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Benthic foraminiferal oxygen isotope (δ18Obf) records trend toward high-er values throughout the late Pliocene warm period (1), culminating in higher amplitude of glacial-interglacial (G-IG) δ18O variability (Fig. 1) associated with a dramatic increase of Ice Rafted Debris (IRD) in sedi-ments around the Arctic at ~2.73 Ma [Marine Isotope Stage (MIS) G6], and signaling a major intensification of Northern Hemisphere glaciation (NHG) (2–4). During the same period, proxy records indicate a long-term sea surface temperature (SST) cooling in most regions (5–8), which is mimicked in bottom water temperature (BWT) records from the deep North Atlantic (9–11). Here, we reconstruct the BWT history of the deep Pacific at 2-5 kyr resolution from ODP Site 1208 (36.13°N, 158.20°W, 3350m water depth, fig. S1) on Shatsky Rise in the northwest Pacific Ocean (12), and determine ice volume changes during the 400 kyr inter-val preceding the NHG intensification (3.15-2.75 Ma, henceforth pre-NHG).

The Pliocene δ18Obf record at Site 1208 is consistent with the global δ18Obf stack (LR04) (3) (Fig. 1). Beginning at glacial MIS G6, the ampli-tude of G-IG δ18Obf variability (Δδ18OG-IG) increases from ~0.5‰ to >0.8‰ with more extreme glacial maxima and interglacial minima (fig. S2).

The BWT record at Site 1208 was reconstructed from Mg/Ca meas-urements of the infaunal foraminifera, Uvigerina spp. We determined a Mg/Ca-temperature sensitivity, 0.067 ±0.016 mmol/mol/°C, for this species by calibrating the Holocene-LGM Δ(Mg/Ca) to the Δδ18Obf at Site 1208 (12, 13). Note that, unlike epifaunal species, Cibicidoides wuellerstorfi, used to generate a North Atlantic BWT record (9), Uviger-ina is apparently not affected by seawater carbonate ion saturation, which can alter Mg/Ca independently of temperature (12, 14). Pacific BWTs exhibit an average Pliocene G-IG range of ~2.1 ± 0.8°C, (1 S.D.), similar to the average Holocene-LGM range of 2.6°C (12) in contrast with oscillations of δ18Obf (Fig. 1), suggesting the orbital-scale tempera-ture variability of the Pliocene Pacific was somewhat decoupled from the ice volume. A notable change occurred at ~2.73 Ma, when following ~4°C deglacial warming, average Pacific BWT became ~1.5°C warmer and G-IG temperature changes exhibit slightly lower amplitude (Fig. 1

and fig. S3). Spectral analysis shows that pre-

NHG, our δ18Obf record is dominated by 41 kyr pacing consistent with a strong response to obliquity variations, which is attributed to ice sheet dynam-ics (3). In contrast, Pacific BWT and benthic δ13C exhibit strong 100 kyr periodicity during that interval (fig. S4), suggesting that BWT and circulation responded differently to orbital forcing than ice. The appearance of a secondary 100 kyr beat in the latter part of our δ18Obf record provides evidence for stronger synchronization in the re-sponse of these climate-system compo-nents to orbital forcing as NHG intensified (12).

Site 1208 BWTs show no signifi-cant long-term trend over the 400 kyr pre-NHG interval (Fig. 2). Therefore, we interpret the significant δ18Obf in-crease, 0.21 ± 0.04‰ (1 S.E.), at Site 1208 over this interval (Fig. 2) to pri-marily reflect an increase in the extent of continental ice (12). Assuming an average 0.1 ± 0.02‰ Δδ18O/10 m sea-level (15), we estimate 21 ± 10 m (2

S.E.) sea-level equivalent of “permanent” ice growth (12) pre-NHG. Because seawater Mg/Ca was likely ~20 ± 10% lower than modern dur-ing the Pliocene, e.g., (16), BWTs at Site 1208 might have been higher than present by ~1°C (12) (fig. S3). However, this uncertainty has a negligible effect on our estimates of sea-level change and the G-IG vari-ability of BWT and δ18Osw during the late Pliocene interval leading to the NHG intensification (12) (fig. S5). Using the long-term trends also minimizes uncertainties due to inaccurate temperature corrections or differences in the primary orbital beats of the isotope and temperature records. Thus, our reconstruction of continental ice provides only infor-mation regarding mean sea-level position and does not reflect glacial lowstands or interglacial highstands, which presumably occurred at or-bital frequencies over this 400kyr interval (12).

