Authors / CoAuthors
Exon, N.F. | Kennett, J.P. | Malone, M.
Abstract
The recent drilling of ODP Leg 189 sheds new light on what happened as Gondwana broke up, Australia drifted northward from Antarctica, and the Tasmanian Gateway opened . The drifting contributed to the change in global climate from relatively warm early Cenozoic Greenhouse conditions to late Cenozoic Icehouse conditions. It isolated Antarctica from warm gyral surfacee currents from the north, and provided the critical deepwater conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans. One critical element in the global climate changes was the opening of the Tasmanian Gateway between Australia and Antarctica at ~33.5 Ma (Kennett, Houtz et al., 1975), which coincided with the onset of global cooling near the Eocene-Oligocene boundary. The other critical element was the opening of the Drake Passage between South America and Antarctica, which probably occurred somewhat later. Progressive cooling at high latitudes eventually formed major ice sheets, initially on Antarctica from ~33.5 Ma (Kennett, 1977; Miller et al., 1987), and later in the Northern Hemisphere from ~3 Ma (Shackleton and Opdyke, 1977; Ruddiman et al., 1989). High southern latitude development of glaciers triggered the formation of the cold deep ocean (psychrosphere) and intensified thermohaline circulation. The expansion of the Southern Ocean during the Cenozoic contributed to changes in the Earth's environmental system and in oceanic biogeographic patterns. Early DSDP drilling of the Tasmanian Gateway provided a basic framework of paleoenvironmental changes associated with the opening, but was inadequate to fully test the interrelationships of plate tectonics, circum-polar circulation and global climate. However, Kennett, Houtz et al. (1975) proposed that climatic cooling and an Antarctic ice sheet (cryosphere) developed as the Antarctic Circumpolar Current (ACC) progressively isolated Antarctica thermally. This current was initiated by the opening of the Tasmanian Gateway, and its history was a major focus of Leg 189. Leg 189 continuously cored sediments on foundered continental blocks (west Tasmanian margin, South Tasman Rise, East Tasman Plateau - Fig. 2), which were formerly part of the Tasmanian land bridge between Australia and Antarctica. The land bridge separated the Australo-Antarctic Gulf (AAG) in the west from the proto-Pacific Ocean to the east. The continuous coring has documented in detail what happened during the early and very slow separation of Australia and Antarctica from 75 Ma to 43 Ma, the fast separation between 43 Ma and 37 Ma while the land bridge still existed, the initial current break-through south of the South Tasman Rise (STR) from 37 to 33.5 Ma, and Australia's independent fast movement northward thereafter. This region with its always relatively shallow waters is one of the few places where well-preserved and almost complete Cenozoic marine sequences could be drilled in the Southern Ocean. Leg 189 exceeded our expectations in testing, refining and extending the above scenario, and will thus greatly improve the understanding of Southern Ocean evolution and its relationship with Antarctic climatic development during the last 70 m.y. (early results in Exon, Kennett, Malone et al., 2001). We have better documented the climatic and oceanographic consequences of the gateway opening during the transition from warm Eocene climates to cool Oligocene and Neogene climates. The sequences cored reflect the evolution of a tightly integrated and dynamically evolving system, involving the lithosphere, hydrosphere, atmosphere, cryosphere, and biosphere.
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nonGeographicDataset
eCat Id
38151
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- External PublicationArticle
- ( Theme )
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- palaeoclimatology
- ( Theme )
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- marine
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- AU-TAS
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2001-01-01T00:00:00
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