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Bipolar modulation of millennial-scale West African monsoon variability duringthe last glacial (75,000e25,000 years ago)

Syee WeldeabDepartment of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA

a r t i c l e i n f o

Article history:Received 8 September 2011Received in revised form15 February 2012Accepted 23 February 2012Available online xxx

Keywords:Africa’s paleoclimateWest African monsoonHeinrich eventsLast glacialBipolar modulation

a b s t r a c t

Time series of planktonic foraminiferal d18O and Ba/Ca-based sea surface salinity (SSS) estimates from theeastern Gulf of Guinea (eastern equatorial Atlantic) indicate changes in runoff that reflect variability ofspatially integrated precipitation over the equatorial West African monsoon area. Millennial-scale andrecurring runoff-induced SSS rises and declines in the range of 1.5 and 2 psu (practical salinity unit)reveal rapid oscillation between dry and wet phases. The timing of decreased runoff coincides withoscillation of DansgaardeOeschger stadials and Heinrich events, the most severe monsoon weakeningcorrelating with the latter. d18Oresidual time series, derived by removing temperature, ice volume, andsalinity components from the foraminiferal d18O record, suggest that weak monsoon precipitation duringstadials and Heinrich events was accompanied by significant shifts in d18Oprecipitation toward highervalues. Furthermore, d18O analysis of individual tests of Globigerinoides ruber pink (d18Oindiv) during dryepisodes show a total range and variance of 2.3& and 0.25 (n! 121), indicating that seasonal contrast ofsea surface freshening was significantly reduced during Heinrich events relative to that of interstadialswhich show a total range and variance of 3.35& and 0.42 (n! 140). On the basis of the timing andmagnitude of changes in the monsoon record, it is evident that northern high latitude climate was themost dominant control on the West African monsoon variability. However, a southern high latitudeimprint is also apparent during some episodes. This centennially resolved climate record demonstratesthat the equatorial West African monsoon experienced profound changes in the amount, seasonalcontrast, and moisture source of summer monsoon precipitation during the last glacial. The mostplausible mechanism is a large-scale southward displacement of the monsoon trough, most likelyinitiated by large-scale reorganization of atmospheric circulation in response to northern high coolingand southern high latitude warmth.

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1. Introduction

Growing lines of evidence indicate that Africa’s climate wassensitive to rapid and recurring ice sheet instabilities during thelast glacial (deMenocal et al., 2000; Johnson et al., 2002, 2011;Stager et al., 2002, 2011; Adegbie et al., 2003; Weldeab et al.,2005, 2007a,b, 2011; Shanahan et al., 2006; Brown et al., 2007;Scholz et al., 2007, 2011; Mulitza et al., 2008; Tjallingii et al.,2008; Castañeda et al., 2009; Niedermeyer et al., 2010; Sangenet al., 2011). Episodes of cold air temperature over Greenland,known as DansgaardeOeschger stadials (Dansgaard et al., 1993;Blunier and Brook, 2001), and meltwater flux into the NorthAtlantic during Heinrich events (Heinrich, 1988; Vidal et al., 1997;McManus et al., 2004) correlate with rapid decline in precipita-tion over much of Africa. Climate amelioration over the African

continent coincides with relatively warm interstadial in northernhigh latitudes and vigorous Atlantic meridional ocean circulation(AMOC) (McManus et al., 2004), suggesting a strong atmosphericlinkage.

Key aspects concerning the mechanisms of climate linkage,timing, and magnitude of precipitation changes are, however,insufficiently understood. For instance, the lack of quantitativeproxies for precipitation presents a serious obstacle to quantifyingwetedry oscillations and assessing the severity of proposed “mega”droughts (Scholz et al., 2007; Mulitza et al., 2008; Stager et al.,2011). Age model uncertainty prevents the investigation of leadand lag patterns needed to infer whether tropical climate wasmerely responding to high latitude climate instabilities or wasinfluencing the latter (Brown et al., 2007). Large-scale shifts in theintertropical convergence zone (ITCZ) are often invoked to explainthe tight link between the African monsoon and northern highlatitude climate. While the ITCZ hypothesis provides a viablemechanism for the northern and central parts of the AfricanE-mail address: [email protected].

