The breakup of the world's original supercontinent, coupled with the breakdown of massive amounts of volcanic rock, plunged Earth into the deepest freeze it has ever experienced, new research shows.
In a paper appeared on March 17 in the journal Nature, a group of scientists that includes a University of Florida geologist argue that the breakup of Rodinia, the first supercontinent and the mother of all modern continents, accelerated the breakdown of then-common volcanic rock, stripping carbon dioxide from the atmosphere and causing "Snowball Earth" -- the massive and mysterious chill that froze the oceans and sent glaciers creeping across the equator 750 million years ago.
UF geologist Joe Meert said he and other scientists have long been puzzled by Snowball Earth because it occurred in an era of intense volcanic activity. Volcanoes burp enormous amounts of heat-trapping carbon dioxide into the atmosphere, yet to trigger a Snowball Earth, the amount of carbon dioxide in the atmosphere had to dip to about half that of today.
"Intuitively, the release of carbon dioxide via volcanic activity should warm things up, but what we get here as Rodinia comes apart is a geochemical cycle that actually cools things down," said Meert, a UF assistant professor of geology and one of five authors of the paper.
The sun also played a role. Because stars become hotter as they age, the sun was about 6 percent weaker than it is today, which probably accounted for some of the cooling, Meert added.
Snowball Earth is one of at least three periods in the planet's history during which it was entirely covered by glaciers. All took place in the Proterozoic eon, from 2.5 billion to 543 million years ago, before the dawn of modern life when only simple organisms, such as blue-green algae, were present.
Some scientists have argued that the end of these Snowball or "Slushball" events -- the latter applying to the not-quite-so-cold freezes before and after Snowball Earth -- set the stage for the Cambrian radiation, an explosion of life that occurred about 543 million years ago.
Seeking a new approach to figuring out why these ancient temperatures dipped, Meert and four French scientists merged computer models of climate change, continental drift and carbon dioxide change during Snowball Earth, running the results simultaneously to see what patterns emerged.
No one had previously thought to put all the pieces together, Meert said. "This was a synthesis of existing data sets and a new way of thinking about the problem."
The results revealed the breakup of Rodinia occurred over the same geological time period as carbon dioxide declined and temperatures fell. That, Meert said, pointed to Rodinia's breakup as the cause of Snowball Earth.
Meert's explanation: Just as shattering a rock into pieces increases its exposed surface area, so breaking up the supercontinent increased the amount of land exposed to the elements and resulted in a more active hydrological cycle. Because of the abundant volcanic activity, much of this exposed land consisted of volcanic rock or basalt, which erodes easily. When the rain, acidified by the volcanoes' eruptions, fell on this rock, it rapidly dissolved, trapping the carbon dioxide in the land and surrounding waters, he said.
"You take carbon dioxide in the atmosphere, combine it with water, and basically produce carbonic acid or acid rain," he said. "That rain then weathers the continent and the carbon dioxide gets bound back up in the rock and soil. So you're removing carbon dioxide from the atmosphere with an active hydrological cycle."
Despite the release of the greenhouse gases from volcanic activity, the process was powerful enough to draw down the carbon dioxide level, triggering the deep freeze, he said.
Meert said the current proliferation of life probably would make it impossible for Snowball Earth-type events to recur, even though the continents continue to drift or break up. Plants take in carbon dioxide and release oxygen, while animals do the opposite -- processes that would ameliorate the geochemical cycles seen before the advent of modern life, he said.
While some might be tempted to think of the Snowball Earth as a radical solution to global warming, Meert said that notion is a dead end.
"I don't think you can say that we're ever going to see this again," he said. "At the same time, we've shown that the breakup of continents into smaller pieces does influence global temperatures and results in significant drawdown of carbon dioxide."
Gregory Jenkins, an associate professor of meteorology at Pennsylvania State University and an expert on early Earth climate simulation, called the research "good and exciting work" showing that the Earth's climate is capable of massive and rapid change.
"The techniques applied in this paper will cause other researchers to rethink some of their ideas," he said.
The research was funded by a grant from the Centre de la Reserche Scientifique in Gif sur Yvette, France. The other authors of the paper are Yannick Donnadieu and Gilles Ramstein, of the Laboratoire des Sciences due Climat et de l'Environnement; and Yves Godderis and Anne Nedelec, of the Laboratoire des Mecanismes et Transferts en Geologie, Universite Paul Sabatier, both in France.
University of Florida
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London - Apr 08, 2004
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