherty Geological Observatory contribution glaciation process. 3926.

CONCLUSIONS REFERENCES

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•A. C. Mix, College of Oceanography, Oregon State Sciences,Queens College, City University of New York, University, Corvallis, OR 97331. NY. W. F. Ruddiman, Lamont-Doherty Geological Observ- atory, and Department of Geology, Columbia University, (ReceivedSeptember 4, 1985; Palisades, NY 10964. revised November 6, 1985; 2A. Mcintyre, Department of Earth and Environmental acceptedNovember 8, 1985.)