INTRODUCTION

This paper is a contribution to the Neogene history of Antarctic Peninsula glaciation, as recorded in sediments of the continental rise, drilled during Ocean Drilling Program (ODP) Leg 178. The overall aims of the leg were given by Barker and Camerlenghi (1999). The continental rise was considered a particularly important province to drill because its fine-grained sediments contain a record of glacial history much more continuous than that on the continental shelf, which has been overridden and partly eroded many times by grounded ice. Prior to ODP drilling, the rise west of the Antarctic Peninsula had been thoroughly surveyed using single- and multichannel seismic reflection profiles, 3.5-kHz and TOPAS acoustic profiles, and piston cores up to 11 m long (Rebesco et al., 1997, 1998; Canals et al., 1998; Pudsey and Camerlenghi, 1998; Pudsey, 2000). Oceanographic data including results from three near-bottom current meter moorings were also available (Camerlenghi et al., 1997; A. Camerlenghi and A. Crise, unpubl. data).

The continental rise includes a number of large mounds interpreted as sediment drifts (Rebesco et al., 1997) deposited under the influence of weak southwest-flowing bottom currents. Site 1095 on Drift 7 is the most distal of the continental rise sites, some 180 km from the base of the slope and only 100 m above the floor of the adjacent channel (Fig. F1) (Shipboard Scientific Party, 1999). A long (600 m), near-continuous section was recovered from the Holocene down to nearly 10-Ma sediments at the base. These sediments were composed of a mixture of fine-grained turbidites, hemipelagites, and muddy contourites (Shipboard Scientific Party, 1999). During shipboard core description, meter-scale lithologic cyclicity was evident within many cores and some long-term depositional trends were noted over tens to hundreds of meters. Specific questions to be addressed at Site 1095 (Barker and Camerlenghi, 1999) included the following:

  1. Is the present depositional system, documented from work on piston cores, a plausible analog for the older depositional environment reflected within the cored section?
  2. Was deposition cyclic within the lower part of the drift section? If so, what are the cycle frequencies? And what does this cyclicity represent?
  3. Can the onset of the present stage of continental glaciation (involving regular ice sheet excursions to the shelf edge) be recognized in the drift sediments? Is there a relationship between drift development and continental glacial history?

In this paper, I describe and interpret sediment composition (proportion of biogenic silica) and texture (sand percentage and grain size of the fine fraction) in the upper 300 m of the cored section in Drift 7, representing 0-7 Ma. The magnetostratigraphy measured during the cruise (table T38 in Shipboard Scientific Party, 1999) is used as a timescale. It is recognized that the dating for Site 1095 may improve with the completion of a magneto-biostratigraphic synthesis, but this was not available at the time of writing. Long-term trends (over several million years) and shorter-term cycles (at approximately orbital frequencies) in sediments are discussed. Although lithologic cyclicity (particularly as reflected in physical properties) includes all the sediment types recovered, sampling concentrated on the hemipelagic and contouritic parts of the section. Evidence is thus adduced for paleoceanographic conditions as well as the state of glaciation of the continent.

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