オーストラリア・タスマニア島南方海域の古海洋
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概要
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Paleoceanographic and tectonic studies of the uppermost Cretaceous to the Quaternary around the Tasmania region based on ODP Leg 189 were reviewed from approximate 70 papers. The Tasmania Land Bridge blocked the seawater connection between the Pacific and Indian oceans by 35.4 Ma (timescale of CK 95), although a very shallow water path was already present from approximate 40 Ma. Prior to the opening of the Tasmanian Gateway, the Tasmanian region was strongly influenced by the 3rd order of the global eustaic change. After the Tasmanian Gateway substantially opened at 35.4 Ma, the bottom current strengthened between 33.5 and 30.2 Ma, and this gate opened fully after 30.2 Ma when nannofossil-ooze with planktonic foraminifers started to accumulate on the seafloor around the region. The paleoceanographic and paleoclimatic conditions underwent a major change in the latest Cretaceous (Maastrichtian at least), the earliest Paleocene to the latest Eocene, the latest Eocene to the early Oligocene, the late Oligocene to the early Miocene, the early to the late Miocene, and the Quaternary. This region was influenced by global climatic and paleoceanographic changes. A significant deepening of the Tasmanian Gateway occurred 1.7 my earlier than the Eocene-Oligocene boundary, and the surface water temperature rose during the significant opening of the gate. In the late Eocene, the climate in the summer of the Southern Hemisphere was relatively cool based on floral changes, although this cool condition did not affect to the marine faunal ecology. Bottom water around the Tasmania region was common in this region by 28 Ma, whereas the sea-surface condition was different before 22.8 Ma. The full development of the Antarctic Circumpolar Current that flow over the seafloor was estimated at 23.95 Ma. Global paleoceanographic events such as the middle Miocene Climatic Optimum, Monterey Excursion, the middle Miocene climatic transition, and carbonate crash (=late Miocene carbon shift) were detected in this region.