THE OCEAN GATEWAY AT DRAKE PASSAGE

The opening of a deep-water gateway between the Pacific and Atlantic oceans at Drake Passage had profound implications for the Earth system. Removal of a major, perhaps final, barrier to circumpolar deep-water flow had a critical effect on Late Neogene thermohaline circulation, and is commonly associated with Antarctic glaciation and biogeographical changes in the oceans and on land. However, the timing of initial opening and the sequence of events which led to the establishment of a deep-water pathway are poorly known. The conclusion of Barker and Burrell (1977), that a deep-water connection was established at the Paleogene/Neogene boundary, can now be shown to be inaccurate due to insufficient data. Since that publication, new data and techniques have become available to help constrain the kinematic history of plate fragments in the embryonic Scotia Arc, including satellite-derived, free-air gravity, new magnetic anomaly profiles and improved magnetic reversal timescales, the ability to create and manipulate large datasets, and new inversion techniques.

A series of new reconstructions is presented, starting at 54 Ma (~Paleocene/Eocene boundary), when a fragmentary connection between South America and the Antarctic Peninsula is thought to have precipitated a vicariant event in mammal evolution. The 37 Ma (~Late Eocene) reconstruction features an intermediate-depth ocean gateway, which can be related to the appearance of a proto-polar oceanographic front. In contrast to earlier models, our preferred model shows a deep water gateway by 26.5 Ma (mid-Late Oligocene), just seven million years after the opening of the Tasman gateway to similar depths, and the onset of glaciation in East and West Antarctica. At this point, a true Antarctic Circumpolar Current could have started, growing in importance as Drake Passage widened prior to the onset of modern-type ice-sheet glaciation of Antarctica in the Middle Miocene, at about 15 Ma.