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monsoon, the extent of southward ITCZ displacement duringHeinrich events and stadials is not well constrained. Furthermore,an alternative model such as proposed by Nicholson (2009) may berequired to explain the climate mechanisms responsible for dryconditions in areas south of the mean summer position of the ITCZ.Nicholson (2009) showed that summer West African monsoonprecipitation consists of a relatively weak rainfall zone at north-ernmost boundary of the monsoon area, which is associated withthe ITCZ position, and an intense rain belt that is linked toascending air masses sandwiched between the African Easterly Jet(AEJ) and Tropical Easterly Jet (TEJ) (Nicholson, 2009). Furthermore,modern data also show that during Sahel droughts the main rainbelt retreated several hundred kilometers southwards, while theweak rainfall zone linked to the ITCZ remained over the northernSahel (Nicholson, 2009). Most recent spatio-temporal reconstruc-tion of West African monsoon (Weldeab et al., 2011) appears tosupport this model. In this contribution, we present a centenniallyresolved record of West African tropical hydrology, revealing rapidclimate oscillations that were not discovered in a lower-resolutiondata set of the same core material (Weldeab et al., 2007a). Therecord covers the time interval between 75 and 25 thousand yearsbefore present (kyr, BP), and presents two independent proxies forhydrological changes whose differences in magnitude, timing, andpace provide crucial insights into regional climate patterns andtheir possible link to bipolar thermal oscillation.

2. Background

Riverine discharge of large West African river systems into theeastern Gulf of Guinea provides a spatially integrated record ofrainfall over the tropical and equatorial West African monsoon area(Fig. 1). Though runoff from the Niger River in the north and the

Ogooué River in the south also contributes to the low sea surfacesalinity (SSS) in the Gulf of Guinea, the annual riverine discharge ofapproximately 86 km3 from Sanaga, Nyong, and Ntem rivers, whichdrain the southern margin of the West African monsoon area, hasthe most impact on SSS (Antonov et al., 2010), isotope composition(LeGrande and Schmidt, 2006), and trace element budget of theseawater in the eastern Gulf of Guinea (Weldeab et al., 2007a).Dissolved Ba in runoff is, on average, 3 times higher (60 mg/l) thanthe Ba concentration in seawater (20 mg/l) (Turner et al., 1980). Theconcentration of dissolved Ba in sea surfacewater in coastal regionsadjacent to large rivers is strongly controlled by the amount ofrunoff, and thus tightly anti-correlated with runoff-induced SSSchanges as demonstrated by Ba and SSSmeasurements off Amazon,Congo, Ganges-Brahmaputra, and Mississippi rivers (Edmond et al.,1978; Carroll et al., 1993; Moore, 1997). Laboratory experimentsshow that the amount of Ba uptake into foraminiferal calcite islinearly correlated to the dissolved Ba concentration in seawater inwhich the foraminifers calcify, and the Ba incorporation is notinfluenced by changes in calcification temperature, pH, carbonateion concentration, and total alkalinity (Lea and Spero, 1994;Hönisch et al., 2011). Thus, the temporal variability of runoff,reflecting precipitation of river basins, can be reconstructed byanalyzing Ba/Ca and d18O in the calcite tests of low salinity-tolerating, surface-dwelling planktonic foraminifer Globigerinoidesruber pink (Weldeab et al., 2007a).

3. Methods

MD03-2707 core sediment was recovered from the easternGulf of Guinea (02"30.110N, 09"23.680E, 1295 m water depth)(Fig. 1) and presents a high-resolution sequence covering the last155 kyr BP (Weldeab et al., 2007a). In this study, we focus on the

Fig. 1. Map of West Africa showing basins of Niger, Sanaga, Nyong, Ntem, and Ogooué rivers, and the location of MD2707 in the eastern Gulf of Guinea. The inlet shows annual seasurface salinit (Antonov et al., 2010). The salinity map presents an extrapolation of large area, including the location of MD2707 for which no salinity measurements exist in theWA09.

S. Weldeab / Quaternary Science Reviews 40 (2012) 21e2922


interval spanning from 75 to 25 kyr BP. For trace element analysis,30e40 individuals of G. ruber variety pink were selected from the250e300 mm fraction for each analysis. Shell samples were gentlycrushed and cleaned using the UCSB standard foraminiferacleaning procedure (Martin and Lea, 2002) that includesmechanical (sonication), oxidative, and reductive steps. Forami-niferal trace elements in dissolved samples were analyzed by theisotope dilution/internal standard method (Martin and Lea, 2002)using a Thermo Finnigan Element2 sector field ICPeMS. Analyticalreproducibility of Ba/Ca, assessed by analyzing consistency stan-dards matched in concentration to dissolved foraminifera solu-tions and analyzed over the course of entire study (578 samples),is estimated at 1.8%. To assess the removal of silicate and coatingphases, Al/Ca, Fe/Ca, Mn/Ca, and other diagnostic elements (REEs/Ca and U/Ca) were also analyzed. Out of 578 trace elementmeasurements, 12 (2%) samples were disregarded due to anom-alously high Ba/Ca values, most of which were also accompaniedby either high Al/Ca, Fe/Ca, or Mn/Ca ratios. We establisheda record of Ba/Ca-based SSS estimates using the foraminiferal Ba/Ca time series, an estimate of practical partition coefficient ofDBa! 0.149# 0.05 [Ba/Caforam shell!DBa * Ba/Caseawater] (Lea andSpero, 1994; Hönisch et al., 2011), and a linear relationshipbetween modern Ba/Caseawater and SSS off Congo River [SSS(psu)!$1.1 * Ba/Caseawater% 37.45 (#1.06); r2! 0.98] (Edmondet al., 1978). Error propagation in the SSS estimate results to anuncertainty of #1.2 practical salinity unit (psu).

