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'Snowball Earth': Glaciers, ice packs once met at Equator

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Macdonald says that glaciers on land at the Equator means solid pack ice at sea in the tropics, too.

"Climate modeling has long predicted that if sea ice were ever to develop within 30 degrees latitude of the equator, the whole ocean would rapidly freeze over," he says. "So our result implies quite strongly that ice would have been found at all latitudes during the Sturtian glaciation."

Even on Snowball Earth, however, it's thought that there must have been "refugia" where liquid surface water and at least occasional sunlight remained, as proto-animal life above the bacterial level is believed to have arisen and survived at around the time of the first, Sturtian snowballing. Macdonald considers that the big freeze may actually have stimulated early, primitive life to get its act together and begin the long process which has led to the present-day biosphere.

"The fossil record suggests that all of the major eukaryotic [other than microbial] groups, with the possible exception of animals, existed before the Sturtian glaciation," says the prof. "The questions that arise from this are: If a snowball Earth existed, how did these eukaryotes survive? Did the Sturtian snowball Earth stimulate evolution and the origin of animals?"

"From an evolutionary perspective," he adds, "it's not always a bad thing for life on Earth to face severe stress."

Macdonald and his colleagues aren't sure what triggered the Sturtian snowball effect, but note that a lot of lava came to the earth's surface at around that point, suggesting that the big chill could have been kicked off by volcanic dust darkening the skies.

The Harvard team publish their results this week in hefty boffinry mag Science. ®

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