CURRENT-CONTROLLED SEDIMENT DEPOSITION FROM THE SHELF TO THE DEEP-OCEAN - THE CENOZOIC EVOLUTION OF CIRCULATION THROUGH THE SW PACIFIC GATEWAY

The circulation of cold, deep water is one of the controlling factors of the Earth's climate. Forty per cent of this water enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal to abyssal depths, east of the N ew Zealand microcontinent. South of latitude 50 degrees C, the DWBC is intimately linked with the Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to underst anding ocean and climate history, and global ocean hydrography. The ev olution of the DWBC system, and of related circum-Antarctic currents, has taken place since 30-25 Ma when plate movements created the first oceanic gaps south of Australia and South America. The stratigraphic r ecord preserved in sediment drifts of the Southwest Pacific, in easter n New Zealand, is the best available for deciphering the Neogene histo ry of Southern Ocean water masses, and of the circulation of the ACC, DWBC and their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene (Hoiho Dr ift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift; ACC). During the Neogen e the eastern South Island continental shelf built seawards by accreti on at its outer edge of large Miocene current drifts up to tens of kil ometres long and hundreds of metres thick (Canter bury drifts). Also c ommencing in the mid-Cenozoic, but in depths >2000 m, the DWBC emplace d large deep-water sediment drifts. Rates of drift deposition accelera ted considerably in the late Neogene, when climatic change (and partic ularly glacial sea-level falls) caused the delivery of large volumes o f turbiditic sediment into the path of the DWBC via the Bounty and Hik urangi channels.