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Quaternary Science Reviews 86 (2014) 24e34

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Quaternary Science Reviews

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CO2 radiative forcing and Intertropical Convergence Zone influenceson western Pacific warm pool climate over the past 400 ka

Kazuyo Tachikawa a,*, Axel Timmermann b, Laurence Vidal a, Corinne Sonzogni a,Oliver Elison Timmb,c

aAix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545 Aix en Provence, Franceb IPRC, SOEST, University of Hawaii, Honolulu, HI 96822, USAcDAES, University at Albany, Albany, NY 12220, USA

a r t i c l e i n f o

Article history:Received 4 August 2013Received in revised form9 December 2013Accepted 18 December 2013Available online

Keywords:Mg/Ca-SSTTransient model simulationWestern Pacific warm poolCO2 forcing

* Corresponding author.E-mail address: kazuyo@cerege.fr (K. Tachikawa).

0277-3791/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.quascirev.2013.12.018

a b s t r a c t

The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation andinfluences climate conditions worldwide. Estimating WPWP sensitivity to past radiative forcing per-turbations is important for understanding the magnitudes and patterns of current and projected tropicalclimate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction overthe past 400 ka from the Bismarck Sea, off Papua New Guinea, along with benthic foraminiferal d18Orecords and a transient intermediate complexity earth system model simulation. The Mg/Ca-SST recordexhibits a close similarity with atmospheric CO2 content for the whole study period. Our model analysisdemonstrates that greenhouse gas forcing is the primary driver for glacial/interglacial SST changes in theentire WPWP region. Mg/Ca-SST in the Bismarck Sea also includes a weaker precessional component,which covaries with reconstructed and simulated local precipitation, and simulated surface currents. Wepropose that orbitally driven latitudinal shifts of the Intertropical Convergence Zone and oceanic heatadvection are responsible for this residual SST variability. On glacial timescales the reconstructed WPWPsurface temperature changes over the past 400 ka are highly correlated with East Antarctic air tem-perature variations. The strong effect of greenhouse gas forcings on both records and on global meantemperature variability allows us to determine a scaling factor of 1e1.5 between reconstructed WPWPtemperature anomalies and estimates of the global mean temperature.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Climate-model based projections of future climate change in thePacific show an enhanced tropical sea surface temperature (SST)rise relative to the subtropical regions in response to increasedgreenhouse gas concentrations, in particular along the equator (Xieet al., 2010). However, the simulated magnitude of this responseover the next century is comparable to biases in tropical SSTs ofstate-of-the art climate models run under present-day conditions(Widlansky et al., 2012). This raises the question of how robust thecurrent regional climate change projections are in the tropical Pa-cific. Further independent evidence for an amplified sensitivity ofequatorial SST to radiative perturbation changes could be obtainedfrom past climate archives.

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On glacial/interglacial timescales, tight relationships betweenthe tropical SST and atmospheric CO2 concentration have beenreported based on modelling and proxy reconstructions (Lea,2004; Hansen et al., 2008; Clark et al., 2012; Hargreaves et al.,2012). However, tropical SSTs are sensitive not only to CO2 radi-ative forcing but they also respond to changes in orbital param-eters (Timmermann et al., 2007) and the presence of ice-sheets inthe Northern Hemisphere (Timmermann et al., 2004). Further-more on regional scales, local oceanographic effects, air-sea in-teractions and the potential seasonality of proxy producers(Schneider et al., 2010) need to be taken into account wheninterpreting SST proxy data. Separating these individual effects inpast SST records can be facilitated by comparing proxies withtransient climate-model sensitivity experiments that use differentcombinations of external forcings (Timm et al., 2008;Timmermann et al., 2009). The centre of the western Pacificwarm pool (WPWP), where current annual mean temperaturesexceed 28 �C (Fig. 1), is one of the targeted areas because stronglocal effects such as equatorial upwelling and frontal movements

Fig. 1. Core location and modern climatology: (a) modern annual mean SST (Locarnini et al., 2010) in the Western Pacific and the location of core MD05-2920 (2�51048S, 144�32004E,water depth 1843 m, this study) together with previously studied cores from the central WPWP (SST� 29 �C) and around the Indonesian Archipelago: ODP806B (0�19.10N,159�21.70E, 2520 m; (Lea et al., 2000), MD97-2140 (2�020N, 141�460E, 2547 m; (de Garidel-Thoron et al., 2005), MD98-2162, (4�41.330S, 117�54.170E, 1855 m; (Visser et al., 2003),MD01-2378 (13�4.950S, 121�47.270E, 1783 m; (Xu et al., 2008) and MD06-3067 (6�310N, 126�300E, 1575 m; (Bolliet et al., 2011). (b) Present-day monthly SST variation in the upper50 m at sites closest to the core locations (Locarnini et al., 2010). (c) Observed climatological SST (shading) using AVHRR SST data in 9 km resolution from Jan 1985 to Dec 2002(Vazquez-Cuervo et al., 1998) and simulated 10-year climatology of the surface zonal current (contours) for January and for July from a 50-year hindcast simulation conducted withthe NCEP-wind forced Ofes/MOM3 model (Masumoto et al., 2004). Contour interval is 10 cm/s. Red solid and blue dotted contours show eastward and westward currents,respectively. SEC¼ South Equatorial Current, NGCC¼New Guinea Coastal Current, NECC¼North Equatorial Counter Current. Star indicates core location MD05-2920 (2�51048S,144�32004E, water depth 1843 m). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e34 25

are absent. Using a new high-resolution SST reconstruction fromthe WPWP in combination with transient climate-model experi-ments, we will determine the effects of greenhouse gas forcingand regional ocean dynamical processes on glacial/interglacial SSTvariability.

