Onset of ‘Pacific-style’ deep-sea sedimentary carbonate cyclesat the mid-Pleistocene transition

Philip F. Sexton a,b,⁎, Stephen Barker b

a Centre for Earth, Planetary, Space & Astronomical Research, The Open University, Milton Keynes, MK7 6AA, UKb School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 February 2011Received in revised form 15 December 2011Accepted 29 December 2011Available online 3 February 2012

Editor: P. DeMenocal

Keywords:AtlanticPacificcarbonate cyclesPleistoceneglacialocean circulation

The calcium carbonate (CaCO3) content of deep-sea sediments in the Pacific Ocean increases during glacialsof the late Pleistocene in comparison to interglacials, whereas records of sedimentary CaCO3 in the AtlanticOcean show an anticorrelated pattern across glacial-interglacial cycles. Here we show that this anticorrela-tion in inter-ocean CaCO3 cycles arose comparatively recently, at ~1.10 million years ago (Ma), during themid-Pleistocene transition. Before this time, we show that the CaCO3 content of Pacific and Atlantic Oceansediments experienced in-phase cyclicity, both having ‘Atlantic-style’ phasing with respect to glacial–inter-glacial cycles. The onset of anticorrelated cyclicity at 1.10 Ma involved a twofold switch in the Pacific'sCaCO3 cycles: glacial CaCO3 preservation became consistently better while interglacial preservation becameconsistently worse. We demonstrate that the cause of this glacial–interglacial ‘mirror imaging’ of PacificCaCO3 dissolution cyclicity at 1.10 Ma was a switching over of the relative ventilation state of abyssalSouth Pacific waters between glacials and interglacials. Specifically, we suggest that a consistent strengthen-ing of deep water ventilation within the Pacific sector of the Southern Ocean during glacials drove glacial Pa-cific CaCO3 dissolution to diminish, while a contemporaneous weakening of very well-ventilated ‘upstream’

North Atlantic Deep Water during interglacials drove interglacial Pacific CaCO3 dissolution ‘downstream’ tointensify. We propose that the increased mean alkalinity of the global deep ocean (driven by the geograph-ically and bathymetrically vast Pacific Ocean) during glacials after 1.10 Ma may explain part of the drawdownin glacial atmospheric CO2 levels hypothesised to have been linked to the increased severity of these latePleistocene glacials.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Sediments accumulating on the deep seafloor are predominantlybiogenic in origin, and by far the greatest component of these sedi-ments is calcium carbonate (CaCO3). Owing to greater removal of al-kalinity from the ocean (via biological production) than addition ofalkalinity into the ocean (via river run-off from the continents), andin order to meet the need for these fluxes to balance, only a fractionof CaCO3 produced in the sunlit surface ocean is permanently re-moved from the ocean via seafloor sedimentary burial. These budget-ary constraints dictate that most of the deep seafloor below about4.5 km depth is prohibitive to survival of CaCO3 — at this depth(termed the carbonate compensation depth [CCD]) all CaCO3 dis-solves, returning alkalinity back to the ocean. However, the depth atwhich the CCD is located varies between ocean basins (~4 km in thePacific versus ~5 km in the Atlantic [Archer, 1996; Berger andWinterer, 1974]), primarily as a result of the ‘age’ and ‘preformed

nutrient content’ (acidity) of the dominant water mass bathing eachbasin (Berger, 1968, 1970; Broecker and Peng, 1982).

In today's oceans, the Atlantic is favourable to CaCO3 sedimenta-tion because of production in its northern reaches of the ‘youngest’and least corrosive (owing to very low preformed nutrient concentra-tions) deep waters globally (North Atlantic Deep Water, NADW). Incontrast, Pacific sediments are relatively CaCO3-poor owing to the‘upstream’ injection of deep waters with high preformed nutrientconcentrations (e.g. Circumpolar Deep Water, CDW) and this basin'sposition toward the end of a long deep ocean ‘conveyor’ characterisedby progressive ‘ageing’ (acidification) of waters along its length (e.g.Berger, 1968, 1970; Broecker and Peng, 1982; Crowley, 1985;Ruddiman and Heezen, 1967). This ageing process is a consequenceof cumulative remineralisation at depth along the conveyor of thecontinual rain of organic carbon from the sunlit upper ocean. Conse-quently, deep-sea CaCO3 sediments are a sensitive monitor of therelative corrosivity (acidity) of competing deep water masses.

Curiously, this pattern of relative propensity of CaCO3 depositionin each basin reverses during glacials of the late Pleistocene. Sedi-mentary CaCO3 content in the Pacific and Indian Oceans increasesduring glacials in comparison to interglacials (e.g. Anderson et al.,

Earth and Planetary Science Letters 321-322 (2012) 81–94

⁎ Corresponding author at: Centre for Earth, Planetary, Space & AstronomicalResearch, The Open University, Milton Keynes, MK7 6AA, UK.

E-mail address: P.F.Sexton@open.ac.uk (P.F. Sexton).

0012-821X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2011.12.043

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2008; Arrhenius, 1952; Berger, 1970, 1973; Broecker, 1971; Farrelland Prell, 1989; Hays et al., 1969; Le and Shackleton, 1992;Ninkovich and Shackleton, 1975; Peterson and Prell, 1985;Thompson and Saito, 1974), whereas Atlantic CaCO3 contents are typ-ically lower during glacials (e.g. Bé et al., 1976; Berger, 1973;Broecker, 1971; Crowley, 1985; Curry and Lohmann, 1990; Damuth,1977; deMenocal et al., 1997; Gardner, 1975; Hodell et al., 2001;Olausson, 1965; Philippi, 1910; Verardo and McIntyre, 1994; Volatet al., 1980) (Fig. 1; see Fig. 2 for site locations). Although Fig. 1shows the existence, to a first order, of an anticorrelation betweenAtlantic versus Pacific CaCO3 cycles, we note that this ‘classic view’of inter-basin CaCO3 cycles may be a slight over-simplification.When viewed in greater detail, some of the finer structure of CaCO3

variance between the Atlantic and Pacific appears to have occurredin parallel (Crowley, 1985; Rickaby et al., 2010).

