Australia: The Land Where Time Began

A biography of the Australian continent 

Climate cycles in a Cooling Australia

The global climate has been dominated over about the last 2 million years by glacial cycles. Glacial cycles occur in the northern hemisphere, as glaciers expanded and contracted, in the Southern Hemisphere the glacial phases are more cool, dry and windy than glacial. In Australia glaciation only occurs in the high country in the south east of the mainland and in the high country of Tasmania. About 700,000 years ago a long term trend of climate cooling stopped. After about 700,000 years ago the temperature cycle range increased due to lower temperatures at glacial maxima. After 450,000 years ago there was another change, this time due to warmer interglacials. After about 700,000 years ago cycle lengths increased in which cold phases increased in length from about 40,000 years to about 100,000 years. The last 4 cycles have formed a very constant pattern. The temperature drops abruptly at the start of each cycle after which the temperature fluctuated for about 100,000 years, temperatures dropping gradually, the cycle ending in an extreme cold phase near the end of the cycle, followed by an abrupt increase in temperature. The warm interglacials usually lasted only about 4,000 years, meaning the present warm phase, that has lasted 10,000 years, is well past the time when it should have reverted to a glacial phase.

Between about 105,000 years ago and about 75,000 years ago, within the last glacial cycle, the temperatures were moderate. At about 70,000 years ago there was a cool phase, then between about 60,000-40,000 years ago there was another warm phase with moderate temperatures. Beginning about 35,000 years ago, the last glacial maximum (LGM) was the coldest part of the glacial phase, reaching its most extreme low temperatures a bit before 20,000 years ago, after which it ended about 18,000 years ago. From the interglacial to the LGM, in southern Australia the temperature change was about 10o C (Miller et al., 1997; Petit et al., 1999), but change was less in the equatorial regions (Barrows & Juggins, 2005).

Through the various phases of the climate after about 2 Ma the atmospheric CO2 levels have changed with the temperature, being high in interglacials and low in interglacials. The causes for the changing CO2 levels, though varying dramatically from peak levels in glacial phases, dropping by more than 35 % at the LGM,  is not well known (Francois, 2004). After about 430,000 years ago both temperature cycles and CO2 cycles increased in magnitude (Sieghenthaler et al., 2005).

During glacial cycles rainfall declines as a result of lower temperatures of the atmosphere and the sea surface. It was under these conditions that cool deserts formed over vast areas of the inland, and these arid conditions expanded towards the coasts from central regions (Nanson et al., 1992). This view is based on the evidence of sand dune activity over big areas of the inland between about 30,000 and 20,000 years ago (Munyikwa, 2005). During this very dry period the dunes reached as far as the Blue Mountains to the west of Sydney. Marine sediments to the southeast and northwest of the continent received large amounts of blowing dust from mainland Australia at this time. The large amounts of dust that accumulated in these sediments indicate that such high rates of dust export usually occurs over relatively short periods near the end of each glacial cycle. The dry phase near the end of the second last glacial phase differs, indicating an unusually long cold phase. The million-year E39.75 sediment core from the Tasman Sea show low, constant dust concentrations before an increase at 380,000 BP. Hesse (1994) suggested that this 'marks the transition to true aridity' in Australia (Johnson, 2006).

Between about 70,000 to 50,000 years ago an episode of aridity coincided with a drop of temperature of the atmosphere. Between 50,000 and 35,000 BP the climate is believed to have been mild and moist, it was a time when some of the large inland lakes, in particular Lake Eyre was permanently full for some time (Munyikwa, 2005). Sparse vegetation over much of inland Australia is indicated by the high level of dust export and the high rate of dune building that occurred between 30,000 and 20,000 BP.

Overall, aridity increased across wide areas towards the end of the glacial cycle, but the availability of water was different in southern and northern Australia (Hesse et al., 2004; Kershaw, 1995). One of the results of the dry phase was the failure of the summer monsoon, and this led to widespread drying of lakes and rivers across northern Australia that depended on the monsoon for their water. The other source of water for them is from the occasional cyclone that crosses the west, north or east Australian coast to become a rain depression that dumps large amounts of water on the affected areas. This lack dry season occurs annually in non-glacial times, but the dry season became unbroken until the end of the dry phase. Lake Eyre gets all its water from long, slow, widespread river systems that bring the water all the way from northwestern Queensland where the headwaters of the rivers are situated. The huge area that water spreads as it travels slowly across the vast, gently sloping  flood plains takes months to reach the lake, evaporating and infiltrating the dry sandy soil all along the way, so it requires very heavy, persisting rain to produce enough water to reach as far as Lake Eyre. This makes Lake Eyre an indicator of the strength of the monsoon.

In contrast, there was actually more available water in southern Australia during the LGM than before or since. During the LGM, Lake George, which is north of Canberra, held much more water than in historic times. Also during the LGM, it is believed that up to 4 times the present volume of water was flowing into Murrumbidgee River system (Hesse et al., 2004).

There are several factors involved that can explain why the southern parts of the country had more water during the LGM. The presence of glaciers in the high country meant that summer meltwater from ice and snow was available to the rivers. It is believed this stabilised water levels in the river systems, such as the Murrumbidgee, and also into some arid country lakes. About 60,000 BP,  glaciers on the Kosciusko massif expanded, and this coincided with increased water levels on Lake Mungo. At about 30,000 BP, a rise in the Lake Mungo water levels coincided with advancing glaciers in the Kosciusko massif (Barrows et al., 2001; Bowler et al., 2003). After a brief period, Lake Mungo levels dropped but the lake didn't completely dry out until about 20,000 BP (Bowler, 1998; Bowler et al., 2003).

Another reason for the presence of more water in the south of the continent is that lower temperatures would lead to low evaporation rates, any rainfall recharging groundwater more effectively, leading to water entering river systems. In the caves of South Australia, high calcite deposition rates during the LGM could be explained by this increased infiltration of rainwater (Ayliffe et al., 1998).

As well as lower evaporation rates, it is believed the rainfall regime at this time was reliable, the type of conditions that would lead to the formation of wetlands that were dry before and after the LGM. In valleys in the Flinders Ranges of LGM age, fine-textured, well-stratified sediments contain the remains of snails, diatoms and swamp grasses. This indicates perennial wetlands and slow flowing streams in places where at present flash floods following heavy rain scoured the streams reducing the water retention of the system.

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Sources & Further reading

  1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
Author: M. H. Monroe
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Last Updated 21/03/2010

 

 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading