Australia: The Land Where Time Began
Ice Age Biotas - Middle Pleistocene to start of the Last Glacial cycle
Ice Ages occur about every 300 million years, the latest one beginning in the Quaternary - the Pleistocene and the Holocene epochs. The conditions the biota have experienced since the beginning of the last ice age haven't been experienced since the previous ice age between the Late Carboniferous and the Permian. This Ice Age has drastically altered the landscape of Australia and moulded the flora and fauna to its present unique state. It produced the largest arid zone within the borders of a single country, the total desert areas being as large of those of North Africa and the Middle East combined. It wasn't a continuous process, but a series of stops and starts, as the interglacials interrupted the glacials for short periods. It was this alternation of wet and dry phases that fine tuned the flora and fauna to survive in this most extreme of continents.
Seasonal aridity and widespread droughts were well-established in central Australia by early in the Pleistocene. Evidence for this long-lasting regime of aridity has been found in the form of pollen of dry land plants like grass, daisy chenopod, and Casuarina. In the less arid parts eucalypt-dominated communities occupied seasonally dry areas. Semi-deciduous rainforest and non-eucalypt savanna occupied the tropical north, including the Arafura Plain that connected Australia to New Guinea. Parts of the eastern ranges, from New Guinea to Tasmania, were the only places were there was closed forest.
The climate fluctuations from 700 000 BP put great selection pressure on the flora and fauna. The more specialised plants and animals were put under even more pressure than the less specialised one. There were many extinctions during the Quaternary. The less-specialised species took advantage of the changing ecosystems to spread and adapt to occupy new niches.
With each glacial phase complete ecosystems and many local populations disappeared, and when better times returned the vacated spaces were repopulated from the refugia areas. During the difficult times of the glacial periods the species that survived in the refugia were isolated from the other populations of their species in refugia in other parts of their former range. This separation into smaller populations with no possibility of gene flow between the isolated populations encouraged speciation. By the time the climate changed and allowed the spread from the refugia many populations would have changed genetically enough to form new species. Many of these new species would have been able to take advantage of the niches that were were opening up.
Acacia is a good example of what could be achieved by an adaptable genus when the conditions are changing so much over a long period. A few species of Acacia has grown into 640 species during the Pleistocene. There was also the development of disjunct populations, when species could not repopulate their original range.
The value of refugia has been clearly demonstrated during the difficult Pleistocene climatic swings. Refugia remain very important in present-day Australia, with continuing seasonal swings from drought to flood and with large changes made to the landscape by human activity. With no areas where the flora and fauna of a particular habitat can survive during hard times the biodiversity would suffer as only those species that could adapt rapidly enough to the changed conditions would survive. In the past there must have been refugia in all the major vegetation provinces for the flora and fauna to have continued through the fluctuating climate of the Pleistocene.
The existence of migration routes would also help vulnerable species to survive hard times, allowing them to escape the worst of the bad conditions. Valleys containing alluvial soils provided a path for species of animals associated with gallery forests, better soils, swamps and permanent water to spread to other areas. The Darling River system connects well-watered areas in the northeast across dry inland stretches to the southern edge of the continent. This probably formed a corridor for migration during the Quaternary. Western Australia didn't have any migration connections between its wetter areas. In the Miocene the West Australian inland rivers stopped flowing and especially the southwestern region, was isolated and developed a unique flora, many of which are endemics.
700 000 BP a meteorite crashed into the side of Mt Darwin, south of Queenstown, in the wast of the Tasmania. It was smaller than the much larger bolide that landed in Kampuchea at the same time. The resulting Darwin Crater is about 200 m deep and 1000 m wide. It eventually formed a lake that accumulated sediments which ultimately filled the lake completely, so that the area looks like a part of the natural landscape. These sediments proved very useful in finding the vegetation of the area over the time the sediments were forming.
The strewnfield of Darwin glass that formed as a result of the impact became the centre of trade in Darwin glass throughout Tasmania, as it was prized for making stone implements valued by the Pleistocene and later hunter gatherers. The Darwin glass is spread over an area of about 400 km2.
The Tasmanian high country was glaciated from a time prior to the Linda Glaciation of 700 000 BP to the end of the last glacial cycle. Dating of the glacial stages is uncertain. The Henry Glacial Stage has been dated to about 200,000 BP. The Governor Glacial Stage predates the Henry. Within these glacial stages the glaciers advanced and retreated many times.
Sometime after 700,000 BP windy, arid glacial stages there was amplification of climatic fluctuations, resulting in the start of formation of dune and sandsheets, creating widespread and extensive sand ridge and sandplain deserts. Dating of the sediments over this period is difficult because of the repeated reworking of sand dunes and wind-blown sediments.
The dating of the later dunefileds from the latest glacial cycle are better known because they haven't been disturbed by being redistributed or covered during later glacial cycles.
The Woorinen Beds, an east-west linear dune sequence beneath the dunes produced in the Mallee during the last glacial stage, in the Lake Bungunnia area, deposited later than the Blanchtown Clay Formation of the lake, shows evidence for the beginning of dune formation. These beds correlate with the saline beds of the Tyrell Formation, which show the beginning of arid salt lake conditions at Lake Tyrell. The Woorinen Beds show 4 episodes of dune formation. They are thought to date from about 500 000 BP. They were believed to be the oldest dunes so far detected, but other older dunes have now been found.
