Australia: The Land Where Time Began

A biography of the Australian continent 

Rising Seas Beginning About 18,000 BP

At the close of the glacial era the climate warmed and the seas began rising as the ice sheets and glaciers melted. At this time the climate of Australia began to be dominated by tropical influences, with the monsoons intensifying and ocean currents being stimulated by warm trade winds resulting in warmer temperatures and increased rainfall. Deglaciation began about 18,000 years ago and the ice disappeared rapidly, being virtually complete by 15,000 years ago and by 10,000 years ago scrublands had replaced forests. By 14,000 years ago the monsoon had returned, peaking about 8,000-6,000 years ago resulting in high rainfall and summers that promoted the growth of sclerophyll forests and rainforests across the northern, eastern and southern parts of the continent. In the Kimberleys tropical grasslands and rich woodlands returned, and by 11,000 BP boab trees were growing there. Coastal rivers in Arnhem Land became estuaries and the shores were choked by mangroves which reached their maximum extent about 6,000 BP.

At this time the northern rivers began to flow again and as the glacial meltwaters declined the southern rivers began slowing. beginning about 14,000 BP there was an increase in the southwesterly winds, though they declined again by 11,000-7,500 BP, which brought warmer, wetter conditions across most of southeastern Australia, lake levels increased. Herb fields, shrubland and grasslands were replaced by Eucalyptus and Casuarina across temperate Australia, with open woodland returning to the Murray-Darling Basin. In western Tasmania rainforests grew and there was an increase in fire everywhere.

There were fluctuating conditions in the deserts with temperatures rising a few degrees above those of the present about 9,000 BP. By 8,000 BP the climate had stabilised, reaching an optimum where it remained for the following 3,000 years. At this time the watertable was replenished and woodland and grassland communities were re-established. Along the coast of the Nullarbor trees grew, and inland lakes began filling again with the advent of increased monsoonal flows, more rain in winter and a higher watertable. About 12,000 BP Lake Eyre was a shallow lake and Paraku (Lake Gregory) was semi-permanent from 14,000 BP to about 6,000 BP. Over this time period the great deserts that had existed during the glacial era had their size reduced by about half.

In the later part of the Holocene, which was distinct, the climate changed again, the weather becoming unstable as the climate dried once again which resulted in a decrease of the summer rain, increasing winter rain, and after 5,000 BP, temperatures that were decreasing. The Australian continent became dry more often at this time of climate variability during which there were increased floods and droughts.

Lake levels rose and fell in southeastern Australia. From 6,000 BP to 5,000 BP southeast Australia was substantially drier, lake levels being lower than at present. For a time the climate became warmer and wetter before drying more severely between 3,500 BP to 1,000 BP, then in the more recent past it became warmer and wetter again. Burning across southeastern Australia increased generally, though after 6,000 BP fires decreased across southern Tasmania.

As monsoons weakened and there was increased rainfall variability the desert became drier. Across the north the estuaries were choked with sediment leading to them becoming the extensive floodplains of the present. Most of the desert lakes became ephemeral, dunes were mobilised and woodlands and tall shrubs changed to more open scrub. Over the next 2,000 years the grasslands recovered and the vegetation of the present had become established by 1,500 BP, a desert that was subject to increasing seasonality and variability.

The rising sea level was the most immediate result of climate change across the Holocene that continued for 13,000 years. The magnitude of flooding was incremental with the rate of rise being very variable and overall the sea rose at a rate of 1 cm/year. This relentless flood level totaled about 1 m over the lifetime of a person, which meant that many kilometres of flat coastal plain was lost with every generation. The sea level rise did not occur in regular increments, the rising water moving imperceptibly slowly at times while at other times it rose disconcertingly fast. There were also times when great swathes of land was lost as people watched, and at other rimes it rose so slowly that the flood might have appeared to have stopped.

At the beginning of the sea level rise was quite slow, then becoming more rapid and spasmodic, rising at a rate of 1 m/100 years between 19,000 BP and 14,600 BP, after which the rate increased to 5 m/100 years for the next 300 years, at which time it was about 80 m below the current level. The rise then slowed until  13,000 BP, by that time reaching 60-64 m below the present level. By 10,000 BP it had risen by another 20 m and was continuing to rise, by 7,000 BP it had reached about 1 m above the present level. The rate of rise then oscillated significantly with greater variability around the continent and by 6,500 BP it reached the level of the present. It then rose about 1 m about 3,600 BP, after which it fell to the present level by 1,500 BP.

    By the time the rise of the sea was complete about 25 million km2, or about 25 % of the area of Greater Australia had been submerged over a period of 13,000 years, the loss averaging more than 190 km2 of land per year. Across the northern coastline of the continent at least 1.6 million km2 was submerged. Between 8,500 BP and 6,500 BP the land bridge beneath Torres Strait was submerged, leaving New Guinea a somewhat foreign island, as the sea had invaded Lake Carpentaria by 12,000 BP, turning the lake into a salty embayment over the following 2,000 years. Tasmania was isolated about 10,000 BP when the Bassian Plain was submerged. By 9,000 BP the extensive coastal plain seaward of the Pilbara had been submerged, the sea flooding across the Nullarbor Plain up to the base of the escarpment, the Nullarbor Cliffs of the present, by 11,500 BP. By 9,500 BP Kangaroo Island had been separated from the mainland, and between 12,000 BP and 7,000 BP the present islands along the coast of Western Australia were formed as the surrounding land was flooded.

Sources & Further reading

  1. Cane, Scott, 2013, First Footprints: The epic story of the first Australians, Allen & Unwin

Scott Cane has included in his book, written as a companion to the ABC TV series of the same name, a number of stories from his days living among Aboriginal people in the desert and moving around with them.

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  13/12/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading