The Antarctic ice sheet and the adjacent Southern Ocean act together to form the Antarctic ocean-cryosphere system. Paleoceanographers, climatologists, and geochemists have recognized over the last decade that processes occurring in the Southern Ocean have played a vital role in defining Earth's climate, yet many questions remain about the region's paleoenvironmental evolution (e.g., Kennett and Barron, 1992). The Southern Ocean is an extraordinarily important region for several reasons:

1.The Antarctic cryosphere represents the largest accumulation of ice on Earth's surface and should it melt, sea level would rise by 50 to 60 m. The development and evolution of the Antarctic Ice Sheet and sea-ice field has had a profound influence on global sea-level history, 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 Antarctic Bottom Water (AABW) that mainly originates in the Weddell Sea region. 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., North Atlantic Deep Water [NADW]). 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 (ACC), a 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 biogenic sedimentary provinces that are characteristic of the Southern Ocean. Upwelling of nutrient-rich water results in primary productivity that constitutes nearly one-third of the oceanic total (Berger, 1989). About two thirds of the silica supplied annually to the ocean is removed by siliceous microorganisms in the Southern Ocean. This leads to high accumulation rates of biogenic opal between the Polar Frontal Zone (PFZ) and the northern seasonal limit of sea ice (e.g., DeMaster, 1981; Lisitzin, 1985).

4.Surface waters in the circumantarctic are also important globally because upwelling of deep water and sea-ice formation link the thermal and gas compositions of the ocean's interior with the atmosphere through air-sea exchange. As a result, in most paleogeochemical models atmospheric CO2 is highly sensitive to changes in nutrient utilization and/or alkalinity of Antarctic surface waters (e.g., Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984; Knox and McElroy, 1984; Broecker and Peng, 1989).

The importance of Antarctica and the Southern Ocean is well known, yet many questions remain regarding the paleoceanographic and paleoclimatic history of this remote region of the world's oceans. The body of quantitative paleoceanographic data from the Southern Ocean is small relative to the climatic importance of the region.

To improve the present latitudinal and bathymetric coverage in the Southern Ocean, seven sites in the high latitudes of the southeast Atlantic Ocean were drilled during Leg 177. Leg 177 represented the return of the JOIDES Resolution to Antarctic waters for the first time in 10 years, since the last major Antarctic drilling campaign in 1987-1988 (Legs 113, 114, 119, and 120). After departing Cape Town on 14 December 1997, a latitudinal transect of sites was drilled beginning at 41°S near the southern Subtropical Zone, extending across the Polar Front Zone from 47° to 50°S, and ending at 53°S in the northern Antarctic region (Fig. 1). The water depth of sites ranged from 1976 to 4624 m, intersecting most of the major deep- and bottom-water masses in the Southern Ocean (Fig. 2). Specific sites were targeted that contain expanded Quaternary, Neogene, and Paleogene sequences that had not been recovered adequately at these depths and latitudes by past drilling. As such, the sediments recovered during Leg 177 fill a critical gap in the distribution of ocean drilled sites and constitute an invaluable archive of cores needed to extend our understanding of Southern Ocean paleoceanography.

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