Cold Water Tossed on 'Snowball Earth' Theory

Glacial deposits support snowball earth — Maroon iron-rich glacial deposits are visible the Ogilvie Mountains of the Yukon Territory. These were deposited by glaciers about 716.2 million years ago, when the planet may have been covered in ice, according to a theory known as Snowball Earth. Back then, these rocks, and the glaciers, would have been located much further south. Evidence like this suggests that glaciers reached into the equatorial region.

(Image credit: Francis Macdonald)

Although increasing levels of greenhouse gases in the Earth's atmosphere these days are seen as a harbinger of doom, millions of years ago they may have rescued the planet from a deep freeze.

Some researchers believe that at points in our planet's history — at least two, possibly three times — ice blanketed its surface, down to the equator and across the oceans, forming a "Snowball Earth."

But new research raises questions about whether a surge in carbon dioxide — one of the greenhouse gases responsible for modern, human-caused global warming — could have been responsible for the big thaw that followed the most recent Snowball Earth, about 635 million years ago. And if there wasn't a greenhouse effect big enough to melt the thick veil of ice, perhaps, the researchers suggest, Earth may not have been a big, icy snowball at the time.

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The primary evidence for these icy times, particularly the one about 635 million years ago, comes in the form of deposits of rocks ground up and carried by glaciers. These have been found around the world at locations which, about 635 million years ago, would have been located near the equator. These deposits have another layer of rock on top of them, called cap carbonates, which was believed to have formed as the glaciers melted or shortly afterward.

The normal processes that would pull the carbon dioxide out of the atmosphere were blocked by ice, which prevented carbon dioxide exchange between the atmosphere and ocean. The cold would also have prevented natural rock weathering reactions from pulling carbon dioxide, in the form of carbonic acid, out of the atmosphere and turning into bicarbonate. This would have allowed for an intense buildup of the greenhouse gas.

To find out how much carbon dioxide was around at the time, researchers analyzed the chemical composition of rocks taken from one of these deposits in Brazil, and the organic matter fossilized within them. The researchers also looked at data from samples from elsewhere in the world. They looked at ratios of carbon isotopes, molecules of carbon that have different atomic weights, in both the rocks and the organic matter fossilized within them. [Big Freeze: Earth Could Plunge into Sudden Ice Age]

Both the rocks and the organic matter — mostly algae — form using carbon from carbon dioxide dissolved in the ocean. However, a lower concentration of carbon dioxide causes the algae to take up more of the heavy version of carbon. The ratio of carbon isotopes picked up by the carbonate rocks, however, doesn't change, regardless of the carbon dioxide concentration. So by comparing the ratios from the two sources, the scientists could get an idea of what the concentration of carbon dioxide was in the ocean, and hence the atmosphere, at the time.

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Wynne was a reporter at The Stamford Advocate. She has interned at Discover magazine and has freelanced for The New York Times and Scientific American's web site. She has a masters in journalism from Columbia University and a bachelor's degree in biology from the University of Utah.