Analyses of d18O in bulk (250e300 mm) and individual(300e400 mm) G. ruber pink tests from MD03-2707 down coresamples were madewith ThermoMAT 253mass spectrometer at theauthor’s stable isotope lab, UCSB. The mass spectrometer is coupledonline to a Kiel Carbonate Device (Type IV) for automated CO2 prep-aration. Samples were reacted by individual 100% phosphoric acidaddition. Results were corrected using NBS19 standard and are re-ported on the Peedee Belemnite (PDB) scale. d18O estimate of localseawater (d18Oseawater) is obtained by removing the temperature andice volume components from the d18OG.ruber record. The temperature

component is reconstructed using Mg/Ca-based calcificationtemperature of G. ruber pink (Weldeab, under review) and theTed18Ocalciteed18Oseawater equationT ("C)! (16.5# 0.2)$ (4.8# 0.16) *(d18Ocalcite$ d18Oseawater) (Bemis et al., 1998). The ice volume compo-nent is obtained using high-resolution sea-level record (Siddall et al.,2003) that is digitally resampled and converted to a global d18Oseawaterrecord assuming d18Oseawater changes of 0.0085& per meter eustaticsea-level change (Waelbroeck, 2002). Uncertainty in the recon-structed d18Oseawater record arises due to errors in the d18OG.rubermeasurements (#0.07&), sea-level reconstruction(#12mz#0.102&), conversion of d18OG.ruber into d18Oseawater(#0.16&), and errors in the Mg/Ca-based estimate of calcificationtemperature (#1.2 "Cz#0.25&), resulting to an error estimate of#0.32&. In a further step, we applied the Ba/Ca-based SSS record andmodern SSSed18Oseawater relationship (Schmidt et al., 1999)[d18Oseawater! 0.1713 * SSS$ 5.32, r2! 0.8, n! 477] to separated18Oseawater variability related to runoff-induced SSS changes (d18OSSS-

changes) from d18Oseawater related to changes in the isotopecomposition of precipitation and other processes (d18Oresi-

dual! d18Oseawater$ d18OSSS-changes). The modern d18Oseawater and SSSdata, used to establish the SSSed18Oseawater relationship, are fromtropical Atlantic (8.5"Ee80"W and 28.8"Ne27"S, water depth:0e50 m, SSS: 20e40 psu) (Schmidt et al., 1999). The estimate of errorpropagation in the reconstructed d18Oresidual time series accounts for#0.45&.

The agemodel for theMD03-2707 record has been developed byaligning the planktonic foraminiferal d18O record in MD03-2707 tothe d18O record of GISP2 ice core (Blunier and Brook, 2001) (Fig. 2).The rationale for this approach is based on (i) the observation thatchanges in the 14C-dated d18O MD03-2707 record (Weldeab et al.,2007a,b) are synchronous, within age model uncertainty, with airtemperature variationoverGreenland, as reflected in d18Oof ice corerecords (Blunier and Brook, 2001); and (ii) based on the assumptionthat this linkage holds throughout the millennial-scale last glacialclimate instabilities. A constant deposition rate based on the tiepoints, lends support to the above assumption.

Fig. 2. Time series of d18O (B) and Ba/Ca (C) analyzed in Globigerinoides ruber pink (250e300 mm) from MD2707 core sediment plotted versus calendar age model. The age model ofMD2707 section shown here has been established by tuning the d18O MD2707 record to the GISP2 d18O record (A) (Blunier and Brook, 2001). Black triangles along the d18O recordshow tie points of the age model. Numbers above the GISP2 record denote to DansgaardeOeschger interstadials and gray bars indicate time interval of Heinrich events and stadials.

S. Weldeab / Quaternary Science Reviews 40 (2012) 21e29 23


4. Results

We generated 590 new d18O and 320 new Ba/Ca measurementson bulk G. ruber pink (250e300 mm) at 2 cm interval. This samplinginterval corresponds to a time resolution of 40 and 250 years with270 Ba/Ca measurements based upon a previous study (Weldeabet al., 2007a). We also analyzed 489 new d18O measurements onindividual G. ruber pink (300e400 mm) from nine samples withinHeinrich event 5, interstadial 12, and stadial 11. The new high-resolution d18Ocalcite and Ba/Ca data sets, and the derived d18Osea-

water and d18Oresidual time series provide a centennial- tosubmillennial-scale record of changes in West African monsoon(Fig. 2).

