Site 1096 (Fig. F1A) is the more elevated and proximal of two sites on a sediment drift on the continental rise off the northwestern Pacific margin of the Antarctic Peninsula. This site lies in 3152 m of water close to the top of the gentle, northeast-facing slope of the drift (Drift 7). It was drilled to sample the shallow part of the stratigraphic section within the drift, where it is thick and undisturbed. The deeper section was drilled 75 km to the northwest, on the northwestern flank of Drift 7 at Site 1095 (3840 m), where the youngest sediments are thinner.
Site 1096 was located near the crest of the drift (Fig. F2), on a gentle slope (<1º) elevated more than 700 m above the large, dendritic channel systems that develop at the foot of the Antarctic continental slope and extend northwest to the abyssal plain. The channel systems are maintained by turbidity currents originating on the continental slope, loaded with the unstable component of unsorted, largely terrigenous sediment, glacially transported to the continental shelf edge. We anticipated that the difference in elevation between the site and the turbidity-current channels would limit direct turbidite deposition. This would allow the mainly fine-grained terrigenous component, which had been suspended and entrained as a nepheloid layer within ambient bottom currents, to accumulate. According to the results of shallow piston coring, a barren, terrigenous silty clay representing glacial deposition would alternate downcore with mainly biosiliceous, pelagic silty clay deposited during interglacials.
The sedimentary cover, more than 3 km thick, rests on oceanic basement dated at ~ 40 Ma by magnetic anomalies (Barker, 1982). The magnetic profile obtained on passage between Sites 1095 and 1096 has allowed basement at both sites to be dated more precisely (Fig. F3). Basement is dated at 36.7 Ma at Site 1096 and at 42.7 Ma at Site 1095 (using the time scale of Cande and Kent, 1995). Upon crossing the site, the 3.5-kHz sub-bottom profile (Fig. F2) showed up to 75 ms two-way traveltime (TWT) penetration (corresponding to 50-60 m) through continuous reflectors parallel to the seafloor. The alternation of stronger reflectors and acoustically transparent sediment suggested the occurrence of three depositional cycles in the upper part of the section, possibly reflecting glacial-interglacial variation.
Multichannel seismic (MCS) profiles across the site (I95-135, I95-130A, and IT92-109 [Figs. F4, F1B; also see "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter]) show that the sedimentary sequence corresponding to Units M1 and M2 of Rebesco et al. (1997) comprise continuous and parallel reflectors with a change in amplitude across the unit boundary: low amplitude in Unit M1 above 400 ms and high amplitude in Unit M2 beneath. The total thickness of Units M1 and M2 here is 740 ms, compared with 300 ms at the distal drift Site 1095. The deeper Units M3 and M4 cored at Site 1095, underlying the two upper units, here lie below a bottom-simulating reflector (BSR) that we have interpreted provisionally as the result of silica diagenesis (opal-A to opal-CT). This might adversely affect diatom preservation.
Drilling at Site 1096 was intended to examine the variation of this hemipelagic component of glacially derived sedimentation in a highly expanded Pliocene-Pleistocene section. Penetration to ~650 m would sample only the upper part of the drift (Units M1 and M2, the "drift maintenance" stage of sedimentation), presumably extending back about 5 m.y. The principal objectives were
To see if the pronounced cyclicity apparent in the topmost 5-10 m of drift deposits (and thus accessible to piston coring [Camerlenghi et al., 1997; Pudsey and Camerlenghi, 1998]) would persist downsection and therefore be capable of providing information about the glacial state of the adjacent continent; and If so, to use the drift sediment record to examine at high time resolution the Pliocene-Pleistocene history of Antarctic glaciation.
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Ms 178IR-106