First, we present a new reconstruction of SST for the last 400 kabased on Mg/Ca of the surface-dwelling planktonic foraminiferaGlobigerinoides ruber (white variety), together with the d18O re-cords of two species of benthic foraminifera, Cibicidoides wueller-storfi and Uvigerina peregrina. Only fewMg/Ca-SST records from the

Table 1Foraminiferal Mg/Ca ratio and relative abundance of the two morphotypes ofG. ruber in core MD05-2920.

Sample name Age (ka BP) Proportion Mg/Ca (mmol/mol)

G. ruber (s.s.)-1 5.7 69% 4.960G. ruber (s.s.)-2 5.7 5.245G. ruber (s.l.)-1 5.7 31% 5.093G. ruber (s.l.)-2 5.7 5.020G. ruber (s.s.)-1 21.3 74% 3.884G. ruber (s.s.)-2 21.3 3.900G. ruber (s.l.)-1 21.3 26% 3.958G. ruber (s.l.)-2 21.3 3.876G. ruber (s.s.) 23.9 Not determined 3.908G. ruber (s.l.) 23.9 Not determined 4.288

During Holocene (5.7 ka), s.s. morphotype presents 69% (total counted individualnumber n¼ 240) of the G. ruber (white) population and 74% (n¼ 183) during LGM(21.3 ka).

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western equatorial Pacific cover several glacial cycles with hightemporal resolution (Fig. S1). The studied core MD05-2920(2�51048S, 144�32004E, water depth 1843 m, total core length of36 m, Fig. 1) was retrieved off the mouth of the Sepik River to theNorth of Papua New Guinea. This location has a relatively highsedimentation rate of about 10 cm/ka in the WPWP because of theenhanced input of river particles associated with precipitation overnorthern Papua New Guinea (Tachikawa et al., 2011), and containswell-preserved foraminiferal tests (Fig. S2).

Second, we analyze output from a transient w400 ka-long nu-merical modelling experiment (abbreviated as TR400) conductedwith a coupled 3-dimensional ice-sheet climate model (iLOVE). Ourobjective here is not to reproduce SSTs that quantitatively matchwith the proxy reconstructions but rather to provide further in-sights into the driving mechanisms of glacial-scale variability in theWPWP, and examine whether potential seasonalities in foraminif-eral production could bias Mg/Ca-SST reconstructions. Further-more, the numerical experiments are free of the specificchronological uncertainties of the MD05-2920 core and can hencebe used to cross-validate the age scale of the paleoclimate re-constructions. Using TR400, we will further estimate the CO2-induced SST signal and explain the potential origin of residual SSTvariability in the WPWP record. The combined paleo-data andearth system model analysis will provide information on themechanisms that contributed to WPWP SST variability.

2. Regional setting

The studied core MD05-2920 (2�51048S, 144�32004E, waterdepth 1843 m, core length of 36 m, Fig. 1a) was collected during theMD148 IMAGES XIII PECTEN cruise and is located in an area withweak present-day seasonal SST variability (28.5e29.3 �C in theupper 50 m, Fig. 1b) (Locarnini et al., 2010). To document therelation between climatological SST variability and regional andlarge-scale ocean currents we show in Fig. 1c the observed clima-tological January and July SST (shading) using AVHRR SST data in9 km resolution (Vazquez-Cuervo et al., 1998) along with a simu-lated 10-year climatology of surface zonal currents (contours) froma 50-year hindcast simulation conducted with the NCEP-windforced eddy-resolving OfES/MOM3 model (Masumoto et al.,2004). The annual cycle in local SST is generated by the seasonalcycle of local solar insolation, resulting airesea interactions,remotely-driven monsoon winds, associated heat flux variationsand corresponding changes in the strength of the New GuineaCoastal Current (NGCC) (Kuroda, 2000), a western boundary cur-rent branch of the South Equatorial Current (SEC) (Fig. 1c). Duringaustral summer (January, Fig. 1c) the northwestern winds push theITCZ to its southern position and the wind-driven NGCC flowseastward, transporting cooler water to the core site by advectionand a subtle weak coastal upwelling along New Guinea (Kuroda,2000). During austral winter (July, Fig. 1c), the southeasternwinds shift the ITCZ northward and the NGCC flows westwardalong the coast of Papua New Guinea. The Island of New Britainserves as a topographic barrier and prevents intrusion of coolerwater to the Bismarck Sea.

On interannual timescales the El Niño-Southern Oscillationphenomenon exerts only weak influences on SST (Fig. S3a). Seasurface salinity in the upper 50 m ranges between 34.2 and 34.8,with a mean annual value of 34.5 (Fig. S3b). The studied area isoligotrophic with a mean chlorophyll-a concentration of 0.29 mg/m3 (SeaWiFS Project, http://oceancolor.gsfc.nasa.gov/). A moderateincrease of chlorophyll-a occurs during austral autumn (MarcheApril) coinciding with a period of the weak coastal upwelling(Fig. S3c). It was suggested that the nutrients brought in by riverrunoff have little impact on local productivity in the Bismarck Sea

(Higgins et al., 2006). Currently, the bottom water at the corelocation is made up of a mixture of upper Circumpolar Deep Water(Tsimplis et al., 1998; Sokolov and Rintoul, 2000; Kawabe and Fujio,2010) formed at southern high latitudes and old Pacific DeepWater,which is linked to Antarctic Bottom Water.

