Leg 177 sites are situated along a transect across the ACC (Fig. 1). The ACC consists of a cold, surface-water mass that rings Antarctica and contains complex fronts and upwelling/downwelling cells. In the South Atlantic, the ACC has a strongly banded velocity field and can be divided into three distinct zones separated by frontal boundaries (Peterson and Stramma, 1991). In the north, the ACC is bounded by the Subtropical Front that marks the northward limit of the Subantarctic Zone. Sites 1088, 1090, and 1091 are located in the northern Subantarctic Zone between the Subtropical Front and the Subantarctic Front to the south. Sites 1091 and 1092 are located in the PFZ that is bounded by the Subantarctic Front to the north and the Polar Front to the south. The average width of the PFZ in the South Atlantic off Africa is 670 km, and it is centered at 45°S with a span of roughly ±2.5° latitude (Lutjeharms, 1985). The PFZ separates cold, nutrient-rich Antarctic surface water to the south from warmer, less nutrient-rich Subantarctic Surface Water to the north. The PFZ also represents a transition zone in sediment lithology from diatom ooze near the Polar Front to a mixed siliceous-calcareous ooze near the Subantarctic Front. South of the Polar Front is the Antarctic Zone that is marked by cold, silica rich Antarctic surface water. Site 1093 is located at ~50°S in the northern Antarctic Zone close to the present-day Polar Front, and about 5° north of the average winter sea-ice edge. Site 1094 is located south of the Polar Front in the ice-free Antarctic Zone, close to the average winter sea-ice edge.
The water depths of the sites range from 1976 to 4620 m. The bathymetric distribution of sites intersects all major deep- and bottom-water masses in the Southern Ocean, including upper and lower Circumpolar Deep Water (CPDW), NADW, and AABW. Two partial depth transects were drilled on the Agulhas Ridge and Meteor Rise. Sites 1088, 1090, and 1089 form a depth transect at 2083, 3718, and 4620 m, respectively, from the crest of the Agulhas Ridge to the deep Cape Basin. On the Meteor Rise, Sites 1092, 704, and 1091 form a partial depth transect at 1976, 2532, and 4378 m, respectively.
Sites 1088 (2083 m) and 1092 (1976 m) are the shallowest sites and are positioned near the interface of upper NADW and upper CPDW. Site 1090 (3718 m) is near the interface of lower NADW and CPDW. The remaining Sites 1089, 1091, 1093, and 1094 are positioned within lower CPDW, and fall along a linear trend marked by decreasing salinity and temperature toward the south, which may reflect decreasing input of NADW (Fig. 8).
To 177 Drilling Strategy
To 177 Table of Contents