SCIENTIFIC OBJECTIVES

 Paleogene History
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., 1997a) 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 siliceous microfossils of colder-water character, and varves in some intervals, suggesting strong seasonality perhaps related to the onset of glacial conditions in the Antarctic region. The middle Eocene to upper Oligocene sequences are crucial to understanding the opening of the Tasmanian Seaway, 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 circum-polar circulation both across and south of the STR (about 65°S at that time). The opening of this 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 this 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 WT-1A, WSTR-1B, and WSTR-2A will provide data about the Indian Ocean paleoenvironment prior to opening (middle to late Eocene), whereas ETP-2A will provide information about South Pacific paleoenvironments prior to opening of the Tasmanian Seaway. All sites will address the initial shallow-water breakthrough (late Eocene) and most will address the deep-water breakthrough to some extent (early-middle Oligocene?).

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), may 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 are 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 isotope stratigraphies should provide a chronology of sufficient resolution. Specific stratigraphic boundary events (e.g., Eocene/Oligocene, Miocene/Pliocene) will be analyzed at high resolution.

Neogene and Quaternary History
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, oscillations in ocean temperatures, migration of ocean fronts, 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 will fill a key geographic gap. For example, the sites are expected to 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 circumpolar symmetry of Southern Ocean paleoclimate change and interbasin circulation patterns that influence the ocean's dissolved carbon and alkalinity budgets.


To 189 Proposed Sites

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