Here we demonstrate that the first order anticorrelation in inter-ocean CaCO3 cycles was initiated at 1.10 million years ago (Ma)during the ‘mid-Pleistocene transition’ from moderate glacial cyclesof 40 kyr in duration to extreme cycles 100 kyr-long. We show that,prior to 1.10 Ma, glacial–interglacial variations in sedimentaryCaCO3 were in-phase between the Atlantic and Pacific, with higherCaCO3 contents occurring in both basins during interglacial periodsrelative to glacials. The onset of anticorrelated cyclicity at 1.10 Mainvolved a twofold switch in Pacific CaCO3 cycles. CaCO3 (and sedi-mentary coarse fraction) contents during glacials consistently in-creased at the same time as they consistently decreased duringinterglacials. This glacial–interglacial ‘mirror imaging’ of PacificCaCO3 cycles at 1.10 Ma from their pre-1.10 Ma pattern producedno discernable change in mean CaCO3 (or coarse fraction) contentacross glacial–interglacial cycles, which partly explains why this

Fig. 1. Records of sedimentary CaCO3 from the deep Atlantic and Pacific oceans. a. CaCO3 content of sediment from central North Atlantic DSDP Site 609 (3880 mwater depth) (Rud-diman et al., 1989). Age model based on tuning benthic foraminifer δ18O record from this site (Ruddiman et al., 1989) to the LR04 benthic δ18O stack (Lisiecki and Raymo, 2005). b.CaCO3 content of sediment from central Pacific site W8803B-51GC (4400 m water depth) (LaMontagne et al., 1996). Age model based on tuning δ18O of the lower thermocline-dwelling planktic foraminifer G. tumida (LaMontagne et al., 1996) to the LR04 benthic δ18O stack.

Fig. 2. Location of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill sites (and a piston core from the Pacific) discussed here. Map from http://www.planiglobe.com/omc_set.html.

82 P.F. Sexton, S. Barker / Earth and Planetary Science Letters 321-322 (2012) 81–94

switch in CaCO3 cyclicity has escaped notice for so long. We demon-strate that the likely cause of this mirror imaging of Pacific CaCO3 cy-clicity was a switching over of the relative ventilation strength ofdeep South Pacific waters between glacials and interglacials. Specifi-cally, we suggest that a consistent strengthening of deep water venti-lation within the Southern Ocean Pacific sector during glacials causedglacial Pacific CaCO3 preservation to improve, while a coeval weaken-ing of very well-ventilated ‘upstream’ NADW during interglacialscaused interglacial Pacific CaCO3 preservation ‘downstream’ to deteri-orate. The increase in Southern Ocean Pacific sector deep water pro-duction during glacials at 1.10 Ma was contemporaneous withincreased northward penetration of glacial Southern Ocean watersinto the deep Atlantic (Raymo et al., 1997; Venz and Hodell, 2002)which, contrary to its ameliorating effect on glacial Pacific CaCO3

preservation, intensified glacial CaCO3 dissolution in the Atlantic(relative to that prevailing during interglacials when supersaturatedNADW predominates), thereby amplifying the inter-ocean anticorre-lation in CaCO3 cycles.

2. Dating the onset

A number of major questions about the Atlantic–Pacific anticorre-lation in CaCO3 cycles remain unanswered. For example, has thisanticorrelated relationship always existed? If not, when did it firstarise? Was the appearance of anticorrelated cycles gradual or abrupt?A number of studies in the early 1980s attempted to answer thesefundamental questions. CaCO3 cycles in Pacific sediments that yielded‘Atlantic-style’ patterns (i.e. low CaCO3 content during glacials) wereinferred for the late Pliocene (Vincent, 1981) and convincingly dem-onstrated for a ~400 kyr interval of the late Miocene (Dunn et al.,1981). An ‘Atlantic-style’ pattern was also found for Pacific CaCO3

cycles during the early Pliocene (Dunn, 1982) and it was suggestedthat the switch to a ‘Pacific-style’ pattern occurred during the mid-Pliocene (Dunn, 1982). However, independent analyses on the samePliocene sediment core through the same interval of geologic timeyielded conflicting conclusions (Prell, 1985). These inconsistenciesare likely to be symptomatic of the typically low resolution of theseearly records and their occasional resort to the use of planktic forami-niferal δ18O instead of benthic foraminiferal δ18O to depict glacial–interglacial cycles. A further limitation of this early work is that nocontemporaneous records from the Atlantic were available withwhich to properly test for parallelism (or otherwise) in inter-basinCaCO3 cycles. What is needed are long, continuous, high resolution re-cords of CaCO3 content and benthic δ18O, spanning contemporaneousintervals of geologic time in both ocean basins, and possessing detailedand accurate astronomically calibrated age models.

In Fig. 3 we show a long, high resolution record of sedimentaryCaCO3 content from the deep (3430 m) central North Atlantic(DSDP Site 607) versus benthic δ18O from this same site. FromFig. 3a–c it is clear that this Atlantic site has exhibited the classic‘Atlantic-style’ pattern (i.e. low CaCO3 content during glacials,1 de-fined here as ‘in phase’ behaviour; see phase wheels in Fig. 3) contin-uously throughout the past 3 million years. However, when we turn