We compare the Pacific with North Atlantic DSDP Site 607 (41°N, 32°W, 3427 m water depth) records (9) and find pre-NHG Atlantic BWTs, generated using C. wuellerstorfi, are on average 4 ± 1.4°C warmer than at Site 1208 (9) (Fig. 2). New discrete measurements of Uvigerina Mg/Ca from Site 607 range from 1.1-1.6 mmol/mol, well above values recorded by Uvigerina in the Pacific at the same time (fig. S6), supporting the interpretation of warm North Atlantic BWTs during the Pliocene (12). Over the pre-NHG interval, Site 607 δ18Obf records a 0.48 ± 0.04‰ increase, more than double that observed at Site 1208, and BWTs cool by ~1.3 ± 0.4°C (Fig. 2). After accounting for BWT cooling, we estimate ~15 ± 14 m (1 S.E.) sea-level equivalent of “permanent” ice growth from the residual Δδ18Obf at Site 607 (12), consistent with the estimate from Site 1208. As the uncertainty in the Pacific sea-level rec-ord is less than the Atlantic estimate due to lack of temperature correc-tion, we conclude that it is very likely (>95% confidence) that baseline sea-level fall exceeded 11 m and possibly reached 31 m pre-NHG.

The gradual increase in Pliocene δ18Obf records has previously been attributed to a prolonged onset of NHG (17, 18). Ice-rafted debris pro-vides evidence of ice on Greenland since the Miocene and an augmented ice sheet extent ~3.3 Ma (19). Although tectonic uplift in high northern latitudes surrounding the Barents Sea likely promoted regional ice growth as early as 4 Ma (20), the alpine glaciers were not large enough

Antarctic role in Northern Hemisphere glaciation Stella C. Woodard,1* Yair Rosenthal,1,2 Kenneth G. Miller,2 James D. Wright,2 Beverly K. Chiu,2 Kira T. Lawrence3 1Rutgers University, Institute of Marine and Coastal Sciences, 71 Dudley Rd, New Brunswick, NJ 08901, USA. 2Rutgers University, Department of Earth and Planetary Sciences, 610 Taylor Rd, Piscataway, NJ 08845, USA. 3Lafayette College, Department of Geology and Environmental Geosciences, 730 High St, Easton, PA 18042, USA.

*Corresponding author. E-mail: woodard@marine.rutgers.edu

Earth’s climate underwent a major transition from the warmth of the late Pliocene, when global surface temperatures were ~2-3°C higher than today, to extensive Northern Hemisphere glaciation (NHG) at ~2.73 Ma. We show that North Pacific deep waters were significantly colder (4°C) and likely fresher than North Atlantic deep water prior to the intensification of NHG. At ~2.73 Ma, the Atlantic-Pacific temperature gradient was reduced to <1°C suggesting the initiation of stronger heat transfer from the North Atlantic to the deep Pacific. We posit that increased glaciation of Antarctica, deduced from the 21 ± 10 m sea-level fall from 3.15-2.75 Ma, and the development of a strong polar halocline, fundamentally altered deep ocean circulation, which enhanced inter-hemispheric heat and salt transport thereby contributing to the NHG.

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to reach sea-level until MIS G6 (20), and so probably account for negli-gible sea-level equivalent ice pre-NHG (12). In light of the little evi-dence for widespread glaciers in the Northern Hemisphere until ~2.73 Ma (4, 19, 20), we suggest that at least half of the 21 ± 10 m sea-level fall pre-NHG was due to ice added to Antarctica. Furthermore, because the Greenland Ice Sheet (GIS) and West Antarctic Ice Sheet (WAIS) can accommodate at most ~12 m ice equivalent of sea-level (21, 22), it’s likely that the extent of the East Antarctic Ice Sheet (EAIS) was less than modern during the late Pliocene, consistent with records of a dynamic Antarctic margin (23–26).

