The influence of Antarctica and the Southern Ocean on Late Pliocene global climate reconstructions has remained ambiguous due to a lack of well-dated Antarctic-proximal, paleoenvironmental records. Here we present ice sheet, sea-surface temperature, and sea ice reconstructions from the ANDRILL AND-1B sediment core recovered from beneath the Ross Ice Shelf. We provide evidence for a major expansion of an ice sheet in the Ross Sea that began at ∼3.3 Ma, followed by a coastal sea surface temperature cooling of ∼2.5 °C, a stepwise expansion of sea ice, and polynya-style deep mixing in the Ross Sea between 3.3 and 2.5 Ma. The intensification of Antarctic cooling resulted in strengthened westerly winds and invigorated ocean circulation. The associated northward migration of Southern Ocean fronts has been linked with reduced Atlantic Meridional Overturning Circulation by restricting surface water connectivity between the ocean basins, with implications for heat transport to the high latitudes of the North Atlantic. While our results do not exclude low-latitude mechanisms as drivers for Pliocene cooling, they indicate an additional role played by southern high-latitude cooling during development of the bipolar world.
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The development of the first continental-scale ice sheet on Antarctica occurred at ∼34 Ma, coincident with a ∼1.5‰ increase in benthic foraminiferal δ18O (1, 2), which is interpreted as a 4 °C cooling in deep ocean temperature (3) with 80 m of sea level equivalent ice volume on the Antarctic continent (4, 5). Direct geological evidence of a continental-scale ice sheet calving at the coastline by the earliest Oligocene came from ice-rafted debris in ocean drill cores from Prydz Bay (6, 7) and marine grounding-line deposits in Western Ross Sea drill cores (8). Proximal geological drill cores (9) and high-resolution deep-sea δ18O (10) records imply the existence a dynamic and highly variable, orbitally paced East Antarctic Ice Sheet (EAIS) that drove global sea-level changes of ∼40 m (11) up until ∼14 Ma. A ∼1.2‰ increase in benthic foraminiferal δ18O at ∼14 Ma and cooling of Southern Ocean surface waters by up to 7 °C is associated with the development of a more permanent EAIS (1, 12, 13). Terrestrial glacial deposits at high elevations in the Transantarctic Mountains (TAM) indicate a transition from wet-based to dry-based glaciation at this time, and using the preservation of delicate terrestrial plant fossils, ancient landscapes, and volcanic ashes, it has been argued that the volume of the EAIS remained relatively stable since ∼14 Ma (14, 15). While terrestrial glacial and glaciomarine deposits indicate marine incursions into fjords coupled with thinning and recession of the low elevation parts of the EAIS along Western Ross Sea and in Prydz Bay during the Pliocene (16, 17), widespread deglaciation of the EAIS at this time (18) is unsupported by ice sheet models (19, 20), benthic δ18O records (21) and glacial deposits at high elevation in the TAM (22).
The development of an ephemeral West Antarctic Ice Sheet (WAIS) is thought to have occurred around 34 Ma (5) coincident with the first ice sheets in East Antarctica, but it was not a permanent feature until much later (23). Glacial unconformities observed in seismic profiles in the central Ross Sea, correlated to dated horizons in Deep Sea Drilling Project Site 270, indicate that periods of an extensive grounded marine ice sheet within the Ross Sea embayment have occurred since the Early Miocene (24). During the warmest intervals of the Pliocene (4.5–3.0 Ma) Earth’s average surface temperature was ∼2–3 °C warmer than present, atmospheric pCO2 was ∼400 ppmv, and equator to pole temperature gradients were weaker (25–27). During this peak Pliocene warmth, the largely marine-based WAIS and the Greenland Ice Sheet were reduced in extent (19, 28, 29) and global sea level is estimated to have been between 5–40 m above present with most reconstructions converging on 20–25 m (30). Subsequent cooling, which led to the onset of major Northern Hemispheric glaciation by ∼2.7 Ma (31), has been variously attributed to declining pCO2 (32), changing orbital geometries (33), tectonic influences (34), increased oceanic stratification and precipitation in northern high latitudes (35), and reduced zonal sea-surface temperature (SST) gradients in the equatorial Pacific Ocean (36). Until now, the role of Antarctica in Late Pliocene global cooling has been unclear. The Antarctic Drilling Program’s (ANDRILL) AND-1B core contains a series of well-dated sedimentary cycles documenting ice sheet advance and retreat that correlate with the global marine oxygen isotope and southern, high-latitude insolation time series (Fig. 1) (19, 28).