At Lake Amadeus in Central Australia, saline and aeolian deposits date from about 900 000 BP, about 400 000 years before the deposition of the Lake Tyrell beds. It now seems that parts of Central Australia were probably experiencing typical arid zone conditions that led to the formation of salt lakes, gypsum deposits and wind blown dunes for as much as 1 million years. The evidence for this date of arid conditions was found in drill cores from Auger Island in Lake Amadeus.
TL dating of floodplain sediments from the Channel Country has found vast sandsheets indicating much higher river activity during the interglacials from 250 000 BP to 200 000 BP and from 120 000 BP to 100 000 BP. The phases of increased sand deposition are believed to indicate that they occurred during floodout in semi-arid conditions similar to those present today, when the rivers only flow at times of heavy rain upstream in the wetter areas. Mud layers above the sandsheet deposits indicate much drier times from 85 000 BP onwards. The fact that the rivers no longer carry a heavy sand load suggests that the present interglacial is not as humid as previous interglacials.
It now seems likely that large areas of Australia have been arid, with wind blown sand dunes and sandsheets forming during the heights of the arid glacial stages. So it seems the biota of the majority of Australia have probably been drought-adapted for a very long time, and most likely becoming even more drought-resistant with each glacial phase. Of every 100 000 years of the glacial stage, climate regimes prevailed that brought arid conditions, with seasonal aridity and frequent droughts, and very likely increased fires, for 70 000 years, the brief interglacials would have been too short for the biota to lose its drought hardiness before the next drying stage.
Pollen in cores from Lake George sediments show the vegetation changes on the southern tablelands near Canberra between 700 000 and 130 000 BP. For the last 350 000 years of this period the record is very detailed. The vegetation of 2 interglacials, from 347 000-297 000 BP and 251 000-195 000 BP, were dominated by CASUARINACEAE, with some cool-temperate rainforest species. In the intervening glacials. many herbaceous groups, grasses and daisies predominated with a sub-alpine component as a minor component.
During the glacials temperatures were 8-10o C cooler than present, with substantially reduced rainfall. At about 130 000 BP the vegetation pattern suddenly changed, there was a big increase in Eucalyptus pollen and a very large increase in the amount of carbon particles from fires. The frequency and severity of fires must have increased substantially. The scientists who carried out the Lake George project, Dr Singh and co-workers, suggested that this was evidence for the arrival for the arrival of firestick-wielding Aboriginals in the area. Their interpretation was not widely accepted in the scientific community, particularly as all the available evidence pointed to an arrival of the Aborigines at about 60 000 BP or a bit earlier. Since then new evidence from north Queensland sea-bed cores has found a large increase in carbon at 140 000 BP. The possibility of the arrival of the Aborigines has again been raised, especially as no other plausible explanation has been put forward.
The time of 140 000 BP for the Queensland increase would probably fit with the 120 000 BP of the Canberra area. Based on other evidence it seems the first colonists moved along the coasts and up river valleys before they spread out into the more arid areas. 20,000 years looks like a plausible time to move from north Queensland to Canberra.
The increased burning in the Late Pleistocene and Holocene, whatever its cause, had a huge effect on the vegetation, with a decrease in diversity of the flora, fire tolerance became a prerequisite for much of the Australian flora. The Casuarina-dominated fire-sensitive sclerophyll communities at Lake George, as well as the temperate rainforest patches, had survived the climate with infrequent lightning strikes that caused burning and the rate of litter accumulation. During glacials the carbon usually disappeared from cores, insufficient litter accumulated to sustain fires. The increase in fire frequency led to the replacement of fire-sensitive sclerophylls and cool temperate forests with fire-promoting eucalypts. Their flammability and their production of flammable litter allowed the maintaining of high fires frequencies even into the following glacial.
There were only minor fluctuations at Lake George between open forest and sclerophyll woodland from 130 000 to 75 000 BP. Casuarina and cool-temperate species made a small comeback during the last interglacial, they are now extinct regionally. Soil salinity increased due to hydrological changes under the less stable vegetation cover, possibly leading to the increase in chenopod saltbush and eucalypts.
Effects of low atmospheric CO2 levels in the glacial phases
The lower rainfall and cooler temperatures during glacial phases led to a drier, more open type of vegetation. Also affecting the vegetation at these times was the low CO2 levels in the atmosphere. It is believed the low atmospheric CO2 levels in glacial phases had a significant effect on the vegetation. Trees would have been affected because they require CO2 to produce the wood necessary to make their stems strong enough to allow them to grow tall, with the result that trees and shrubs were probably stunted compared with the height they would grow under higher atmospheric CO2 levels. Understorey plants would have been doubly disadvantaged, already being at a disadvantage because of the shading by taller shrubs and trees, and the lower CO2 concentrations would have affected them as well.
C3 plants, temperate grasses and most shrubs and trees, are affected more by low CO2 levels because this type of photosynthesis requires more CO2 than the C4 photosynthetic pathway. The result is that low CO2 levels would favour an open type of vegetation composed predominantly of C4 plants, even without any other effects of the environment in glacial periods (Ehleringer & Monson, 1993).
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|