The expansion of Antarctic ice sheets during the late Pliocene ap-pears to surpass an important climatological threshold, which contribut-ed to the intensification of NHG. The present deep Pacific δ18Osw value, ~-0.15‰, reflects the mixture of NADW (δ18Osw ~0.2‰) and AABW (δ18Osw ~-0.5 to -0.3‰) forming circumpolar deep water (CDW) (27). The pre-NHG deep Pacific δ18Osw was similar to modern AABW, aver-aging -0.43‰ and fluctuating by as much as 0.4-0.8‰ on G-IG time-scales (Fig. 3). During this interval Pacific δ18Osw reached near modern values only during glacial periods with much lower values during inter-glacials. The δ18Osw reconstruction at Site 607 shows no significant long-term change over the entire interval for which there is data overlapping with our Pacific record (9) (Fig. 3). Although the Site 607 Pliocene G-IG amplitude of Δδ18Osw was similar to the Pacific, average δ18Osw was higher (0.28‰), with interglacial values not much different from the modern North Atlantic. Thus, the low Pacific δ18Osw cannot be attributed to lower continental ice volume but must reflect a hydrographic differ-ence.

The persistence of an average ~0.7 ‰ δ18Osw gradient between Site 607 and Site 1208 (Fig. 3B), suggests the two sites were not ventilated by the same water mass pre-NHG. During the same interval, Pacific δ18Obf and BWTs were offset from the North Atlantic (Fig. 2). The rela-tively faster long-term δ18Obf increase at Site 607 compared with Site 1208 brings the records together by MIS G6 (Fig. 2A), and the Atlantic-Pacific BWT gradient was reduced from ~4°C to <1°C over one IG-G cycle, MIS G6-MIS G5 (Fig. 2B), by cooling at Site 607and subsequent warming at Site 1208. We interpret these data to reflect hydrographic changes, which initiated stronger heat and salt transfer from the North Atlantic to the Pacific due to a change in deep ocean circulation, perhaps setting up a CDW mixing pattern similar to modern.

Today most AABW production occurs in the Weddell Sea (28), in-creasing opportunities for mixing between northern and southern sourced deep-water masses due to its location in the Atlantic sector of the Southern Ocean. If the formation regions for Pliocene southern sourced deep water were different, an entirely different pattern of mixing and CDW formation was possible. Detailed reconstructions of the late Pliocene from the proximal ANDRILL AND-1B core indicate numerous episodes of near total WAIS collapse (25), and diatom assemblages sug-gest open marine conditions were accompanied by deep-mixing in the Ross Sea (26). Increased Pliocene deep-water production in the Pacif-ic/Indian sector of the Southern Ocean (e.g., Prydz Bay, Ross Sea, Amundsen Sea), potentially allowed AABW to influence the deep Pacif-ic more directly, consistent with recent model results (29).

Past studies, which attribute Pliocene warmth to increased oceanic heat transport via the thermohaline conveyor (30–32), cite weak meridi-onal δ13C gradients (30, 31) and increased North Atlantic sediment drift accumulation (32) as evidence of enhanced NADW production. Howev-er, their interpretation of ocean circulation based upon δ13C hinges on the use of modern NADW and AABW end-members that are related to surface nutrient content and atmospheric exchange, and thus may not reflect Pliocene values. Evidence for poorly stratified (33, 34) and more productive (24, 25) Southern Ocean surface waters with less sea ice during the Pliocene, makes a higher preformed δ13C AABW end-member plausible (35). While there is evidence for NADW production

during the late Pliocene based on drift deposits (32), we have no direct way to quantify deep-water formation relative to the modern or to AABW. Both AABW and NADW production may have increased sim-ultaneously, as suggested by modeling (29), with the end result of lower contribution of NADW to CDW.

Regardless, the absence of a long-term temperature trend in our Pa-cific BWT record over the period when NADW cooled by ~1.3°C attests to inefficient heat transport from the North Atlantic to the deep Pacific via the Southern Ocean before 2.73 Ma. We contend that NADW was not a major contributor to CDW and that deep water formed in the Southern Ocean was the primary source of deep water to the Pacific pre-NHG. This interpretation is also supported by a ~0.6‰ increase in Site 1208 δ18Osw, which effectively eliminates the Pacific-Atlantic δ18Osw gradient by ~2.7 Ma (Fig. 3). The proposed change in deep ocean circu-lation was almost certainly tied to high latitude processes which altered the temperature or salinity of either the northern or southern deep water mass or both. Presumably, high latitude cooling steepened equator-to-pole temperature gradients throughout the Pliocene (6, 7) and triggered the expansion Antarctic glaciation in the Southern Hemisphere (26), as supported by our sea-level estimates from Site 1208. Dust records sug-gest a northward shift in southern hemisphere wind belts about the time of NHG intensification (36), which would have allowed expansion of sea ice and promoted the development of a stronger halocline (34), reducing deep water ventilation around Antarctica (26, 35).

