Ocean Drilling Program, United States

Leg 177 will core sediments in the southeast Atlantic sector of the Southern Ocean to study the paleoceanographic history of the Antarctic region on short (millennial) to long (Cenozoic) time scales. Six primary sites are located along a latitudinal transect across the Antarctic Circumpolar Current (ACC) from 41 degrees to 53 degrees S, including two sites (TSO-6A/B, TSO-7C/B) within the circum Antarctic siliceous belt. The sites are also arranged along a bathymetric transect ranging from 2100 to 4600-m water depths, intersecting all of the major deep- and bottom-water masses in the Southern Ocean. The general goals of Leg 177 are two-fold: (1) to augment the biostratigraphic, biogeographic, and paleoceanographic history of the earlier Cenozoic, a period marked by the establishment of the Antarctic cryosphere and the ACC; and (2) to target expanded sections of late Neogene sediments, which can be used to resolve the timing of Southern Hemisphere climatic events on orbital and suborbital time scales and which can be compared with similar records from other ocean basins and with ice cores from Greenland and Antarctica. Drilling the proposed sites will provide the sedimentary sequences needed to address a number of first-order objectives and questions in southern high-latitude paleoclimatology and stratigraphy, including (1) the evolution of the ACC and past changes in the position of the Polar Front Zone and the Antarctic sea-ice field; (2) the evolutionary history and stability of the Antarctic cryosphere; (3) changes in Southern Ocean productivity, nutrient cycling, and pCO (sub 2) and their role in global biogeochemical cycles and climate evolution; (4) changes in the mixing ratio of various deep- and bottom-water masses in the Antarctic (e.g., North Atlantic Deep Water); (5) the response of the Southern Ocean to orbital forcing and the phase relationships to climatic changes in low and northern high latitudes; (6) documentation of abrupt climate change on millennial time scales in the Southern Ocean and their correlation with ice and sediment core records from other areas; and (7) recovery of the first continuous sequences from the circum-Antarctic siliceous belt that will provide insight into early low-temperature chert diagenesis.