Australia: The Land Where Time Began
Last Glacial Maximum in
During the glacial maximum the climatic system produced intensified pressure systems, leading to high speed winds that reactivated the dunefields covering much of the arid zone, that had formed during previous stages. The dunes had been stabilised by vegetation during the previous wetter phase. As the drying of the arid zone reached extreme levels, the vegetation was unable to cope with the increasing drought, and increased fire frequency and severity in many areas that would have occurred in the increasingly dry and windy conditions.
When the lakes were full during the wet phase, sand dunes formed on the down-wind sides of lakes and rivers. When the wind picked up, these dunes were blown across the land, forming new dunes and sandsheets. The salts and sediment of dry salt lakes were moved down wind. The first deposits were clay-rich, gypsum and salt being concentrated in these beds. They were then dispersed by the wind to be deposited as layers in dunes that were being formed at the time. Saltwater discharge areas were a feature of drainage systems of central Australia. When dry, the salt and fine dust from the bed of the lake was spread widely across the land. This salt contributed significantly to the salinity of the groundwater. In places where the fine salty dust accumulated it formed parna, a red silty clay. Loess deposits formed at this time in Europe, America and China.
At this time, a vast area of Australia, from the Gulf of Carpentaria to northern Tasmania, was covered by dunefields. In the sedimentary basins, vast quantities of sand were present, within these basins the sand contributed to the building of the dunefields of the Strzelecki Desert, Simpson Desert, Great Sandy Desert and the Mallee. The other dunefields are in topographical areas that are more complicated. The Tanami Desert dunefields spread between low hills and duricrusted tablelands, over areas of low relief, and over the floodplains, both adjacent and intervening. The dunefield in the north of the Gibson Desert is an extension of that of the Great Sandy Desert, but in the south, the dunefields lie between the ridges and low tablelands. The dunefield of the Great Victoria Desert is similar to that of the southern part of the Gibson Desert. The dunefield of the western part of the Gibson Desert extends on to the Yilgarn Plateau, where it is in the valleys between the rocky outcrops, and the breakaways that have tops covered by duricrust. At this time the sea level was lower, and the continental shelves were exposed. Dunefields had formed on them, sand moving from them on to the continental margins.
Tasmania was connected to the mainland by the Bassian Plain. The Bass Strait Islands are the remnants of the dunefields that formed on the plain when the sea level rose at the end of the most recent glaciation. Flinders Island is one of the remnants of mountainous areas in the eastern part of the plain, now Bass Strait. Bass Strait presently overlies the beds of the lakes and rivers that contributed to these dunefields.
There is evidence from 20,000 years ago that Aborigines inhabited the edges of the Bassian Plain, and this occupation continued throughout the glacial maximum, following the changing shoreline as the sea level rose. When the sea entered the Bass Basin between King Island and the mainland, the King Promontory jutted out from the northwestern tip of Tasmania, and in the east, the northeastern tip of Tasmania was connected to Victoria. About 13,000 years ago the Bass Strait Aborigines became Tasmanian Aborigines. As sea level continued rising, the Furneaux Promontory became the Furneaux Islands. Archaeologists have found that on Furneaux Island a population of Aborigines persisted for several thousand years, but eventually there were no more signs that the island was inhabited. Aboriginal artefacts have been found in the dunes of King Island.
The present-day coastal plain of Victoria received sand from the continental shelf of the Bassian Plain during the glacial period. Prominent parallel limestone (calcarenite) ridges, trending west-northwest and east-southeast, are formed from this sand. These ridges are separated by interdune flats. These ridges are called the Bridgewater Formation, and extend throughout the Mt Gambier coastal plain.
Kangaroo Island is 14.5 km from the South Australian coast. During the later part of the latest glacial phase it was connected to the mainland, a low range of hills on the continental shelf. The Murray River flowed for 70 km across the shelf in a southwesterly direction, from the present mouth. It passed within 10m km of the eastern end of the present island, entering the sea 20 km to the south.
At the glacial maximum sealevels were about 130 m lower than present, the continent was surrounded by vast areas of plains, the exposed continental shelf. Two thirds of the continent was covered by dunes extending onto the continental shelf on the western and southern margins. It is believed that at this time the rainfall of central Australia was half or less of the present levels.
