Evidence for late Oligocene establishment of the Antarctic Circumpolar Current

Removal of the last impediment to circum-Antarctic flow - opening of the Drake Passage between Antarctica and South America - is essential to establishment of the ACC, and has been placed in both the earliest Oligocene and earliest Miocene on the basis of paleomagnetic reconstructions. The Tasman Gateway between Antarctica and Tasmania notably deepened following ∼33.5 Ma. Completion of the ACC circuit has been seen as the key to major growth of the East Antarctic Ice Sheet by thermal isolation of the continent. An alternative argument attributes ice sheet growth to changes in atmospheric pCO2. We present evidence on current strength and water-mass properties suggesting a clear increase in flow speed and homogeneity of Southern Ocean water masses in the latest Oligocene after about 23.95 Ma, marking establishment of the ACC, attributable to deep opening of Drake Passage. The Mi-1 glaciation may be a consequence. Establishment of a permanent, low altitude ice sheet does not appear to take place prior to mid-Miocene cooling.