The area between Australia's southernmost prolongation (Tasmania and the South Tasman Rise) and Antarctica is key to understanding global Cenozoic changes in climate and current patterns, involving

•   The breakup of Gondwana between 130 and 30 Ma (Fig. 1);

•   The drifting of Australia northward from Antarctica;

•   Initiation of the Paleogene to early Neogene Circum-Polar Current and the meridional expansion of the Southern Ocean with concomitant thermal isolation of the Antarctic continent and development of its cryosphere during the Paleogene and Neogene (Kennett, Houtz, et al., 1975b); and

•   The role these processes have had on global cooling (Fig. 2) and biotic evolution.

The opening of the Tasmanian Seaway between Australia and Antarctica and the only other important constriction in the establishment of the Circum-Polar Current, the Drake Passage, had enormous consequences for global climate. These consequences came in part by isolating Antarctica from warm gyral surface circulation of the Southern Hemisphere oceans, and also by providing the necessary conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans. Both factors, in conjunction with other global changes, have been crucial in the development of the polar cryosphere, initially in Antarctica in the Paleogene and later in the Northern Hemisphere in the late Neogene. The relatively shallow region off Tasmania (mostly above the present carbonate compensation depth [CCD]) is strategically well located for studies of the opening and later expansion of the Tasmanian Seaway. It is also one of the few places where 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 (Fig. 3).

The geographic position of the Tasmanian offshore region makes this a unique location to study the effects of Eocene-Oligocene Australia-Antarctic separation on global paleoceanography. Australia and Antarctica were still locked together in the Tasmanian area in the late Eocene, preventing the establishment of circum-Antarctic circulation (Fig. 1). At that time, and earlier, the water masses were separated on either side of the barrier in the southern Indian and Pacific Oceans and must have exhibited distinct physical, chemical, and biological properties. An understanding of Cenozoic climate evolution clearly requires better knowledge of the timing, nature, and responses of the opening of the Tasmanian Seaway during the Paleogene (Figs. 1, 2).

Furthermore, the continued expansion of the Southern Ocean during the Cenozoic because of the northward flight of Australia from Antarctica, has clearly led to further evolution of the Earth's environmental system and of oceanic biogeographic patterns. The Tasmanian region is also well suited for the study of post-Eocene development of Southern Ocean climate development, including the formation and variation of high-latitude climate zones. This region is one of the few ideally located in the Pacific sector of the Southern Ocean for comparison with the models of Cenozoic climate development and variation in the Indian Ocean and the South Atlantic.

An outstanding question therefore is whether paleoceanographic variability known from the Atlantic and Indian sectors is characteristic of the entire circum-polar ocean, or whether there are zonal asymmetries in the Southern Ocean and, if so, when these developed. The meridional spread of proposed sites (Fig. 4) on the South Tasman Rise (STR) is well suited to monitoring the migration of frontal zones through time, analogous to transects on the Southeast Indian Ridge (SEIR) (Howard and Prell, 1992). Here the total meridional displacement of fronts is expected to be somewhat less than on the SEIR because the STR is a shallower topographical barrier to the Circum-Antarctic Current. The East Tasman Plateau (ETP) site is ideally located to monitor paleoceanographic changes in the interface between the East Australian Current and the Circum Polar Current, since the East Australian Current transports heat into the Southern Ocean, an important "gateway" objective.

The sites cored during Leg 189 will also provide high-quality paleoclimatic and paleoceanographic records of Neogene age. These sequences will be employed to examine the development of surface water productivity, oscillations in subtropical and polar fronts, changing strength of the east Australian Current, and changes related to further expansions of the Antarctic cryosphere during the middle Miocene and late Neogene.

Quaternary records are expected also to be of high value for studies of the southern temperate and subantarctic regions. The Southern Ocean participation in the glaciation cycles of the past 500 k.y. appears to track well-known Northern Hemisphere indicators of cryospheric, atmospheric, and oceanographic variability (Imbrie et al., 1992; Imbrie et al., 1993). The Southern Ocean paleoceanographic record, manifested in its temperature response (Howard and Prell, 1992) and carbon cycling (Howard and Prell, 1994; Oppo et al., 1990) mirrors that of the Northern Hemisphere. However, on the Milankovitch band there appears to be a lead in the Southern Ocean, perhaps reflecting the importance of this region. For example, the potential role of Southern Ocean paleoproductivity changes on global climate remains a topic of considerable debate.

In summary, major questions to be addressed are

•   How did the Circum-Antarctic Current develop, and what were the roles of the opening of the Tasmanian Seaway (~30 Ma) and Drake Passage (~20 Ma)?

•   When did the Tasmanian Seaway open to shallow water, and how did this affect east-west biogeographic differences, isotopic differences relating to changing climatic regimes, and geochemical differences?

•   When did the seaway open to deep waters, and how did this affect surface- and deep-water circulation?

•   How is circum-Antarctic circulation related to changes in Antarctic climate?

•   How did the East Antarctic cryosphere develop in this part of Antarctica, and how does it compare to other sectors?

•   What was the nature of the adjacent Antarctic climate in the Greenhouse period in the middle to late Eocene?

•   How did sedimentary facies change as the Tasmanian region moved northward, circum-Antarctic circulation became important, and upwelling commenced?

•   How did Antarctic surface waters develop in terms of temperature, the thermocline, and oceanic fronts?

•   How did intermediate waters evolve during the Neogene, and how was this evolution tied to Antarctic cryosphere development?

•   How did Australia's climate change as the continent moved northward?

•   How were changes in the marine biota tied to changes in the oceanographic system?

To 189 Background

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