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doi:10.2204/iodp.sp.318.2008

Introduction

Polar ice is an important component of the modern climate system, affecting global sea level, ocean circulation and heat transport, marine productivity, and planetary albedo, among other things. The modern semipermanent ice caps are, geologically speaking, a relatively young phenomenon. Since mid-Permian (~270 Ma) times, parts of Antarctica became (re)glaciated only ~34 m.y. ago, whereas episodes of major Northern Hemisphere continental ice began ~3 m.y. ago (e.g., Zachos et al., 2001) (Fig. F1). In a broad sense, the record of Antarctic glaciation from the time of first ice sheet inception (around the Eocene/Oligocene boundary; Oligocene isotope event 1 [Oi-1] glaciation) through the significant periods of climate change during the Cenozoic, such as the Oligocene/Miocene boundary, the Miocene isotope event 1 [Mi-1], the mid-Miocene climatic optimum, late Neogene cooling, early Pliocene warming events, the Quaternary glacial–interglacial cycles, and the concomitant biotic and paleocenographic evolution, is not only of scientific interest but also is of great importance for society. State-of-the-art climate models (e.g., DeConto and Pollard, 2003a, 2003b; Huber et al., 2004; DeConto et al., 2007) combined with paleoclimatic proxy data (e.g., Pagani et al., 2005) suggest that the main triggering mechanism for initial inception and development of the Antarctic ice sheet were the decreasing levels of CO₂ concentration in the atmosphere (Fig. F2), and that the opening of critical Southern Ocean gateways (e.g., Kennett, 1977; DeConto and Pollard, 2003a, 2003b; Huber et al., 2004; Barker and Thomas, 2004) played only a secondary role. With current rising atmospheric greenhouse gases resulting in rapidly rising global temperatures (Intergovernmental Panel on Climate Change [IPCC], 2007), studies of polar climates are prominent on the research agenda. Understanding Antarctic ice sheet dynamics and stability is of special relevance because, based on IPCC 2007 forecasts, CO₂ doubling or 1.8°–4°C equivalent for the end of this century is expected. These conditions were not experienced in our planet since 10–15 Ma, when only the Antarctic ice sheet existed (Fig. F3).

Since their inception, like the Northern Hemisphere ice sheets, the Antarctic ice sheets appear to have been very dynamic; waxing and waning in response to global climate change over intermediate and even short (orbital) timescales (e.g., Wise et al., 1991; Zachos et al., 1997; Barker, Camerlenghi, Acton, et al., 1999; DeConto and Pollard, 2003a, 2003b). Yet, not much is known about the nature, cause, timing, and rate of processes involved, notably of Antarctic glaciation. Past ocean drilling into the Antarctic continental shelf and basins in Prydz Bay and the Ross Sea (i.e., Ocean Drilling Program [ODP] Legs 119 and 188, Deep Sea Drilling Program [DSDP] Leg 28, and Cape Roberts Project) indicates two basic states of the Antarctic ice sheet(s): (1) an early phase lasting ~20 m.y. with a less stable ice cover characterized by strong cyclic waxing and waning (Zachos et al., 1997; Wade and Pälike, 2004; Pälike et al., 2006), and (2) a later (from ~14 Ma to recent) phase when deep-sea isotope records (e.g., Miller et al., 1985; Flower and Kennett, 1994) indicate that the Antarctic ice sheet(s) became a quasipermanent, and more stable, feature sustaining polar climates. However, even the "stable" ice sheets may have varied considerably in size, perhaps by as much as 25 m of sea level equivalent (SLE) (Kennett and Hodell, 1993). Of the two main ice sheets, the West Antarctic Ice Sheet (WAIS) (Fig. F4) is mainly marine based and is considered less stable. The EAIS, which overlies continental terranes that are largely above sea level, is considered stable and is believed to respond only slowly to changes in climate. However, reports of beach gravel deposited 20 m above sea level in Bermuda and the Bahamas from 420 to 360 ka indicate the collapse of not only the WAIS (6 m of SLE), and the Greenland ice sheet (6 m of SLE), but possibly also 8 m of SLE from East Antarctic ice sources (Hearty et al., 1999). Therefore, it is indicated that during episodes of global warmth, with likely elevated atmospheric CO₂ conditions, the EAIS may contribute just as much or more to rising global sea level as the proverbial unstable Greenland ice sheet. In the face of rising CO₂ levels (IPCC, 2007) a better understanding of the EAIS dynamics is therefore urgently needed from both an academic as well as a societal point of view.

A key region for analysis of the long- and short-term behavior of the EAIS is the eastern sector of the Wilkes Land margin, located at the seaward termination of the largest East Antarctic subglacial basin, the Wilkes subglacial basin (Figs. F5, F6) (Drewry, 1983; Ferraccioli et al., 2001, 2007). The base of the portion of the EAIS draining through the Wilkes subglacial basin is largely below sea level, suggesting that this portion of the EAIS is potentially less stable than the rest of the EAIS. Numerical models of ice sheet behavior (e.g., Huybrechts, 1993; DeConto and Pollard, 2003a, 2003b) (Fig. F2) provide a basic understanding of the climatic sensitivity of particular Antarctic regions for early ice sheet formation, connection and expansion, and eventual formation of the entire ice sheet. For example, in these models glaciation is shown to have begun in the East Antarctic interior, discharging mainly through the Lambert Graben to Prydz Bay. These models imply that the EAIS did not reach sea level in the Wilkes Land margin until a later stage. These models must now be validated through drilling and obtaining concrete evidence found in the sedimentary record. Sediments from Prydz Bay cores drilled during Leg 188 (O'Brien, Cooper, Richter, et al., 2001; Cooper, O'Brien, and Richter, 2004) contained the record of the first arrival of ice sheets to that margin. There, the onset of glaciation is dated to the latest Eocene–earliest Oligocene (~34 Ma). The timing and mode of the onset of glaciation at the Wilkes Land continental margin is still unknown but is essential for providing age constraints for the models of EAIS development and changes in its volume. Moreover, detailed portrayal of the subsequent Cenozoic history and dynamics of the Antarctic glacial cycles at Wilkes Land will provide further constraints for model experiments and future predictions about EAIS stability.

Conceivably even more important than the history of the Antarctic glaciations are past lessons of deglaciations. Seismic surveys and pilot studies indicate that the Wilkes Land margin also includes sites of ultrahigh accumulation rates of sediments recording the Holocene deglaciation. Recovery and analysis of these unique ~200 m thick series of "tree ring" type, annually layered sediments predominantly consisting of phytoplankton remains constituting one of the worlds most expanded archives of recent environmental change is one of the tasks ahead to ensure palaeoenvironmental reconstruction with unprecedented detail.

During Expedition 318, we will drill key sites along a shelf-rise-abyssal plain transect at Wilkes Land, Antarctica (Figs. F5, F6; Table T1), to determine the timing and mode of the onset of Antarctic glaciation at the Wilkes Land continental margin, to determine the subsequent Cenozoic history and dynamics of the Antarctic glacial cycles, and to recover and reconstruct the dynamics of the youngest deglaciation in detail.

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