GNS Science, New Zealand

The timing of the origin of the Southern Ocean is important for studies of Antarctic biologic evolution and for understanding past climate change. The long standing theory that separation of Australia from Antarctica at the end of the Eocene allowed the development of a circumpolar ocean circulation, isolating the Antarctic continent and causing ice-sheet growth at the Eocene-Oligocene boundary, has recently been challenged by ODP Leg 189 drilling south of Australia. Based on these new cores it has been proposed that separation and Southern Ocean formation had already occurred within the late Eocene, approximately 2 my before the Antarctic became glaciated. This proposal however extrapolates data from a limited area to the history of a large circumpolar ocean region. To better determine Southern Ocean history we have compiled data from a large number of locations around Antarctica on opal accumulation and have analyzed circum-polar radiolarian faunas for development of endemism and evolutionary turnover. Our results show that opal deposition was widespread in the late Eocene Antarctic oceans with concentrations similar to that of the early Oligocene; substantial endemism was already present in late Eocene radiolarian faunas, and that most evolutionary turnover, in particular the origin of taxa characteristic of the early Oligocene, had already taken place within the late Eocene (ca 35 Ma). We conclude that there is a significant (ca 2 my) gap between a late Eocene Southern Ocean origin and later, Eocene-Oligocene boundary Antarctic glaciation.