The opening or closing of major oceanic gateways or seaways through plate tectonics can significantly change surface and/or deep ocean circulation. This clearly has led to fundamental changes in the Earth's environmental system, global climate, and marine and terrestrial biogeography. Ocean circulation changes resulting from gateway development can significantly affect global heat transfer, and thus climate. However, these climatic effects 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, and later Seaway expansion, led to critical changes in Southern Hemisphere Ocean circulation resulting from the development of the Antarctic Circumpolar Current (ACC). Gateway opening initiated thermal isolation of Antarctica leading to the crossing of a major global climatic threshold and to significant Antarctic ice expansion. Much of this climate change resulted, not from circulation changes alone, but through environmental feedback mechanisms associated with ice expansion and cooling. These included increased albedo, ice sheet elevation, atmospheric changes, increased Southern Ocean productivity, and intensification of thermohaline circulation leading to expansion of deep cold waters. Cooling of the deep ocean and the continents also likely led to decreased atmospheric greenhouse gases CO (sub 2) and CH (sub 4) that, in turn, contributed to pronounced cooling in the earliest Oligocene. Antarctic circumpolar circulation continued to strengthen during the Oligocene through early Neogene in response to further Seaway expansion, increasing thermal isolation of Antarctica and related development of the Antarctic System. (modif. j. abstr.)