Philip Allen

Hard evidence.
This granite cobble was dropped into fine-grained sediment by floating ice, indicating glacial activity.

New Round in Snowball Fight

The debate over whether Earth ever became a giant ice ball has heated up again. A new study looking at isotopes and minerals in sedimentary rocks has found evidence of warming and cooling cycles during a period of time some claim Earth should have been encased in massive glaciers. The finding, say the researchers, deals a blow to the so-called "Snowball Earth" theory.

According to the Snowball Earth theory, advancing glaciers covered so much of the planet's surface 600 million to 700 million years ago that they pushed Earth's climate past a tipping point, causing thick layers of ice to form over the land and oceans. The planet was so frozen that it took 5 million to 10 million years for CO2 from volcanoes to build up enough heat to thaw the ice. Proponents of the theory point to evidence such as variations in carbon isotopes, which suggest a massive marine die-off during the supposed Snowball Earth period. But critics cite significant photosynthesis near the equator at that time--proof, they say, that oceans near the tropics had little or no ice.

To plow further, Philip Allen, a geologist at Imperial College London, and colleagues looked at changes in the composition of mudstone in outcrops in Oman dating back 650 million years. They analyzed how these rocks, which formed beneath Earth's surface, changed chemically when they reached fresh air. In reacting to oxygen, water, CO2, or other compounds in the environment, such rocks serve as a record of the contemporary climate. The team also examined patterns in the sediments deposited by glaciers.

The evidence indicated that glaciers were advancing and retreating in cycles of warming and cooling--something the researchers argue should not have happened on a fully frozen Earth. Earth may have come "dangerously close" to the tipping point during this massive glaciation, says Allen, but it never fully froze. The team reports its findings in the April issue of Geology.

"There's no doubt whatsoever that very significant glaciation episodes occurred during this period," agrees physicist and climate modeler Dick Peltier of the University of Toronto in Canada, who notes that Earth was probably more of a "slushball" at this time than a snowball. But Harvard geochemist Daniel Schrag isn't convinced. Snow and sublimation of ice would keep a hydrologic cycle going on land during a Snowball Earth, he argues, allowing glaciers to flow and generating enough water to affect rocks in a way similar to retreating ice. In addition, he says, CO2 levels were likely rising during most of the glaciation, and the acidic water that resulted may have weathered rocks in ways similar to warm, humid conditions.

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