The area between Australia's southernmost prolongation (Tasmania and the South Tasman Rise [STR]) and Antarctica is a key to understanding global Cenozoic changes in climate and current patterns, involving
The opening of the Tasmanian Gateway between Australia and Antarctica and the only other important constriction in the establishment of the Antarctic Circumpolar 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 positive feedbacks and other changes in the global system, have been crucial in the development of the polar cryosphere, initially in Antarctica during the Paleogene and early Neogene and later in the Northern Hemisphere during the late Neogene. 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 Earth's environmental system and of oceanic biogeographic patterns.
The geographic position of the Tasmanian offshore region makes this a crucial 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 during the middle 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 most likely exhibited distinct physical, chemical, and biological properties. The Tasmanian region is also well suited for the study of post-Eocene development of Southern Ocean climate development, feedbacks that contribute to ice sheet development and increased stability, and 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. Therefore, an outstanding question is whether paleoceanographic variability, known from the Atlantic and Indian sectors, is characteristic of the entire circum-Antarctic ocean or whether there are zonal asymmetries in the Southern Ocean and, if so, when these developed.
The meridional spread of the sites on the STR (Fig. 3) is well suited for monitoring the migration of frontal zones through time, analogous to transects on the Southeast Indian Ridge (SEIR) (Howard and Prell, 1992). The total meridional displacement of fronts on the STR is expected to be somewhat less than on the SEIR because the STR is a shallower topographical barrier to the Antarctic Circumpolar Current. The East Tasman Plateau (ETP) site is ideally located to monitor paleoceanographic changes at the interface between the East Australian Current and the Antarctic Circumpolar Current because the East Australian Current transports heat into the Southern Ocean, an important "gateway" objective.
The sites cored during Leg 189 also provide high-quality paleoclimatic and paleoceanographic records of Neogene age, including the Quaternary, from the southern temperate and subantarctic regions. These sequences are being 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 and late Miocene.
Previous investigations have demonstrated that the Southern Ocean late Quaternary 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. This suggests similar cryospheric, atmospheric, and oceanographic variability in Southern Ocean climate during the past 500 k.y. compared with that of the Northern Hemisphere (Imbrie et al., 1992; Imbrie et al., 1993). 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 significant interest. Despite the excellent documentation of Southern Ocean paleoclimate history of the latest Pleistocene, where variability is dominated by 100-k.y. cycles, there is a lack of fully cored sections to address the mid-Pleistocene (900 ka) transition from obliquity-dominated cycles (40-k.y. periodicity) to eccentricity-dominated (100-k.y. periodicity) cycles (Ruddiman et al., 1989) in this region. A documented Southern Ocean section over this "transitional" period was based on poor recovery (Hodell and Venz, 1992), so this important transition in global climate remains to be properly documented. However, two giant piston cores taken on the STR (Marion Dufresne, 1997) provide excellent records back to 900 ka including this transition. Sedimentation rates were up to 2.2 cm/ky. The STR Ocean Drilling Program (ODP) sites will add to this record and complement subantarctic South Atlantic transect sites (Leg 177) in documenting this transition.
Major questions addressed during Leg 189 include the following:
An understanding of Cenozoic climate evolution has required better knowledge of the timing, nature, and responses of the opening of the Tasmanian Seaway during the Paleogene (Fig. 1, Fig. 2). Early ocean drilling in the Tasmanian Seaway (Deep Sea Drilling Project [DSDP] Leg 29) provided a basic framework of paleoenvironmental changes associated with its opening but was of insufficient quality and resolution to fully test the hypothesis of potential relationships among the development of plate tectonics, circumpolar circulation and global climate. Until now, the timing of events has remained insufficiently constrained.
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 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. 4).
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