Synthesis Antarctic Climate Evolution Since 3Ma

11.6.1. Late Pliocene Cooling, Ice Sheet Expansion and the Development of a Marine Ice Sheet Margin Around Antarctica

• In the Weddell Sea, Kennett and Barker (1990) were the first to identify a cooling step marked by a reduction in sedimentation rate, lower diatom abundance and the presence of sea-ice diatoms during the Late Pliocene.

• Along the Pacific margin of the Antarctic Peninsula, seismic data correlated into ODP Leg 178 drill cores, documented a change in depositional style associated with reduced sediment supply and meltwater volume as the Antarctic Ice Sheets expanded and began cooling towards their present state from around 3-2.5 Ma (Rebesco et al., 2006). A large spike in IRD abundance at 2.8 Ma recorded in Antarctic Peninsula continental rise Site 1101 core also supports the development of a marine ice terminus along the edge of the WAIS (Cowan, 2002).

• In the Ross Sea, Bart (2004) interpreted a major glacial unconformity in seismic reflection data of inferred Late Pliocene age as evidence for widespread westward expansion of the WAIS ice streams into the Ross Sea. Although some uncertainties surround the chronology of DVDP cores in the mouth of Taylor Valley, and CIROS-2 cores in Ferrar Fiord, expansion of WAIS-sourced ice into the Dry Valleys also appears to have occurred about this time (McKelvey, 1981; Powell, 1981).

• Based on the above and new evidence from the ANDRILL AND-1B record (Naish et al., submitted) it appears that between 3.0 and 2.5 Ma, high-latitude climate cooling drove both the WAIS and EAIS towards their present expanded cold polar state. Relatively warm and often terrestrially based ice margins were replaced by more permanent marine termini and the development of ice shelves.

11.6.2. Late Pliocene-Early Pleistocene G-I Climate Cycles and the Role of Northern Hemisphere Glacio-Eustasy

• Direct physical sedimentary records of Antarctic Ice Sheet variability (e.g. IRD, proximal glacimarine cycles) and more distal ocean records of sea-ice distribution (e.g. diatom palaeoecology), thermohaline circulation (e.g. grain size), ocean temperatures (e.g. d18O), frontal dynamics and surface circulation (e.g. faunally derived assemblages and SSTs) all show a strong covariance with the 41 kyr cycle in Earth's obliquity prior to the last 800 kyr.

• It has been proposed that long-term variations in the duration of summer insolation controlled by orbital obliquity (Huybers, 2006), generally dominates over the influence of the intensity of NH insolation (e.g. Raymo et al., 2006) prior to the Mid-Pleistocene. Marine Isotope Stage 31 may be the exception (Scherer et al., 2008). Strong coupling between the different elements of the SH ice-sheet-ocean-climate system, supports a primary obliquity influence on G-I variability between 3 and 1 Ma.

• Glacial periods result in the northward expansion of seasonal sea ice, SSTs up to 6°C colder than now, equatorward migration of frontal zones by — 5-10° latitude (Howard and Prell, 1992; Gersonde et al., 2005), equatorward displacement and intensification of zonal westerly winds (Shulmeister et al., 2004; Toggweiler et al., 2006), invigorated surface circulation (ACC) and intensified abyssal currents (e.g. Hall et al., 2001). All of these processes appear to have occurred 3-7 kyr before the 818O ice volume maximum (e.g. Crundwell et al., 2008) when Antarctic Ice Sheets are fully extended onto the continental shelf. The converse is true for interglacials.

• Stratigraphic analysis of 41 kyr duration, Late Pliocene glacimarine cycles in the AND-1B core straddling the Gauss-Matuyama (G-M) polarity transition (ca. 2.6 Ma) display abrupt transitions between ice-proximal diamictites and open marine biosiliceous deposits, implying rapid oscillations between glacial and marine environments with significant volume changes in the WAIS (Naish et al., 2008). Intriguingly, the facies also imply a cooler style of ice sheet compared with the Early Pliocene cycles, indicating reduced amounts of subglacial meltwater and terrigenous sediment (McKay et al.. accepted). From -2.6 Ma polar Antarctic ice volume changes may have been controlled by the effect of NH glacio-eustasy on its marine margin, and that this mechanism accounts for much of the orbital variability since 2.6 Ma (cf. Raymo et al., 2006).

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