2. An Approach to Antarctic Glacial History: the Aims of Leg 178¹

Ocean Drilling Program (ODP) Leg 178 was proposed partly as an examination of Antarctic Peninsula glacial history and partly as a test of the strategy of determining this history by sampling glacially transported sediments at the continental margin. If successful, it could lead to a program of two or three other legs around Antarctica that, in combination, might determine the long-term history of the entire Antarctic Ice Sheet. Leg 178 also had a second objective: to obtain a long, high-resolution record of Holocene climate from Palmer Deep, an isolated deep basin on the inner continental shelf.

The proposal that became Leg 178 was therefore based on several assumptions, described below. Briefly, they are:

1. The Antarctic Ice Sheet is now and has been throughout its existence an important component of the Earth's climate engine, which it is necessary to document and understand.
2. An understanding of the ice sheet's function and what controls its development cannot be obtained until its history is known.
3. Existing knowledge of Antarctic glacial history, derived largely from sparse onshore data and low-latitude climate proxies, is inadequate and ambiguous; the proxies themselves disagree. Furthermore, continued use of the same proxies and onshore data is unlikely to resolve present ambiguities and disputes.
4. The unsorted sediments transported beneath the ice sheet to the outer continental shelf and slope, and their derivatives in sediment drifts on the continental rise, contain complementary records of ice-sheet history that are accessible to drilling. The shelf/slope record is direct but relatively crude (discontinuous and difficult to recover). The rise drift record is continuous, high resolution, and easier to recover but is indirect, and therefore needs the shelf record to aid its interpretation.
5. By sampling this record in three or four places around Antarctica and by combining the data from different places using numerical models of ice-sheet behavior, it should be possible to obtain a direct estimate of ice-sheet history.
6. The Antarctic Peninsula is the best place to begin because both sediment distribution and the glacial environment are well defined there. Also, glacial history is probably relatively short. There is the opportunity to refine the technique and understand the depositional environments better before tackling what might be a more difficult margin elsewhere.
7. By elucidating Antarctic glacial history, drilling will encourage studies of the causes of glaciation and will simplify the future interpretation of proxy data (in particular, studies of global eustatic sea-level change, for which variations in grounded ice volume remain the one generally accepted rapid-action, repeatable cause).

This strategy was proposed by the Antarctic Offshore Acoustic Stratigraphy initiative (ANTOSTRAT, a group comprising mainly seismic stratigraphers and marine geologists working around the Antarctic margin), reviewed and adopted by an ODP Detailed Planning Group, reviewed further by thematic panels, and formally endorsed by the JOIDES Planning Committee (now Science Committee/Operations Committee). Despite the additional cost of an ice support ship and the inflexibility of the Antarctic ice and weather window, the proposed strategy has led to the drilling program described here that, if successful, will in turn lead to drilling in Prydz Bay and elsewhere around the Antarctic margin.

In this introduction to the objectives, activities, and achievements of Leg 178 drilling, we set out the regional tectonic environment, what is known of past and present Antarctic climate and ocean circulation, and an account of predrilling knowledge of the depositional environments that we aimed to sample during the leg. We include also a brief account of the longer term rationale for drilling, showing how the results of drilling at three or four locations around Antarctica, coordinated through numerical models, might elucidate Antarctic glacial history.