The Earth has rarely experienced ice age conditions during its long evolution. The Cenozoic Era is thus distinctive in its development of major ice sheets in both polar regions. Progressive cooling at high latitudes during the Cenozoic led to the development of the polar cryosphere, initially on Antarctica and later in the Northern Hemisphere. In the early 1970s, a hypothesis was proposed that climatic cooling and cryospheric development resulted from plate tectonic changes that progressively thermally isolated the Antarctic continent as the Circum-Polar Current developed. The development of circum-polar circulation resulted from the opening of the "gateway" of the Tasmanian Seaway south of Tasmania in the Paleogene and the Drake Passage in the earliest Neogene. Circum-polar circulation continued to expand during the Cenozoic with northward migration of Australia and expansion of Drake Passage.

It is likely that these paleoceanographic changes played a fundamental role in the development of Cenozoic climate evolution, associated paleoenvironmental changes such as sea level, and in terrestrial and biotic evolution. Thus, the opening of the Tasmanian Seaway appears to have been vital to the development of Cenozoic global evolution of the Earth's system. Early ocean drilling in the Tasmanian Seaway (Deep Sea Drilling Project Leg 29) provided a basic framework of paleoenvironmental changes associated with the opening of the seaway. Yet, the information obtained during Leg 29, nearly 25 years ago, is of insufficient quality and resolution to fully test the hypothesis of potential relationships among the development of plate tectonics, circum-polar circulation, and global climate. The timing of events remains insufficiently constrained.

During Leg 189, a suite of sites will be drilled to test the hypothesis that Antarctic cryospheric evolution resulted from the isolation of Antarctica by the Circum-Antarctic current. Five sites, located in water depths from 2475 to 3580 m, will be drilled to document paleoceanographic and paleoclimatic changes associated with the opening of the Tasmanian Gateway as Australia moved northward from Antarctica during the Cenozoic. Most sites are designed to penetrate to middle to upper Eocene strata, recording the middle Eocene-Quaternary climatic development preserved in carbonate sediments. Changes in sedimentation are expected to have resulted from the Eocene Gondwanan rifting, onset of circum-Antarctic surface water circulation (70°S), and the development of deep-water circulation (60°S). In addition, the sites will document meridional variations in geochemistry and water-mass temperatures in high-southern latitudes and environmental and biotic differences during the Paleogene between the Indian and Pacific Oceans because of isolation by the South Tasman Rise. Three sites (WT-1A, WSTR-1B, WSTR-2A) are located at different latitudes in the Indian Ocean, one site (ETP-2A) is in the Pacific Ocean, and one site (STR-2A) is located between the Indian and Pacific Oceans. Four alternate sites (Table 2) have also been selected.

Leg 189 drilling targets new sites designed to greatly improve understanding of Circum-Polar oceanographic and climatic evolution. For example, the relatively shallow region off Tasmania is one of the few places where well-preserved and almost complete marine middle Eocene to Holocene carbonate-rich sequences can be drilled in present-day latitudes of 40°-50°S, and paleolatitudes of up to 70°S.

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