5. Discussion

5.1. Millennial-scale oscillation of monsoon precipitation

The foraminiferal d18O and Ba/Ca records over the last glacialreveal millennial-scale shifts between relatively weak and strongmonsoon episodes (Fig. 2). The d18Oseawater record, obtained byremoving the temperature and ice volume components from d18O inG. ruber, shows numerousmillennial-scale oscillations (Fig. 3B). Timeintervalswith the highest d18Oseawater values coincidewith the timingand duration of Heinrich events H3, H4, H5, H5a, and H6. Further-more, every stadial in the GISP2 ice core record has also its counter-part in the MD03-2707 sequence, with relatively high d18Oseawatervalues. Conversely, episodes synchronous with northern high

latitude interstadials are characterized by markedly light d18Oseawatervalues. The amplitude of these negative and positive d18Oseawaterexcursions declines significantly toward full glacial conditions; this ismost evident during interstadials 3, 4, and H6 (Fig. 3).

Due to the proximity of the core site to the river mouths and thestrong impact of runoff on the salinity and isotope composition ofseawater in the eastern Gulf of Guinea (Fig. 1) (LeGrande andSchmidt, 2006), temporal oscillations of reconstructed d18Oseawaterin MD03-2707 most likely reflect changes in the amount of runoffand/or variation of oxygen isotope composition of precipitationover the riverine basins. This view is fully corroborated by the Ba/Ca-based monsoon record, which provides independent insightsinto variations in runoff and, therefore, changes in precipitationover the river basins (see Background and Methods section). Downcore Ba/Ca values in G. ruber pink vary between 0.75 and 1.5 mmol/mol. Foraminiferal Ba/Ca-based SSS estimates (see Methodssection) suggest that runoff-induced SSS varied between 32 and27 psu. For comparison, modern annual SSS over the core siteaccounts for 29.5 psu, seasonally varying between 28.5 and31.5 psu (Antonov et al., 2010). Episodes of elevated Ba/Ca-basedSSS estimates coincide and overlap with the duration of DO-stadials and Heinrich events, indicating reduced amounts ofrunoff and weak monsoon precipitation at those times. In the Ba/Carecord some of the short-lived interstadials are present in mutedamplitude (D/O 10) or completely absent (D/O 9, and 11). Becausethe mean oceanic residence time of Ba (w10,000 years) (Turneret al., 1980) is significantly longer than the duration of these briefD/O interstadials and foraminiferal Ba/Ca is sensitive to brief

Fig. 3. 5-point running average of d18Oseawater (B) and Ba/Ca-based sea surface salinity (SSS) estimates (C) compared to the d18Oice records from Greenland (GISP2) (A) and Antarctica(Byrd) (E) based on common methane synchronized age model (Blunier and Brook, 2001). (D) Variation of solar insolation over 5"N (July, 21) is also shown (Berger and Loutre, 1991;Paillard et al., 1996). Uncertainties in the d18Oseawater and Ba/Ca-based SSS estimates account for #0.32& and #1.2 psu, respectively.


Syee Weldeab

Syee Weldeab


climate excursions such as 8.2 kyr events (Weldeab et al., 2007b), itis not clear why the imprint of D/O 9 and 11 is not recorded in theBa/Ca series. Regardless, the two independent runoff proxies showthat monsoon precipitation in equatorial Africa was marked bynumerous millennial-scale oscillations.

The overall patterns of last glacial West African Monsoon vari-ability that emerge from d18O and Ba/Ca records are consistent withother components of the global monsoon as manifested in the d18Orecords of Arabian (Burns et al., 2003; Fleitmann et al., 2011), Indian(Kudrass et al., 2001; Sinha et al., 2005; Cai et al., 2006), and eastAsian (Wang et al., 2001) monsoons, suggesting that large-scaleatmospheric reorganization occurred in the low latitudes intandem with ice sheet instabilities in both northern and southernhigh latitudes (Dansgaard et al., 1993; Blunier and Brook, 2001).However, the lack of a proxy that enables the separation of thed18Ocalcite records into the amount effect of precipitation andd18Oprecipitation changes related to source, pathway, and recycling ofmoisture prevents nuanced insights into regional climate variation,as suggested by modeling studies (Lewis et al., 2010; Pausata et al.,2011). In the following chapters, we use Ba/Ca-based runoff-induced SSS variation and its temporal relationship to the recon-structed d18Oresidual to assess non-mount changes in d18Ocalcites andits implication for understanding the severity of past monsoondeclines.

