Previous deep-sea drilling in the Southern Ocean, especially cores recovered with the hydraulic piston corer (HPC), advanced hydraulic piston corer (APC), and extended core barrel (XCB) systems (Deep Sea Drilling Project [DSDP] Leg 71, Ocean Drilling Program [ODP] Legs 113, 114, 119, and 120, Fig. 5), have provided a basic understanding of the paleoceanographic and paleoclimatic evolution of the southern high latitudes during the Cenozoic. The history of this region is closely related to paleogeographic changes (i.e., Gondwana breakup) that opened the Tasman Seaway and Drake Passages and permitted the establishment of the Antarctic Circumpolar Current (Kennett, 1977). Isotopic and microfossil evidence suggests that ice sheets were established and sea ice expanded during the earliest Oligocene (Kennett, 1982; Webb, 1990), but little agreement exists on the presence of ice sheets during the Eocene, particularly during the early middle Eocene (Barron et al., 1991, Wise et al., 1992). Prior to the growth of large Northern Hemisphere ice sheets during the late Pliocene, the Southern Hemisphere cryosphere is implicated as a major driving force for global climate and sea level fluctuations. There are large differences of opinion, however, regarding the details of Antarctic cryospheric evolution, arising primarily from differences in the interpretation of continental, marine isotopic, and sea-level records (Fig. 6). For example, the cause of a rapid fall in sea level ~30 Ma is enigmatic because it was not accompanied by a significant increase in benthic oxygen isotopic values. Major differences also exist in the interpretation of the extent and volume of the Antarctic ice sheet during the early-late Pliocene (during the Gauss Chron). There are those who assume an essentially stable, combined East and West Antarctic ice sheet since the early Pliocene (Kennett and Barker, 1990; Clapperton and Sugden, 1990), and those who envision a highly dynamic Antarctic ice sheet during the early and early-late Pliocene (Webb and Harwood, 1991; Hambrey and Barrett, 1993). Strongly related to these controversies is the unresolved question of how to partition the temperature, salinity, and ice volume signals embedded in the Cenozoic oxygen isotopic record (see discussion in Wise et al., 1992) and how these signals are related to sea-level changes inferred from sequence stratigraphic analysis (Fig. 6). The former problem might be addressed by the reconstruction of latitudinal oxygen isotopic gradients across the Southern Ocean (see Zachos et al., 1992) in conjunction with patterns of biogeographic distribution of microfossil assemblages, which reflect changes in water mass properties. In addition, tandem measurements of oxygen isotopes in diatoms and foraminifers may deconvolve the temperature from the ice-volume signal in oxygen isotopic records, although this novel approach has only been attempted thus far in the latest Pleistocene (Shemesh et al., 1992). The main stumbling block for reconstructing latitudinal gradients in the Southern Ocean has been the lack of suitable core material. Drilling during ODP Legs 113, 119, and 120 was concentrated in the Antarctic Zone south of the Polar Front, and most of the ODP Leg 114 and previous DSDP sites that used modern drilling techniques are aligned around 50°S in the Atlantic sector of the Southern Ocean (Fig. 5). As a result, a true latitudinal transect of cores does not exist to study the response of surface water masses in the Southern Ocean to the glacial evolution on the Antarctic continent.

Another deficiency in the distribution of ocean-drilled cores is the lack of Quaternary, Neogene, and older sequences from the southern high latitudes that would permit the generation of high resolution stratigraphic and paleoenvironmental signals. Compared to the superb records now available from the North Atlantic Ocean (ODP Legs 94, 154, 162, and 172), the Southern Ocean has relatively few sites that are suitable for high-resolution paleoclimatic studies. One of the fundamental tasks in paleoclimatology today is documenting and explaining phase relationships between climatic proxies from different oceanographic regions. Cores with high sedimentation rates are needed to study the timing and response of the southern high latitudes to orbital forcing and the phase relationships to climatic changes in the high-latitude Northern Hemisphere (Imbrie et al., 1989, 1992). Targeting ultra-high-resolution sequences is also important to study rapid climate change on suborbital (millennial) time scales and to understand the nature of abrupt triggering and feedback processes in the ocean/sea-ice/atmosphere system.

Previous ODP drilling in the Southern Ocean has resulted in only a few sites that recovered Pleistocene and Neogene sections with high-sedimentation rates (Site 704, Meteor Rise; Site 594, Chatham Rise; Site 695, east of South Orkney; Site 514, Subantarctic southwest Atlantic). Sediment drifts and the region of the circum-Antarctic biogenic silica belt in the South Atlantic are ideal targets for the recovery of sediments deposited at high and ultra-high sedimentation rates. On Leg 177, we expect to recover expanded late Neogene sections with good biocalcareous and biosiliceous preservation at several sites (SubSAT-1B, TSO-6A, and TSO-7C) between 41° and 53°S. Piston cores at these sites indicate sedimentation rates in excess of 20 cm/k.y. In addition, the high flux of organic materials to the seafloor in the circum-Antarctic biogenic silica belt may lead to the formation of laminated biosiliceous sediments, which develop when oxygen is depleted periodically in sediment pore waters. Biosiliceous sediments deposited above the carbonate compensation depth (CCD) in the biogenic silica belt also generally contain sufficient calcareous microfossils (foraminifers) for stable isotopic analysis. Together with quantitative reconstructions of paleoenvironmental conditions using statistical methods developed recently for diatoms (Pichon et al., 1987; Zielinski, 1993; Zielinski and Gersonde, 1997) and radiolarians (Brathauer, 1996; A. Abelmann, pers. comm., 1997), these records can be used to reconstruct surface water hydrography, nutrients, and productivity. In addition, sea-ice distribution can be deciphered using diatom taxa that are indicative of sea ice (Gersonde et al., 1996; Zielinski and Gersonde, 1997).

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