Fig. 1.
Summary stratigraphic log of lithofacies in AND-1B. The glacial proximity curve is based on interpretation of the lithofacies and tracks the relative position of the grounding line [ice-contact (I), ice-proximal (P), ice-distal (D), and marine (M)], providing a proxy for ice-sheet extent (28). Chronostratigraphy is derived by magnetostratigraphy, constrained by biostratigraphy and tephrochronology (28). Diatom assemblages, derived SST (with light purple calibration error envelope), and bulk sediment stable isotope data are from interglacial deposits, and record a cooling trend that is coincident with increased variance toward glacial values in the global δ18O benthic stack (42).
Here, we describe a major phase of ice sheet expansion and cooling in coastal Antarctic waters at ∼3.3 Ma following a ∼1.2 Myr-long period of warmer-than-present marine conditions accompanied by a diminished marine-based ice sheet in the Ross Embayment during the Early Pliocene (∼4.5–3.4 Ma). This cooling involved the expansion of the Antarctic ice sheets onto the continental shelf, increased sea ice extent and duration, and altered Southern Ocean circulation. Our multiproxy dataset indicates: (i) a progressive increase in ice sheet extent and variability, and reduced glacial meltwater, and (ii) a general cooling of the coastal Antarctic seas based on evidence from sedimentary facies, diatom assemblages, (tetraether index of lipids consisting of 86 carbon atoms) sea-surface temperature reconstructions, and δ13C and δ15N of bulk sedimentary organic matter.
Results
AND-1B was drilled beneath the McMurdo Ice Shelf, an extension of the Ross Ice Shelf at its northwest margin. The provenance of clasts within subglacially deposited diamictites in AND-1B is consistent with transport by glacial ice from EAIS outlet glaciers to the south of the drill site (37), indicating deposition by grounded ice sheets within the Ross Embayment. Ice sheets that occupied the Ross Embayment and the AND-1B drill site during past glacial maxima were grounded several hundreds of meters below sea level and separated from the land-based sector of the EAIS by the TAM. Glacial retreat of these marine ice sheets is considered to be controlled by variations in ocean temperature and marine ice sheet instability processes associated with the reverse bed slope of the Ross Sea continental shelf (19, 38). This implies that, on glacial to interglacial timescales, once retreat was initiated for past configurations of the ice sheet, it was likely to have been widespread across the Ross Embayment, similar to the patterns documented for the last deglaciation (e.g., 39, 40). Therefore, we argue that the AND-1B record represents the overall state of the marine-based ice sheet in the Ross Embayment, and that the conditions that led to open water deposits at this location also require partial to complete collapse of the WAIS (19, 28).
In AND-1B, advances and retreats of the ice sheet grounding line (Fig. 1) are constrained by glaciomarine cyclic stratigraphy (28, 41). Each cycle begins with a sheared glacial surface of erosion (SI Text), overlain by subglacial and ice-proximal glaciomarine diamictites (poorly sorted deposits of gravel, sand, and mud), mudstones, and sandstones, passing upward into interglacial diatomites deposited under open-ocean conditions. During glaciations, the ice sheets had laterally extensive marine termini that extended beyond the AND-1B drill site onto the Ross Sea continental shelf (28). In general, our glacial proximity curve (Fig. 1) shows that WAIS expansion into the Ross Sea occurred during the Late Pliocene and Early Pleistocene when benthic marine δ18O was more positive than Holocene values (42) and ice volumes exceeded the modern Antarctic volume of 27 × 106 km3.
