Australia: The Land Where Time Began
Ice Age Australia
The northern ice cap formed about 2.4 Ma, resulting in a very arid phase in Australia. At this time Australia had only remnants of the closed canopy rainforests it had at the time it separated from Gondwana. The remnant forests were in places where the conditions were unchanged or had migrated into suitable areas that emerged as the climate changed during the Tertiary, as the continent dried out. The present day distribution of these remnant forests and fossil evidence indicates that the positions of the refugia didn't change during the Quaternary Period. The boundaries of these refugia, and the ecosystems contained within them, have changed over time, expanding and contracting with the changes of climate during the Pleistocene and Holocene.
Overall, Australian forests and woodlands became much more restricted during glacial times, grasslands replacing many of them. For much of the continent, this was not the result of the land being in the grip of sub-alpine conditions. The continent lay in mid latitudes, with very little high ground, so the temperatures during the Ice Age didn't drop too drastically. The great expansion of grasslands, and changes in ecosystems that appeared as though they were the result of extreme cold, were actually more likely to be the result of seasonal aridity, with accompanying high winds and increased evaporation, and to the increased fires that accompanied the drying out near the glacial peaks.
By 2.4 Ma many dryland vegetation types had been established. In places where topography, soil type and salinity produced extreme aridity, drought-adapted plants had already been selected, pre-adapting them to fill the niches that increasingly became available as aridity spread over the land. During the glacials and interglacials, the timescale were changing from millions of years to thousands of years. As the dry times came and went, as the glacial climate fluctuated, stopping increased aridity in some parts and slowing its spread in others, the boundaries of the various vegetation types expanded and contracted in sync with the climate.
In the southern warm temperate parts of the continent the climate was settling on a pattern of wet winters and dry summers. The northern section was becoming subject to the tropical monsoonal belt, with wet summers and dry winters. In between, the climate was becoming close to one long dry season that lasted the whole year, the water that arrived was down the streams from the northern monsoonal regions.
Tasmania and the highlands in the southeast of the continent had a cool temperate climate. During glacial stages, as well as increased aridness, there were increases in wind speed, fewer cyclones in the north, more wind-blown sand and cooler air and oceans, and snow and ice in Tasmania and Mt Kosciusko.
The sea level rose in the interglacials and fell during the glacials, so that the amount of change depended on the severity of the stages of the glaciation. The cyclical onslaught of the aridity produced by the sequence of glacial periods gradually converted more of Australia to desert. Each glacial period adding more of the continent to the zone of permanent desert, after each dry period more of the land was unable to return to its previous state.
Fire became an increasing factor as the drying progressed. Looked at as a whole, over the period of the last 2.4 million years, it seems Australia has been becoming more arid, even from the beginning of the Ice Age 15 Ma. The drying process was lifted a notch with the arrival of the first Australians about 60,000 years ago and up a further notch with the arrival of the new Australians 200 years ago. Looks like its on the way to being the biggest desert island in the world.
It was during the last 1.6 million years the climate patterns and variability of Australia gradually became established to what they are today.
Over large areas of southern and inland Australia are deeply weathered profiles of Early Pliocene age, acidic and often cemented with iron to form duricrust, that are often overlain by sediments rich in calcium and alkaline soils. Wind transport is the agency for deposition of these fine-grained deposits.
Sedimentation had been occurring in shallow lakes in Central Australia throughout the Miocene and the Early Pliocene. The Etadunna Formation and Namba formation, among others, are examples of these sediments. These lake sediments are often capped by clay layers. The arid, windy times when the lakes were dry are evidenced by the fine wind-blown material that was blowing, deflating, from the dry lake beds, the continual evaporation of the water concentrating their mineral continent, the water becoming progressively more saline and gypsum-rich.
The sedimentation regime changes in the Upper Tirari Formation where a gypsum-cemented layer occurs above the Etadunna formation. The start of desertification is marked by deposition of large scale aeolian deposits and the development of alkaline soils in the Late Pliocene. It appears 2.4 Ma was also the time when this large-scale change occurred.
