Sinian World

Geoscientists in Scotland say they have evidence to disprove the controversial "Snowball Earth" theory - the idea that the planet was completely encased in ice just over 600 million years ago.

The team, from the University of St Andrews, has published its findings in the scientific journal Geology, after studying rocks in the west of Scotland, Ireland, Namibia and California.

Drs Dan Condon, Tony Prave and Doug Benn say they have found evidence of sedimentary material, which could only have been derived from floating ice on open oceanic waters.

This, they believe, indicates that the Earth's oceans could not have been frozen during the snowball years.

Life 'explosion'

The controversial theory that for millions of years the Earth was entirely smothered in ice, up to one kilometre thick, has been kicked around for more than 50 years.

It rests on the apparently anomalous evidence of past glaciation in places that should have been much too hot - very near the equator. Even during the most severe ice age, many scientists thought that the ice only reached as far down as Northern Europe and the middle of the USA.

"Snowball Earth" supporters claim that the supposed freeze would have caused severe environmental stresses upon early life.

This would have resulted in repeated mass extinctions followed by an "explosion" of more complex (multi-cellular) lifeforms on Earth once the thaw came.

But Drs Condon, Prave and Benn say their work casts severe doubt on the Snowball Earth idea.

Floating ice

The team examined a suite of rocks called Port Askaig Tillite, south west of Oban in Scotland and directly north of Jura, which is supposed to record Snowball Earth glaciations.

However, they found that the rocks similarly contained evidence that the Earth's oceans remained unfrozen during the Snowball period.

Dr Prave said: "What is interesting about the Port Askaig Tillite, is that it was the first rock unit ever described (in 1871) to be attributed to these ancient glaciations, thus the roots of the Snowball Earth model are firmly grounded on Scottish soil."

Dr Condon said: "We are claiming that in reality there was not a totally frozen snowball Earth and that even during the coldest conditions large regions remained ice-free.

"If the Earth's oceans were not totally covered in thick sea-ice, there would have been areas of open seas which would have offered refuge for early marine life forms.

Different approach

"This means that there would have been places in which marine micro-organisms lived and survived during the glaciation."

Dr Condon said that others had approached the debate with a geochemical argument by examining isotopes of carbon while the St Andrews team had looked at the glacial rocks themselves.

The team examined rocks in Namibia upon which the Snowball Earth theory is based, as well as samples from County Donegal, Ireland, and Death Valley in California, which would have formed during the glaciations.

Dr Condon added: "What is important and different about our findings is that we have found physical evidence for the glacial rocks being deposited in an environment where there were areas of open seas, contradictory to the 'hard' Snowball model."

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