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 leaves the history unresolved and limits the credibility and usefulness of both proxy data sets. For example, there is a 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. Similarly, changes in grounded ice volume provide the only generally accepted repeatable, rapid-acting cause of global, eustatic sea-level change, yet the timing and amplitudes of sea-level change adduced from low-latitude margin sediments are disputed. Changes also occur at times when there is no independent evidence for substantial volumes of grounded ice on Antarctica or elsewhere. Further, the isotopic and sea-level estimates of grounded ice volume disagree substantially at both long and short periods through most of the Cenozoic. Onshore Antarctic evidence of glacial history is sparse and at present controversial at present: the argument continues as to how stable the early Pliocene Antarctic Ice Sheet has been (Webb and Harwood, 1991; Denton et al., 1993).
Deep and intermediate Southern Ocean waters generally corrode the carbonate tests used in isotopic analysis. Therefore, despite the production of high-resolution data sets (e.g., Tiedemann et al., 1994; Shackleton et al., 1995), the problems of using distal proxy data will persist. Antarctic margin sediments hold a direct record of ice-sheet fluctuation that can determine ice-volume change and clear the way for a more useful interpretation of high-latitude isotopic and sea-level data in the future.
The five linked Antarctic Offshore Acoustic Stratigraphy initiative (ANTOSTRAT) drilling proposals are intended to estimate size variations of the Antarctic Ice Sheet through the Cenozoic. This will include warmer periods when the ice sheet was much smaller, reaching the margin in only a few places, with fluvial sediment transport and deposition elsewhere. The proposals aim to sample both the East and West Antarctic margins and to distinguish an interior ice sheet, barely reaching the margin, from an ice sheet with a large coastal ice budget. Leg 178, the first of these proposals funded, succeeded in sampling the Pacific margin of the Antarctic Peninsula at nine sites (Figs. F1, F2, F3). Selection of these site locations and those in the other proposals was aided by numerical models that indicate patterns of past glaciation. For example, Figure F4 (from Huybrechts, 1993) shows a glaciological model of ice sheets that cover only parts of the continent during warmer conditions. Some regions are clearly more sensitive to particular stages of ice-sheet volume change than others, and no single region will provide a complete history. The models allow data from different regions of the Antarctic margin to be combined in a complete history of ice-sheet development.
The collaborative interpretation of Antarctic margin seismic data through ANTOSTRAT (Cooper et al., 1994, 1995; Barker and Cooper, 1997) and the simplicity of the modern Antarctic glacial regime have led to rapid emergence and application of a unifying model of glacial-sediment transport and deposition (Alley et al., 1989; Larter and Barker, 1989; Bartek et al., 1991; Cooper et al., 1991; Kuvaas and Kristoffersen, 1991). Briefly, almost all ice transport to the ice-sheet margins takes place within broad, rapidly moving ice streams. Rapid flow is enabled by low basal friction, whose main source is an overpressured and undercompacted, unsorted, shearing basal till. The shear ensures that ice transport is accompanied by till transport, and virtually all of the transported till is melted out/dropped/deposited close to the grounding line, where the ice sheet becomes an ice shelf before calving into icebergs. The ice stream erodes and transports inshore of the grounding line and deposits offshore, in a high-latitude analogue of the low-latitude subaerial erosion/shoreline/marine deposition system. Further, the grounding line advances and retreats under the influence of upstream ice provision and basal sediment supply (and sea-level change), all climate related. The very large prograded sediment wedges beneath the Antarctic margin were developed during glacial maxima, when the ice sheet was grounded to the continental shelf edge (Fig. F5).
The glacial sedimentary regime has other characteristics. Progradation is usually focused into broad "trough-mouth fans" opposite the main ice streams, and the shelf is overdeepened (generally to between 300 and 600 m, but in places much deeper) and inward sloping. Continental slopes are often steep, and in places turbidity-current transport of the unstable component of slope deposition (with downcurrent deposition of suspended fines) has produced large hemipelagic sediment drifts on the continental rise (e.g., Kuvaas and Leitchenkov, 1992; Tomlinson et al., 1992; McGinnis and Hayes, 1995; Rebesco et al., 1996, 1997; Fig. F6). Sediment supply to the slope is highly cyclic, with large quantities of unsorted diamict deposited during glacial maxima and very little deposited during interglacial periods.
Three depositional environments are recognized: shelf topsets and slope foresets of the prograded wedge, and proximal hemipelagic drifts on the continental rise. Of these, the shelf record is potentially the least continuous. There, sediment is preserved mainly by slow subsidence, from cooling and from flexural response to the topset and foreset load, and the sediment is prone to re-erosion during the next glacial advance (e.g., Larter and Barker, 1989; Vanneste and Larter, 1995). Topsets tend to mark only the major changes in glacial history, so the more continuous foreset record is complementary. Before Leg 178, the drifts proximal to the rise were sparsely sampled (see Camerlenghi et al., 1997) but potentially contained an excellent record, closely related to that of the upper-slope foresets from which they are derived. Existing seismic data and drill sites from around Antarctica had demonstrated the coarse- (but not yet the fine-) scale climate record in continental rise sediments and the likely climate sensitivity of margin-wedge geometry (Barker, 1995). They had also revealed the partial nature of the shelf topset record (Hayes, Frakes, et al., 1975; Barron, Larsen, et al., 1989). During Leg 178, the glacial shelf prograded wedge was sampled at Site 1097 and the linked transect Sites 1100, 1102, and 1103; the rise drifts were sampled at Sites 1095, 1096, and 1101.
