Geologists say 'snowball Earth' likely 750 million years ago

I wonder what Farmerman will think of this theory?---BBB

Geologists say 'snowball Earth' likely 750 million years ago
Supercontinent's breakup may have forged global glacier
David Perlman, San Francisco Chronicle Science Editor
Thursday, March 18, 2004

While the Bay Area basks in a heat wave and New York freezes in the snow, a team of geologists and climate experts looking back in time has renewed an old debate about the possibility that the entire surface of the globe was once frozen solid.

Some 750 million years ago, according to some experts, the planet now being subject to global warming may have been a glacier-covered "snowball Earth."

The geologists have created a computer model of a time more than a billion years ago when much of Earth's surface was welded into a single massive continent, known as Rodinia. When it eventually broke into smaller continental fragments, the result was a wave of volcanic eruptions throughout the world's equatorial regions.

Those eruptions, say the scientists, filled the atmosphere with carbon dioxide, the notorious greenhouse gas whose rising levels may have warmed the planet for a time, as is occurring now from the carbon dioxide released from fossil fuel combustion.

But the volcanoes also loosed torrential rains, the researchers suggest, which weathered and degraded deep layers of Earth's rocky surface. That would have greatly enhanced the capacity of the planet to absorb all the carbon dioxide in the atmosphere and may have brought on a time of deepening cold.

At the same time, the scientists say, the sun's radiance was some 6 percent fainter than it is today, and as glaciers spread southward, they grew to monstrous size until they covered the entire Earth from pole to pole -- producing the "snowball Earth" that scientists have argued about for a century or more.

This latest account of what may have happened during a time that scientists call the Proterozoic Era is being published today in the journal Nature by a team headed by Yannick Donnadieu of France's Climate and Environmental Science Laboratory and including Joseph Meert, a geologist who researches evidence for ancient climates at the University of Florida.

In a telephone interview Wednesday, Meert conceded that in this snowball debate among scientists he has been known as highly skeptical that the events ever occurred at all.

"The models proved so persuasive, and the evidence we obtained from magnetic changes in the Earth's crust and the geochemistry of the ancient rocks seemed so convincing, that I finally said, 'Well, so be it, it's a highly plausible scenario, and now it's up to others to test it' -- that's science," Meert said.

In fact, Meert said, it's highly likely that the Earth became a glacier- coated virtual snowball not just that one time with the breakup of Rodinia 750 million years ago, but it may well have happened at least two or three times since then as tectonic forces assembled the Earth's surface into later supercontinents and then broke them apart, too.

"In any event," Meert said, "I think our model will keep the pot boiling for quite a while."

The boiling may well have begun already.

During an interview Wednesday, geology Professor Grant Young at the University of Western Ontario said he saw many flaws in the report by the French-American team.

Young said he had been researching the question all over the world for the past three decades and had found evidence for major glacial advances during the Proterozoic Era in only three places: in rocks along the north shore of Lake Huron in Ontario, in southeastern Wyoming and in northern South Africa.

"I've found no evidence of global glaciation," Young maintained, "and it's a wild idea."

Young always has been highly skeptical of computer models, he said, because as a field geologist, he needs to see the direct evidence in the rocks. "Nature is very complicated," he said, "and this model doesn't fit the known evidence we have."