3. Material and methods

3.1. Samples and age model

The WPWP core MD05-2920 (Fig. 1) is composed of non-laminated greyish olive clay with dispersed bioclasts and forami-niferal tests. The two major components of the sediments arerepresented by a terrigenous fraction and calcium carbonates(Tachikawa et al., 2011). Individual tests of the surface-dwellingplanktonic foraminifera G. ruber (white), as well as the benthicforaminifera C. wuellerstorfi and U. peregrina, were picked at every4e10 cm interval from the 250e355 mm size-fraction. The twoG. ruber morphotypes are considered to represent slightly differentcalcification depths: G. ruber sensu stricto (s.s.) is calcifying in theuppermost 30 m of the water column, while G. ruber sensu lato (s.l.)lives at depths below 30 m (Wang, 2000). We compared the Mg/Caratio and relative abundance of the two morphotypes for severalselected samples (Table 1). G. ruber (s.s.) was dominant and there isno systematic and significant Mg/Ca offset between the morpho-types (Table 1), which is consistent with a deep thermocline (about150 m) in the studied region (2.5S, 144.5E) (Locarnini et al., 2010).Therefore, the difference between morphotypes is not taken intoaccount in our study.

Because of the narrow salinity range, with absolute values lowerthan 35 (Fig. S3b), the influence of salinity on the present-dayforaminiferal Mg/Ca thermometer can be considered negligible(Arbuszewski et al., 2010).

The age model (Tachikawa et al., 2011) is based on ten AMS 14Cdates of G. ruber (white), and a correlation of the benthic forami-niferal d18O record with the LR04 reference benthic stack (Lisieckiand Raymo, 2005). The deglacial change in benthic foraminiferald18O of core MD05-2920 might slightly lag the continental icevolume because of diachronous deepwater temperature variations(Skinner and Shackleton, 2005; Lisiecki and Raymo, 2009) and adelayed deep ocean propagation of the glacio-eustatic d18O signalduring glacial terminations (Ganopolski and Roche, 2009; Friedrichand Timmermann, 2012).

For the given age model, the sedimentation rate varied from6 cm/ka to 13 cm/ka, with a mean value of 10 cm/ka (Tachikawaet al., 2011). The mean temporal resolution of the Mg/Ca analysesis about 450 yr over the last 140 ka. For the older part of the core,

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e34 27

the mean resolution is 1100 yr, but with a better resolution of about600 yr around the terminations.

3.2. Foraminiferal Mg/Ca and stable isotope analyses

Prior to themulti-step cleaning forMg/Ca analysis, thirty tests ofG. ruber (250e355 mm) were weighed to assess the potential in-fluence of test dissolution onMg/Ca ratios (Fig. S2). All the weighedtests were gently crushed and subjected to the Mg cleaning pro-cedure (Barker et al., 2003). After dissolution of tests, the solutionswere analyzed by an ICP-OES instrument (Jobin Yvon Ultima C) atCEREGE. The accuracy of our measurements was checked by in-ternational calibration (Rosenthal et al., 2004; Greaves et al., 2008).The mean external reproducibility and accuracy of Mg/Ca are betterthan 0.5%. Mg/Ca is converted into SST using Mg/Ca (mmol/mol)¼0.38 e0.09T(

�C) calibration (Anand et al., 2003). The overall uncer-tainty of the temperature estimate is �1 �C based on the scatter ofcalibrations (Anand et al., 2003).

To assess the potential bias of Mg/Ca-SST in relation to partialtest dissolution, individual foraminiferal test weight and test lossduring cleaning were quantified. The test weight varies from 8.2 to14.5 mg/individual for G. ruber, with a mean value of 11.5�1.1 mg/individual (1s, n¼ 651, Fig. S2). Even though a subtle decreasingtrend is observed for the older part of the core, the correlationbetween G. ruber test weight and age is weak (r2¼ 0.26), and thereis no systematic change on glacial/interglacial timescales (Fig. S2). Ithas been already shown that the change in test weight does notnecessarily modify foraminiferal Mg/Ca (Tachikawa et al., 2008).The proportion of test loss during cleaning is estimated at 18e75%,with a mean value of 34� 8% (1s, n¼ 651). For the whole core, theloss is clearly smaller than the critical value of 80% (Tachikawa et al.,2008). The present calcite saturation depth in the Western Equa-torial Pacific is approximately 2500 m (Feely et al., 2012), deeperthan the water depth of the core location (1843 m).

The benthic foraminiferal d18O measurements were carried outat CEREGE using a mass spectrometer (Finnigan Delta Advantage)equipped with a carbonate preparation device. The measured iso-topic values are normalized against NBS19. Mean external repro-ducibility is better than 0.05&.

3.3. Transient climate-model experiment

To further study the effects of time-varying greenhouse gasconcentrations, ice-sheet changes and orbitally-induced insolationchanges on tropical Pacific, we conducted a 406 ka transient modelsimulation with a 3-dimensional coupled ice-sheet climate model,which is based on the ice-sheet LOVECLIM (iLOVE) modellingframework.

The model has been developed from the previous version ofLOVECLIM(V1.1) (Goosse et al., 2010) and the community ice-sheetmodel GLIMMER(V1.7.1) (Rutt et al., 2009). The spectral atmo-spheric component uses T21 horizontal resolution (approx. gridresolution 5.625� � 5.625�) and 3 vertical layers. Atmospheric dy-namics are governed by the quasi-geostrophic potential vorticityequation and ageostrophic forcing terms that are diagnosed fromthe vertical motion field (Opsteegh et al., 1998). A set of physicalparameterizations of diabatic processes (radiative fluxes, sensibleand latent heat fluxes) is included into the thermodynamicequation.