(Fig. 4) to a similarly long, high resolution CaCO3 record from thedeep (3310 m) Pacific (ODP Site 846, eastern equatorial Pacific), wefind that, in the late Pliocene (3 to 2 Myr ago, Fig. 4a), this Pacificsite also shows the ‘Atlantic-style’ pattern of CaCO3 variability (i.e.low CaCO3 content during glacials; compare orange vector in phasewheel in Fig. 4a to that in Fig. 3a). In fact, Pacific CaCO3 content is,within error, precisely in phase with both benthic δ18O and AtlanticCaCO3 content (Figs. 3a, 4a). Figs. 4a, 5a and 6a demonstrate thatthe existence of this ‘Atlantic-style’ pattern in the Pacific is also seenin contemporaneous late Pliocene records of the sedimentary coarse(>63 μm) fraction content (a proxy for bottom water corrosivity2;i.e. greater dissolution intensity [lower % coarse fraction] during gla-cials) (red vectors on phase wheels) from both the western and east-ern Pacific. It is therefore apparent that, during the late Pliocene (3 to2 Myr ago), CaCO3 cycles (likely controlled primarily by variable dis-solution intensity) in the Atlantic and Pacific basins were in-phasewith each other (low % CaCO3 and low % coarse during glacials, butwith a slight (3 to 6 kyr) lead of % coarse over benthic δ18O(Table 1); compare phase wheels in Figs. 3a, 4a and 5a). Figs. 3b, 4b,5b and 6b show that this in-phase behaviour between the two basinsin their CaCO3 cycles (both ‘Atlantic-style’) also existed during theearly Pleistocene (2 to 1 Ma; compare phase wheels; although witha slight Pacific lead over the Atlantic of a few kyrs [Table 1]). YetFigs. 3c, 4c, 5c and 6c show that the Atlantic and Pacific display out-of-phase behaviour throughout the last 1 Myr (with the Pacific sitesswitching to their classic late Pleistocene ‘Pacific-style’ pattern;phase wheels show Pacific CaCO3 and coarse fraction contents[Figs. 4c, 5c and 6c] almost 180° anti-phased with respect to Atlantic[Fig. 3c]). The switch in phasing of Pacific CaCO3 cycles with respect toglacial cycles (and with respect to the Atlantic) must therefore liesomewhere near 1 Myr ago, and is consequently a relatively recentpalaeoceanographic phenomenon. Closer scrutiny of records fromthe three Pacific sites across 1 Myr ago reveals that the switch inphasing of Pacific CaCO3 cycles with respect to glacial cycles occurredbetween 1.10 and 1.05 Myr ago (Figs. 4d, 5d and 6d), during the earlyphase of the mid-Pleistocene transition (defined as spanning the in-terval 1.2 to 0.7 Ma Clark et al., 2006). Because the long Pacific recordsof CaCO3 and coarse fraction contents consistently show high valuesin tandemwith one another across climate cycles (and correspondinglow values for available foraminifer fragmentation data [Fig. 5a]), andrecords from all three Pacific sites display an identical and simulta-neous switch in phasing at ~1.10 Ma, we suggest that the predomi-nant common control on these two parameters is CaCO3 dissolution.This conclusion is in line with those from other studies that have sug-gested that, even in the biologically productive equatorial Pacific, thedominant control on Pacific sediment CaCO3 and coarse fraction var-iability is CaCO3 dissolution (Anderson et al., 2008; Hagelberg et al.,1995; Lalicata and Lea, 2011).

3. Precise nature of the change in Pacific CaCO3 phasing

Before we explore mechanisms that may be responsible for theappearance of anticorrelated inter-ocean CaCO3 cycles, we need to es-tablish precisely how the phasing of Pacific CaCO3 cycles changed at1.10 Ma. This phase change involved a twofold switch in PacificCaCO3 cycles. First and foremost, we would expect to see a consistentswitch to higher CaCO3 content (and higher coarse fraction) duringglacials (e.g. Fig. 1b), which is precisely what the Pacific data show(Fig. 4c, d; Fig. 5c, d; Fig. 6c, d). However, at the same time, the data

1 We acknowledge that variability in sedimentary CaCO3 content in the Atlanticacross glacial-interglacial cycles may incorporate changes in CaCO3 production and di-lution, as well as CaCO3 dissolution. During the Last Glacial Maximum (LGM, 21 ka),the lower CaCO3 contents found in the Atlantic at sites b4 km water depth have beeninterpreted to result from lower CaCO3 production, coupled with higher terrigenousinput (dilution) (Bacon, 1984; Francois et al., 1990; Francois and Bacon, 1991). Yet,while dissolution indicators (e.g. foraminifer fragmentation, normalized foraminifershell weight) reveal intensified dissolution at the LGM at Atlantic sites >4 km waterdepth (Crowley, 1983a, b, 1985; Francois et al., 1990; Barker et al., 2004), reducedabyssal [CO3

2−] (relative to Holocene) is found at water depths up to ~3 km (Yu etal., 2008, 2010). This implies a ~1 km depth discrepancy at the LGM between the depthat which the apparent CaCO3 saturation state (relative to Holocene) declines signifi-cantly versus the depth at which the sedimentary imprint of intensified CaCO3 dissolu-tion becomes apparent.

2 Although the ‘% coarse fraction’ parameter can clearly be influenced by the rain ra-tio of, for example, foraminifers to coccolithophorids, and sediment pore water chem-istry that may differ from that of overlying bottom waters, it is widely believed that thepredominant control is foraminifer fragmentation driven by bottom water corrosivity(Broecker and Clark, 1999; Broecker et al., 1999; Lalicata and Lea, 2011; Peterson andPrell, 1985).

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show a contemporaneous and consistent switch to lower CaCO3 con-tent (and lower coarse fraction) during interglacials (i.e. % CaCO3 and% coarse values during interglacials post-1.10 Ma are approximatelythe same as those seen during glacials pre-1.10 Ma; Figs. 4b–d, 5b–d

and 6b–d). This glacial-interglacial ‘mirror imaging’ of Pacific CaCO3

and coarse fraction cycles at 1.10 Ma from their pre-1.10 Ma patternproduced no discernable change in mean CaCO3 or coarse fractioncontent across glacial-interglacial cycles (Fig. 4b–d, 5b–d, 6b–d),

Fig. 3. a–c, Plio-Pleistocene records of sediment CaCO3 content (brown) and benthic δ18O (blue) from the deep central North Atlantic (spliced sequence from core V30-97 and DSDPSite 607 [same location and water depth, ~3400 m]) (Raymo, 1992; Raymo et al., 1989; Ruddiman et al., 1989). Age model based on tuning benthic δ18O from this site to the LR04δ18O stack (Lisiecki and Raymo, 2005). Scales of y-axes in a to c are identical. To left are log-power spectra (Ghil et al., 2002) of CaCO3 content (brown) and benthic δ18O (blue) foreach 1 Myr time interval. Before analysis, a Gaussian notch filter was applied to remove periods longer than ~400 kyr. Thin lines represent 99% confidence level for respective spec-tra. To right are phase wheels depicting Blackman-Tukey cross-spectral phase estimates (Paillard et al., 1996) for CaCO3 content relative to benthic δ18O at the particular frequencyband dominating the global climate system during that interval (i.e. the 41 kyr frequency from 3 to 1 Ma, ~96 kyr frequency from 1 to 0 Ma). Vectors in the 12 o'clock positiondenote CaCO3 content being in phase with benthic δ18O. Phase lags increase in the clockwise direction (i.e. 3 o'clock denotes a 90° lag of CaCO3 content relative to benthic δ18O,6 o'clock denotes an anti-phased response, and 9 o'clock denotes a 90° lead). Vector arc represents the 2σ phase error.

