INTRODUCTION
The
Antarctic ice sheets and the adjacent Southern Ocean act together
to form the Antarctic ocean-cryosphere system. Over the last
decade, paleoceanographers, climatologists, and geochemists have
recognized 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). To study the paleoceanographic
history of the Southern Ocean, Ocean Drilling Program (ODP) Leg
177 drilled seven sites along a north-south transect from 41° to
53°S (Fig. F1 ) and in water
depths ranging from 1974 to 4620 m (Fig. F2 ). The Southern
Ocean is an extraordinarily important region for several reasons:
- 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 have 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.
- The
Southern Ocean is one of the primary sites of
intermediate-, deep-, and bottom-water formation. For
example, almost two-thirds of the global 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
all ocean basins (Fig. F3 ).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];
Charles and Fairbanks, 1992). 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.
- The
Antarctic continent is thermally and biogeographically
iso-lated from the subtropics by the Antarctic
Circumpolar Current (ACC), a ring of cold water that
contains complex frontal features and
upwelling/downwelling cells (Fig. F4 ). 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 biological
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 Front Zone (PFZ) and the northern seasonal
limit of sea ice (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 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 (Figs. F1, F4; Table T1 ). 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; Fig. F4). After departing
Cape Town on 14 December 1997, a north-south transect of sites
was drilled beginning at 41°S near the southern Subtropical
Zone, extending across the PFZ from 47° to 50°S, and ending at
53°S in the northern Antarctic region, close to the present
winter sea-ice edge (Fig. F1). The water depths
of sites range from 1974 to 4620 m, intersecting most of the
major deep- and bottom-water masses in the Southern Ocean (Fig. F2). 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. Leg 177, which recovered a total of 4046
m of sediment core, ended on 6 February 1998, in Punta Arenas
(South Chile).