Grounding-line oscillations are absent in AND-1B during a composite interval referred to as Cycle 32 (4.6–3.4 Ma; Fig. 1), when ∼80 m of diatomite was deposited (Fig. 1). The upper 60 m of diatomite accumulated between 3.6–3.4 Ma, when some of the lowest ice volumes and highest sea levels of the Pliocene occurred based on correlation to the benthic δ18O stack (28, 42). Below the 60 m section, a hiatus of ∼0.6 million years is associated with a debris flow of volcanic-rich diatomaceous sediment ∼1 m-thick and a carbonate-cemented diatomite ∼0.6 m-thick, which overlies 20 m of uncemented diatomite containing a shorter-duration hiatus at ∼453 m below seafloor (Fig. 1). These hiatuses represent nonglacial erosion or nondeposition, possibly due to currents invigorated by a modest ice sheet expansion that did not override the AND-1B drillsite (43). Ice sheet retreat in the Ross Embayment was also accompanied by retreat of the EAIS marine margin, as recorded from Prydz Bay at 4.7–4.3 Ma (16).
Although many of the diatom taxa in the Pliocene-early Pleistocene interval of AND-1B are now extinct, numerous species are still extant. Based on the present-day areal distribution of the species in the Southern Ocean (44–46), and therefore their sensitivity to modern SST and sea ice extent as it exists today (43, 47), we have grouped assemblages of extant diatom species preserved in AND-1B into three ecologically circumscribed categories. We term these groups “subantarctic,” “polar open ocean and seasonal sea ice tolerant,” and “sea ice” (SI Text). Only two extinct species are included; Rouxia antarctica and Shionodiscus tetraoestrupii are assigned to the “polar open ocean and seasonal sea ice tolerant” and “subantarctic” groups, respectively.
Diatom assemblages in the lower part of Cycle 32 contain abundant “subantarctic” flora that today live north of the Polar Front, whereas taxa that today occur south of the Polar Front, the “polar open ocean and seasonal sea ice tolerant” group, are a relatively minor constituent (Fig. 1). Combined with the very low abundance of diatom taxa that have maximum abundances in the modern zone of seasonal sea ice (Fig. 1), this indicates that sea ice was either absent or restricted to coastal areas during this prolonged period of ice sheet withdrawal in the Ross Embayment (43). Furthermore, δ13C and δ15N curves from Cycle 32 (Fig. 1) show low variance and limited coherence, indicating that Ross Sea surface conditions were substantially different from modern conditions and suggesting relatively invariant and moderate levels of summer productivity. In the modern Ross Sea, water column particulates and sea floor sedimentary organic matter display more negative δ13C values in the deeply mixed polynya center than at the melt-stratified western margin (48), whereas δ15N, which tracks nitrate utilization resulting from primary production, shows increased uptake of the total nitrate pool (more positive δ15N values) in more stratified water (49).
Diatomite in Cycle 32 is overlain by four progressively more ice-proximal sequences (Cycles 31–28) containing diamictites and their associated glacial surfaces of erosion, which represent the first of two major cooling steps in the Late Pliocene section of the AND-1B core. These four cycles formed during a 0.8‰ increase in benthic δ18O during glacial intervals, and correspond to an increase in modeled Antarctic ice volume from +7 m to -1 m equivalent sea-level relative to present (19). The base of Cycle 28 coincides with the base of the Mammoth Subchron at ∼3.3 Ma (Fig. 1). This surface is directly overlain by a ∼20 m thick diamictite deposited at the grounding zone during the glacial advance associated with Marine Isotope Stage (MIS) M2, the largest positive δ18O excursion in the Early Pliocene (Fig. 1). The MIS M2 glaciation marks the beginning of a long-term trend toward progressively more positive δ18O excursions culminating in MIS 100 at ∼2.5 Ma (Fig. 1). Subglacial sediments incorporated in Cycles 31–28 are the first evidence for grounded ice in the Ross Sea after ∼1 million years of open ocean conditions, and mark the termination of significantly warmer than present conditions in southern high latitudes. Subsequent glacial/interglacial grounding-line oscillations between 3.3–2.0 Ma (Cycles 27 to 16) reflect orbitally paced advance and retreat of a marine-based ice sheet across the AND-1B drill site. An up-core transition into cycles with successively thinner terrigenous glaciomarine facies (e.g., mudstone) between subglacial and open-ocean facies occurs in units between Cycles 32 and 21 (Fig. 1). This observation implies a reduction in subglacial meltwater outwash during glacial retreat, and reflects progressive cooling and drying of the subglacial regime from 3.3 to 2.9 Ma.