5.2. Phase relationship between the Ba/Ca and d18O records

Leads and lags between reconstructed changes in the forami-niferal Ba/Ca and d18O are independent from of age model errorbecause the records were generated from aliquots of the sameG. ruber sample. Because the MD03-2707 d18O record is tied to theGISP2 d18O age model (see Methods section), deviations in thetiming, excursion, and pace of changes in the Ba/Ca record fromthose of the d18O record are real climate signals and indicatemodulation of Ba/Ca by processes and influences other thannorthern high climate variability. Temporal variability of Ba/Ca inMD2707 is assumed to reflect changes in runoff. However, exposureof large shelf areas to erosion during sea-level low stands (Giresseet al., 1995; Ngueutchoua and Giresse, 2010) and reduced vegeta-tion cover in the catchments during relatively dry phases (Dupontet al., 2000) may contribute to high rate of dissolved Ba input(Fleitmann et al., 2007). Considering age model uncertainties,a comparison of the Ba/Ca record with the sea-level record (Siddallet al., 2003) does not reveal any coherent patterns. A large sea-leveldrop and reduced vegetation cover over the river basins duringmarine isotope stage 4 (Dupont et al., 2000) is paralleled by low Ba/Ca, suggesting that the timing, pace, and magnitude of Ba/Ca arelargely determined by variations in runoff. A comparison betweenthe Ba/Ca and d18Oseawater records reveals differences in the timing,pace, and magnitude of climate excursions (Fig. 3). The onset ofWest African monsoon weakening at the end of interstadials 21,13,and 8, as suggested by the Ba/Ca record, predates the timing ofchanges manifested in the MD2707 d18O record. The termination ofstadials 20, 7, H5, and H4 in the Ba/Ca record also leads those of thed18O record. We suggest that the early and gradual onset of Ba/Cadecline reflects a gradual climate deterioration in equatorial Africapredating the abrupt and severe decline of precipitation docu-mented in the d18O record. If our interpretation is correct, thetiming and pace of changes in the Ba/Ca record suggests thatmillennial-scale West African monsoon variability was alsomodulated by tropical and southern high latitude climateprocesses. The degree of modulation by these climate processesthroughout the last glacial must have varied, as its influence is notdiscerned in every monsoon shift. Examples that may indicate aninfluence of southern high latitude in West Africa monsoon, as

recorded in the Ba/Ca, include D/O 14. The onset of an abruptdecrease in Ba/Ca-based SSS estimates during D/O 14 lags byw450years air temperature rise over Greenland and coincides witha precipitous decline in air temperature over Antarctica as revealedin the Byrd d18O record (Fig. 3E). Gradual increase of SSS toward theend of D/O 14 is also paralleled by a gradual rise in air temperatureover Antarctica and leads air temperature cooling over Greenland(Fig. 3). Similar patterns can be observed during D/O 16 and 8(Fig. 3). One possible mechanism for the linkage between WestAfrican Monsoon and Southern high latitude climate is provided bymodel studies (Broccoli et al., 2006; Chang et al., 2008). Thesemodels suggest that relatively warm (cool) southern high latitudesinstigate a southward (northward) displacement of the ITCZ due tochanges in low and mid latitude heat exchange, thereby weakening(strengthening) monsoon precipitation. A further possibility, assuggested by declining Ba/Ca-based SSS, is that the onset of climateamelioration in equatorial West Africa occurred concomitant withwarmest phases over Antarctica and leads the termination of H6,H5, H4, and stadial 7 (Fig. 3). This is (seemingly) in contradictionwith the aforementioned mechanism. Though somewhat specula-tive, one way to reconcile this observation with the concept of ITCZdisplacement is that the southernmost ITCZ displacement and itsspatially limited seasonal migration is instigated by the peakwarmth in the southern high latitude and in the eastern Gulf ofGuinea (Weldeab, in press), leading to more annually averageprecipitation in the coastal area. Indeed, a modeling study simu-lating Heinrich event-like conditions predicts such rainfall focusingover coastal areas of the Gulf of Guinea (Chang et al., 2008). Overall,while not every feature in the Ba/CaeSSS estimates is explainableon the basis of interhemispheric modulation, invoking southernhemisphere influence provides an additional tool to better under-stand the timing, pace, and magnitude of African monsoonprecipitation during the last glacial. Our finding lends strongsupport to the emerging lines of evidence that suggest that recordsof millennial-scale last glacial climate variability in equatorialAfrica (Mulitza and Rühlemann, 2000; Brown et al., 2007), IndianSummer Monsoon (Cai et al., 2006), and East Asian monsoon (Caiet al., 2006; Rohling et al., 2009) harbor a significant climateimprint of southern high latitude origin.