The sea ice-ocean component “Coupled Large-scale Ice-Ocean”(CLIO) (Goosse and Fichefet, 1999) consists of a primitive equationocean general circulation model with 3� � 3� resolution on a partlyrotated grid in the North Atlantic. Vertical resolution consists of 20unevenly spaced levels. Mixing along isopycnals, the effect ofmesoscale eddies on diapycnal transports and mixing (Gent and

McWilliams, 1990), and downsloping currents at the bottom ofcontinental shelves are parameterized.

The terrestrial vegetation model “VEgetation COntinuousDEscription” (VECODE) (Brovkin et al., 1997) consists of two plantfunctional types and non-vegetated desert zones. Each grid cellassumes a partial coverage by these three land cover typesdepending on the annual mean temperature and rainfall amountand variability.

The land-ice model GLIMMER is a three-dimensional thermo-mechanical ice-sheet model for grounded ice. The ice dynamics aresimulated using the shallow-ice approximation (Rutt et al., 2009).In the current configuration we simulate the entire NorthernHemisphere domain (approx 30�Ne90�N) in a polar stereographicprojection with 100�100 km grid resolution. Eleven sigma-levelsare used to represent the ice-sheet dynamics and thermody-namics. At the bottom interface between bedrock and ice no-slipconditions are applied. In this simulation we adopted a dailypositive-degree-day (PDD) scheme using daily-mean climatologicaltemperature and precipitation data.

We developed an asynchronous northern hemisphere couplingstrategy between LOVECLIM and GLIMMER similar to the oneapplied in CLIMBER (Ganopolski et al., 2010). To account for thelonger timescales of ice-sheet dynamics relative to atmosphereeoceanesea iceevegetation dynamics, the coupling intervals for theice-sheet model are typically longer (1000 model years) than thecoupling intervals for LOVECLIM (50 model years). Each time a 50-year block of LOVECLIM integration is finished, the updated dailyclimatologies of precipitation and surface temperature are passedto GLIMMER. In GLIMMER these surface temperatures are used tocalculate PDDs after horizontal interpolation to the finer ice-sheetgrid and vertical interpolation using a lapse rate 0.5 K/1000 m.The surface mass balance is obtained from PDD and the simulatedLOVECLIM precipitation field. After completion of a 1000-year-longGLIMMER simulation segment the resulting ice-sheet height andextent is used to update the orography and surface albedo inLOVECLIM for the subsequent 50-year-long model simulation.Slowly time-varying boundary conditions due to astronomical andgreenhouse gas forcing (CO2, CH4, N2O) are implemented asaccelerated forcings into LOVECLIM (Timm et al., 2008;Timmermann et al., 2013).

Choosing an acceleration factor of for instance 20, 50 years ofLOVECLIM simulation are subject to the external forcing history of a1000-year interval. With this method, only LOVECLIM experiencesaccelerated orbital and greenhouse gas forcing, whereas the ice-sheet model GLIMMER experiences unaccelerated forcings andboundary conditions. A simulation covering the forcing history ofthe last 406 ka will then result in 20,300 LOVECLIM model yearsand 406,000 GLIMMER model years. More details on the advan-tages and disadvantages of the orbital acceleration technique canbe found in Timm and Timmermann (2007) and Timm et al. (2008).

The initialization of the transient coupled simulation starts ininterglacial conditions at 406,000 BP.Without proper knowledge ofthe exact ice-sheet conditions at this time, we chose the initialconditions of the Eemian period at 125,000 BP, which was obtainedfrom previous uncoupled interglacial simulations with LOVECLIMand GLIMMER. The iLOVE present-day climate sensitivity amountsto 3.2 K for a CO2 doubling.

4. Results

4.1. WPWP SST reconstructions

Over the past 400 ka, Mg/Ca values in G. ruber varied between3.43 and 5.68 mmol/mol (Fig. S2), which corresponds to an SSTrange between 24.4 and 30.1 �C, according to a commonly-used

Fig. 2. MD05-2920 time series compared with other records. (a) LR04 stack (Lisiecki and Raymo, 2005). (b) Benthic foraminiferal U. peregrina and C. wuellerstorfi d18O records of coreMD05-2920 over the last 400 ka (this study). A correction is applied for the empirical bias of C. wuellerstorfi d18O (þ0.64&). (c) G. ruber Mg/Ca-SST of core MD05-2920 (this study)compared to atmospheric CO2 variability based on Antarctic EPICA Dome C (Monnin et al., 2001) and Vostok ice cores (Petit et al., 1999; Pepin et al., 2001; Raynaud et al., 2005)using the EDC3 chronology. Note that CO2 concentration is shown with logarithm-scale. (d) Zoom on Terminations I, II, III and IV, showing the variability of Mg/Ca-SST and benthicforaminiferal d18O (this study). Blue zones represent glacial periods and numbers along upper x-axis indicate marine isotopic stages. Black diamonds and white triangles along theupper x-axis indicate 14C dating points and benthic d18O tie points (Tachikawa et al., 2011). (For interpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e3428

calibration (Anand et al., 2003) (Fig. 2c). The core-top value of28.7 �C agrees well with the present-day annual mean temperatureof 28.9� 0.3 �C (1s) in the upper 50 m from World Ocean Atlas2009 (Locarnini et al., 2010) (Figs. 1b and 2c). The SST at the corelocation during the Last Glacial Maximum (LGM, 23e19 ka BP) isestimated at 25.5� 0.4 �C (1s, n¼ 31), which corresponds to acooling of 3.1�0.5 �C (1s) relative to the value of the late Holocene(2e0 ka BP) of 28.5� 0.3 �C (1s, n¼ 4) (Fig. 2c). The glacial/inter-glacial SST amplitude of Termination III is comparable to theTermination I value whereas it attains values of up to 4 �C for Ter-minations II and IV (Fig. 2d). OurMD05-2920Mg/Ca record exhibitsvery pronounced glacial/interglacial variability over the past400 ka, with characteristic orbital periodicities of 100 ka (eccen-tricity), 41 ka (obliquity) and 23 ka (precession) (see Fig. S4). It

shares the general glacial/interglacial features with other recordsfrom the central WPWP but with more detailed variability forTerminations III and IV (Fig. S1).

