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No evidence for a global ice sheet or “Snowball Earth”

No evidence for a global ice sheet or “Snowball Earth”

(December 13, 2001)

The provocative “snowball Earth” hypothesis suggests that the oceans were covered with a sheet of ice on several occasions ~650 million years ago. The last of these “snowball” events appears to have immediately preceded the first appearance of complex organisms in the fossil record, and the widespread destruction of ecosystems led to a rapid diversification and evolution of the first animals in Earth history.

But in the December issue of Geology, Martin J. Kennedy of the Department of Earth Sciences at the University of California, Riverside, and colleagues from Columbia University and the University of St. Andrews argue that life in the oceans during the snowball event continued as usual, thus challenging the notion of a completely ice-covered ocean. They present data that suggests that a healthy and productive marine ecosystem prevailed during the glaciation.

“Coverage of the oceans by ice would have significantly altered geochemical cycles that maintain a habitable biosphere,” says Kennedy. “It would almost certainly have resulted in a profound and long-term destruction of marine ecosystems.”

Noting that carbon isotope values in the ocean and in the mantle would need to be the same in the absence of photosynthesis or life in the ocean, the authors find no isotopic evidence that registers the impact on marine production or synthesis during the snowball event.

“It appears to have been business-as-usual as far as marine organisms are concerned,” Kennedy adds.

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