Fossilised iceblocks shed light on early life

The discovery of blocks of gravel which sank to the bottom of the sea trapped in ancient icebergs has sparked a new understanding of a bizarre group of creatures.

The research published today in the Australian Journal of Earth Sciences, has also forced a rethink of the conditions that existed more than 500 million years ago.

Associate Professor Victor Gostin and colleagues at the University of Adelaide found evidence of ancient icebergs mixed in with volcanic rocks which were spewed out when an asteroid hit the earth between 635-542 million years ago.

The 4.7-kilometre asteroid left a 90-kilometre crater in what is now Lake Acraman in the Gawler Ranges of South Australia.

Gostin, who first discovered rocky traces of the asteroid 'ejecta' almost 30 years ago, says the asteroid impact occurred during a period of extreme cold.

This contradicts previous ideas which suggest the impact of the asteroid actually precipitated a period of glaciations, Gostin says.

He says evidence of the ancient icebergs is embedded in the fine grained shale found several hundred km from Lake Acraman.

"Icebergs carry coarse debris, boulders and grains," says Gostin. "As the iceberg melts it is dragged to the bottom by the weight of the debris, then the ice melts and the surrounding mud eventually buries the debris, creating this sort of fossilised iceblock.

"So when you find coarse boulders embedded in fine deposits, something must have rafted it there - in this case, an iceberg."

'Double whammy'

In a commentary to be published in the The Australian Geologist, Professor Malcolm Walter of the University of New South Wales says the research resolves issues around the timing and record of the glaciations, the impact and the rise of the Ediacaran biota.

He describes the effect of the icy climate and asteroid impact as a "double whammy" that paradoxically led to the "flourishing of exotic plankton and the first macroscopic animals".

"Did the Acraman impact and coincidentally coeval glaciation help trigger one of the great biological revolutions in earth history? Time will tell," writes Walter.

The Ediacaran fauna, which were the first large complex life forms to appear. They resembled do jellyfish, sponges, marine worms, plus other organisms whose body plan resembles nothing alive today.

Gostin is keen to explore other glacial deposits that formed around the same time in other parts of the world to see if there is further evidence of the asteroid's impact on the microscopic creatures at that time.