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Antarctic and Southern Ocean Influences on Late Pliocene Global Cooling

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Antarctic and Southern Ocean Influences on Late Pliocene Global Cooling Antarctic and Southern Ocean influences on Late Pliocene global cooling Robert McKaya,1, Tim Naisha, Lionel Cartera, Christina Riesselmanb,2, Robert Dunbarc, Charlotte Sjunneskogd, Diane Wintere, Francesca Sangiorgif, Courtney Warreng, Mark Paganig, Stefan Schoutenh, Veronica Willmotth, Richard Levyi, Robert DeContoj, and Ross D. Powellk aAntarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand; bDepartment of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305; cDepartment of Environmental Earth Systems Science, Stanford University, Stanford, CA 94305; dAntarctic Marine Geology Research Facility, Florida State University, Tallahassee, FL 32306; eRhithron Associates, Inc, Missoula, MT 59804; fDepartment of Earth Sciences, Faculty of Geosciences, Laboratory of Palaeobotany and Palynology, Utrecht University, U3584 CD Utrecht, The Netherlands; gDepartment of Geology and Geophysics, Yale University, New Haven, CT 06520; hNIOZ Royal Netherlands Institute for Sea Research, Department of Marine Organic Biogeochemistry, 1790 AB Den Burg, Texel, The Netherlands; iGNS Science, Lower Hutt 5040, New Zealand; jDepartment of Geosciences, University of Massachusetts, Amherst, MA 01003; and kDepartment of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115 Edited by* James P. Kennett, University of California, Santa Barbara, CA, and approved February 28, 2012 (received for review August 2, 2011) 18 The influence of Antarctica and the Southern Ocean on Late δ O records (21) and glacial deposits at high elevation in the Pliocene global climate reconstructions has remained ambiguous TAM (22). due to a lack of well-dated Antarctic-proximal, paleoenvironmental The development of an ephemeral West Antarctic Ice Sheet records. Here we present ice sheet, sea-surface temperature, and (WAIS) is thought to have occurred around 34 Ma (5) coincident sea ice reconstructions from the ANDRILL AND-1B sediment core with the first ice sheets in East Antarctica, but it was not a per- recovered from beneath the Ross Ice Shelf. We provide evidence manent feature until much later (23). Glacial unconformities for a major expansion of an ice sheet in the Ross Sea that began observed in seismic profiles in the central Ross Sea, correlated at ∼3.3 Ma, followed by a coastal sea surface temperature cooling to dated horizons in Deep Sea Drilling Project Site 270, indicate of ∼2.5 °C, a stepwise expansion of sea ice, and polynya-style deep that periods of an extensive grounded marine ice sheet within the mixing in the Ross Sea between 3.3 and 2.5 Ma. The intensification Ross Sea embayment have occurred since the Early Miocene of Antarctic cooling resulted in strengthened westerly winds and (24). During the warmest intervals of the Pliocene (4.5–3.0 Ma) EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES invigorated ocean circulation. The associated northward migration Earth’s average surface temperature was ∼2–3 °C warmer than of Southern Ocean fronts has been linked with reduced Atlantic present, atmospheric pCO2 was ∼400 ppmv, and equator to pole Meridional Overturning Circulation by restricting surface water temperature gradients were weaker (25–27). During this peak connectivity between the ocean basins, with implications for heat Pliocene warmth, the largely marine-based WAIS and the Green- transport to the high latitudes of the North Atlantic. While our land Ice Sheet were reduced in extent (19, 28, 29) and global sea results do not exclude low-latitude mechanisms as drivers for Plio- level is estimated to have been between 5–40 m above present cene cooling, they indicate an additional role played by southern with most reconstructions converging on 20–25 m (30). Subse- high-latitude cooling during development of the bipolar world. quent cooling, which led to the onset of major Northern Hemi- spheric glaciation by ∼2.7 Ma (31), has been variously attributed ∣ ∣ ∣ glacial history West Antarctic Ice Sheet Late Neogene to declining pCO2 (32), changing orbital geometries (33), tec- paleooceanography ∣ paleoclimate tonic influences (34), increased oceanic stratification and preci- pitation in northern high latitudes (35), and reduced zonal sea- he development of the first continental-scale ice sheet on surface temperature (SST) gradients in the equatorial Pacific TAntarctica occurred at ∼34 Ma, coincident with a ∼1.5‰ Ocean (36). Until now, the role of Antarctica in Late Pliocene increase in benthic foraminiferal δ18O (1, 2), which is interpreted global cooling has been unclear. The Antarctic Drilling Program’s as a 4 °C cooling in deep ocean temperature (3) with 80 m of sea (ANDRILL) AND-1B core contains a series of well-dated sedi- level