Fig. 4. a–c, Plio-Pleistocene records of sediment CaCO3 content (orange), coarse fraction content (red) and benthic δ18O (blue) from the deep eastern equatorial Pacific (ODP Site846) (Mix et al., 1995a, b; Shackleton et al., 1995; M. Hall, personal communication). Age model based on tuning benthic δ18O from this site to the LR04 δ18O stack (Lisiecki andRaymo, 2005). Scales of y-axes in a to c are identical. Coarse fraction data are on logarithmic scale because variance appears to be non-linear (possibly owing to location of Site846 [3300 m water depth] near to the lysocline). Power spectra and phase wheel representations as in Fig. 3, apart from phase wheel estimates in b calculated from 2.0 to1.1 Ma (to exclude interval from 1.1 to 1.0 Ma where glacial–interglacial phasing switches). Large error on CaCO3 content phase vector (orange) at 96 kyr frequency (in c) arisesfrom the prominent ‘mid-Brunhes’ ~400 kyr cycle (0.2 to 0.6 Ma; linked to an increase in CaCO3 productivity [Barker et al., 2006]), overwhelming power at the higher ~100 kyrfrequency. d. Expansion of mid-Pleistocene transition showing switch in phasing of CaCO3 and coarse fraction contents with respect to glacial–interglacial cycles.

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which may partly explain why this switch in CaCO3 cyclicity has es-caped notice for so long. We thus require mechanisms to explainnot only the consistent improvement in apparent Pacific CaCO3

preservation during glacials but also the simultaneous deteriorationin apparent preservation during interglacials.

4. Onset of ‘Pacific-style’ CaCO3 cycles triggered by a switch inSouthern Ocean overturning

Given that the origin of the late Pleistocene inter-ocean anticorre-lated CaCO3 cycles appears to lie with a switch in phasing of thePacific's CaCO3 cycles, it makes sense to first explore possible drivingmechanisms for this switch that originate within this basin. Duringthe late Pleistocene, coincident with the Pacific experiencing highersedimentary CaCO3 contents during glacials, an increasing body of ev-idence points to the abyssal Pacific having been better ventilated dur-ing glacials than interglacials. First, δ18O and δ13C data from depthtransects in the South West, western equatorial and North WestPacific indicate that, below 3500 m depth, a cold, very saline, relative-ly well-ventilated deep water mass (corresponding to Lower CDW)dominated the Pacific during the LGM (summarised in McCave etal., 2008). This water mass was likely sourced from the Pacific sectorof the Southern Ocean, probably the Ross Sea (McCave et al., 2008).Second, stronger ventilation of Lower CDW in the deepest Pacificduring the last glacial is also in agreement with a gradient of in-creasing [CO3

2-] below about 3 km water depth (Anderson andArcher, 2002; Broecker and Clark, 2001) and with higher [CO3

2-] (rela-tive to Holocene) at Lower CDW-depth (4.1 km) in the Indian Ocean(Yu et al., 2010). Third, Pacific δ13C records though the late Pleistocene(last 800 kyr) show that the deepest site (ODP 849, 3850 m, withinLower CDW) yields δ13 C values during glacials that are consistentlyhigher than those from shallower sites (ODP 846 at 3300 m and ODP806 at 2500 m) (Lisiecki, 2009), suggesting that the well-ventilatedLower CDW found during the last glacial (McCave et al., 2008) alsodominated this basin during earlier glacials. Fourth, sedimentological(sortable silt) and isotopic (benthic δ13C difference, Δδ13C, betweentwo drill sites, one high latitude, the other low latitude) evidencefrom the deep South Pacific indicate a coherent pattern of faster flowwithin the deep western boundary current (at the depth of LowerCDW) and stronger ventilation of Lower CDW during glacials

throughout the late Pleistocene (Hall et al., 2001). The sortable siltand Δδ13C records are statistically coherent with each other (at the90% confidence interval) at the ~100 and 41 kyr frequencies (Hall etal., 2001).

Here we extend this same isotopic (Δδ13C) proxy for South PacificLower CDW ventilation back to the late Pliocene (~3 Ma). This proxycomprises the difference in δ13C (Δδ13C) of benthic foraminifera be-tween ODP Sites 1123 (3300 m water depth, located at the Pacificentry point of modern Lower CDW inflow) and 849 (3850 m waterdepth, eastern equatorial Pacific). During the late Pleistocene, SouthPacific Δδ13C values decrease during glacials at the same time that thesortable silt proxy indicates faster flow speeds of Lower CDW (Hall etal., 2001). The link between this South Pacific Δδ13C proxy and ventila-tion presumably involves faster transit times of Lower CDW betweenthe two drill sites during glacials, with consequently less time forwater mass ageing to diverge their δ13C signatures. Fig. 7 shows the ex-tended Δδ13C record along with the LR04 benthic δ18O stack (to whichδ18O records from Sites 1123 and 849were independently tuned). Dur-ing the late Pliocene (3 to 2 Myr ago), inferred ventilation strength ofLower CDW weakens (higher Δδ13C) during glacials (Fig. 7a) (definedhere as ‘in phase’ behaviour; see phase wheels in Fig. 7), the oppositepattern from that seen during the late Pleistocene (Hall et al., 2001)(e.g. compare phase wheels in Fig. 7a and c). Fig. 7b demonstratesthat this late Pliocene regime (weaker ventilation during glacials) con-tinued into the early Pleistocene (2 to 1 Ma; with Δδ13C displaying asimilar phasing with respect to δ18O [Fig. 7b] as do contemporaneousPacific CaCO3 and coarse fraction records [Figs. 4b, 5b, 6b], see phasewheels). Fig. 7c then shows that, throughout the last 1 Myr, Pacificdeep water ventilation displays the converse pattern of stronger venti-lation during glacials (e.g. Hall et al., 2001) (phasewheels in Figs. 7c and7a are 180° anti-phased). In a similar manner to Pacific CaCO3 cycles(Figs. 4, 5 and 6), it appears that a switch in the phasing of Pacificabyssal ventilation with respect to glacial cycles must therefore liesomewhere near 1 Myr ago. Closer scrutiny of the Pacific Δδ13C recordacross 1 Myr ago reveals that the switch in phasing of Pacific LowerCDW ventilation strength with respect to glacial cycles occurred 1.10Myr ago (Fig. 7d), in tandem with the switch in phasing of PacificCaCO3 cycles (Figs. 4d, 5d and 6d).

