During Ocean Drilling Program (ODP) Leg 178, three sites were drilled on two of eight or nine fine-grained sediment drifts on the upper continental rise off the Pacific margin of the Antarctic Peninsula (Barker, Camerlenghi, Acton, et al., 1999; Barker et al., in press). The main aim was to recover a continuous, high-resolution record of the state of glaciation of the adjacent continent. The drifts experienced mainly pelagic deposition during interglacials, comprising biogenic debris, ice-rafted detritus, and possibly other minor terrigenous material, usually bioturbated. During glacial periods, when an ice sheet was grounded to the continental shelf edge and depositing unsorted sediment on the upper continental slope, drift deposition was mainly of barren, laminated (i.e., unbioturbated) clays from the suspended fine-grained component of turbidity currents originating in the unstable part of upper slope deposits (with some fine silts, also of turbidity-current origin). The alternation of thicker laminated and thinner bioturbated, mainly fine-grained terrigenous deposits extended to the base of the deepest hole at the shallower, more proximal sites (Sites 1096 and 1101) and to 9 Ma at Site 1095. The preservation of cyclicity within the drifts suggested that the residence time on the upper slope of the unstable component of glacially transported material was short compared with the length of a glacial cycle.
Biogenic debris recovered from the drifts was largely diatomaceous, but biogenic carbonate (mainly Neogloboquadrina pachyderma [s] and nannofossils) was found at intervals in the younger drift sediments at shallower sites and was most abundant within the period from 0.8 to 2.1 Ma at Sites 1096 and 1101. Biogenic carbonate seemed to occur mainly in narrow intervals of bioturbated sediment, a few of which could be defined as foraminiferal ooze. Calcareous benthic foraminifers and other planktonic species were sparse, but again were most abundant in these narrow intervals. In sediments older than 2.1 Ma, calcareous tests were not preserved, possibly as a consequence of a higher biosiliceous productivity.