“Subantarctic” taxa are rare in the diatom assemblages of Cycles 27 and 23 (∼3.2 and 3.0 Ma; Fig. 1), and together with increased abundances of Chaetoceros resting spores suggest the occurrence of spring blooms in a well-stratified upper ocean influenced by seasonal sea ice melt. The abundance of the “subantarctic” assemblage increases again in Cycles 21–20 (∼3.0 to 2.6 Ma) but “polar open ocean and seasonal sea ice tolerant” and “sea ice” taxa also increase significantly. These observations indicate that while surface ocean conditions warmed relative to Cycles 27 and 23 they remained cooler than peak warmth recorded in Cycle 32 (Fig. 1). SSTs reconstructed from analysis (Fig. 1) support the cooling inferred from diatom assemblage analysis and facies interpretations despite recognized uncertainties of ± 4 °C associated with the high-latitude calibration (50) (SI Text). The -derived temperature for the inferred warm interglacial characterized by Cycle 32 is ∼5 °C in the lower half of this unit, but gradually cools to 2 °C in the later part of this interglacial. Cycle 32 contains species which are part of a “subantarctic” diatom-assemblage that occurs today between the Subantarctic and Polar Fronts where modern SSTs are ∼3–8 °C, which is broadly consistent with the derived-SST for Cycle 32.
Cycles 21 to 16 diatom assemblages contain a relatively large proportion of “polar open ocean or seasonal sea ice tolerant” and “sea ice” species (47). Today, these diatoms live in waters between the summer sea ice edge, where SSTs are as low as -1.5 °C, and the Polar Front, where SSTs rise to ∼3 °C (44–46). Although the abundance of extinct species in Pliocene AND-1B diatom assemblages hampers the development of a quantitative SST and sea ice reconstruction using modern analogues or transfer functions, the general trend of decreasing “subantarctic” species, and increasing “polar open ocean and seasonal sea ice tolerant” and “sea ice” species is consistent with the lower -derived SSTs in the interglacials following Cycle 32.
In all diatomites younger than 3.3 Ma, δ13C and δ15N generally co-vary (Fig. 1), suggesting a shift toward modern oceanic conditions in the Ross Sea, which supports conclusions drawn from other environmental proxies outlined previously. Peak interglacial δ13C is consistently less than -27‰ after ∼2.6 Ma (Cycle 20). Similar values today are only observed in the deeply mixedcentral Ross Sea polynya (48), which is an important source area for sea ice and deep-water formation. Such negative values in the Pliocene and Pleistocene likely reflect enhanced polynya-style deep wind mixing of the surface ocean. In contrast, more positive δ13C and δ15N values at the start and end of interglacial periods suggest increased stratification due to sea ice melt in the absence of a well-developed polynya. A notable feature of the data in most AND-1B diatomites younger than 3.3 Ma is that the derived SSTs are ∼2–4 °C warmer during the glacial retreat and advance phases than during the middle of the interglacial (Fig. 1). As the TEX86 proxy is likely to be sensitive to variations in water column structure (SI Text), it may be influenced by meltwater processes. Increased stratification, and therefore warming of the upper water column, would be expected to occur when the ice sheets were more proximal to AND-1B or there was increased sea ice duration/extent. Such stratification is indicated by the more positive δ13C and δ15N values at the start and end of these interglacials (Fig. 1). However, during the peak interglacial the lower δ13C and δ15N values suggest a well-mixed water column, and therefore the temperatures would be expected to be more consistent with mixing of colder waters from the subsurface and temperatures of ∼0–2 °C (Fig. 1). A similar pattern has been noted in a TEX86 record from a Holocene diatom ooze in the Antarctic Peninsula, with TEX86-derived SSTs 3–4 °C warmer during the deglacial (12–9 ka), compared to those later in the Holocene (< 9 ka; 51), and is also attributed to increased meltwater stratification. Although there is uncertainty in the use of calibration to obtain absolute SST values in high polar latitudes (50), the general trend of cooling in the Pliocene interval of AND-1B is consistent with the sedimentological, diatom, and stable isotope interpretations of cooling.
