Frozen. A true Snowball Earth would have coated the globe with ice.

Cosmic Dust Supports a Snowball Earth

Dick writes about Earth and planetary science for Science magazine.

An element that rains down on our planet from space is providing researchers with new evidence that, at one time, Earth was frozen solid for 12 million years.

Answering questions about Earth's climate of more than half a billion years ago can be a challenge--even questions as stark as whether land and sea were completely coated by ice from pole to pole. Indeed, the revival of the Snowball Earth hypothesis almost 7 years ago has bogged down of late, as paleoclimatologists have failed to turn up unequivocal evidence that ice enrobed our planet.

But reporting 8 April in Science, a group of geochemists offers a new snowball marker: the element iridium, which continually rains down on us from space. Geochemists Bernd Bodiselitsch and Christian Koeberl of the University of Vienna, Austria, and their colleagues analyzed three cores drilled by copper miners in Zambia and the Democratic Republic of the Congo. The team figured that on an iced-over world, the iridium-rich meteoritic dust that rains onto Earth would accumulate until the snowball ended in a sudden meltdown. All the iridium accumulated in the ice would then be deposited in a single, thin layer of marine sediment. The more iridium deposited at the end of a snowball, the longer the snowball had gone on. Anything less than a snowball--such as a partially glaciated world with lingering ice-free ocean waters—and the iridium could not build up.

In the first few centimeters of sediment laid down on top of glacial sediments, Bodiselitsch and colleagues indeed found sharp spikes in the abundance of iridium. A spike showed up in all three cores at the end of the Marinoan glaciation about 635 million years ago and in two cores at the end of the earlier Sturtian glaciation about 710 million years ago. If meteoritic material was falling to Earth 635 million years ago at anything like the rate it has during the past 80 million years, the group calculates, Snowball Earth lasted 12 million years, give or take 3 million years.

Geochemists are excited but naturally cautious. "Iridium is a strong indicator of extraterrestrial material," says Bernhard Peucker-Ehrenbrink of the Woods Hole Oceanographic Institution in Massachusetts. "However, it is just one of a series of useful tracers." He and others, he expects, will be pursuing other extraterrestrial tracers such as isotopes of helium and of osmium to test the claim of a Snowball Earth.

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