Australia: The Land Where Time Began
Pleistocene Ice Age
As they were gathered around the North Pole, the northern continents were the most severely affected by glaciation during the Pleistocene ice age. The southern continents, being collected together in Gondwana were most affected by the previous ice age in Late Carboniferous to the earliest Permian. During the Carboniferous-Permian ice age the South Pole was situated on Gondwana while the northern continents were separated from the North Pole by an expanse of ocean.
About 16,000 years ago the northern ice cap reached its maximum area and inn places was up to 3 km thick. At this time it is believed there were probably about 40 million cubic kilometres on land, covering Canada and down to New York and the Ohio River and along the Rockies and Cascades. In Europe it covered Scandinavia, spreading as far as Britain, and covered Germany and European Russia. Treeless plains surrounding the ice sheets to the south. Further south were Boreal forests of Birch, Pine and Spruce. The broadleaved forests had retreated to North Africa and the Near East. In America that had retreated to Florida and the southern edge of the Gulf States. In Siberia and North Alaska were dry corridors in the ice cap.
In America grew near the margins of some of the glaciers. The ice had spread further that the climate would have caused it to form because of the massive weight of the ice that squeezing it, causing it to spread out, spreading faster than it melted, in some cases into areas where the climate was warm enough to allow forests to grow. The large area of the northern ice cap of this time was the result of the spreading of the glaciers across the land rather then being formed in the warmer areas. The situation at the South Pole is different. The ice cap is centred on Antarctica, as glaciers spread out at its edges they enter deep water so are constantly trimmed by the wave action and melting as they enter warmer water.
The North Atlantic was an Arctic sea during this ice age, being frozen between Greenland and Britain, with sea ice joining Cape Hatteras with Spain. The warm Gulf Stream crossed the Atlantic directly from Florida to Spain. The North Pacific was less affected by the cold conditions, though as a result of the cooler equatorial currents seas that are now tropical would have had temperatures closer to those of the modern North Sea in summer.
By 11,000 years ago the climate of Europe and Britain had become mild once the ice melted and Birch and Pine woods replaced the tundra. about 10,500 years ago there was a sudden climate change over a period of about 100 years, tundra returned to Britain and Low Countries, and reindeer returned, for a period of about 700 years, after which the climate returned to mild. European summers were almost as warm as the present by about 9,000 years ago. England and Germany were again covered by Oak and Hazel. After about 4,000 years there was another little ice age, starting a more variable climate with mostly cooler and wetter conditions. Greenland and Antarctica have retained the iced age conditions.
Another "little ice age" has occurred in historical times, between 1650 and 1850, the badly affected parts of Europe.
Australia wasn't affected by glaciers as the northern continents had been, but as usual, when the northern continents freeze Australia dries. A variety of deposits were laid down in Australia during the quaternary. They include sediments deposited on the continental shelf, those spread in river deltas and floodplains, and others that are the deposits formed of the rock debris that accumulated as glaciers ground across the high country, and vast windblown sand dunes formed into ridges, all resulting from sediment accumulation in different ice age environments. As the sealevel fell and rose the continental shelves were covered and exposed, the high ground was glaciated, and arid phases alternated with pluvial phases.
During this Period Australia was tectonically quiet, only experiencing volcanic activity in 2 regions between the Pliocene and Holocene, in northern Queensland, where basalt flows covered 13,600 sq km, and in parts of Victoria and adjacent areas of South Australia where basalt flowed over 26,000 sq km. 4850 years ago the latest of the volcanic activity occurred in the Mt Gambier region in South Australia, and about 4500 years ago at Tower Hill in Victoria. At Tower Hill, dingo bones and an Aboriginal grinder have been found beneath volcanic rocks, so there is no doubt humans and dogs were present at the time of these eruptions.
On the continental shelf off the Eucla and Canning Basins marine deposits from this time have been found, and marine deposits were laid down in southern basins and the northern Carpentaria Basin at times of high sealevel. In Western Australia, mostly carbonate sediments were deposited on the Sahul Shelf on the ocean side of Bonaparte Basin. On the major platform area off the Queensland coast, carbonate deposition had been taking place since the Miocene, and continued in the Quaternary. The Great Barrier Reef started growing in the area about 8000 years ago, at a time when sealevels had stabilised. The adjacent Queensland Plateau had been the largest area in the world to support a reef before this time. The sealevel rose faster than the reef could grow, resulting in the death of the organisms from which the reef was composed. A similar fate could befall the Great Barrier Reef if sealevels rise too fast due to global warming, though the reef could be killed by bleaching before rising water could kill it.
Surface-type deposits of sand and soil covered the land, spread by rivers during the quaternary. The inward-draining system of the Lake Eyre Basin formed this type of deposit from the rivers draining its catchment. The mesh of rivulets, and sand and mud, of the modern Channel Country, alternating between the usual arid state and the floodplain when the rivers flooded, continue the regimes that spread sediment as occurred throughout the Quaternary. Arid phases in which the vegetation cover was decreased led to increased erosion rates in wetter times as there was less cover to protect the land surface. See Dunes
Glaciation of the Australian mainland was very limited during the Pleistocene. Around Mt Kosciusko in southern New South Wales about 50 km2 of the highest country was glaciated permanently throughout the last, most severe glacial phase. In New Guinea the high peaks of the mountains, though on the Equator, were glaciated. This suggests that on the mainland of Australia the reason there was no glaciation away from the high country was that the vast majority of the area of Australia is flat and featureless. There was snow and ice on other highland areas of southeast Australia, but it was apparently present only seasonally.