A significant cooling is represented by grounding-line advance and substantial erosion at the base of Cycle 20, with loss of up to ∼300 kyrs of record immediately below the Gauss-Matuyama transition at 2.6 Ma (Fig. 1). The “subantarctic” diatom assemblage is mostly absent in the overlying diatomite of Cycle 19, and is replaced by taxa more consistent with the presence of sea ice or cold, open waters (47) (SI Text). Although AND-1B diatomites reveal marked Late Pliocene cooling, they do not record the transition to late Pleistocene-Holocene Ross Sea diatom assemblages, which most likely occurred during additional cooling steps sometime after 2.0 Ma (52, 53), possibly after 1.0 Ma when the ice sheets in the Ross Embayment cooled to such extent that the Ross Ice Shelf, rather than marine conditions, persisted at the AND-1B drillsite throughout interglacial periods (41, 47). Rather, the Late Pliocene diatom assemblage in AND-1B likely represents a cooling of the surface ocean that favored increased duration and extent of sea ice into the summer months.
Discussion
Ice sheet expansion and development of sea ice in Antarctica’s coastal regions between 3.3 and 2.6 Ma appear to be accompanied by major changes in the Southern Ocean that affected heat transport farther afield (Fig. 2). Foraminiferal δ13C and δ18O from tropical Atlantic sediments suggest only minor input of Antarctic deep waters above 3,000 m depth prior to 3.7 Ma, even during glaciations (54). This supports a strong Atlantic Meridional Overturning Circulation (AMOC) that enhanced the supply of North Atlantic Deep Water (NADW), and was accompanied by reduced Antarctic Bottom Water (AABW) and Lower Circumpolar Deep Water (LCDW) input, reflecting diminished Antarctic ice sheet extent. A decrease in benthic δ13C during MIS Mg4 (∼3.4 Ma) at Ocean Drilling Program (ODP) Site 925 (54) in the equatorial Atlantic, recording the first Late Pliocene expansion of AABW/LCDW and mixing with NADW at ∼3,000 m depth. Stable isotopes and increased abundances of the benthic foraminifera Nuttallides umbonifera (55) highlight an increased northward influx of AABW/LCDW into the Atlantic Ocean between ∼3.2–2.9 Ma (e.g., Figs. 2 and 3). This incursion presumably reflected enhanced bottom and deep water production in the Weddell Sea, Ross Sea, and other source areas following the MIS Mg4 to M2 glacial advances, and the subsequent polynya and sea ice development recorded in AND-1B (Fig. 1 and 2). At ∼2.8 Ma, δ13C of deep Southern Ocean water decreases significantly compared to the North Atlantic and Pacific Ocean (Fig. 2C; 56–58), which is attributed to reduced Southern Ocean ventilation due to increased sea ice cover and surface water stratification (56).
Fig. 2.
Southern Hemisphere climate system feedbacks and responses, and their relationship to Antarctic climate evolution between 4–2 Ma. Steps in the deterioration of Pliocene climate at ∼3.3 Ma and ∼2.6 Ma are highlighted by blue dashed lines. (A) Atmospheric CO2 concentration derived from boron-(purple) and alkenone- (green) based marine proxies (26), compared with present (red line) and preindustrial (blue line) concentrations. (B) Benthic foraminiferal δ18O proxy for ice volume and temperature (δ18O; 42). (C) Influence of southern sourced bottom waters on deep ocean ventilation at ∼2.8–2.6 Ma shown by comparison of benthic foraminiferal δ13C records from South Atlantic/Southern Ocean ODP Sites 704/1090 (56), equatorial Pacific Ocean ODP Site 849 (57), and North Atlantic DSDP site 607 (58). (D) Atmospheric circulation (relative westerly wind strength) from dust mass accumulation rates for ODP Site 1090 (59). (E) Southern Ocean primary productivity based on biogenic opal mass accumulation rates at ODP Site 1091 shows a sharp increase coincident with increased windiness and nutrient supply by Fe-rich dust at ∼2.6 Ma (53, 59). (F) Onset of Antarctic sea ice at ∼2.6 Ma marked by a decline in primary productivity recorded in biogenic opal mass accumulation at Antarctic Peninsula ODP Site 1096 (53). (G) Summary of ocean, sea ice, and ice sheet evolution in Ross Embayment based on the AND-1B record. Note the cooling in SST and the return of periodic grounded ice sheets to western Ross Embayment occurs at ∼3.3 Ma, ending a ∼1.2 Ma period of relatively warm, ice free, open ocean conditions, and the appearance of sea ice and the development Ross Sea polynya between 3.2 and 2.6 Ma. Drill site locations are shown in Fig. 3.
