Ocean Drilling Program, United States

During Ocean Drilling Program (ODP) Leg 189, five sites were drilled in bathyal depths on submerged continental blocks in the Tasmanian Gateway to help refine the hypothesis that its opening near the Eocene/Oligocene boundary led to formation of the Antarctic Circumpolar Current (ACC), progressive thermal isolation of Antarctica, climatic cooling, and development of an Antarctic ice sheet. A total of 4539 m of largely continuous upper Maastrichtian-Holocene marine sediments were recovered with a recovery rate of 89%. The sedimentary sequence broadly consists of shallow-marine mudstones until the late Eocene, glauconitic siltstones during that time, and pelagic carbonates thereafter. The microfossils in the mudstones and siltstones are largely palynomorphs and diatoms, and those in the carbonates are largely nannofossils and foraminifers. During the Late Cretaceous, northward movement of Australia away from Antarctica commenced, forming the Australo-Antarctic Gulf (AAG). However, a Tasmanian land bridge at 70 degrees -65 degrees S almost completely blocked the eastern end of the widening AAG until the late Eocene; there is no evidence of extensive current circulation across the ridge until the earliest Oligocene. Prior to the Oligocene, muddy marine siliciclastic sediments were deposited in temperate seas. During the late Eocene, the northeastern AAG was warmer and less ventilated than the gradually widening southwest sector of the Pacific Ocean, which was affected by a cool northwesterly flowing boundary current-a difference that may have existed since the Maastrichtian. In the late Eocene ( approximately 37 Ma), the Tasmanian land bridge and its broad shelves began to subside, currents swept the still-shallow offshore areas, and condensed glauconitic siltstones were deposited. Palynological and diatom evidence suggest a general cooling. The southwestern South Tasman Rise finally separated from Antarctica at the time of the Eocene/Oligocene boundary ( approximately 33.5 Ma), the rise subsided, and the continental margin of Tasmania collapsed. The Tasmanian Gateway opened to deep water, disrupting oceanic circulation at high southern latitudes and leading to one of the major climatic shifts of the Cenozoic. Thereafter, a marked reduction in siliciclastic supply, as well as the flow of warm currents from northern latitudes, favored deposition of carbonate. At the eastern sites, deposition of Oligocene bathyal carbonates directly followed an unconformity caused by the onset of the ACC, but change was more gradual in the west. In contrast, siliceous biogenic sediments typified the Antarctic margin, now isolated from warm water by the ACC. Steady northward movement kept the Tasmanian region north of the Polar Front throughout the Neogene, and pelagic carbonates accumulated.