Tasmania was badly affected by what appears to be about 4 phases of major glaciation. It is believed that a single ice cap covered about 4,000 sq km of the northwestern part of the central plateau, as well as other smaller ice caps and valley glaciers. In Tasmania, deep lakes exist in glacial valleys. The passage of glaciers, and the cycle of advance and retreat of glaciers, is seen in the erosion features and moraines that remain.
The fluctuations of the Tasmanian glacial and interglacial phases in indicated by the pollen record. During the glacial phases Beech forests were replaced by alpine grassland. The main components were grasses and Asteraceae (Daisy Family), with some Plantago, Gentieanella and Oreomyrrhis (Umbelliferae) usually present. There were alpine and subalpine heath in some areas, included in these were dwarf conifers Diselma (Cupressaceae), and Microstrobus and Micochrys (Podocarpaceae), and there were many Epacridaceae. In interglacial phases the forests returned rapidly.
During the Quaternary there were often dramatic sea level fluctuations. In the Pleistocene falls of as much as 200 m are thought to have occurred in some parts of the world, and vast areas of continental shelf were often exposed at time of low sea level. Over the past few million years mainland Australia has been connected to Tasmania and New Guinea by exposed shelf more than they have been separated by sea over the flooded shelf. At times of low sea level the connection to New Guinea via Torres Strait didn't seem to pose a problem for migration. The distance between the mainland and Tasmania is greater and deeper than that across Torres Strait, so is more of an obstacle to migration when the continental shelf is flooded. During low sea level times the Bassian landbridge was covered by grasslands and areas of scrub heath. The Tasmanian Aboriginal People simply walked to Tasmania during a glacial phase, but were trapped there when sea level rose during the last, present, interglacial.
Australian coastlines have been shaped by sea level fluctuations. High sea level stands are indicated by terraces, and drowned valley systems are the result of rivers cutting though the bedrock at times of lower sea level and later being flooded as the sea level rises. Sydney Harbour is an example of a coastal feature that resulted from this type of evolution.
It has been found that at Broken Bay the lowest point which coincides with the maximum extent of the ice cap and the lowest level of the sea indicate that at this time the sea level was 125 m lower than the present. That is the level to which the deepest channel has cut in the bedrock, the Hawkesbury River had to trave much further to reach the sea at that time. Studies have found that the coastline from that time had deep irregular embayments that don't exist at the present.
It was found that at times of maximum ice cap coverage, so minimum sea level, there was a coastal plain of up to 20 km wide between the present coast and the sea, that would have gradually emerged and become vegetated as the ice caps grew. At times of low sea level there would have been coastal strips of varying widths around the entire continent. These coastal plains would have been covered by dunes, lagoons, river deltas with areas of coastal scrub, heathland and grassland on sandy soil. The same applied to all other continents.
During the Quaternary the pollen record shows an essentially present-day flora with changes in composition. The pollen record of the preceding Tertiary documents the change to the modern-type flora. The Pleistocene megafauna of the time, on the otherhand, were still largely different from that at present. The megafauna included giant Kangaroos, Wallabies and Wombats as well as forms that didn't leave descendants such as Diprotodon, giant Running Birds and a giant Python.
Quaternary pollen assemblages indicate some plant communities that were similar to some of the present, while others show communities that no longer exist. It is hard to be sure how much influence the climate has had on the selection of fire-tolerant floras because of the fire-stick farming of the Aboriginal People that has been present from some time after their arrival about 60,000 years ago, which has undoubtedly had an enormous effect on the composition of Australian floras.
Evidence from Kin Valley, West Coast Ranges, Tasmania
The King Valley in the West Coast Ranges of Tasmania has been found to contain evidence of 4 periods of glaciation and 2 interglacial phases, based on evidence from geomorphology, geology and Palynology. The earliest deposits in the valley are believed to predate the start of glaciation, probably dating from the Late Tertiary. Overlying this are the sediments deposited during the Linda Glaciation. Intense chemical weathering occurred during this period, the deposits is believed to date from before 730,000 BP. These highly weathered till deposits are overlain by deposits from the Regency Interglacial phase during which montane scrub rainforest gave way to lowland rainforest. This was followed by the Moore Glaciation during which the King Glacier advanced, as well as glaciers from the West Coast Range. An interstade that occurred during this glaciation is indicated by a fluvial deposit containing pollen that are believed to have formed between 730,000 BP and 390,000 BP. The next glaciation was the Henty Glaciation that probably occurred prior to 130,000 BP. During this period there were a number of advances of the King Glacier, and during an interstitial period a large lake formed. Following this was the Pieman Interglacial Phase during which organic fine sands and silts formed deposits that indicate a lowland scrub rainforest. In small areas, that were restricted to the northern section of the valley, the final deposits formed, the Margaret Glaciation, older deposits of which suggest a possible early-last glaciation ice advance. The most recent ice advance culminated after about 19,000 BP. The evidence from the King Valley provides some of the most complete records of glaciation in the Southern Hemisphere. When added to evidence from Westland, New Zealand and South America, the King Valley evidence suggests the middle latitude areas were probably glaciated simultaneously during the last glaciation (2).
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|