The effects of sea level fluctuations that occurred as the glacials and interglacials waxed and waned had a big effect on the Australian coastline. At times of high sea level large areas of the continental margins were covered and marine deposits laid down. One result of these high sea levels, during interglacials, is the presence of dune strand lines and shell beds, in places far inland from the present coast. The continental shelves were exposed in many places at times of low sea level, glacials. At these drier times coastal rivers cut deep channels through the deposits laid down during the times of high sea level. These drowned river valleys form the coastline of present-day Australia.
During glacial times when the continental shelf was exposed the sand, that was the ocean floor, was blown inland in the windy, dry conditions. The sand formed dunes that were often blown into sand sheets. This process formed the Western Australian coastal strip, the onshore section of the Perth Basin. This coastal belt has been relatively stable. Events occurring in the Late Pliocene to Pleistocene have mostly been preserved. Near Perth, in the central section of the basin, marine sequences from the Pliocene to the Quaternary, there are deposits parallel to the present coastline, bounded to the east by the Darling Fault. This fault separates the Perth Basin from the Yilgarn Block, the edges of which form the Darling Scarp. These marine sequences increase in elevation and age towards the scarp, representing different stages in the Pleistocene. During the Pleistocene, uplift of the Basin and the Yilgarn Plateau occurred, raising the various deposits to higher elevations, and in the same period, the crustal shelf margin was subsiding.
A complex of sediments deposited at ancient shorelines, the Yoganup Formation and Ascot Formation, and the Bassendean Sand, on the inner shelves, and dunes. The Pliocene-Pleistocene units comprising the Ascot Formation, are sub-surface, forming one of the water-mound aquifers that Perth depends on for its water supply. These are quartzite sands.
The plant fossil record of the Late Pliocene and Early Pleistocene was fragmentary. At this time there was rapid change. The sorting of the flora occurring as the area covered by forests decreased and it was being replaced by Sclerophyll forests, which were becoming widespread and aridity was tightening its grip on central Australia. The Ice Age began to make its presence felt in Tasmania, with large areas of glaciation occurring during the Ross Glaciation of the Late Pliocene and Early Pleistocene. Though the exact time of the beginning of this glaciation is unknown it it thought it may coincide with the sharp drop in global temperature 900,000 BP. The effects were also affecting Africa, and are believed to have contributed to the disappearance of robust Australopithecines and the spread of Homo erectus into Eurasia.
As a result of the continuation of the cooling trend, by 700,000 BP the polar ice caps had reached a new maximum. The Linda Glacial Stage in Tasmania has been dated to this time, about 730,000 BP. The deposits from this glacial stage are extremely weathered and overlain by those from later glaciations, so they are not well known. At Regatta Point on Macquarie Harbour, western Tasmania, macrofossils and pollen have been found that indicate a Late Pliocene to Pleistocene age for the deposit.
The rainforest type community was present, but it differed from the one that covered the area in the Oligocene at Pioneer. There was lower density and smaller leaf size in the younger rainforest, as well as lower diversity. In this rainforest there were Nothofagus cunninghamii, Eucryphia (leatherwood), Atherosperma mochatum (sassafras), Quintinis (possum wood), (now extinct in Tasmania), Acacia, Lagarostrobos franklinii (Huon pine) and Athrotaxis seleginiodes (King Billy pine) and A cupressoides (pencil pine), Podocarpus, Tasmannia (WINTERACEAE) and the tree-fern Dicksomnia antarctica. There was also a sclerophyll community, similar to the one that is there at the present, composed of Eucalyptus, Casuarina, Banksia, epacrids and tea trees.
The list of temperate rainforest plants from the Pliocene-Pleistocene differs very little from the species comprising the modern beech forests of Tasmania. In exception is Quintinia, which is now apparently extinct, though it remained a part of the Tasmanian rainforest at this location much longer than it did in the rainforests of the southern parts of the mainland.
