SCIENTIFIC OBJECTIVES (continued)
Neogene and Quaternary History
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, 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 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 circum-Antarctic symmetry of Southern Ocean paleoclimate change and interbasin circulation patterns that influence the ocean's dissolved carbon and alkalinity budgets.
Most 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 circum-Antarctic 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 sectors 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 will be extended southward almost to a present latitude of 50°S by four of our holes. East of the STR, the results of the Lord Howe Rise Leg 90 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).
Principal Results-Site 1168 | Table of Contents