In a very similar manner to the glacial-interglacial ‘mirror-imaging’seen in the Pacific's CaCO3 and coarse fraction records, glacial Δδ13C

Table 1

Phase relationships for datasets shown in Figs. 3 to 7 relative to benthic δ18O from the same drill sites.

Time interval(Ma)

Time lag(kyrs)

DSDP 607CaCO3 (%)

ODP 846CaCO3 (%)

ODP 846coarse (%)

ODP 806coarse (%)

ODP 1241coarse (%)

ODP 1123-849Δδ13C

3.0 to 2.0 (41 kyr freq.) −0.72 −0.15 −6.60 −3.89 – 0.80(0.52) (1.92) (1.13) (0.61) – (2.54)

2.0 to 1.1 (41 kyr freq.) −1.1 −4.85 – −6.81 −5.69 −5.25(0.44) (0.91) – (0.52) (1.30) (6.53)

1.0 to 0 (41 kyr freq.) 7.90 −27.40 −33.25 −30.46 −34.98 −48.71(1.20) (7.68) (3.10) (1.62) (1.41) (2.43)

Note: Negative values denote a lead of CaCO3 or coarse fraction content over benthic δ18O.Note: Values in parentheses at base of each pair denote the 2 sigma phase error.

Coherency estimates between datasets shown in Figs. 3 to 7 and benthic δ18O from the same drill sites

Time interval(Ma)

Coherency

DSDP 607CaCO3 (%)

ODP 846CaCO3 (%)

ODP 846coarse (%)

ODP 806coarse (%)

ODP 1241coarse (%)

ODP 1123-849Δδ13C

3.0 to 2.0 (41 kyr freq.) 0.93 0.52 0.76 0.89 – 0.38(0.07) (0.38) (0.20) (0.14) – (0.38)

2.0 to 1.1 (41 kyr freq.) 0.93 0.79 – 0.91 0.72 0.21(0.09) (0.23) – (0.11) (0.23) (0.21)

1.0 to 0 (41 kyr freq.) 0.91 0.33 0.71 0.86 0.91 0.75(0.11) (0.33) (0.23) (0.17) (0.09) (0.26)

Note: Values in parentheses at base of each pair denote the 2 sigma phase error.

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Fig. 5. a–c, Plio-Pleistocene records of sediment coarse (>63 μm) fraction, foraminifer fragmentation and benthic δ18O from the western equatorial Pacific (ODP Site 806). δ18O datafrom Bickert et al. (1993, 1997) and Karas et al. (2009). Sediment coarse fraction from T. Bickert and E. Jansen (unpublished data) and foraminifer fragmentation data from Anders-son (1998). Age model based on tuning benthic δ18O from this site to the LR04 δ18O stack (Lisiecki and Raymo, 2005). Power spectra and phase wheel representations as in Fig. 3,apart from phase wheel estimates in b calculated from 2.0 to 1.1 Ma (to exclude interval from 1.1 to 1.0 Ma where glacial–interglacial phasing switches). d. Expansion of mid-Pleistocene transition showing switch in phasing of coarse fraction with respect to glacial–interglacial cycles.

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Fig. 6. a–c, Plio-Pleistocene records of sediment coarse (>63 μm) fraction and benthic δ18O from the eastern equatorial Pacific (ODP Site 1241) (Lalicata and Lea, 2011; Steph et al.,2006; Tiedemann et al., 2007). Age model based on tuning benthic δ18O from this site to the LR04 δ18O stack (Lisiecki and Raymo, 2005). Power spectra and phase wheel represen-tations as in Fig. 3, apart from phase wheel estimates in b calculated from 2.0 to 1.1 Ma (to exclude interval from 1.1 to 1.0 Ma where glacial–interglacial phasing switches). Nophase wheel shown from 3 to 2 Ma because the continuous coarse fraction time series is too short and yields low coherence with benthic δ18O. d. Expansion of mid-Pleistocenetransition showing switch in phasing of coarse fraction with respect to glacial–interglacial cycles.

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Fig. 7. a–c, Plio-Pleistocene records of South Pacific deep water ventilation versus benthic δ18O. Δδ13C (green) is the difference in δ13C of benthic foraminifera between ODP Sites1123 (3300 m water depth (Hall et al., 2001; Harris, 2002) and 849 (3850 m water depth (Mix et al., 1995c) (e.g. Hall et al., 2001). δ18O (blue) is the LR04 benthic δ18O stack(Lisiecki and Raymo, 2005). Age model based on tuning benthic δ18O from Sites 1123 and 849 (Hall et al., 2001; Harris, 2002; Mix et al., 1995c) to the LR04 δ18O stack (Lisieckiand Raymo, 2005). Power spectra and phase wheel representations as in Fig. 3, apart from phase wheel estimates in b calculated from 2.0 to 1.1 Ma (to avoid interval from 1.1to 1.0 Ma where glacial–interglacial phasing switches). Large error in phasing of Δδ13C with respect to δ18O from 2 to 1 Ma (b) may result from a low signal-to-noise ratio in benthicδ18O from site 1123 from 1.3 to 1.5 Ma, causing uncertainties in precise correlation to the LR04 δ18O stack for that specific 0.2 Ma interval. d. Expansion of mid-Pleistocene transitionshowing switch in phasing of Δδ13C with respect to glacial-interglacial cycles.

