The Antarctic Ice Sheet is both a major system component (involved in global deep- and bottom water formation and sea-level change) and a source of "noise" in the oxygen isotopic record that limits the value of this record to other studies through most of the Cenozoic. The proposal on which this leg is based is one of four or five linked proposals intended to extract Antarctic Cenozoic glacial history from the sediments of its continental margin. Leg 178 will drill eight sites (Figs. 1, 2, 3) on the continental margin of the Antarctic Peninsula. Sites include a transect of the outer continental shelf and complementary holes in a hemipelagic drift on the continental rise, both extending back 6-10 m.y., and a shallow hole on the inner continental shelf that will provide an ultra-high resolution Holocene record.

At present, the history of the Antarctic Ice Sheet is unknown. It has been inferred from low latitude proxy data such as oxygen isotopic measurements on deep-ocean benthic foraminifers and the record of eustatic sea-level change adduced from sediments on low-latitude margins (Miller et al., 1987; Haq et al., 1987). However, these inferences are ambiguous and in disagreement (Sahagian and Watts, 1991; Barker, 1992), which not only leaves the history unresolved, but also limits the credibility and usefulness of both sets of proxy data. For example, there is dispute over whether the principal increases in Antarctic ice volume, which affect the benthic isotopic record, occurred at about 35 Ma, at 16-13 Ma, or only after 3 Ma. Within these various hypotheses, assumptions that may be incorrect have been made about the constancy of equatorial surface temperatures or the high-latitude surface origins and temperatures of intermediate to deep waters at low latitudes. Similarly, changes in grounded ice volume provide the only generally accepted repeatable, rapid-acting cause for global eustatic sea-level change, yet the timing and amplitudes of sea-level change adduced from low-latitude margin sediments are disputed, and changes also occur at times when there is no independent evidence for the existence of substantial volumes of grounded ice on Antarctica or elsewhere. Further, the isotopic and sea-level estimates of grounded ice volume disagree substantially with each other at both long and short periods through most of the Cenozoic. Onshore Antarctic evidence of glacial history is sparse and is also presently controversial. Argument continues as to how stable the Antarctic Ice Sheet has been (Webb and Harwood, 1991; Denton et al., 1993).

Deep and intermediate waters of the Southern Ocean have generally been corrosive to the carbonate microfossil tests that are almost exclusively used in isotopic analysis. Therefore, the problems in using distal proxy data to make indirect estimates of ice volume will persist. Some progress may be made by detailed analysis at very high resolution of carbonate sections from a large number of lower latitude sites, but the solutions will remain ambiguous. The Antarctic margin sediments hold a direct record of Antarctic ice-sheet fluctuation that can help resolve the ambiguities of ice-volume change and clear the way for more useful interpretation of isotopic and sea-level data in the future.

The ultimate aim of the four or five linked Antarctic Offshore Acoustic Stratigraphy (ANTOSTRAT) drilling proposals is to provide an estimate of variations in size of the Antarctic Ice Sheet through the Cenozoic. This will necessarily include warmer periods when the ice sheet was much smaller than today, reaching the margin only occasionally and in a few places, with significant fluvial sediment transport and deposition elsewhere. It is therefore necessary for drilling to sample both the East and West Antarctic glacial history, and to distinguish a small interior ice sheet, barely reaching the margin, from a much larger ice sheet with a large coastal ice budget. This means making use of numerical models to suggest what might have been the patterns of past glaciation and using the modeling results or other relevant information to select drilling locations in different regions. For example, Figure 4 (from Huybrechts, 1993) shows a glaciological model of ice sheets that cover only parts of the continent during warmer conditions. It is clear that some regions will be more sensitive to particular stages of ice-sheet volume change than others, and that no single region will provide a complete history. The models provide the means of combining data from different regions of the Antarctic margin into a complete history of ice-sheet development.

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