Stronger and more permanent stratification of Southern Ocean sur-face waters is indicated by decreased opal accumulation beginning around 2.73 Ma (33) (Fig. 3C). Such stratification probably reduced heat exchange between the atmosphere and upwelling Southern Ocean waters [e.g., (37)], trapping heat transported via NADW in the subsurface and causing the warming observed in our Pacific BWT record. A decreasing SST gradient between the North and South Atlantic (Fig. 3D) further signifies the transfer of heat from the high northern latitudes. Thus, we argue that the expanded glaciation of Antarctica during the late Pliocene culminated in a fundamental change in deep-water circulation, which contributed to the intensification of NHG by facilitating the redistribu-tion of heat at Earth’s surface.

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Acknowledgments: The authors thank N. Venti and K. Billups for providing samples, J. Crisicone, N. Abdul and R. Mortlock for lab assistance. Funding was provided by NSF grants EAR-1052257 and OCE-1334691. B.K.C. thanks Rutgers Aresty Undergraduate Research Fellowship for its support.

Supplementary Materials www.sciencemag.org/cgi/content/full/science.1255586/DC1 Materials and Methods Supplementary Text Figs. S1 to S8 References (39–84)

5 May 2014; accepted 10 October 2014 Published online 23 October 2014 10.1126/science.1255586

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Fig. 1. Stable isotope and Mg/Ca data for the deep Pacific Ocean, Site 1208, spanning the Holocene-LGM (0-25 ka, left) and late Pliocene (3.3-2.5 Ma, right). Holocene-LGM data from V19-30 (eastern tropical Pacific) (38) and the Pacific δ18O benthic stack (39) are included for comparison. (A) Uvigerina spp. δ13C records from the Pacific Ocean. (B) Global benthic δ18O stack (LR04) (3). (C) Uvigerina spp. δ18Obf record from Site 1208. (D) Uvigerina spp. Mg/Ca record from Site 1208, temperature scale is given for reference. Raw data (points) were smoothed using a 15 kyr low-pass filter (line); gray shading indicates 1 S.E. of temperature estimate. The box identifies the pre-NHG interval and gray shaded box highlights the period following NHG intensification. Important Pliocene Marine Isotope Stages (MIS) are labeled on the LR04 benthic δ18O stack.

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Fig. 2. Benthic δ18Obf (A) and BWTs (B) comparing Pacific (blue, this study) with the North Atlantic [red, (9)] records. Gray shading highlights the interval after Pacific and Atlantic δ18Obf and BWT records come together around MIS G6. Long-term trends spanning the 400 kyr pre-NHG interval (3.15-2.75 Ma) are highlighted. Raw data (points) are smoothed by 15 kyr low-pass filter (line). Blue (Site 1208) and red (Site 607) shading gives 1 S.E. of filtered BWTs. Dashed lines are least squares regressions. The slopes of significant trends are given in boxes. Marine Isotope Stages (MIS) discussed in the text are labeled on panel (A).

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Fig. 3. (A) Calculated δ18Osw for Pliocene deep Pacific Site 1208 (blue, this study) and the North Atlantic Site 607 [red, (9)]. Shaded area indicates 1 S.E. of filtered δ18Osw. Dashed line indicates modern deep Pacific δ18Osw, -0.15‰. (B) Pacific minus Atlantic δ18Osw showing a reduction in the inter-basinal gradient at ~2.7 Ma. (C) Southern Ocean opal mass accumulation rates (ODP Site 1096) (33). (D) Evolution of North (DSDP Site 607 and ODP Site 982) (7) and South Atlantic (ODP Site 1090) (8) high latitude SST gradients. Data were interpolated to even 2 kyr spacing before differencing (thin lines); thick lines are 50 kyr-smoothed curves.