Texas A&M University, United States

The Cenozoic Era is unusual in its development of major ice sheets. Progressive high-latitude cooling during the Cenozoic eventually formed major ice sheets. initially on Antarctica and later in the Northern hemisphere. In the early 1970, a hypothesis was proposed that climatic cooling and an Antarctic cryosphere developed as the Antarctic Circumpolar Current progressively thermally isolated the Antarctic continent. This current resulted from the opening of the Tasmanian Gateway south of Tasmania during the Paleogene and the Drake Passage during the earliest Neogene. The five Leg 189 drill sites, in 2463 to 3568 m water depths, tested, refined, and extended the above hypothesis, greatly improving understanding of Southern Ocean evolution and its relation with Antarctic climatic development. The relatively shallow region off Tasmania is one of the few places where well-preserved and almost-complete marine Cenozoic carbonate-rich sequences can be drilled in present-day latitudes of 40 degrees -50 degrees S and paleolatitudes of up to 70 degrees S. The broad geological history of all the sites was comparable, although there are important differences among the three sites in the Indian Ocean and the two sites in the Pacific Ocean, as well as from north to south.