OPENING OF THE TASMANIAN GATEWAY: IMPLICATIONS FOR CLIMATIC AND BIOGEOGRAPHIC CHANGE DURING THE EOCENE/OLIGOCENE TRANSITION

The opening or closing of major oceanic gateways or seaways through plate tectonics can significantly affect changes in surface and/or deep ocean circulation. This clearly has led to fundamental changes in the Earth’s environmental system, global climate change and marine and terrestrial biogeography. Paleocirculation changes resulting from gateway evolution can significantly affect global heat transfer, and thus climate. However, the climatic effects of circulation changes due to gateway evolution should be considered within an Earth System context involving a variety of integrated environmental feedbacks. The opening of the Tasmanian Gateway during the Eocene/Oligocene transition created paleoceanographic changes associated with the development of the Circum-Antarctic Current. This contributed towards thermal isolation of Antarctica leading in turn to a rapid global climatic threshold associated with the first major cryospheric expansion of the continent. Much of this climate change resulted, not from circulation changes alone, but through environmental feedback mechanisms associated with thermal isolation. These include increased albedo, ice sheet elevation, atmospheric changes, increased ocean productivity, and intensification of thermohaline circulation leading to expansion of deep cold waters. Cooling of the deep ocean and the continents and associated expansion of the methane hydrate reservoir also likely led to decreased atmospheric greenhouse gases CO2 and CH4 that in turn contributed to major cooling in the earliest Oligocene. The paleobiogeography of planktonic microfossil assemblages continues to play a vital role in better understanding oceanic circulation changes associated with gateway evolution. In the region of the Tasmanian Gateway, late Eocene planktonic microfossil assemblages exhibit biogeographic differences due to separation of the Australo-Antarctic Gulf and the southwest Pacific prior to Gateway opening. Also, assemblages reflect fundamental changes in ocean circulation during gateway opening over the Eocene/Oligocene transition. Changes in calcareous microfossil assemblage distribution and stable isotopic gradients appear consistent with the hypothesis that surface-ocean heat transfer towards Antarctica was fundamentally affected by opening of the Tasmanian Gateway.