Australia: The Land Where Time Began

A biography of the Australian continent 

At the start of the Eocene Gondwana had been splitting up for some time, Australia and South America were still attached to Antarctica, though only the final link, Tasmania, kept Australia connected to Antarctica, which straddled the South Pole, but was at this time ice free. Australia was connected to South America by an unbroken belt of rainforest. The Australian Peninsula was still close to the South Pole, but it was mostly covered with warm, wet rainforests in which the broad-leafed angiosperms, a characteristic type being Southern Beech, were replacing the gymnosperms. A broad rift valley had formed between Australia and Antarctica as they drifted apart. the early Southern Ocean continuing to widen throughout the Eocene. The last connection between Tasmania and Antarctica was finally flooded at about 45 Ma. Until the end of the Eocene it remained a shallow shoaling area, at which point it was finally covered by deeper water that became a free flowing part of the sea. The warmth of the water at the time of separation is indicated by the composition of the carbonate ooze that was accumulating on the floor of the widening Southern Ocean. The Nullarbor plain was the floor of the Eucla Basin before it was flooded. The limestone of the Nullarbor Plain were the oozes deposited on the floor of the Eucla Basin before it was uplifted.

In the Early Eocene India collided with Asia about 50 Ma. The Indian Plate then fused with the Australian Plate to become the Australia-India Plate, one of the major plates at the present.

Early in the Eocene the temperature of the surface water on the Campbell Plateau, that lies adjacent to Antarctica, was about 20^o C. Global circulation patterns were changing throughout the Eocene as the southern continents moved away from Antarctica and the ocean basins widened, and by the end of the Period there had been a marked cooling episode. Based on the foram data obtained from cores drilled during the Oceanic Drilling Program, it has been found that at the start of the Eocene, the climate was usually warm globally (4). The ocean circulation was sluggish, warm saline water extending almost to the seafloor. As a result, the climate was unusually warm up to high latitudes. There was a time of increased tectonic activity with rapid spreading from the mid-ocean ridges. It has been suggested (4) that the increased amount of volcanic activity associated with this period of rapid spreading may have released large amounts of CO² into the atmosphere, with more being added by massive flood basalt eruptions in the North Atlantic. There may also have been a contribution from India ploughing into Asia at the beginning of the closure of the Mediterranean Tethys.

About 50 Ma, at the transition from the Early to Middle Eocene, the atmospheric CO² was greatly reduced, most apparently being sequestered in the coal deposits forming at this time. It also coincided with a reduction of tectonic activity, so there would have been less CO² being added to the atmosphere by associated volcanic activity. Prothero suggests that as the atmospheric CO² levels dropped there would have been a "Monterey"- type response, a cooling being indicated by oxygen isotopes. As the temperature dropped silica would have precipitated in the oceans in what has been called a "silica burp". Prior to the cooling, that has been called a reverse greenhouse effect, in the warm climate of the Early Eocene the amount of silica dissolved in the oceans was increased by a combination of factors that included high levels of volcanic activity, deep weathering on the continents, extending to high latitudes, and the silica being released into a warm, sluggish ocean.

About 38-37 Ma, at the transition from the Middle to Late Eocene, further cooling resulted in the glaciers first beginning to form in Antarctica. It has been suggested that while Antarctica may have been cold enough to form glaciers before this time it was prevented from doing so by the lack of moisture in the atmosphere above it. This moisture may have become available when the ocean filled the widening gap between Antarctica and Australia, the increased available moisture falling as snow that began the accumulation of the glaciers. This was also the time of the largest extinction events of the Cainozoic. There were 4 possible impacts on the earth at this time, but they don't appear to have had a significant effect on the climate or extinctions.

About 36-35 Ma, in the mid-Late Eocene, shallow water circulation between Antarctica and the South Tasman Rise began, the beginning of the Antarctic Circum Polar Current.

About 34 Ma, the terminal Eocene event, that Prothero describes as a non-event, resulting in only minor extinctions of planktonic foraminferans, with very little effect on other organisms. Prothero suggests these events were leading ot the real "event", the Early Oligocene deterioration of 33 Ma.

Among the animals, Dinosaurs and their relatives from the oceans and the air had gone extinct, their place being taken by mammals - monotremes, marsupials and placentals and frogs, horned turtles, giant flightless birds and snakes and lizards. In the Late Eocene deposits at Blanch Point Formation, Whitton Bluff near Noarlunga, South Australia, giant penguins, more than 1 m tall, with larger flippers than other known penguins and long necks, appearing to be closer to the ancestral flighted birds than any other known penguin. They are also found in New Zealand and Seymour Island in the Antarctic.

There was a rich flora in Australia during the Eocene, as indicated by the pollen assemblages from such places as the Gippsland Basin and Otway Basins in Victoria, Nerriga, New South Wales, West Tasmania and the Perth Basin in Western Australia. Some components of this flora were Myrtaceae, Sapindaceae, Malvaceae, Bombacaceae (Baobab family), Beauprea (Proteaceae), Anacolosa (Olacaceae), Nipa (the Mangrove Palm), Nothofagus (low proportion, as it was in the Palaeocene, and Conifers. New Zealand also has Nipa pollen, which indicates that both Australia and New Zealand had inherited the ancestral stock of the palm from Gondwana. Palms were among the earliest flowering plants, forming a Palm Province in low latitudes, and were also present in high latitudes before the polar regions cooled too much.

Eocene-Oligocene Boundary Cooling

See Australian Plant Communities – Early Eocene

Sources & Further reading

1. After the Greening, The Browning of Australia, Mary E. White, Kangaroo Press, 1994
2. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
3. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
4. Donald R. Prothero, The eocene-Oligocene Transition-Paradise Lost, Columbia University Press, New York, 1994

Links

Silica burp in the Eocene ocean