To the east of the dunefields there was an arc of semi-arid savannah stretching to the Great Escarpment in eastern New South Wales and north over a large area of exposed continental shelf on the Sahul Shelf and Arafura Plain. Mallee, Callitris, acacia and Casuarina woollands invaded the semi-arid lands between the central dunefields and the coastal ranges in the east. Amongst the eucalypt woodlands that presently occupy this area there are pockets of relict stands of this vegetation type.
Over 85 % of the continent there were no trees, they were restricted to woodlands, with pockets of rainforest, in the strip along the east coast between the Great Dividing Range and the coast, at that time extending much further east on the exposed continental shelf. As Fraser Island was connected to the mainland, allowing the spread of rainforest to the island and to become established before the sealevel rose again. It is the only rainforest growing on pure sand.
In the south, woodland extended down the western margin of the Bassian Plain to Tasmania. Some were also present in the southwestern corner of Western Australia.
In the southeastern corner, the high country was cold steppe and alpine grassland, no trees being present all the way to the coast. With the snowline lowered, snow gums were growing in the valleys at low altitude. The river red gums of the inland had been lost from the southern floodplains because of frost and salinity, and Casuarina had gone from the upland rivers. The cold steppe grasslands extended across the Bassian Plain to Tasmania, with the exception of the western margin of the Plain.
There must have been scattered refuge areas around the continent for the vegetation types from the major vegetation zones to survive the time when most of their former habitat was covered by dunefields. There must also have existed refuge area for the aquatic life of the then extremely dry inland. Evolving as they had, to survive in extreme conditions of salinas, with changing salinity and water levels, desiccation, even before the glacial maximum, they would have been at least partially prepared for the really hard times, so probably didn't need as much protection as the they would without such preadaptations. The plants and smaller animals also had very good dispersal mechanisms for the egg, cysts and seeds, being dispersed on the feet of birds, and even in the deflation of the dry surface by the wind.
It is believed that mound-springs played a major part in the survival of enough of the aquatic organisms to allow them to recolonise their normal habitat when the wetter times returned. These springs would have continued whatever the local conditions were like as they are powered by the pressure of the water that was being added at their sources in the distant ranges. These mound-springs have a highly specialised endemic fauna of gastropods, crustaceans and isopods, other forms appear to have migrated there from lakes that have probably disappeared.
It is believed some water bodies survived the worst of the glacial maximum. Ruppia maritima is a cosmopolitan species with a patchy distribution pattern, being found in salt lakes, mound springs and around artesian bores. This species is part of the relict aquatic flora that in wetter times was widespread across the arid regions of central Australia. Fossil Ruppia seeds have been found in cores of sediment from Lake Frome. It no longer grows at Lake Frome, even when the lake fills. Because of habitat fragmentation and isolated ecosystems that survive in refugia, the same type of disjunct distribution pattern also occurs with terrestrial biotas.
Present-day Australia is only about 10,000 years from the driest, most desert covered, the continent has ever been, an insignificant length of time in geological terms. Prior to the glacial maximum, there had been a progressive drying, increasing salination of soils and groundwater, vast areas of the continent being affected. Increasing areas of wind-blown sand and alteration of drainage systems and river behaviour, the continued weathering and leaching of the soils, already ancient and depleted. Fire had been altering environments ever since the rainforest of Gondwana retreated, and increased with the coming of Aborigines, but with the arrival of Europeans the devastation was taken to a new level. At the time humans arrived, the continent had been recovering from it driest ever period for less than 10,000 years. The fire practices of the Aborigines changed the recovery to preserve the habitats that suited their hunting preferences, but at least it was sustainable. With the coming of Europeans with their agricultural practices that were unsuitable for the Australian environment, the fire frequency was increased by about 3.5 times that of the Aboriginal fire practices. Increased fire frequency favours elements of the Pleistocene communities such as Acacia and tussock grass that expand, while inhibiting the growth of trees. Areas that are over-burnt or over-grazed return to the open vegetation with the low diversity of glacial times.
The process of turning Australia into a vast desert was begun by nature, but now humans are doing their bit to make sure it never recovers from its "big dry". Increasing population and "development" are making things even worse, as more land is cleared for agriculture, the spread of cities, with roads dividing what habitat remains, isolating to varying degrees inhabitants of those divided habitats.
The increased demand for housing resulting from increasing population is now leading to increasing areas of what little productive land is available in Australia being turned over to housing and industrial use.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|