Relative variations of C. wuellerstorfi and U. peregrina benthicforaminiferal d18O records of core MD05-2920 closely match thestacked LR04 composite record (Lisiecki and Raymo, 2005) withtypical glacial/interglacial amplitudes of 1.3e1.6& (Fig. 2a and b).Considering that the current bottom water at the core location is amixture of water masses with different origins, changes in tempera-ture and/or seawater d18O of specific water masses might have onlysecond-order influenceonthebenthic foraminiferald18O. Thebenthicd18O records of core MD05-2920 can be thus considered to primarilyreflect the global variability in response to continental ice volume andto intermediate/deep water temperature on orbital timescales.

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e34 29

The G. ruber Mg/Ca-SST (Fig. 2c), and C. wuellerstorfi andU. peregrina benthic foraminiferal d18O records (Fig. 2b) of coreMD05-2920 show that WPWP SSTs varied almost synchronouslywith the global d18O change inferred by the benthic foraminiferald18O records for the last four terminations (Fig. 2d).

Comparison between the MD05-2920 SST reconstruction andthe simulated mean annual SST in experiment TR400 at the corelocation reveals good phase correspondence (Fig. 3a) and amaximum cross-correlation of 0.78 at lag 0 ka. The observed in-phase relationship between externally-forced model variabilityand the MD05-2920 SST record on orbital timescales provides anindependent support for the age model of MD05-2920, which wasderived from a benthic foraminiferal d18O stack. However, we findconsiderably smaller amplitudes of glacial/interglacial SST changein the model solution of onlyw1.7 �C for the LGM, compared to thereconstructed range of 3.1�0.5 �C. It is worth putting these esti-mates into the context of the LGMmulti-model uncertainties. Usingoutput from the Paleomodel Intercomparison Project 2 (PMIP2)multi-model ensemble (Braconnot et al., 2007) (see Fig. 3a, col-oured bars) we find glacial/interglacial SST differences in theWPWP ranging from 1.7 �C (IAP-FGOALS model) to 4.0 �C (GFDLCM2.1 model).

To estimate the potential effect of seasonal biases in foraminif-eral production on the reconstructed SSTs (a possible source for themismatch between model and SST reconstruction) we calculated aforaminifera production-weighted average SST time series,assuming that (1) seasonal G. ruber production follows the present-day climatology of euphotic zone chlorophyll concentrations(Fig. S3c) and (2) the production patternwas constant thorough thestudied period. Multiplying the proportion of seasonal production

Fig. 3. Comparison between Mg/Ca-SST and simulated SST records. (a) G. ruber Mg/Ca-SSTColoured bars along SST axis indicate LGM cooling range obtained by Paleomodel Intercblue¼GFDL CM2.1 model (�4.0 �C), light blue¼ IPSL (�2.4 �C), violet¼UKMO (�2.0 �C),Comparison between the mean annual SST simulated with TR400 (black curve) and forasensitivity experiment for the last 130 ka is also shown (dark blue curve) as SST anomalynorthern hemisphere ice-sheet forcing. (For interpretation of the references to colour in th

by TR400 seasonal WPWP SST time series yielded the weightedaverage SST time series.

The result indicates that the production-weighted average SSTclosely matches the mean annual TR400-SST in terms of bothtiming and amplitude (Fig. 3b). This suggests that seasonality ef-fects are small in the Mg/Ca-based SST reconstruction and are un-likely to explain the difference between simulated andreconstructed WPWP SSTs.

4.2. The WPWP SST and atmospheric CO2

One of the most noticeable features of the Mg/Ca-SST record isthe similarity to past atmospheric CO2 forcing changes (CO2 vari-ations plotted on a log-scale in Fig. 2c) inferred from Antarctic icecores (EPICA Dome C and Vostok) (Petit et al., 1999; Monnin et al.,2001; Pepin et al., 2001; Raynaud et al., 2005). The lag correlationcoefficient between Mg/Ca-SST and logarithm of CO2 attains amaximum value of 0.85 at lag 0 ka. The corresponding linear rela-tion Mg/Ca-SST¼ 7.7 ln(CO2)� 15.0 explains about 72% of the totalSST variance. The simulated SST in TR400 (Fig. 3a) is correlatedwiththe CO2 radiative forcing attaining values of 0.91 at lag 0 ka,yielding a simple regression model TR400-SST¼ 3.7 ln(CO2)þ 7.6,with an explained variance of 0.84, but a smaller slope parameterthan for the SST reconstructions.