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values consistently decrease at 1.10 Ma at the same time as interglacialΔδ13C values consistently increase. Taken together, these records sug-gest that strengthened ventilation of the abyssal Pacific during glacialscan explain the consistent improvement in glacial CaCO3 preservation,while weakened ventilation of the abyssal Pacific during interglacialscan explain the consistent deterioration in interglacial CaCO3 preserva-tion.We suggest that, in linewith findings from the last glacial (McCaveet al., 2008), stronger glacial ventilation in the South Pacific after1.10 Ma arose because of a consistent increase in glacial deepwater pro-duction within the proximal Southern Ocean Pacific sector (probablythe Ross Sea). We argue that weaker interglacial ventilation in theSouth Pacific after 1.10 Ma arose because of a consistent decrease inthe ‘upstream’ interglacial production of very well-ventilated (andsupersaturated with respect to CaCO3) NADW. Evidence for a reductionin NADW contribution during interglacials after ~1.10 Ma is seen inFig. 8a,which shows the difference in δ13C (Δδ13C) of benthic foraminif-era between Atlantic ODP Sites 982 (1145 m water depth) and 607(3430 m water depth), a measure of changes in the vertical watermass structure of the deep North Atlantic (Venz and Hodell, 2002). In-terglacial Δδ13C (982 δ13C–607 δ13C) values increase after 1.10 Ma(dashed lines, Fig. 8a), indicative of a switch to a greater vertical gradi-ent in δ13C during interglacials. Interglacial weakening of NADW (andconsequent northward penetration of southern-sourced abyssalwaters) as an explanation for this increased vertical Δδ13C gradient issupported by other δ13C data (Raymo et al., 1997). Furthermore, atrend toward cooler interglacial sea surface temperatures at thesesame two northern Atlantic drill sites after 1.10 Ma (dashed lines inFigs. 8b, c) is consistent with our inferred reduction in overturning-driven northward ocean heat transport during interglacials after themid-Pleistocene transition.

In addition to an increase during interglacials, the Atlantic's Δδ13C(982–607) record also shows substantial increases during glacialsafter ~1.10 Myr ago (Fig. 8a). Sedimentological evidence from theAtlantic's Southern Ocean Polar Front zone (Diekmann and Kuhn,2002) suggests that the cause of this consistent glacial increase in ver-tical water mass δ13C structure was increased northward penetration

of southern-sourced abyssal waters (e.g. Clark et al., 2006; Raymo etal., 1997). This glacial increase in southern water influence withinthe Atlantic, together with our inferred Southern Ocean source forthe strengthened glacial ventilation of Pacific LCDW, appears to be in-dicative of an increased influence of Southern Ocean-sourced watersin the global deep ocean during glacials post-1.10 Ma (e.g. Lawrenceet al., 2010). This switch to greater Southern Ocean influence on gla-cial deep water properties may have intensified glacial CaCO3 dissolu-tion in the deep Atlantic, while at the same time diminishing glacialdissolution in the deep Pacific (Figs. 4, 5 and 6; owing to our inferredproximal deep water source in the Southern Ocean Pacific sector dur-ing glacials, Fig. 7), thereby giving rise to the inter-ocean anticorrela-tion in CaCO3 cycles.

Our inferred source (i.e. the Southern Ocean) for the strongerventilation in the deep Pacific during glacials after 1.10 Myr ago(Fig. 7b-d) that appears to have driven the simultaneous onset of‘Pacific-style’ CaCO3 cycles (Figs. 4b–d, 5b–d and 6b–d) is supportedby a recent prediction regarding Southern Ocean stratification acrossthe mid-Pleistocene transition (MPT). Inconsistencies in the relation-ships between various palaeoceanographic records (e.g. benthic δ18O,Pacific benthic δ13C, Southern Ocean SST) before, in comparison toafter, the MPT are best explained in a numerical model by a post-MPT reduction in Southern Ocean stratification and increase inwater column mixing rates (Kohler and Bintanja, 2008). A recently-hypothesised change to Antarctic ice sheet dynamics across the MPTsuggests an explanation for how this switch in Southern Ocean circu-lation might have arisen. It has been hypothesised that an expansionof the East Antarctic Ice Sheet occurred at the MPT, creating a marine-based ice sheet margin in place of one that was previously restrictedto the Antarctic continent (Raymo et al., 2006). Based on evidencefrom the last glacial, when expansion of Antarctic marine-based iceshelves (Anderson et al., 2002; Denton and Hughes, 2000) and seaice (CLIMAP, 1981; Gersonde et al., 2005) both occurred in tandemin the Southern Ocean Pacific sector (e.g. Ross Sea), it is likely thatthe hypothesised appearance of a marine-based ice sheet margin atthe MPT (Raymo et al., 2006) would have been accompanied by sea

Fig. 8. Plio-Pleistocene evolution of the vertical water mass structure of the deep North Atlantic. a. Difference in δ13C (Δδ13C) of benthic foraminifera between ODP Sites 982(1145 m water depth; Venz and Hodell, 2002; Venz et al., 1999) and 607 (3430 m water depth; Raymo, 1992; Raymo et al., 1989; Ruddiman et al., 1989). Note reversed y-axisscale. b. North Atlantic SST, ODP Site 982 (Lawrence et al., 2009). c. North Atlantic SST (winter), DSDP Site 607 (Ruddiman et al., 1989). Grey shaded envelopes denote 1σ varianceabout the mean interglacial Δδ13C (a) and SST (b, c) (black dashed lines) before versus after 1.10 Ma.

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ice expansion, particularly during glacials. Brine rejection associatedwith increased Southern Ocean sea ice cover at the LGM has beenshown to explain 80% of a predicted contemporaneous order of mag-nitude increase (versus modern) in formation rates of Antarctic Bot-tom Water (Shin et al., 2003). Expanded Southern Ocean sea icecover at 1.10 Myr ago may therefore provide an explanation for themodel prediction (Kohler and Bintanja, 2008), and our own indepen-dent inference, of a strengthening (or instigation) of deep waterventilation in the Southern Ocean Pacific sector (Fig. 7b–d) duringglacials after this time.