Fig. 3.
Schematic representation of ocean surface circulation and intermediate/deep water mass formation responses to Late Pliocene Antarctic cooling. Dotted lines represent reduced water mass formation or current strength. (Upper) The warm Pliocene was characterized by a reduced WAIS volume and sea ice extent and duration during interglacials. Such conditions during Pleistocene glacial-interglacial transitions are associated with southward migration of Southern Ocean fronts (60, 61), allowing for greater connectivity of the Pacific, Indian, and Atlantic subtropical gyres, and may have promoted enhanced AMOC (61, 62). (Lower) Antarctic Ice Sheet expansion and sea ice growth in the Late Pliocene, and associated northward migration of the Southern Ocean frontal systems restricts water exchange between ocean basins, and reduces the AMOC (61). Increased sea ice extent during these steps resulted in increased AABW formation (55) (blue arrows) and the intensified frontal systems likely promoted AAIW and SAMW formation (66).
An observed increase in southern mid-latitude dust deposition after 2.8 Ma suggests enhanced atmospheric circulation, or an expanded dust source (59; Fig. 2D), and coincides with an increase in primary productivity at the Subantarctic Front (53) (Fig. 2E). This transition also appears to be associated with the development of polynya-style mixing and a higher abundance of the sea ice tolerant diatom group in the Ross Sea (Fig. 2G) and off the Antarctic Peninsula. The latter experienced declining opal production between ∼3.2 Ma and 2.3 Ma as a consequence of a longer duration of summer sea ice and a resultant reduction in light availability (53) (Fig. 2F). We suggest that extensive Antarctic sea ice may have reduced the warming effect of the Southern Hemisphere subtropical gyres on high latitudes through northward migration of ocean fronts, including reducing the Agulhas inflow and its influence on the AMOC (e.g., Fig. 3) as has been inferred for Late Pleistocene glacial periods (60–62). Additionally, tectonic modification of the Indonesian Seaway between ∼3–4 Ma potentially reduced and cooled the westward inflow of Pacific water into the Indian Ocean, in turn exacerbating the reduced heat transport via Agulhas leakage and heat transport to higher latitudes (34). The influence of upwelling NADW would also be reduced along the Antarctic margin, a process potentially enhanced by expanded ice sheets and sea ice. As noted earlier, such conditions favored local formation of AABW/LCDW, which along with a reduced AMOC would have affected the global heat transport.
Another consideration is that these ice sheet/sea ice/ocean changes between 3.3 and 2 Ma coincided with increased productivity and upwelling of cooler waters in the eastern equatorial Pacific (63) and the southeastern Atlantic (64) oceans. The intensification of the eastern Pacific upwelling and productivity at this time (65) has been associated with termination of “permanent El Niño-like” conditions in the Pacific Ocean, itself a potentially important global cooling mechanism (36). In the Southern Ocean, changes in wind fields have been linked to increased production of Antarctic Intermediate Water (AAIW) near the Polar Front, as postulated for the LGM (66). Thus, expansion of sea ice in the Southern Ocean may have driven increased upwelling of cooler water in the lower latitudes, with models simulating a potential 44% reduction of heat flux into the eastern equatorial Pacific under increased Southern Hemisphere sea ice (67).
Atmospheric teleconnections to equatorial regions and high northern latitudes, based on model simulations, indicate that major ice sheet and sea ice expansion in either hemisphere significantly alters the global Hadley circulation (68). Although the development of northern hemisphere ice sheets in the Late Pliocene complicates the exact nature of this feedback, altered Hadley circulation may initiate wholesale latitudinal shifts in the position of the Intertropical Convergence Zone, trade winds, and southern midlatitude westerlies—all of which influence wind-driven upwelling and the dynamics of frontal systems, and Southern Ocean ventilation rates (68, 69).
