Leg 177 will core sediments in the southeast Atlantic sector of the Southern Ocean to study the paleoceanographic history of the Antarctic region on tectonic (106 yrs, Cenozoic), orbital (104 to 105 yrs, Milankovitch), and suborbital (102 to 103 yrs, millennial) time scales. Six primary sites are located along a latitudinal transect across the Antarctic Circumpolar Current (ACC) from 41° to 53°S (Fig. 1), which should encompass the dynamic range of past frontal boundary movements within the ACC. Two of the sites (TSO-6A, TSO-7C) are located south of the Polar Front within the circum-Antarctic siliceous belt. Leg 177 sites are also arranged along a bathymetric transect ranging from 2100 to 4600-m water depths, intersecting all of the major deep (Circumpolar Deep Water [CPDW], North Atlantic Deep Water [NADW]) and bottom (Antarctic Bottom Water [AABW]) water masses in the Southern Ocean (Fig. 2). Two deep holes (TSO-2B, TSO-6A) are planned that will recover Cenozoic sequences. Several sites (SubSAT-1B, TSO-6A, and TSO-7C) exhibit average sedimentation rates exceeding 20 cm/k.y. during the late Neogene, offering an opportunity for paleoclimatic studies on millennial time scales.

Paleoceanographers, climatologists, and geochemists have recognized over the last decade that processes occurring in the Southern Ocean have played a major role in defining the Earth's climate system. The Southern Ocean is an extraordinarily important region because

1. The Antarctic cryosphere represents the largest accumulation of ice on the Earth's surface. The development and evolution of the Antarctic ice sheets and sea-ice field has had a profound influence on global sea-level history, the Earth's heat budget, atmospheric circulation, surface and deep-water circulation, and the evolution of Antarctic biota.

2. The Southern Ocean is one of the primary sites of intermediate-, deep-, and bottom-water formation. For example, almost two-thirds of the ocean floor is bathed by AABW that mainly originates in the Weddell Sea region. AABW depresses the temperature of at least 55-60% of the world's ocean volume to below 2°C (Gordon, 1988). In addition, the Southern Ocean represents the "junction box" of deep-water circulation where mixing occurs among water masses from other ocean basins (Fig. 3). As such, the Southern Ocean is perhaps the only region where the relative mixing ratios of deep-water masses can be monitored (e.g., fluxes of NADW production). As one of the primary sites of deep- and intermediate-water mass formation, the geochemical and climatic fingerprint of Southern Ocean processes is transmitted throughout the world's deep oceans.

3. The Antarctic continent is thermally and biogeographically isolated from the subtropics by the Antarctic Circumpolar Current, a circum-global ring of cold water that contains complex frontal features and upwelling/downwelling cells. The zonal temperature, sea-ice distribution, and nutrient structure within the ACC control the biogenic sedimentary provinces that are characteristic of the Southern Ocean. Upwelling of deep, nutrient-rich water in the Southern Ocean results in significant primary productivity and constitutes nearly one-third of total marine productivity (Berger, 1989). As a result, about two-thirds of the silica supplied annually to the ocean is removed as hard parts of planktonic siliceous microorganisms in the Southern Ocean. This leads to high accumulation rates of biogenic opal and the formation of a circum-Antarctic biogenic silica belt, located between the Polar Frontal Zone (PFZ) and the northern seasonally sea-ice covered Antarctic Zone of the ACC (e.g., DeMaster, 1981; Lisitzin, 1985). Surface waters in the circum-Antarctic are also important globally because upwelling of deep water and sea-ice formation link the thermal and gas composition of the ocean's interior with the atmosphere through air-sea exchange. As a result, most paleogeochemical models of atmospheric CO2 are highly sensitive to changes in nutrient utilization and/or alkalinity of Antarctic surface waters. Variations in the export of organic matter from the Southern Ocean and its associated drawdown of atmospheric CO2 have been proposed as a forcing mechanism for past global climate change on both short (Kumar et al., 1995) and long (Pollock, 1997) time scales.
Although Antarctica and the adjacent Southern Ocean represent one of the most important components of the Earth's climate system, significant gaps exist in our knowledge of its paleoceanographic and paleoclimatic history. The main hindrance for improving our knowledge of Southern Ocean paleoceanography has been the lack of continuous deep-sea sedimentary sequences from the region. To improve the present latitudinal and bathymetric coverage in the Southern Ocean, Leg 177 will drill six primary sites in the high latitudes of the southeast Atlantic Ocean (Fig. 1). Specific sites have been targeted that contain expanded Quaternary, Neogene, and Paleogene sequences not adequately recovered at these depths and latitudes by past drilling. On the basis of previous drilling in the region, Leg 177 is expected to recover predominantly calcareous sediments at the northern sites of the transect. At the southern sites, sediments are expected to be biosiliceous during the Quaternary and late Neogene, grading into biogenic calcareous sequences prior to the late Pliocene (Fig. 4). Because of the general paucity of high-quality sedimentary sequences available from the Southern Ocean, Leg 177 is expected to fill a critical gap in the distribution of drilled ocean sites.

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