Australia: The Land Where Time Began

A biography of the Australian continent 

Precambrian Ice Age - Snowball Earth - Marinoan Glaciation

Beginning about 750 million years ago, and lasting until about 580 Ma, the Cryogenian, a major glacial period appears to have affected much of the Earth. Some have called this period "Snowball Earth" (Paul E. Hoffman & Daniel P Schrag, of Harvard), a time when it seems the Earth may have come close to having the newly-emerged life snuffed out before it really got going. Some believe it probably resulted from the photosynthesis being carried out by microorganisms that was adding oxygen to the atmosphere, by removing carbon dioxide in the same process. Glaciers appear to have covered most of the Earth at this time, even in the tropics. The most severe phase, when the tropics may have frozen over, has been suggested to have lasted for about 10 million years. see Cryogenian

It has also been suggested that the glaciation may have been triggered by an impact event at Lake Acraman.

At Chambers Bluff in the Indulkana Range in the far north of South Australia, an outcrop of the glacial deposits can be seen in the form of rounded granite boulders that differ in colour from the other rocks in the area, and also differ in that they are rounded, as are rocks in river beds that have been worn smooth, whereas the surrounding rocks are  hard-edged and heat-shattered. The smooth, rounded rock is a dropstone, dropped from the base of an iceberg floating in the inland sea that covered much of Australia at the time. Another outcrop occurs at Tillite Gorge near Arkaroola. This outcrop is only a glimpse of the vast despots that lie beneath the surface, stretching from the Kimberley in the northwest of Western Australia to the Flinders Ranges in South Australia. At the northern end of the Flinders Ranges the glacial deposit reaches up to 5.5 km thick.

Like other, later, ice ages, the Precambrian one was not a single unbroken period of glaciation, but waxed and waned for about 400 million years before finally giving way to a warmer time.

There is some scattered evidence of an even earlier ice age at about 2.3 Ga.

Rocks from many parts of Australia provide evidence for glaciations prior to the Quaternary. The Gowanda and Sturtian glacial periods were 2 of a number of such periods that have been suggested to have occurred during the Precambrian, as indicated by evidence such as exotic blocks and striated pavements. At 680 Ma a glacial period is indicated by what has been interpreted as periglacial block fields and (infilled) ice wedges that have been exposed in the Mount Gunson copper mine to the north of Port Augusta, South Australia (Twidale & Campbell, 2005).

Through the history of the Earth cyclic perturbations of global carbon are often linked to changes of palaeogeography, glaciation, oxygenation of the oceans, as well as biological diversification. Between 635 and 542 Ma, in the Ediacaran, there was a pronounced excursion of carbonate carbon-isotope that was accompanied by invariant or decoupled organic carbon-isotope values, that according to the authors³ has been explained by a model relying on the ocean having a large reservoir of organic carbon. The authors present data of carbonate and organic carbon-isotope that indicates there was no decupling from about 820-760 Ma, and between the Sturtian and Marinoan glacial events of the Cryogenian Period, about 720-635 Ma. They suggest "Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep ocean conditions facilitated by an increase in the Fe:S ratio of the riverine flux after Sturtian glacial removal  of a long-lived continental regolith."³ 

Life survived Snowball Earth

Sources & Further reading

1. Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993
2. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd.
3. Cryogenian Glaciation and the Onset of Carbon-Isotope Decoupling

links

1. Hoffman, Paul F. & Schrag, Scientific American, January 2000, p 68