5.3. Changes in the d18O of monsoon precipitation

The reconstructed d18Oseawater time series (Fig. 4) representsa composite of signatures related to the amount of riverinedischarge and changes in isotope composition of the runoff (Fig. 4).Using Ba/Ca-based SSS estimates and modern SSSed18Oseawaterrelationship (see Methods section) and assuming that this rela-tionship does not significantly vary throughout the investigatedtime interval (Schmidt, 1999; Rohling, 2007), we removed thesalinity component from the d18Oseawater record and derived theresidual oxygen isotope composition (d18Oresidual) (see Methodssection). d18Oresidual is assumed to reflect oxygen isotope changesof seawater in response to variability of d18O in West Africanprecipitation. We note that this assumption presents an over-simplification of processes and components that shape the d18Ore-

sidual (Fig. 4). Nonetheless, we argue that at this particular core sitechanges in the oxygen isotope composition of precipitation pres-ents the dominant factor in determining large-scale excursions ind18Oresidual. If this interpretation is correct, the d18Oresidual recordsuggests that the severely weakened monsoon precipitation duringHeinrich events and stadials was significantly enriched in heavyoxygen isotope. The enrichment is most likely related to multiple-influences such as changes in moisture source, the degree of rain-out, atmospheric mixing, evapo-precipitation, recycling of mois-ture, and/or changes in precipitation seasonality (LeGrande and



Schmidt, 2009; Lewis et al., 2010). One modeling study simulatingHeinrich event-like conditions suggested that the d18O of precipi-tation in all northern hemisphere monsoon systems experienceda significant shift toward higher values (Lewis et al., 2010). Acommon dominant factor for the d18O shift in all monsoon systemsis most likely the weakening of monsoon coupled with large-scaleITCZ shift. In addition, regional characteristics of monsoon such asproximity to moisture source, orographic barriers, and the relativecontribution of winter and summer monsoon most likely shapedthemagnitude of the d18Oprecipitation shift. Because themain riverinecatchments are located in coastal areas, changes in the amounteffect and increase in the relative contribution of winter precipi-tation are the most plausible mechanisms explaining the inferredshift in the d18Oprecipitation. Invoking an increase in the relativecontribution of winter rainfall is strongly supported by the data anddiscussion in 4.3. Overall the data are consistent with model resultsthat predict a shift of d18O toward heavier values in monsoonprecipitation during Heinrich events (Lewis et al., 2010) andsuggest a fundamental atmospheric reorganization occurred overthe monsoon area, affecting not only the amount (Fig. 3B and C) butalso the isotope signature of precipitation (Fig. 4). This finding hasan important implication for assessing the severity of precipitationdecline based only on d18O of marine and terrestrial archives.

5.4. Contrast in seasonality and shift of rainfall zone over themonsoon area

Large-scale displacement of the mean summer position of theITCZ is often invoked to explaining past variation of proxies ofmonsoonal hydrology. This concept is broadly supported bymodeling studies focusing on orbital and millennial time scales(Kutzbach and Liu, 1997; Chiang et al., 2003; Timmermann et al.,2005; Broccoli et al., 2006; Liu et al., 2006; Chang et al., 2008;Timm et al., 2010). To assess changes in sea surface conditionsthat would relate to southward displacement of the ITCZ over theGulf of Guinea during Heinrich events, we analyzed d18O in indi-vidual tests of G. ruber pink (d18Oindiv) from narrowly spaced timeintervals within Heinrich event 5 (HE 5), stadial 11 (preceding D/O11) and interstadial 12 (Fig. 4). The life span of G. ruber pink isapproximately 2weeks (Hemleben et al., 1989; Spero,1998) and thevariance and range of d18Oindiv measurements from samples withmulti-decadal resolution reflects sub-monthly-to-seasonalcontrasts in mixed layer depth (Spero, 1998; Koutavas et al.,2006). In the eastern Gulf of Guinea, the modern seasonality ofSSS variation is largely determined by runoff related to monsoonprecipitation and current mixing. The ITCZ remains throughout the

year north of the eastewest trending coastal zone and SE tradewinds prevail over the eastern Gulf of Guinea. Significant changesin the monsoon strength and southward shift of NE trade windsover the Gulf of Guinea are expected to leave a strong imprint in thevariance and range of d18Oindiv of G. ruber pink.