To further examine the effect of CO2 forcing on the SST in theWPWP, we evaluated another LOVECLIM sensitivity simulationwhich uses fixed pre-industrial atmospheric CO2 concentrationsbut time-varying ice-sheet topography and albedo from an off-lineice-sheet model simulation conducted with the IcIES model (Abe-Ouchi et al., 2007). In this experiment the range of orbital-scale

and simulated annual SST range from experiment TR400 (at 147.5E/1.25S, this study).omparison Project 2 (PMIP2) multi-model ensemble (Braconnot et al., 2007). Darkorange¼ CCSM3 (�1.9 �C), red¼MIROC (�1.9 �C), green¼ IAP-FGOALS (�1.7 �C). (b)miniferal production-weighted average SST (light blue curve) records. Result of CO2

using constant pre-industrial concentrations and time-dependent orbital forcing andis figure legend, the reader is referred to the web version of this article.)

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e3430

SST variability at the grid close to the core location amounts toabout 0.3 �C (Fig. 3b), which is similar to previous estimates oforbitally-forced warm pool SSTs (Timmermann et al., 2007). Thefact that the non-CO2 effects can only account for 18% of simulatedglacial/interglacial amplitude of TR400-SST clearly demonstratesthat that greenhouse gas forcing is the primary driver for theWPWP temperature variability for the last deglaciation (Clark et al.,2012). This is different from the eastern equatorial Pacific, whereremote effects of the ice-sheets can play a much larger role, ac-cording to past modelling studies (Timmermann et al., 2004).

Regional non-CO2 effects on WPWP SST can be determined bysubtracting the CO2 contribution (7.7 ln(CO2)� 15.0) from the Mg/Ca-SST. The same procedure is applied to TR400 (with the respec-tive regression SST¼ 3.7 ln(CO2)þ 7.6). The resulting residual Mg/Ca-SST records are characterized by dominant precessional vari-ability with a period of 23 ka (Fig. 4a). The residual record based onTR400-SST also has a frequency corresponding to obliquity (notshown in figure). The common feature in the residual SST records,the precessional cycle, may indicate regional effects on the WPWPSST. The governing processes for this type of variability will bediscussed further below.

5. Discussion

Both proxy reconstruction andmodel simulations are consistentwith the previous finding that greenhouse gas radiative forcing isthe primary factor affecting the glacial/interglacial WPWP SSTchanges (Clark et al., 2012; Hargreaves et al., 2012), although withdifferent sensitivities. In the following, we discuss some possiblereasons for the apparent mismatch in the amplitude of simulatedand reconstructed SST variability, as well the origin of precessional-scale residual SST variability. We will also address the relation of

Fig. 4. Residual Mg/Ca-SST record of core MD05-2920 compared with parameters related toannual northward flowing NGCC in model at velocity grid point closest to the New Guinea cderived from the natural logarithm of Ti/Ca concentrations measured in core MD05-2920 an(136�Ee148�E; 8S�e2�S). Note that ln(Ti/Ca) axis and the precipitation axis are reversed tobars mark precession cycle.

WPWP SST variations and estimates of global mean temperaturechanges.

5.1. Possible reasons for different WPWP SST amplitude betweenreconstruction and TR400

The foraminiferal Mg/Ca thermometer is based on a relativelylimited number of organisms (in this study, we used thirty fora-miniferal tests per sample). Considering this point, potential factorsaffecting Mg/Ca-SST could be bioturbation, sampling and mea-surement noise, and ecological effects on foraminifera (Laepple andHuybers, 2013). On the other hand, underestimated feedbacks inthe model could contribute to creating the difference. Below weevaluate each possibility related to the proxy and the model.

Under current conditions, surface salinity influence on forami-niferal Mg/Ca-SST is estimated to be negligible because of thenarrow salinity range and the mean value of 34.5 (Fig. S3b) that islower than the critical salinity of 35 (Arbuszewski et al., 2010).However, during glacial periods, global salinity increased about 1unit because of a 3% reduction of ocean volume, potentially leadingto salinities at the studied site higher than the critical value. Inorder to evaluate the impact of glacial salinity increases, we applythe calibration (Kısakürek et al., 2010) that contains a salinity-correction term, assuming that the glacial salinity at core locationwas 35.5. The estimated SST at MD05-2920 core location duringLGM is 25.0� 0.6 �C (1s, n¼ 31) compared to the original estimateof 25.5� 0.4 �C. The SST of the late Holocene (2e0 ka BP) is29.1�0.4 �C (1s, n¼ 4) with the calibration by Kısakürek et al.(2010) with a salinity of 34.5, which agrees with the original esti-mate of 28.9� 0.3 �C within uncertainty. Consequently, theconsideration of salinity effect is small and rather emphasizes theglacial/interglacial amplitude of Mg/Ca-SST. The glacial salinity

ITCZ for the past 400 ka. (a) The residual Mg/Ca-SST and simulated magnitude [m/s] ofoast; (b) precipitation proxy for northern Papua New Guinea (Tachikawa et al., 2011) asd simulated time series of February/March/April averaged precipitation in New Guineaindicate a more northern (upward) or southern (downward) position of the ITCZ. Grey

Fig. 5. Relationship between Antarctic air temperature anomaly relative to the presentand WPWP SST anomaly for the last 400 ka. Proxy relationship between Mg/Ca-SST ofcore MD05-2920 and the deuterium-based temperature reconstructions fromAntarctica (Jouzel et al., 2007) is characterized by a slope of 2.9 (r2¼ 0.94).

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e34 31

effect, even if present for the core location, is likely not the reasonfor the differences in simulated and reconstructed glacial/inter-glacial SST amplitudes in the WPWP.

Other factors include a potential deepening of the calcificationdepth of G. ruber under glacial conditions, which could lead to anamplification of the glacial/interglacial reconstructed temperaturerange. This possibility, however, seems unlikely, considering theecological preference of this warm water species (Hemleben et al.,1988). Moreover, the two morphotypes of G. ruber calcifying atdistinct water depths present negligible difference of Mg/Ca notonly for Holocene but also for the LGM (Table 1).