equivalent ice volume on the Antarctic continent (4, 5). mentary cycles documenting ice sheet advance and retreat that Direct geological evidence of a continental-scale ice sheet calving correlate with the global marine oxygen isotope and southern, at the coastline by the earliest Oligocene came from ice-rafted high-latitude insolation time series (Fig. 1) (19, 28). debris in ocean drill cores from Prydz Bay (6, 7) and marine Here, we describe a major phase of ice sheet expansion and grounding-line deposits in Western Ross Sea drill cores (8). Prox- cooling in coastal Antarctic waters at ∼3.3 Ma following a imal geological drill cores (9) and high-resolution deep-sea δ18O ∼1.2 Myr-long period of warmer-than-present marine conditions (10) records imply the existence a dynamic and highly variable, accompanied by a diminished marine-based ice sheet in the orbitally paced East Antarctic Ice Sheet (EAIS) that drove global Ross Embayment during the Early Pliocene (∼4.5–3.4 Ma). This sea-level changes of ∼40 m (11) up until ∼14 Ma. A ∼1.2‰ in- cooling involved the expansion of the Antarctic ice sheets onto crease in benthic foraminiferal δ18Oat∼14 Ma and cooling of the continental shelf, increased sea ice extent and duration, and Southern Ocean surface waters by up to 7 °C is associated with altered Southern Ocean circulation. Our multiproxy dataset the development of a more permanent EAIS (1, 12, 13). Terres- trial glacial deposits at high elevations in the Transantarctic Author contributions: R.M., T.N., and R.L. designed research; R.M., T.N., L.C., C.R., Mountains (TAM) indicate a transition from wet-based to dry- 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., based glaciation at this time, and using the preservation of deli- 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. cate terrestrial plant fossils, ancient landscapes, and volcanic analyzed data; and R.M., T.N., L.C., and C.R. wrote the paper. ashes, it has been argued that the volume of the EAIS remained The authors declare no conflict of interest. relatively stable since ∼14 Ma (14, 15). While terrestrial glacial *This Direct Submission article had a prearranged editor. and glaciomarine deposits indicate marine incursions into fjords 1To whom correspondence should be addressed. E-mail: [email protected]. coupled with thinning and recession of the low elevation parts of 2Present address: Eastern Geology and Paleoclimate Science Center, US Geological Survey, the EAIS along Western Ross Sea and in Prydz Bay during the Reston, VA 20192. Pliocene (16, 17), widespread deglaciation of the EAIS at this This article contains supporting information online at www.pnas.org/lookup/suppl/ time (18) is unsupported by ice sheet models (19, 20), benthic doi:10.1073/pnas.1112248109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1112248109 PNAS Early Edition ∣ 1of6 Downloaded by guest on October 1, 2021 L 18 Glacial Diatom TEX 86 Bulk sediment O benthic proximity assemblages (%) SST(°C) stable isotopes stack sea ice Modern Ross Sea 15 WAIS and TEXL derived SST N polar open ocean/ 86 Greenland 13 seasonal sea ice C deglaciated subantarctic Astronomical time scale (2004) 2.5 3.5 3 4.5 4 -19 IPDM -31 7.5 10 2.5 other 0 5 -2.5 Cycle number Biostratigraphic datum Depth (mbsf) Lithology Magnetostratigraphy Chaetoceros 2 Ma >2.01 150 16 >2.21 17 18 19 <2.79 200 20 Pleistocene >2.87 100 21 22 250 23 24 25 3 Ma 26 300 27 M2 Mg2 Mg4 Mg6 28 350 Gi2 29 Gi4 30 Pliocene 31 400 4 Ma 32 <3.56 >4.29 450 <4.78 33 -4 12 diatomite diamictite mudstone volcanogenic 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 L derived by magnetostratigraphy, constrained by biostratigraphy and tephrochronology (28). Diatom assemblages, TEX86 derived SST (with light purple cali- bration 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). indicates: (i) a progressive increase in ice sheet extent and varia- Ross Embayment, and that the conditions that led to open water bility, and reduced glacial meltwater, and (ii) a general cooling of deposits at this location also require partial to complete collapse the coastal Antarctic seas based on evidence from sedimentary of the WAIS (19, 28). L facies, diatom assemblages, TEX86 (tetraether index of lipids con- In AND-1B, advances and retreats of the ice sheet grounding sisting of 86 carbon atoms) sea-surface temperature reconstruc- line (Fig. 1) are constrained by glaciomarine cyclic stratigraphy tions, and δ13Candδ15N of bulk sedimentary organic matter. (28, 41). Each cycle begins with a sheared glacial surface of ero- sion (SI Text), overlain by subglacial and ice-proximal glaciomar- Results ine diamictites (poorly sorted deposits of gravel, sand, and mud), AND-1B was drilled beneath the McMurdo Ice Shelf, an exten- mudstones, and sandstones, passing upward into interglacial dia- sion of the Ross Ice Shelf at its northwest margin.
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