The continental shelf is an area of high biogenic productivity during interglacials. Although long-term sediment preservation on the shelf is limited because grounded ice sheets erode during subsequent glacials, biogenic interbeds will be preserved within sequence groups composed mainly of thick glacial diamict topsets and foresets. In addition, glacially eroded deeps can preserve expanded Holocene sections that may be continuous and essentially biogenic, if the ice-sheet grounding line is remote enough that ice-rafted debris is minor or absent and the section is sufficiently protected from bottom currents. Such sections can provide a record of decadal and millennial variability that can be compared with records from low latitudes and the ice sheet itself. This environment is available on the inner shelf of the Antarctic Peninsula (Domack and McClennen, 1996; Leventer et al., 1996) and was sampled during Leg 178 in Palmer Deep at Sites 1098 and 1099.
Each ANTOSTRAT proposal is focused on the particular contributions its region might make toward understanding Antarctic glacial history. A single region does not offer the best opportunities for drilling in all respects. The particular value of drilling on the Antarctic Peninsula is the simplicity of its environment, together with the existing level of baseline understanding.
All Antarctic margins are extensional (or effectively so) in a thermal and flexural sense, but most are old. Age governs thermal subsidence and rigidity, which controls response to erosion and deposition and to cyclic ice loading. The Antarctic Peninsula behaves as a young passive margin, having subducted a ridge crest (at 50 Ma in the southwest to only 6-3 Ma in the northeast [Barker, 1982; Larter and Barker, 1991a]). The margin undergoes steady thermal subsidence, which means better preservation of topset beds of the prograded wedge than at an older, colder margin and a more local isostatic response to sediment load.
Ridge subduction occurred well before the onset of glaciation in the southwest but not in the northeast. In the northeast, this provides a useful constraint on the maximum age of glacial sediments (which overlie young ocean floor of known age) but also threatens interference between tectonic and glacial events. For the older glacial history, it is prudent to avoid the northeast area of the margin.
Sub-ice geology (resistance to erosion) is a significant variable, to the extent that a till base facilitates ice streaming. The peninsular interior is 2000 m high and is composed largely of Andean-type plutonic and volcanic rocks. Before ridge subduction, the Pacific margin was a well-developed forearc terrain, on which the glacial regime has superposed an extensive prograded wedge (Larter and Barker, 1989, 1991b; Anderson et al., 1990; Larter and Cunningham, 1993; Bart and Anderson, 1995; Larter et al., 1997). The topography and geology of the peninsula vary little along strike, which simplifies the models of erosional and depositional response to climate change. Short cores on the outer shelf show diamict beneath a thin cover of Holocene hemipelagic mud (Pope and Anderson, 1992; Pudsey et al., 1994).
Snow accumulation varies with temperature and is greatest around the continental edge, particularly along the Antarctic Peninsula, which is warmer than East Antarctica (Drewry and Morris, 1992). Snow accumulation governs the required rates of ice transport, hence basal sediment transport. Greater accumulation means an expanded sediment record. Warmer ice means (probably) faster ice flow, which also contributes to a rapid response to climate and an expanded sediment record.
The extent of the ice drainage basin affects the speed of response to climate change and adds the complexity of a distal to a proximal signal (which allows the possibility of seeing the effects of a small, purely inland ice sheet at the coast during less-glaciated periods). The Antarctic Peninsula is a narrow strip of interior upland, dissected by fjords and bordered by a broad continental shelf. It therefore has a low-reservoir, high-throughput glacial regime with only a proximal source and so is both simple and highly responsive to climate change.
Onshore evidence of Eocene and Oligocene glaciation on the South Shetland Islands (northern Antarctic Peninsula) has been published (see Birkenmajer, 1992), but this conflicts with other evidence of regional climate. The Antarctic Peninsula is able to provide a high-resolution record of glaciation back to at least 10 Ma. To go back farther could involve entanglement with the tectonics of ridge-crest collision, making this a problem instead of an asset. However, because of the Antarctic Peninsula's more northerly position, its glacial history may be shorter than East Antarctica's. The record before 10 Ma may be largely nonglacial, or it may reveal a stage of valley glaciation lacking regular ice-sheet extension to the continental shelf edge.
The principal drilling objectives of Leg 178 were (1) to extract and compare high-resolution records of the past 10 m.y. of continental glaciation contained in topset beds (paleoshelf) of the glacial prograded wedge at the Antarctic Peninsula Pacific margin, in foreset beds (paleo-slope) of the same sequence groups, and in a hemipelagic sediment drift on the continental rise; (2) to compile an optimal high-resolution history of grounded ice-volume fluctuation and compare it with low-latitude records of sea-level change and isotopic estimates of ice-volume change over the past 10 m.y.; (3) to assess the main controls on sediment transport and deposition during glacial intervals and use the insights gained to optimize investigation of the longer, more complicated East Antarctic record of glaciation and glacioeustatic sea-level change; and (4) to extract an ultrahigh-resolution Holocene record from a protected basin on the inner continental shelf for comparison with similar records from the ice sheets and lower latitude sites to investigate decadal- and millennial-scale climatic variation.
Leg 178 was drilled at nine sites: three (1095, 1096, and 1101) on continental rise drifts; four (1097, 1100, 1102, and 1103) on the outer continental shelf prograded wedge; and two (1098 and 1099) on an inner-shelf basin, Palmer Deep. For a brief summary of drilling at these sites, see Table T1.