Surface seawater pH and carbonate ion concentrations may alsoaffect foraminiferal Mg/Ca during the calcification (Lea et al., 1999;Russell et al., 2004) but the effect is estimated to be significant onlyfor conditions with pH and carbonate ion concentrations consid-erably lower than the usual open-ocean values (Lea et al., 1999;Russell et al., 2004). Reconstructed surface water pH in the west-ern equatorial Pacific during the LGM was slightly higher thanduring the late Holocene (Palmer and Pearson, 2003), suggestingthat the glacial/interglacial changes of carbonate system did notconsiderably affect G. ruber Mg/Ca during the calcification.

We have already discussed the good preservation of G. rubertests for the whole core (Fig. S2). In addition, carbonate preserva-tion was generally better during glacial periods in the Pacific basin(Farrell and Prell, 1989). To explain the model/data mismatch in theamplitude of glacial SST variability by test preservation, glacial testsshould have been subjected to more dissolution than for theinterglacial periods. This is not the case. Finally, the bioturbationeffect smoothes the signal and reduces the amplitude of glacial/interglacial Mg/Ca-SST changes, which does not explain the model/data mismatch either. Taken together, it seems implausible toexplain the mismatch between the proxy and modelled SST only bybiases related to foraminiferal Mg/Ca.

If the reconstructed temperature anomalies are representativefor a larger open-ocean domain, then the alternative is that themodel’s tropical Pacific SST sensitivity is too low. Simulation TR400underestimates the reconstructed glacial/interglacial SST range bynearly a factor two. The discrepancy is reminiscent of the fact thatunder current climate conditions the warm pool heat budget isdelicately balanced by rather large individual contributions (Liet al., 2000). Missing feedbacks from clouds, underestimation ofwater vapour feedbacks, or the use of a linearized radiative transferparameterization for shortwave and longwave radiation in LOVE-CLIM could contribute to creating a lower temperature sensitivityin the tropical Pacific to greenhouse gas forcings. A systematicanalysis of the individual fluxes in the upper ocean heat balancecould shed further light on the low SST amplitude signal during theglacialeinterglacial cycles. In addition, missing forcings, such asdust, could contribute to the lower simulated SST amplitude.

Here we conclude, that the present mismatch in simulated andreconstructed SST amplitudes is likely due to the uncertainty of theiLOVE model simulation (see Fig. 3a for the range of other PMIPmodels), since the differences can neither be explained by regionalenvironmental parameters nor by ecological foraminiferal re-sponses for the studied site.

5.2. Identification of tropical processes responsible for residual SSTvariability

Monsoon wind variability in the western tropical Pacific is to alarge extent determined by the meridional movements of the ITCZ.At present, when the ITCZ is at its southernmost position in australsummer, the SST at the core location decreases because the weakernorthward boundary current allows cooler surface water to enterthe study site (Fig. 1c). Based on this present-day analogy, we will

examine the hypothesis that the residual SST with precessionalcycle (Fig. 4a) was modulated by past changes in ITCZ position andwind-driven heat transport convergence due to anomalous surfacecurrents.

The past ITCZ position in the studied region is inferred by thereconstruction of precipitation over Papua New Guinea that can betraced by river particle discharge. The river particle inputs, derivedfrom Titanium to Calcium ratio in core MD05-2920 (Tachikawaet al., 2011) indicate a marked precessional variability (Fig. 4b).The residual Mg/Ca-SST is lower when the past precipitation washigher (Fig. 4a and b), the condition corresponding to the southernposition of ITCZ at present (January, Fig. 1c). Using BlackmaneTurkey cross-spectral analysis, we find a high coherence (0.94)between the residual Mg/Ca-SST and log-scale Ti/Ca on preces-sional timescales of 23 ka. The variability of Ti/Ca record agreesqualitatively with TR400 simulated precipitation over Papua NewGuinea (Fig. 4b), supporting the robustness of reconstructed pre-cipitation record related to the ITCZ positions. Finally, the residualMg/Ca-SST variability is highly coherent (coherence of 0.88) atprecessional band with the simulated strength of the wind-drivensurface tropical boundary current off Papua New Guinea in TR400(Fig. 4a). The residual TR400-SST is also correlated with thestrength of the wind-driven current with a high coherence of 0.96at precession periodicity (not shown in figure).

The proxy and model results are qualitatively consistent witheach other, supporting the relationship between precessional var-iations in the position of the ITCZ, northward surface currents, heattransport convergence and residual SST.

5.3. WPWP SST and global mean temperatures

Our results show a strong linkage between CO2 and WPWP SST,and only limited impact of local effects, which occur mostly onprecessional timescales as a result of the mechanisms identified inthe previous subsection. The tight relationship betweenWPWP SSTand atmospheric CO2 is consistent with previous studies that

K. Tachikawa et al. / Quaternary Science Reviews 86 (2014) 24e3432

proposed to use the WPWP SST as an indicator of first orderchanges of the past global mean temperature (Hansen et al., 2008;Masson-Delmotte et al., 2010). A factor of 1.5 was suggested toconvert the WPWP SST change to the global mean temperaturevariability (Hansen et al., 2008). We revisit this approach in light ofthe new WPWP SST record and recent global mean cooling esti-mates during LGM (Schmittner et al., 2011; Shakun et al., 2012;Annan and Hargreaves, 2013).

