Previous sampling has shown that basinal Paleocene history was probably similar in two areas, the west Tasmanian margin and the STR. Clearly, there were differences in Eocene shallow-marine deltaic deposition between the areas, when the northern area experienced more carbonaceous sedimentation while the southern area experienced deposition of siliceous radiolarian-bearing glauconitic mudstones. Overall, results are summarized by Hill et al. (1997b), Exon et al. (1997a), and Exon et al. (1997b). As Australia cleared Antarctica, submarine erosion in both areas formed an Oligocene unconformity, and both areas subsided steadily. However, the southern area sank vertically as a block, whereas the west Tasmanian margin rotated downward from a hingeline near the coast, increasing water depths with increasing distance from Tasmania. In many southern areas, the Antarctic Circumpolar Current removed most Neogene sediments, but thick late Oligocene to Holocene carbonate sediments are present in depocenters off west Tasmania and on the STR.
The stratigraphy of petroleum exploration wells on the west Tasmanian continental shelf was summarized by Moore et al. (1992). The detrital Upper Cretaceous sequence is probably disconformably overlain by Cenozoic strata. Nonmarine to shallow-marine, Paleocene to lower Eocene fining-upward sequences are always present. The middle Eocene to lower Oligocene sequence is more calcareous, consisting of shallow-marine sandstone, marl, and limestone. Above a major unconformity, the late Oligocene and younger sediments are dominantly shelfal marl and limestone. All of the Upper Cretaceous, Paleocene, and Eocene siliciclastic sediments sampled are interpreted as shallow or restricted marine and are commonly deltaic (prograding is marked in the seismic profiles).
Existing stratigraphic and sedimentologic information indicates that middle Eocene sequences are different in the northern sites west of Tasmania (DSDP Site 282; Hill et al., 1997b) and in the south in the STR (DSDP Site 281; Exon et al., 1997b), although shallow-marine and deltaic facies are found in both areas. Northern sequences contain abundant organic matter and calcareous temperate microfossil assemblages. Southern sequences contain more siliceous microfossils of colder water character. One occurrence of varves (Exon et al., 1997b) suggests strong seasonality of the Antarctic climate. The middle Eocene to upper Oligocene sequences are crucial to understanding the opening of the Tasmanian Gateway between Tasmania and Antarctica, initially in shallow and later in deep water. Before the Oligocene, sequences on either side of the STR should have distinctive biogeographic characters.
Study of the uppermost Eocene through Oligocene sequences will be of special importance in examining the timing of the development of the circumpolar circulation both across and south of the STR (~65°S at that time). The opening of the Tasmanian Gateway was such a profound event that biotic, sedimentologic, and geochemical parameters would almost certainly have undergone distinct changes. When studied in detail and in unison, changes in these parameters are expected to provide the crucially needed evolutionary information on the gateway. The dating of unconformities or hiatuses will provide critical information on major current activity during the Oligocene, especially in the shallow sequences, although sites have been selected to minimize the effects of sediment erosion. We are especially interested in the timing of initial shallow-water linkage across the STR and deep-water linkage south of the STR.
Sites 1168 and 1170 will provide data about the Indian Ocean paleoenvironment before the opening of the Tasmanian Gateway (middle to late Eocene), whereas Sites 1171 and 1172 will provide information about South Pacific paleoenvironments before the opening. All sites will address the initial shallow-water breakthrough (late Eocene), and most will address the deep-water breakthrough to some extent (early-mid-Oligocene?).
Results from DSDP Leg 29 suggested that a sequential appearance of marine microfossils, from dinocysts and arenaceous foraminifers (early Eocene) to calcareous nannofossils (middle Eocene) to calcareous benthic foraminifers (early late Eocene) and to planktonic foraminifers (late late Eocene), might well be revealed at most of the sites. The order of appearance of major groups is paleoenvironmentally significant and is expected to provide crucial insights about the evolution of the Southern Ocean biota. The upper middle Eocene to the lower Oligocene sequence, where calcareous microfossils are present and sedimentation rates were expected to be 1.5 to 3 cm/k.y., should provide excellent documentation of tectonic, climatic, and oceanographic changes. Planktonic foraminiferal and calcareous nannofossil biostratigraphy, in conjunction with strontium and oxygen isotopic stratigraphies should provide a chronology of sufficient resolution. Specific stratigraphic boundary events (e.g., Eocene/Oligocene and Miocene/Pliocene) will be analyzed at high resolution.