Analysis of d18Oindiv from four narrowly spaced samples withinstadial “11” show mean d18O values of $1.36&# 0.52(1s) (n! 35),$1.04&# 0.37(1s) (n! 59), $1.3&# 0.59(1s) (n! 64), and$1.22&# 0.39(1s) (n! 71), with a total range and pooled varianceof 2.53& and 0.23 (n! 228), respectively (Fig. 5A). 121 d18Oindivmeasurements from two samples within HE 5 show a narrow totalrange of 2.33&, relatively heavy mean value of $1.06&# 0.62(1s),and a pooled variance of 0.25. In contrast, d18Oindiv measurementsfrom 3 samples within interstadial 12 yield mean values of$1.88&# 0.69 (1s) (n! 39), $1.85&# 0.63 (1s) (n! 48), and$1.75&# 0.64 (1s) (n! 53), with a total range and pooled varianceof 3.35& and 0.42 (n! 140), respectively. The pooled variance andtotal range in HE 5 (stadial 11) samples is lower by 41% (45%) and30% (25%) as compared to that of interstadial 12 samples, indicatingthat during stadials and Heinrich events sub-monthly, intra- andinterseasonal contrast of sea surface conditions were significantlyreduced relative to those of interstadials. Assuming that thed18Cindiv record can be used as indicator of upwelling (Koutavaset al., 2006) and is not overprinted by other unknown processes,the poor correlation between the d18Cindiv and d18Oindiv valuesduring interstadial 12 (r2! 0.0005, n! 140), stadial 11 (r2! 0.04,n! 228), and HE 5 (r2! 0.001, n! 121) indicates that no seasonalupwelling was established during these time intervals (Fig. 5B).This is supported by SST changes of w1 "C (Weldeab, in press) andtherefore, could explain at most, a d18O change of 0.21& (Bemiset al., 1998). The dominant source for a total range of 3.35&,2.54&, and 2.3& is therefore most likely runoff-induced SSSchanges, with 0.27& in d18O per salinity unit (Fairbank et al., 1982).During interstadial 12, the intra- and seasonal contrast was mostpronounced, suggesting a strong summer monsoon precipitationand possibly large spatial extent of monsoon area. In contrast,during HE 5 and, to lesser degree, during stadial 11 monsoonstrength was severely weakened (HE5: mean d18Oindiv:$1.06&# 0.62) and the seasonal contrast in runoff-induced SSSchange was markedly reduced as well (total range: 2.33& andvariance 0.25). It is also noteworthy that the difference between theseasonal contrast in sea surface conditions during HE 5 and inter-stadial 12 arises mainly due to changes in the wet season (Fig. 5A).This observation can be attributed to a weak summer monsoonprecipitation (Fig. 3) and shift in the d18O signature of monsoonprecipitation toward high values (Fig. 4). The amount of winter

Fig. 4. 5-point running average of d18Oresidual as derived by removing the temperature, ice volume, and salinity components from the d18O record analyzed in Globigerinoides ruberpink plotted versus calendar age model. Marked increase in d18Oresidual during Heinrich events and stadial are indicated by gray shaded areas. Uncertainty in the d18Oresidual accountsfor #0.45&.



precipitation during HE 5 was (i) either not significantly reduced(relative to that of the interstadial winter, see Fig. 5A), resulting inan increase in the relative contribution of winter precipitation toannual rainfall, (ii) or winter precipitation was reduced as well butwas accompanied by lighter values in the d18O of precipitation. Amodeling study simulating Heinrich event-like conditions suggestsan increase in the relative contribution of winter precipitation inGulf of Guinea coastal area supporting the first alternative expla-nation (Chang et al., 2008). Whether the observation we made inHE 5, interstadial 12, and stadial 11 can be extrapolated to otherHeinrich events and stadials/interstadials is subject to speculation.We argue, however, that Heinrich events and stadials havingsimilar magnitudes of change and duration to those investigatedmay have experienced comparable seasonal contrasts in sea surfaceconditions.

The extent of southward ITCZ displacement and shift in theamount of seasonal precipitation may have played a crucial role inmodulating moisture source and rainout that affect the oxygenisotope composition of precipitation. Because of the strong link thatexisted between northern high latitude climate and the equatorialWest African monsoon (Fig. 3), it is instructive to look at proposedseasonality of northern high latitude climate during the last glacial.While the African monsoon is a summer phenomenon, tempera-ture drops in the range of 15 "C in the northern high latitude duringthe last glacial (Cuffey and Clow, 1997) have been interpreted toreflecting mainly extreme winter conditions (Atkinson et al., 1987;Denton et al., 2005; Kelly et al., 2008). Model (Barnett et al., 1988)and observational data (Kripalani et al., 2003; Zhao and Moore,2004) indicate that winter and spring snow depth on the TibetanPlateau has a strong effect on the ensuing Indian SummerMonsoonprecipitation and Tropical Easterly Jet that is considered as criticalfor the development of West African monsoon precipitation(Nicholson, 2009). By analogy, the above described linkages mayprovide a viable mechanism for explaining the close co-variation ofmonsoon and high latitude climate during the last glacial, as sug-gested by previous studies (Denton et al., 2005; Weldeab et al.,2011). With regard to the extent of ITCZ displacement, the mark-edly poor correlation between d18Cindiv and d18Oindiv (Fig. 5B) andthe absence of large SST changes (Weldeab, in press) indicate thatduring stadials and Heinrich events no significant changes in thedirection of seasonal trade winds occurred over the Gulf of Guinea.This observation together with evidence of weakmonsoon suggests

that during Heinrich events/stadials seasonal migration of the ITCZwas strongly contracted. The southernmost position of the ITCZ,however, remained north of the eastewest trending coastal area.This interpretation is consistent with previous findings suggestingthat during the Younger Dryas the mean summer position ITCZremained north of the Sanaga and Nyong basins (Weldeab et al.,2011).