5. Role of the Southern Ocean in the mid-Pleistoceneclimate transition

It has long been recognised that the ‘mid-Pleistocene transition’from moderate glacial cycles 40 kyr in duration to higher amplitudecycles 100 kyr-long (e.g. Bintanja and van de Wal, 2008; Lisieckiand Raymo, 2005; Mudelsee and Schulz, 1997; Pisias and Moore,1981; Ruddiman et al., 1989; Shackleton and Opdyke, 1976), in theabsence of any notable change in external orbital forcing (Laskar etal., 2004), indicates a major change internal to the climate system(e.g. Berger et al., 2005; Clark et al., 2006; Huybers and Wunsch,2005; Imbrie and Imbrie, 1980; Marshall and Clark, 2002; Raymo,1997; Rial, 1999). It is widely acknowledged that the higher ampli-tude δ18O cycles associated with late Pleistocene 100 kyr cycles(Fig. 9b) primarily signify a post-MPT increase in ice volume duringglacials (Bintanja and van de Wal, 2008; Mudelsee and Schulz,1997; Ruddiman et al., 1989). In Fig. 9 we compare the timing ofglobal ice volume change to that of our switch in phasing of deepSouth Pacific ventilation (e.g. Fig. 7d) and the associated onset of ‘Pa-cific-style’ (i.e. anti-phased with those in the Atlantic) CaCO3 cycles(e.g. Figs. 4d, 5d and 6d). Fig. 9a shows the standard deviation of a3 Myr-long sea level (global continental ice volume) record calculat-ed using a non-linear transfer function between independentlyreconstructed sea level and benthic foraminiferal δ18O over the lasttwo glacial cycles and for selected intervals of the Pliocene (Siddallet al., 2010). The abrupt increase in sea level variance (global ice vol-ume variance) at 1.05 Myr ago (Fig. 9a) is, given that these data arenot on the same LR04 age model as the records shown in Figs. 4 to7, remarkably similar in timing to the switch to stronger glacial

overturning originating in the Southern Ocean at 1.10 Ma inferredfrom the Pacific and Atlantic Δδ13C records (Figs. 7d and 8a). This in-creased northward penetration of southern-sourced deep waters dur-ing glacials may have, via a glacial-interglacial interhemisphericseesaw in deep overturning (Ganopolski et al., 1998; Stouffer andManabe, 2003; Shin et al., 2003), caused a reduction in glacial deepwater formation in the North Atlantic (Fig. 8a), intensifying theinter-ocean anticorrelation in CaCO3 cycles and providing a positivefeedback to further glacial-age continental ice growth (e.g. from 1.0to 0.8 Ma in Fig. 9a) in the circum-North Atlantic region (Bintanjaand van de Wal, 2008). We note that some of the reduction inNADW during glacials after ~1.10 Ma may also have been the resultof greater freshening of the North Atlantic surface ocean during gla-cial maxima driven by calving of larger ice sheets from the surround-ing continents.

6. Shelf-to-basin CaCO3 fractionation versus atmosphere-to-abysspartitioning of carbon

If we assume steady state in terms of alkalinity input from thecontinents, lower sea level during glacials should drive CaCO3 deposi-tion on the continental shelves to decrease (Kleypas, 1997), whilesimultaneously increasing CaCO3 accumulation in the deep ocean.We would expect evidence of this shelf-to-basin fractionation ofCaCO3 (increased deep-sea CaCO3 accumulation and preservationduring glacials; the reverse during interglacials) to be preserved indeep-sea sediments (e.g. Berger and Winterer, 1974; Berger, 1982;Opdyke and Walker, 1992; Hodell et al., 2001). Large continental icesheets (and presumably associated large sea level falls) have been in-termittently present during glacials since at least 2.5 Ma (Balco andRovey, 2010). Yet before the onset of the inter-ocean anticorrelationin CaCO3 cycles (at 1.10 Ma), the Pacific records (which potentiallyreflect the largest and deepest ocean basin) confound the shelf-to-basin fractionation model by showing consistently poorer apparentCaCO3 preservation during glacials throughout the late Pliocene andearly Pleistocene (3.0 to 1.10 Ma; Figs. 4a, b, 5a, b, 6a, b). This sug-gests, somewhat surprisingly, that the deep ocean was consistentlymore corrosive, on a global scale, during glacials in comparison to in-terglacials before 1.10 Ma. Possible explanations for this characteristiccorrosiveness of the global deep ocean during glacials are that the

Fig. 9. Variance in eustatic sea level (global continental ice volume) since the late Pliocene. a. Standard deviation of a 3 Myr-long eustatic sea level record (Siddall et al., 2010).b. LR04 benthic δ18O stack (from Fig. 6). Vertical dashed line marks the timing of the switch to stronger abyssal South Pacific ventilation during glacials inferred from Fig. 6d.

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deep oceans during these intervals have persistently experienced en-hanced storage of carbon and/or a greater proportion of their volumebeing ventilated by a water mass(es) with relatively high preformednutrient concentrations. This observation further emphasises theanomalous nature of CaCO3 cycles in the deep Pacific during the last1.10 Myr.

7. Implications for evolution of atmospheric CO2 and glacial-ageabyssal CO2 storage

Prior to 1.10 Ma, the largest and deepest ocean basin, the PacificOcean, appears to have experienced maximum CaCO3 preservation(and inferred maximum alkalinity) during interglacials (Figs. 4a, b,5a, b, 6a, b). But after 1.10 Ma, the Pacific witnessed maximum in-ferred alkalinity during glacials (Figs. 4c, 5c, 6c). This datum broadlycoincides with the onset of increasingly severe glacial episodes(Fig. 9b) and growth in the size of glacial-age continental ice sheets(Fig. 9a). We suggest that the increased alkalinity of the mean globaldeep ocean (driven by the geographically and bathymetrically vastPacific Ocean) during glacials (cf. Rickaby et al., 2010) after 1.10 Mamay explain part of the drawdown in glacial atmospheric CO2 levelshypothesised (Raymo and Ruddiman, 1988; Berger et al., 1999;Medina-Elizalde and Lea, 2005) to have been linked to their increasedseverity. Although a boron isotope-based reconstruction of atmo-spheric CO2 levels across the MPT does exist (Hönisch et al., 2009),its temporal resolution (average ~60 kyr) is too low to resolveglacial-interglacial variance in CO2. A definitive test of the hypothesisof a lowering of glacial-age CO2 levels across the MPT awaits the ex-tension of ice core records of atmospheric CO2 levels beyond 0.8 Ma(e.g. Brook et al., 2006; Jouzel and Masson-Delmotte, 2010).

