Sedimentary successions on the Antarctic continental margin are a direct record of Antarctic ice-volume fluctuations and allow the resolution of inconsistencies (Barker, Camerlenghi, Acton, et al., 1999) between records of eustatic sea level changes (Haq et al., 1987, 1988), oxygen isotope ratios in deep-water foraminifers (Miller et al., 1987; Shackleton et al., 1995), and Antarctic glacial history inferred from onshore evidence (Webb and Harwood, 1991; Moriwaki et al., 1992). The Pacific margin of the Antarctic Peninsula is a key area of interest because of the sensitivity of the West Antarctic Ice Sheet (WAIS) to temperature changes (Huybrechts, 1993; Barker, Camerlenghi, Acton, et al., 1999).
Ocean Drilling Program (ODP) Site 1095 (66°59.1´S, 78°29.2´W; 3842 m water depth) was the first of three sites (1095, 1096, and 1101) cored on a hemipelagic sediment drift (Rebesco et al., 1997) on the continental rise off the northwestern Pacific margin of the Antarctic Peninsula (Fig. F1). One of the key issues of Leg 178 was to establish the biochronological framework at the rise sites and apply it to shelf sites to investigate Antarctic ice history (Barker, Camerlenghi, Acton, et al., 1999). Winter and Wise (Chap. 26, this volume) provide calcareous nannofossil biostratigraphy at Sites 1096 and 1101. No age constraints for calcareous fossils, however, have been assigned to sediments at Site 1095.
Four sites (1097, 1100, 1102, and 1103) on the outer continental shelf were drilled during Leg 178 (Fig. F1) to date major changes in depositional geometry and to improve understanding of shelf sedimentation (Barker, Camerlenghi, Acton, et al., 1999). Site 1097 (66°23.5´S, 70°45.3´W) is located on the outer continental shelf of the Antarctic Peninsula at a water depth of 552 m, some 14 km from the continental shelf edge. One hole was drilled at this site, with a total 436.6 m of penetration. Recovery varied from ~2.3% in the uppermost 80 m of Unit S1, where only cobbles were recovered, to 18% in Unit S2 down to 150 mbsf, and to 16% in Unit S3, which had massive diamict, defined as poorly sorted sediment, with clasts supported by a muddy matrix.
Site 1100 (63°53.0´S, 65°42.3´W) is the first site on a shelf transect and is located between Sites 1102 and 1103. Four holes were drilled at Site 1100 with the deepest, Hole 1100D, reaching 110.5 meters below seafloor (mbsf) (recovery = 4.8%). Recovered sediment consists of poorly consolidated massive diamict with a diatom-bearing silty clay matrix throughout the hole, which is assigned to Unit S1. The top of the section has been interpreted as tills and glaciomarine muds, reworked by iceberg grounding (Barker, Camerlenghi, Acton, et al., 1999). A total of four holes were drilled at Site 1102 (63°48.1´S, 65°51.4´W) on the edge of the continental shelf at a water depth of 442 m. However, no sediment samples were recovered at this site.
Site 1103 (63°59.9´S, 65°27.9´W) is located on the inshore end of the shelf transect at a water depth of 494 m. A single hole was drilled to a total depth of 362.7 mbsf. A 250-m-thick section of Unit S1 glacial topset along with deeper Unit S3 glacial sediments were drilled. Recovery from the upper 247 m was only 2.3%, but improved to 34% in the lower 115 m where the matrix became hard. This sediment consists of diamictites, poorly sorted sandstones, and mudstones that are interpreted as sediment gravity flows.
The age of sedimentary sequences on the continental shelf are constrained mainly by diatoms and supported by a radiolarian assemblage with a broader age range (see "Biostratigraphy" sections in each site chapter in Barker, Camerlenghi, Acton, et al., 1999; Iwai, 2000a). Unit S1 is composed of uppermost Pliocene through Pleistocene sediments. The boundary between Unit S2 and the underlying Unit S3 lies within the lower Pliocene Thalassiosira inura diatom biostratigraphic zone and radiolarian biostratigraphic Upsilon Zone. The bottom cores from Holes 1097A and 1103A (Unit S3) are assigned to the upper Miocene Actinocyclus ingens var. ovalis Zone.
The distribution of calcareous nannoplankton is restricted to north of the present Polar Front in the Southern Ocean (Honjo et al., in press) and used as a proxy for the southward oscillation of the frontal system (Burckle et al., 1996; Bohaty and Harwood, 1998). Calcareous nannofossils restrict the age of recycled sediments and the final deposition on the continental shelf. Fossil pollen has been used as a proxy for Antarctic terrestrial vegetation (Webb and Harwood, 1987, 1993; Fleming and Barron, 1996). Fleming and Barron (1996) found the significant increase of Nothofaidites cf. lachlaniae (pollen) in the mid-Pliocene sediments from Deep Sea Drilling Project (DSDP) Leg 28, Site 274. They thought that this pollen was derived from Nothofagus trees that were living in Antarctica during the mid-Pliocene, a result that supports the Pliocene deglaciation hypothesis of Webb et al. (1984). This paper describes the occurrence of calcareous nannofossils, pollen, and spores from the continental rise and shelf along the western Antarctic Peninsula and provides age constraints and implications for subglacial depositional history.