6. Summary and conclusion

The results and discussion of this study provide detailed accountof past West African monsoon variability as summarized below.

1. This study reveals a detailed record of the impact of southernand northern high latitude climates on the hydrology ofequatorial West Africa throughout the last glacial. Heinrichevents and stadials are paralleled by rapid drop in monsoonprecipitation. The northern high latitude climate exerted themost dominant control on the monsoon precipitation.However, thermal fluctuation of the southern high latitudesalso influenced the onset and pace of some events, as revealedin the local salinity proxy (Ba/Ca).

2. During Heinrich events we note a marked shift in oxygenisotope ratio (18O/16O) of precipitation toward higher values,indicating changes in the moisture source, evapo-precipitation,seasonality, and/or amount effect. This finding has an impor-tant implication for assessing the severity of precipitationchanges based solely on the d18O record.

3. d18O analysis of individual tests (d18Oindiv) of surface-dwellingplanktonic foraminifers (G. ruber pink) suggests that duringHeinrich events the seasonal contrast of sea surface conditionswas strongly weakened relative to that of interstadials.Furthermore, the distribution of d18Oindiv during HE 5, stadial,and interstadial 12 suggests that the weakening of seasonalcontrast during HE 5 was mainly due to changes in summermonsoon precipitation that results in an increase in the relativecontribition of winter rainfall to the annual precipitation.

4. Poor correlation between d18Oindiv and d13Cindiv indicates thatduring Heinrich events and stadials no significant changes inthe direction of trade winds occurred over eastewest trendingcoastal area. This result is consistent with the idea that the

Fig. 5. (A) d18O analyzed in individual test (300e400 mm) of Globigerinoides ruber pink (d18Oindiv) plotted versus calendar age model of MD2707. Blue open circles, red open circle,and vertical balk show individual measurements from a sample, mean value, and standard deviation (1s). Bold black line indicates 5-point running average of d18O measurement inbulk tests (250e300mm) of Globigerinoides ruber pink. (B) d18Oindiv plotted versus d13Cindiv analyzed in tests of Globigerinoides ruber pink in samples from HE 5, DO 12, and stadial 11(preceding interstadial 11). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



mean summer position of ITCZ remained over the eastewesttrending coastal area of Nigeria and Cameroon.

The most probable mechanism that can account for themillennial-scale variability of equatorial West African monsoonprecipitation and its link to high latitude climate perturbations isa large-scale shift of mean summer position of the ITCZ. Cooling ofsummer temperature over Greenland in the range of 3.9e6.6 "C, assuggested by Kelly et al. (2008) for the Younger Dryas, may haveinstigated a southward displacement of wind systems including theNE trade winds. Based on the growing lines of evidence thatmillennial-scale drops in temperature over the northern high lati-tude during Heinrich events and stadials reflect largely extremewinter cooling (Denton et al., 2005; Kelly et al., 2008), the climatelink between northern high latitude and West African monsooncould have been established via winter preconditioning that affectlow latitude atmospheric circulation in the ensuing summer. Snowdepth over the Tibetan Plateau is considered to have impacted notonly the Indian summer monsoon (Barnett et al., 1988; Kripalaniet al., 2003; Zhao and Moore, 2004) but also the African monsoonvia the Tropical Easterly Jet (Barnett et al., 1988; Nicholson, 2009).Though the northern high latitude climate exerted the mostdominant control on equatorial West Africa hydrology, we identifypacing and timing in the monsoon record that appear to be linkedto southern high latitude thermal oscillation, lending support toprevious proxy-based (Mulitza and Rühlemann, 2000; Brown et al.,2007) and modeling (Broccoli et al., 2006) works. Regardless of thedetails of the linking mechanism, this study demonstrates thatmillennial-scale interhemispheric climate oscillations had a strongimpact on the strength, moisture source, and seasonal contrast ofWest African monsoon precipitation.

Acknowledgment

We thank Dorothy Pak, Alex Simms, Jim Kennett, and David Leafor discussion and suggestion that helps to improve a previousversion of this paper. We also thank Georges Paradis for ICPeMSoperation. Insightful and constructive comments of two anony-mous reviewers are highly acknowledged. I am grateful to UCSB forthe generous start-up package that facilitated the analysis of thedata presented here.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.quascirev.2012.02.014.

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