In conclusion, this reconstruction of Pliocene and Early Pleistocene Ross Sea climate and Antarctic ice sheets, based on new evidence from the AND-1B core, indicates a smaller WAIS, prolonged marine conditions in the Ross Embayment, and reduced sea ice extent and duration before 3.3 Ma. Subsequent Southern Ocean cooling and increased seasonal persistence of Antarctic sea ice occurred between 3.3 and 2.6 Ma, coinciding with a potential drawdown of atmospheric CO2 from ∼400 to ∼280 ppm (Figs. 1 and 2A) (36). This cooling affected expansion of westerly winds and northward migration of ocean fronts in the Southern Ocean, likely restricting the warm Agulhas inflow into the Atlantic (61, 62). We suggest such a scenario contributed to a slowdown of the interhemispheric AMOC beginning after 3.3 Ma, and helped precondition the Northern Hemisphere for continental glaciation. Furthermore, ice-albedo and oceanic/atmospheric feedbacks in both hemispheres, plus cooling in the upwelling zones of the eastern equatorial Pacific (36, 67) and Southeast Atlantic (64), would have further intensified cooling into the Pleistocene (52). Our reconstruction constrains the timing and nature of Late Pliocene cooling in the Southern Hemisphere and provides new evidence supporting the link between ice-sheet, sea ice, and oceanic processes from polar to equatorial latitudes. These boundary conditions and linkages provide a new context for numerical modeling and understanding of the global cooling that ultimately led to the bipolar-glacial world.
Material and Methods
AND-1B was described using standard sedimentological techniques to produce detailed stratigraphic logs (41). Diatom groupings are based on a synthesis of core top (modern) diatom assemblages in the Southern Ocean (44–46) with minor modifications (SI Text). δ13C and δ15N sediment samples were analyzed at the Stanford University Stable Isotope Laboratory. was analyzed at Yale University and NIOZ Royal Netherlands Institute for Sea Research. Detailed methodology is provided as SI Text. Samples were provided by the Antarctic Marine Geology Research Facility, Florida State University.
Acknowledgments.
The scientific studies for ANDRILL are jointly supported by the US National Science Foundation, the NZ Royal Society of New Zealand Marsden Fund, the Italian Antarctic Research Programme, the German Research Foundation, and the Alfred Wegener Institute for Polar and Marine Research.
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The scientific studies for ANDRILL are jointly supported by the US National Science Foundation, the NZ Royal Society of New Zealand Marsden Fund, the Italian Antarctic Research Programme, the German Research Foundation, and the Alfred Wegener Institute for Polar and Marine Research.
Notes
*This Direct Submission article had a prearranged editor.
Department of Earth Sciences, Faculty of Geosciences, Laboratory of Palaeobotany and Palynology, Utrecht University, U3584 CD Utrecht, The Netherlands;
To whom correspondence should be addressed. E-mail: [email protected].
Author contributions: R.M., T.N., and R.L. designed research; R.M., T.N., L.C., C.R., R. Dunbar, C.S., D.W., F.S., C.W., M.P., S.S., V.W., and R.D.P. performed research; R.M., T.N., L.C., C.R., R. Dunbar, C.S., D.W., F.S., C.W., M.P., S.S., V.W., R.L., R. DeConto, and R.D.P. analyzed data; and R.M., T.N., L.C., and C.R. wrote the paper.
Competing Interests
The authors declare no conflict of interest.
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R. McKay,
T. Naish,
L. Carter,
C. Riesselman,
R. Dunbar,
C. Sjunneskog,
D. Winter,
F. Sangiorgi,
C. Warren,
M. Pagani,
S. Schouten,
V. Willmott,
R. Levy,
R. DeConto,
& R.D. Powell,
Antarctic and Southern Ocean influences on Late Pliocene global cooling, Proc. Natl. Acad. Sci. U.S.A.109 (17) 6423-6428,https://doi.org/10.1073/pnas.1112248109(2012).
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