Australia: The Land Where Time Began

A biography of the Australian continent 

The temperature of the surface water on the Campbell Plateau adjacent to Antarctica had been dropping at the start of the Oligocene. It is thought possible that the west Antarctic ice sheet may have begun to form, leading to the drop in sealevel as it expanded. The significant cooling at the poles increased the temperature gradient from Pole to Equator, water temperature dropping 6-7^o C. Australia had been an independent continent for about 10 million years by the start of the Oligocene, separated from Antarctica by a widening belt of oceanic crust.

About 33 Ma, the Early Oligocene deterioration. According to Protehero, this was the real event that is usually attributed to the "Terminal Eocene Event". It is at this time that there is good evidence of cooling on a massive scale, with some glaciation in Antarctica, the formation of the cold bottom water and, in the Northern Hemisphere, deep marine erosion by bottom waters from the Arctic. Most of the extinctions usually attributed to the End Eocene Event actually occurred at this time. (2)

About 30 Ma, in the Early Oligocene, the Drake Passage between South America and Antarctica opened, removing the last impediment to the formation of the circum-polar current (see Southern Ocean) resulting in Antarctica becoming more isolated and leading to more intense cooling. It is believed the rain-bearing westerly wind systems would probably have moved further north as a result of the cooling. These winds would then have brought large amounts of rain to the parts of Australia that were between the Pole and 40^o S. If this was the case the areas in the north and northwest of the continent would have been much more arid, a condition which could have began preparing the fauna and flora for increased seasonality and eventually to the vast areas of extreme aridity that were to come.

Until the Late Oligocene vegetation was still present in the coastal regions of Antarctica. Nothofagus fusca dominated forests that included Protaceae, Myrtaceae and Podocarpaceae grew in close proximity to the earliest known glaciers in the area of the Ross Sea. At the edge of glaciers in the Andes in present-day Chile the same assemblage of plants is found. Among the Protaceae of these Chilean forests is Embothrium, the closest relative of Oreocallis from northern Australia.

The plant microfossil record from the Australian Oligocene is meagre, and comes mostly from the southeast part of the continent, but what there is indicates a declining diversity as would be expected at a time of cooling. Protaceae, Myrtaceae and some other families are dominated by Nothofagus. Among the 7 types of  Myrtaceae pollen is 1 that resembles that of Eucalyptus, though none of the pollen can be confidently assigned to any living genera. There are no known plant macrofossils that can be positively identified as coming from the Oligocene.

The presence of a high watertable and extensive swamps from this time are indicated by widespread coal and lignite deposits in southeast Australia. The pollen of Dacrydium, a conifer that now grows in moist, cool-temperate regions is common in some coal deposits.

The Etadunna Formation from this time, that was deposited by the inward-draining central Australian drainage system that was active during the Oligocene, contains vertebrate fossils. The Etadunna formation was originally believed to be of Middle Miocene age but has been reassigned to the Oligocene.

A Late Oligocene locality on the north side of the Derwent River in Tasmania was the only known site with a land mammal fauna from the Late Oligocene before the discovery of the Etadunna Formation, but it only contained a few Marsupial bones.

Sources & Further reading

1. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
2. Donald R. Prothero, The eocene-Oligocene Transition-Paradise Lost, Columbia University Press, New York, 1994

Links

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene Miocene boundary