Ocean Drilling Program, United States

The circulation of cold, deep Antarctic Bottom Water (AABW) is one of the controlling factors in Earth's climate. Today, 40% of this water enters the world ocean through the Southwest Pacific Gateway as a thermohaline drive Deep Western Boundary Current (DWBC). South of 46 degrees S, the DWBC is coupled with the wind-driven Antarctic Circumpolar Current (ACC). Understanding the evolution of the Pacific DWBC is fundamental to understanding world oceanic and climatic histories. The evolution of the ACC-DWBC system has taken place since the late Oligocene (32-20 Ma), when plate movements created the first deep-water oceanic gaps south of Australia and South America. An excellent stratigraphic record of these events, and of the development of the modern ACC-DWBC, occurs in sediment drifts east of the New Zealand microcontinental plateau. Seven Southwest Pacific drill sites are proposed to reconstruct the stratigraphy, paleohydrography, and dynamics of the DWBC and related water masses. The proposed sites make up a transect of water depths from 315 m to 4460 m, and span a latitudinal range from 39 degrees S to 51 degrees S. Only one previous hydraulic piston core site is located in this large region (Deep Sea Drilling Project [DSDP] Site 594), and earlier DSDP drilling (Sites 275-276) occurred at sites where Neogene sediment is largely missing because of DWBC erosion. Consequently, our knowledge of Southwest Pacific ocean history, and of the development of the ACC-DWBC system, is extremely poor. Leg 181 drilling will provide the sedimentary sequences needed to study a range of high-priority problems in Southern Ocean Neogene paleohydrography, sedimentology, paleoclimatology, and micropaleontology.