The abyssal Southern Ocean is now believed to have played a majorrole in sequestering atmospheric CO2 during the severe glacials of thelate Pleistocene (e.g. Anderson et al., 2009; Hodell et al., 2003; Sigmanet al., 2010; Skinner et al., 2010; Toggweiler et al., 2006). Yet isotopic ev-idence indicates that the Southern Ocean hosted heterogenous abyssalwater masses during the last glacial (McCave et al., 2008; Ninnemannand Charles, 2002; Rickaby et al., 2010), suggesting circum-Antarcticbarriers to abyssal mixing and/or diverse hydrography within this re-gion. Our findings of improved Pacific CaCO3 preservation (Fig. 4b–d,5b–d, 6b–d) and ventilation strength (Fig. 7b–d) within the abyssalSouth Pacific during glacials post-1.10 Ma present a challenge to effortsto locate the predicted abyssal storage reservoir of CO2. In conjunctionwith the results of Pacific benthic versus planktic foraminifer radiocar-bon age differences down to 4.4 km water depth (Broecker and Clark,2010; Galbraith et al., 2007), our findings suggest that this CO2 storagereservoir is very unlikely to have been in the abyssal Pacific or the Pacif-ic sector of the Southern Ocean. In light of this, we suggest that efforts tolocate the hypothesised glacial-age abyssal storage reservoir of CO2 arebest focussed on the Southern Ocean's Atlantic sector (e.g. Hodell et al.,2003; Ninnemann and Charles, 2002; Skinner et al., 2010).

8. Conclusions

The CaCO3 content of deep-sea sediments in the Pacific Ocean in-creases during glacials of the late Pleistocene in comparison to inter-glacials (e.g. Anderson et al., 2008; Arrhenius, 1952; Berger, 1970,1973; Broecker, 1971; Farrell and Prell, 1989; Hays et al., 1969; Leand Shackleton, 1992; Ninkovich and Shackleton, 1975; Petersonand Prell, 1985; Thompson and Saito, 1974), whereas sedimentaryCaCO3 in the Atlantic Ocean shows an anticorrelated pattern acrossglacial-interglacial cycles (e.g. Bé et al., 1976; Berger, 1973;Broecker, 1971; Crowley, 1985; Curry and Lohmann, 1990; Damuth,1977; deMenocal et al., 1997; Gardner, 1975; Hodell et al., 2001;Philippi, 1910; Olausson, 1965; Verardo and McIntyre, 1994; Volatet al., 1980). Here we show that this anticorrelation in inter-oceanCaCO3 cycles arose comparatively recently, at 1.10 Ma, during the

mid-Pleistocene transition. Before this time, the CaCO3 content ofPacific and Atlantic Ocean sediments both showed ‘Atlantic-style’cyclicity. The onset of anticorrelated cyclicity at 1.10 Ma involved atwofold switch in the Pacific's CaCO3 cycles. Sedimentary CaCO3 andcoarse fraction contents at three sites across the Pacific (here inter-preted as being primarily controlled by CaCO3 dissolution/preserva-tion) consistently increased during glacials at the same time as theyconsistently decreased during interglacials. This ‘mirror imaging’ ofPacific CaCO3 cycles at 1.10 Ma from their pre-1.10 Ma pattern pro-duced no discernable change in mean CaCO3 or coarse fraction con-tents across glacial-interglacial cycles, which partly explains whythis switch in CaCO3 cyclicity has escaped notice for so long.

We demonstrate that the likely cause of this mirror imaging ofPacific CaCO3 cyclicity was a switching over of the relative ventilationstate of abyssal South Pacific waters across glacial-interglacial cycles.Specifically, we suggest that a consistent strengthening of abyssalventilation within the Southern Ocean Pacific sector during glacialsdrove glacial Pacific CaCO3 dissolution to diminish, while a coevalweakening of very well-ventilated ‘upstream’ NADW during intergla-cials drove interglacial Pacific CaCO3 dissolution to intensify. En-hanced deep water production in the Southern Ocean Pacific sectorduring glacials was part of a broader increase in the Southern Ocean'sinfluence on glacial deep waters post-1.10 Ma (e.g. Lawrence et al.,2010) (consistent with published findings for the Southern Ocean At-lantic sector [Raymo et al., 1997; Venz and Hodell, 2002]) that mayhave served to heighten glacial CaCO3 dissolution in the deep Atlantic(relative to that prevailing during interglacials when well-ventilatedNADW predominates), thereby intensifying the inter-ocean anticor-relation in CaCO3 cycles.

We show that, throughout the late Pliocene and early Pleistocene(3.0 to 1.10 Ma), consistently poorer CaCO3 preservation was experi-enced in the largest and deepest ocean basin (the Pacific Ocean) duringglacials. This suggests that the corrosivity of the deep oceanwas consis-tently greater, on a global scale, during glacials before 1.10 Ma, possiblydriven by the deep oceans during these intervals having persistently ex-perienced enhanced storage of carbon and/or a greater proportion oftheir volume being ventilated by a water mass(es) with relativelyhigh preformed nutrient concentrations. Regardless, this observationfurther emphasises the unusual nature of CaCO3 cycles in the deepPacific during the last 1.10 Myr.We propose that the increasedmean al-kalinity of the global deep ocean (driven by the geographically andbathymetrically vast Pacific Ocean) during glacials post-1.10 Ma mayexplain part of the drawdown in atmospheric CO2 levels hypothesised(Berger et al., 1999; Medina-Elizalde and Lea, 2005; Raymo and Ruddi-man, 1988) to have been linked to the increased severity of these latePleistocene glacials. Furthermore, given our findings of improvedCaCO3 preservation and ventilation strength within the abyssal Pacificduring glacials post-1.10 Ma, we suggest that efforts to locate aglacial-age Southern Ocean abyssal reservoir of CO2 (e.g. Anderson etal., 2009; Hodell et al., 2003; Skinner et al., 2010; Sigman et al., 2010)are best focussed on the Southern Ocean's Atlantic sector.

Acknowledgments

We thank Torsten Bickert, Simon Crowhurst, Eystein Jansen andMike Hall for providing unpublished data and Carin Andersson, Jer-oen Groeneveld, Ian Hall, Sara Harris, Joe Lalicata, Alan Mix andMark Siddall for published data. Discussions with Alex Dickson, EricGalbraith, Erin McClymont, Chris Pearce, Maureen Raymo, Daniel Sig-man and Sindia Sosdian were particularly helpful. We also thank twoanonymous reviewers for constructive reviews and comments thatimproved the manuscript and Peter deMenocal for editorial handling.This research used data provided by the Integrated Ocean DrillingProgram (IODP). Financial support for this research was provided bya European Commission Marie Curie Outgoing International Fellow-ship (to P.F.S.) and a Leverhulme Trust Fellowship (to P.F.S.).

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