Several studies have shown that as a result of the overwhelmingeffect of CO2 changes on both Antarctic air temperature and globalmean air temperature, Antarctica temperature reconstructions canbe scaled to obtain first order estimates of global mean temperaturevariations (Masson-Delmotte et al., 2006; Kohler et al., 2010). Wethus at first examine the relationship between the WPWP Mg/Ca-SST (this study) and deuterium-based temperature re-constructions from Antarctica (Jouzel et al., 2007).

Comparing the WPWP SST reconstructions with deuterium-based temperature reconstructions from Antarctica (Jouzel et al.,2007) and owing to the dominant CO2 forcing acting on both, wefind an excellent agreement (r2¼ 0.94, Fig. 5). Glacial/interglacialtemperature variations in Antarctica are about 2.9 times larger thanthose from the WPWP region (Fig. 5). In spite of the smaller overallamplitudes of the glacial variability, a similar polar amplificationfactor is attained for the simulated WPWP and Antarctic temper-ature variations in TR400. The smaller variability of WPWP SSTrelative to the Antarctic continental temperatures can be explainedby the fact that the effect of external forcings is damped by largelatent cooling oceans and deep mixed layers, whereas evaporativedamping is absent for Antarctic temperatures. The high correlationbetween the tropical and polar temperature reconstructions sup-ports the hypothesis that both records are to first order propor-tional to global mean temperature variability.

With CO2 affecting global mean temperatures, WPWP SSTs andAntarctic temperatures on orbital timescales, we can estimate thescaling factor a between WPWP SSTs dTWPWP and global meantemperature variations dTGM¼ adTWPWP. We use a series of esti-mates of LGM global mean temperature changes ofdTGM¼ 2.6� 0.5 �C (Schmittner et al., 2011), dTGM¼ 3.6 �C (Shakunet al., 2012) and dTGM¼ 4.0� 0.8 �C (Annan and Hargreaves, 2013)to determine the approximate value of a. With dTWPWP 3.1�0.5 �C,we obtain a warm pool temperature scaling factor a rangingroughly from 1 to 1.5. However, it should be noted here that thisscaling factor depends also on the estimate of global mean tem-perature at LGM that still contains uncertainty: In spite of intenseefforts, spatial coverage of proxy records are not sufficient for somecontinents (Bartlein et al., 2011) and oceanic regions covered by seaice (MARGO Project Members, 2009).

Compilation of well-dated proxy records combined with model-ling results may further help to constrain the changes in past globalmean temperature and the underlying mechanisms (Schmittneret al., 2011; Clark et al., 2012; Shakun et al., 2012) and resultingclimate sensitivity. However, such efforts have previously focused onthe LGM time slice because of chronological uncertainties of proxyrecords and lack of high-resolution records sufficiently covering thesurface of the earth. Our results are based on one marine core takenfrom a key climate area e the WPWPe, which is sensitive mainly togreenhouse gas forcings. Furthermore, our study underscores theimportance of highly resolved records from targeted CO2 sensitiveareas that will provide deeper insight into the effects of glacialboundary conditions and forcings on global climate.

6. Conclusions

Combining proxy reconstruction and transient model simula-tions, we determined the influences of global CO2 radiative forcing

and regional effects on the SST in the western Pacific warm poolover the past 400 ka by analysis of a marine sediment core MD05-2920 collected off the mouth of the Sepik River to the North ofPapua New Guinea.

G. ruber Mg/Ca-SST of core MD05-2920 exhibits an in-phaserelationship with atmospheric CO2 concentration estimated fromAntarctic ice core measurements for the whole studied period,demonstrating the dominant control of greenhouse gas radiativeforcing on the WPWP SST on orbital timescales. A transient climatesimulation with fixed atmospheric CO2 concentration further sup-ported this result.

Regional effects have limited impact on the SST, and can bequantified as residual SST variability unrelated to the CO2 forcing.This variability is characterized by a 23 ka periodicity. We alsoconcluded that in our study site seasonal production of G. ruber islikely to have a small impact. Frommodern seasonal analogues, anddata-model comparisons, a wind-driven northward surface currentadvection towards the WPWP is determined to be responsible forthe regional SST effects on precessional timescales. This variabilitycan be attributed tomeridional shifts of position of the IntertropicalConvergence Zone.

Our new Mg/Ca-SST record from the WPWP is strongly corre-lated with Antarctic air temperatures for the past 400 ka, sug-gesting that glacial-scale temperature variability in these tworegions is primarily driven by greenhouse gas forcings. Comparingthe LGM temperature change of the WPWP based on the Mg/Cadata of core MD05-2920 with recent estimates of LGM globalcooling, we propose that the scaled WPWP SST reconstructionsprovide a first order estimate of global mean temperature changesfor the past 400 ka with a scaling factor of 1e1.5.

Competing interests

The authors declare that they have no competing financialinterests.

Acknowledgements

We acknowledge support from INSU and the French PolarInstitute IPEV, who provided the RV Marion Dufresne and CALYPSOcoring system used during the MD148 IMAGES XIII PECTEN cruise.We thank Luc Beaufort (Chief Scientist), Yvon Balut and the crew ofthe PECTEN cruise. We appreciate insightful discussions with Drs.D. Paillard, L. Stott and G. Schmidt. Two anonymous reviewers arethanked for their helpful comments and suggestions. This researchwas funded by the European Community (Project Past4Future) toK.T. and L.V., MicroForam (CYBER/LEFE-INSU/CNRS) to K.T. and ANRMAGORB (ANR-09-BLAN-0053-01) to L.V. A.T. and O. T. were sup-ported by grant 1010869 from the US National Science Foundation.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quascirev.2013.12.018.

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