Data from the coring in the Tasmanian region will assist in evaluation of the dynamic oceanographic and climate evolution that continued in the Southern Ocean during the Neogene and Quaternary. Information gained will include that related to climate and ocean evolution, ocean temperature oscillations, ocean front migrations, paleoproductivity, and biotic evolution. This leg is complementary to three recent ODP Neogene paleoceanographic legs: Leg 182 in the Great Australian Bight to the northwest, Leg 177 in the subantarctic South Atlantic, and Leg 181 east of New Zealand. Leg 189 fills a key geographic gap. For example, the sites provide temperate and subantarctic Neogene biostratigraphy of foraminifers and calcareous nannofossils.
In particular, the history of water-mass formation and mixing among Antarctic, Indian, and Pacific sources can be monitored in this area through isotopic and trace metal proxies measured in the abundant planktonic and benthic foraminifers. These sites will complement the Leg 177 South Atlantic subantarctic transect sites in answering questions about the Antarctic circumpolar symmetry of Southern Ocean paleoclimate change and interbasin circulation patterns that influence the ocean's dissolved carbon and alkalinity budgets.
Most recent knowledge of Southern Ocean paleoceanography has been derived from the Atlantic and Indian sectors (Legs 113, 114, 119, 120, and 177). It is usually assumed that the history from an individual site or region represents the "zonal" behavior of the Southern Ocean, but differences among the sectors may have been significant, especially for the Paleogene and early Neogene. Even in the late Pleistocene, when there is no doubt that Antarctic circumpolar flow was fully established, there is some evidence that the Atlantic and Indian Ocean sectors may have had important differences in paleoceanographic variability (Wright et al., 1991; Miller et al., 1991). These differences not only provide useful insights about paleocirculation, but also about meridional heat transport (driving zonal thermal anomalies), in the Pliocene-Pleistocene as well as in the Miocene and Oligocene (Hodell and Venz, 1992).
Intersector differences in heat transport could have important implications for the possible melting history of different segments of the Antarctic ice sheets. For example, if meridional heat transport was greater in the southwest Pacific, the West Antarctic Ice Sheet may have been more vulnerable to melting. Did this ice sheet maintain its present mass balance in the face of such circulation changes? Much work related to the cooling of Earth during the late Neogene ice ages is now focused on the role of oceanic and atmospheric polar heat transport.
Sites cored during Leg 189 will also provide records of the interaction of the Antarctic Circumpolar Current and the Western Boundary Current (East Australian Current). Areas of Western Boundary Current "injection" into the Southern Ocean (the Brazil-Malvinas Confluence and the Agulhas Retroflection) are regions of large-scale heat and carbon dioxide exchange between ocean current systems, and between the ocean and atmosphere that ventilate the main ocean thermocline, and will be one of the key components of the ocean to respond to global warming. Understanding the dynamics of such confluences on a geologic time scale is vital to anticipating their possible response to future climate change (e.g., Trenberth and Solomon, 1994).
The Neogene sites also continue global biostratigraphic transects in middle to upper bathyal water depths toward the south. West of the STR, the results from the mid-latitude Great Australian Bight Cenozoic carbonates drilled during Leg 182 (Feary, Hine, Malone, et al., 2000) will be extended southward by four of our holes to a present latitude of ~50°S. East of the STR, the results of the Lord Howe Rise Leg 90 (Kennett and von der Borch, 1986) will be extended south to the same latitude by two of our holes. An additional advantage of the northern Sites 1168 and 1172 is that these sites may contain pollen from nearby Tasmania, allowing direct terrestrial-marine climate comparisons for the Neogene. So far the only mid-latitude Southern Hemisphere drill site that has yielded such a record is Site 594 east of New Zealand (Heusser and van de Geer, 1994).