Geoscience Reference
In-Depth Information
Australia, which was first evident in the mid-Miocene some 15 Ma ago. Rainforest
gave way to dry sclerophyll woodland, and a eucalypt flora adapted to frequent natural
fires came into being and expanded across the continent. The trend to aridity occurred
in stages, with the first stony tableland landscapes appearing at 4-2 Ma and the first
desert sand dunes appearing by 1 million years ago. Large lakes shrank progressively,
and from about 300 ka onwards, the inland lakes show a progressive reduction in
size, associated with enhanced aridity, culminating in the cold, dry and probably
windy Last Glacial Maximum some 20,000 years ago. Although the Australian flora
is reasonably well-adapted to aridity, it is often highly sensitive to extreme cold. The
two extremes of a glacial-interglacial cycle epitomise the full range of environmental
changes that occurred on the continent during the Quaternary. During interglacials,
the lakes were full and fresh, dunes were vegetated and stable, rivers were perennial,
and rainforest and woodland expanded across the continent. By contrast, full glacial
times were cold and generally dry, dunes were reactivated, lakes dried out or became
saline, dust-storms were more frequent, grassland and shrubland replaced forests
and woodlands, limited mountain glaciers and small ice caps occupied the highest
uplands in the south-east and Tasmania, and previously perennial rivers became more
seasonal or even ephemeral. The initial arrival of prehistoric humans around 45 ka
ago coincides with a wave of faunal extinctions, as well as evidence of an increase in
aridity. The jury is still out as to the precise causes of the faunal extinctions.