Daisies and grasses, as well as Acacia and Casuarina, are prominent members of the vegetation in the Latrobe Valley, Victoria, from the Pliocene-Pleistocene transition. This has been shown from pollen analysis. It is thought to indicate cool or dry conditions, or both.
Generally, by the end of the Tertiary Period rainforest had contracted to the eastern edge of the continent, close to their present range. As the climate fluctuated during the Quaternary, the boundaries of the rainforest also fluctuated, expanding and contracting as the conditions became wetter then drier again.
Late Pliocene to Pleistocene fauna from the Murray-Darling Basin, when part of the region was occupied by Lake Bungunnia, is shown from fossils in the area. Among the fauna were dasyurids (native cats), wombats, potoroos, wallabies, kangaroos and rodents (placentals). There were 2 groups of rodents, one of which now occurs in arid regions. The other is a different kind that is only distantly related and is now extinct. Among the marsupials the hare wallaby (Lagorchestes), survives at the present in arid parts of the country. This indicates that the fauna from the Pliocene-Pleistocene were already arid-adapted.
In younger deposits in the same area there are fish, turtles, Thylacoleo (marsupial lion), and macropods (wallabies and kangaroos) of uncertain types, and diprotodon. To the north, in the Menindee Lakes area there were Euowenia (a protodontid) and the giant kangaroo (Protemnodon). The fossils here were found in bore holes. A lungfish tooth was found at Lake Tandou.
This was the last known lungfish from southern Australia. They are now only found in the Mary River and Burnett River in Queensland. They seem to have disappeared from the southern parts of Australia after the drying out of Lake Bungunnia about 700 000 BP, long before the last major phase of aridity from 18000-15000 BP
The Kanunka Fauna from the Late Pliocene-Pleistocene in stream channel deposits, the Katipiri Sands, in the Lake Eyre Basin, display a diverse assemblage of animals - lungfish, chelid turtles, crocodiles, cormorants, flamingos, other water birds, hare wallabies and nail-tailed wallabies. There were large diprotodontids, Diprotodon and Zygomaturus, the giant goanna Megalania. These sediments were palaeodrainage channels of the Cooper Creek and Warburton River. They merge into wind-blown dune sediments of the latest Pleistocene and Holocene. Fossils from the last glacial maximum occur in these sediments.
Among the fossils found in the Kanunka Fauna were 2 types of flamingo, one of which is now present in Africa, and another that is widespread in tropical and temperate regions. The reason for the local extinction of the now African flamingos is unclear. A suggested reason might be competition from more adaptable filter-feeding ducks that can feed in varying water conditions, and possibly from other water birds. In the later stages of the Pleistocene the water levels were fluctuating as the shallow lakes repeatedly dried out would have made it a difficult time for specialist feeders that couldn't adapt to the varying conditions. Those specifically adapted to a specific niche would have needed to adapt rapidly to other niches or die out. There must have been large amounts of shallow water in Australia between the Miocene and the later Pleistocene to support flamingos.
Among the species of the Malkuni Fauna, and from the Katapuri Sands, from the lower Cooper, were lungfish, crayfish, turtles, crocodiles. There was a diverse bird fauna - grebes, pelicans, cormorants, rails, herons, a crane, ducks and spoonbills. The mammals included Tasmanian devils (Sarcophilus) brushtail possums and bettongs. Among the mammals were extinct forms such as Sthenurus (browsing kangaroos), Phasocolonus (a giant wombat), Protemnodon (a giant kangaroo), Procoptodon (giant browsing kangaroo), 2 diprotodontids, Diprotodon and Zygomaturus. There must have been water present continuously prior to the drying because of the presence of lungfish and crocodiles, they became locally extinct as soon as the drying began.
After the Greening, The Browning of Australia, Mary E. White, Kangaroo Press, 1994
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|