Authors / CoAuthors
Exon, N.F. | Kennett, J.P. | Malone, M.J. | Chaproniere, G.H.
Abstract
As Australia separated from Antarctica and drifted northward the Tasmanian Gateway opened, allowing the Antarctic Circumpolar Current to develop. This current began to isolate Antarctica from the influence of warm surface currents from the north, and an ice cap started to form. Eventually, deepwater conduits led to deepwater circulation between the southern Indian and Pacific Oceans. The existence of these conduits ultimately allowed ocean conveyor circulation. Continuing Antarctic thermal isolation, caused by the continental separation, contributed to the evolution of global climate from relatively warm early Cenozoic ?Greenhouse? to late Cenozoic ?Icehouse? climates. ODP Leg 189 addressed the interrelationships of plate tectonics in the gateway, circum-polar current circulation, climate and sedimentation, and global climatic changes. DSDP drilling had led to a basic framework of paleoenvironmental changes associated with gateway opening, but was not a full test of the various interrelationships. Using the DSDP results, Kennett, Houtz et al. (1975) proposed that climatic cooling and an Antarctic ice sheet (cryosphere) developed from ~33.5 Ma as the ACC progressively isolated Antarctica thermally. They suggested that development of the Antarctic cryosphere led to the formation of the cold deep ocean and intensified thermohaline circulation. Leg 189 gathered data that support this hypothesis. Leg 189 continuously cored sediments in the gateway, which was once part of a Tasmanian land bridge between Australia and Antarctica. The bridge separated the Australo-Antarctic Gulf in the west from the proto-Pacific Ocean to the east. This region is one of the few in the Southern Ocean where almost complete Cenozoic marine sequences could be drilled in paleo-water depths that were shallow enough to allow preservation of calcareous micro-organisms for isotopic studies. The Leg 189 sequences described by Exon, Kennett, Malone et al. (2001) reflect the evolution of a tightly integrated and dynamically evolving system over the past 70 million years, involving the lithosphere, hydrosphere, atmosphere, cryosphere and biosphere. The most conspicuous changes in the region occurred over the Eocene-Oligocene transition (Figure 1) when Australia and Antarctica finally separated. Before the separation, the combination of a warm climate, nearby continental highlands, and considerable rainfall and erosion, flooded the region with siliciclastic debris. Deposition kept up with subsidence. After separation, a cool climate, smaller more distant landmasses, and little rainfall and erosion, cut off the siliciclastic supply. Pelagic carbonate deposition could not keep up with subsidence. Leg 189 confirmed that Cenozoic Antarctic-Australia separation brought many changes. The regional changes included: warm to cool climate, shallow to deep water deposition, poorly ventilated basins to well-ventilated open ocean, dark deltaic mudstone to light pelagic carbonate deposition, microfossil assemblages dominated by dinoflagellates to ones dominated by calcareous pelagic microfossils, and sediments rich in organic carbon to ones poor in organic carbon.
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nonGeographicDataset
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38191
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- External PublicationArticle
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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2004-01-01T00:00:00
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