The opening of the tasmanian gateway drove global cenozoic paleoclimatic and paleoceanographic changes: Results of leg 189

The focus of the expedition was to better document the climatic and oceanographic consequences of the opening of the gateway during the transition from warm Eocene climates to cool Oligocene climates. The relatively shallow region off Tasmania (Exon and Crawford, 1997) is one of the few places where well-preserved and almost complete marine Cenozoic carbonate-rich sequences can be drilled in present-day latitudes of 40-50°S, and paleolatitudes of up to 70°S. The broad geological history of all the sites was comparable. However, the Paleocene-Eocene separation of the three Indian Ocean sites from the two Pacific Ocean sites by the Tasmanian land bridge led to important differences. Depositional conditions in the restricted Australo-Antarctica Gulf (AAG), west of the land bridge, contrasted with those in the more open Pacific Ocean. There are also differences from north to south related to proximity to the opening gateway and to major land masses. In all, 4539 m of core was recovered (overall recovery 89% despite some real Southern Ocean weather), with the deepest core taken 960 m beneath the seafloor. The sedimentary sequence cored is entirely marine, and contains a wealth of microfossil assemblages that record marine conditions from the Late Cretaceous (Maastrichtian) to the late Quaternary, with major terrestrial input until the earliest Oligocene. The drill sites are on submerged continental blocks, which were at polar latitudes in the Late Cretaceous when Australia and Antarctica were still united, although rifts had developed as slow separation and northward movement of Australia commenced. The cores indicate that the Tasmanian land bridge completely blocked the eastern end of the widening AAG until the late Eocene, during both slow and fast (from 43 Ma) spreading phases.