Abstract:
Leg 178 drilled 4 sites on the outer shelf prograded wedge and 3 on upper rise hemipelagic drifts of the Antarctic Peninsula continental margin. Drilling assumed that terrigenous deposition on slope and rise is controlled by periodic extension to the shelf edge of an ice sheet transporting unsorted shearing basal till. Shelf wedge sediments are largely unsorted (thus difficult to recover) and discontinuously preserved, but contain a direct ice-sheet record. Continental rise sediments are sorted (more easily recovered), continuously deposited and provide a higher-resolution but less-direct glacial record. Three shelf sites (1100, 1102, 1103) formed an axial transect of one progradational lobe and Site 1097 sampled an inter-lobe area where access to a deeper sequence S3 of uncertain origin was eased by thinner overlying, clearly glacial sequence groups S1, S2. Recovery (largely in glacial till) was poor if the matrix was too soft to hold the larger clasts, but improved with consolidation. Shelf drilling time was lost because vessel heave exceeded shallow water drilling guidelines. Moderate recovery at 1097 and 1103 and limited penetration at 1100 showed that S1 and S2 are glacial: most of S1 is Pleistocene and the older part of S2 early Pliocene, but the conformable S2/S1 boundary was not sampled. The S3/S2 boundary was dated at 4.7 Ma and the enigmatic sequence S3 is glacial. Deposition at all 3 drift sites was mixed hemipelagic/pelagic muds and fine-grained distal turbidites. All sites show cyclic sedimentation, whatever the dominant depositional mode, reflecting glacial sediment provision to the upper slope by ice sheets grounded periodically to the shelf edge. Sedimentation was fastest on the drift crest (1096) and slowest on the distal flank (1095), and decreased through the Plio-Pleistocene. Turbidite abundance and frequency, bioturbation, colour, magnetic susceptibility, possibly IRD and clay mineralogy all show variability close to orbital periods. The Site 1095 section extended back beyond 10 Ma, and showed cyclicity from about 9 Ma, reflecting a change in the level or nature of onshore glaciation. Although dating of the lower sampled part of S3 is incomplete, sediments as old as 9 Ma are unlikely to have been sampled on the shelf.