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The Mountains That Froze the World

3 November 2009 (All day)
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The rise of the Appalachians plunged Earth into an ice age so severe that it drove nearly two-thirds of all living species extinct. That's the conclusion of a new study, which finds that the mountains' rocks absorbed enough greenhouse gas to freeze the planet.

The Appalachians, a heavily forested mountain range stretching more than 1500 kilometers from Georgia to Maine, were not always so tranquil. In fact, about 460 million years ago during the Ordovician period, they were the site of one of the most violent volcanic events in Earth's history. As the eastern edge of what became the North American continental plate overrode the basin of an ancient ocean, numerous volcanoes sprang up in what are today the Taconic Mountains of New York state and New England. The volcanoes spewed enough lava to form mountains as high and rugged as the Alps. They also belched out more carbon dioxide (CO2) than at any time in Earth's past, creating greenhouse-gas levels as great as 20 times higher than they are today.

That much CO2 should have kept the climate warm for eons. But a mere 10 million years later, atmospheric levels of the gas began to plummet, and 5 million years after that Earth entered a severe ice age.

To find out what happened, geologist Seth Young of Indiana University, Bloomington, and colleagues analyzed three sedimentary rock formations in Nevada. If anything had washed down from the Appalachians 460 million years ago, it might be found there, in that once-shallow seabed.

The rock formations were mostly limestone, a good indication that the Appalachians had sequestered carbon from the atmosphere. That's because high levels of atmospheric carbon dioxide would have produced acid rain rich in CO2; as the rain hit the mountain, the CO2 would have combined with calcium in the volcanic basalt, forming calcium carbonate--i.e., limestone--runoff.

But the real clincher came when the researchers analyzed the ratio of two strontium isotopes in the limestone. Most sediments sport traces of seven parts strontium-87 to 10 parts strontium-86, a ratio of 0.7. But the Nevada limestone showed a ratio of 0.6, the biggest disparity ever recorded. Because basalt is rich in strontium-86, the most likely explanation, the researchers say, is heavy limestone runoff from the Appalachians.

In the October issue of Geology, Young and colleagues propose the following scenario: As CO2-laced acid rain fell on the rocks, it formed limestone that washed into the Nevada sea and locked away huge amounts of carbon from the atmosphere. Then, when the volcanism ended, about 450 million years ago, the sequestering continued, thinning CO2 levels to maybe a few times higher than today. Back then, a dimmer sun couldn't keep the atmosphere warm without CO2's help--hence, the eventual onset of the ice age.

Could these findings explain why, as some scientists think, the rise of the Himalayas about 35 million years ago were also followed by an ice age? Unlikely, says geochemist and co-author Lee Kump of the Pennsylvania State University, University Park. The strontium data from this study are "the opposite of that associated with Himalayan weathering," he says. That's because the Himalayas are primarily composed of granite, not basalt, and granite weathering cannot sequester carbon.

"What is really neat" about the current study, says geochemist Graham Shields of University College London, is that it solves the long-standing mystery about the timing of the ice age and the end of the Ordovician. One idea was that the Appalachian volcanoes themselves sparked the big chill by spewing sun-blocking ash into the air. But that didn't jibe with the fact that the ice age started 5 million years after the eruptions stopped. The new study shows, Shields says, how the cooling effect "really kicks in" when the volcanism ends.

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