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Selasa, 25 Januari 2011

Toxic Ash Clouds Might Be Culprit in Biggest Mass Extinction

Tiny particles embedded in ancient Canadian rocks have provided new clues about what might have triggered Earth's deadliest mass extinction. The ultimate cause, researchers say, might be globe-smothering clouds of toxic ash similar to that spewed by modern-day coal-fired power plants.

The die-off, which occurred worldwide about 250 million years ago at the end of the Permian period, was even more extensive than the one that wiped out the dinosaurs. More than 90% of marine species went extinct, and land-based ecosystems suffered almost as much. Scientists have long debated the reasons. Favorite hypotheses include an asteroid impact, massive volcanic eruptions in Siberia, and toxic oceans. Geochemist Stephen Grasby of the Geological Survey of Canada in Calgary and colleagues report online today in Nature Geoscience a new twist on the volcano notion.

Rocks that now make up the northernmost islands of the Canadian Arctic formed millions of years ago as seafloor sediments off the northwestern coast of a supercontinent called Pangaea. When Grasby and his team analyzed rocks from just before the Permian mass extinction, they noticed unusual microscopic particles. Besides the usual miniscule clumps of organic matter, they also found tiny bubble-filled particles called cenospheres. These frothy little blobs form only when molten coal spews into the atmosphere, the researchers say. Today, the fly ash produced by coal-fired power plants brims with cenospheres, but they are largely trapped by pollution-control equipment before they escape the smokestack. Millions of years ago, they must have been created when massive amounts of molten rock—more than 1 trillion metric tons—erupted through overlying coal deposits in Siberia to form lava deposits known as the Siberian Traps.

Because the late Permian cenospheres are approximately the same size and likely about the same weight as the smallest particles of volcanic ash, they could have easily risen to an altitude of about 20 kilometers in the atmosphere and then been swept around the world by jet stream winds. And like coal ash produced today, the particles would have been loaded with toxic metals such as chromium and arsenic. When the ancient cenospheres eventually dropped into the seas, they would have converted surface waters into a toxic soup, the researchers speculate. Then, they say, after most life died, decomposition would have robbed the water of its dissolved oxygen, smothering many of the survivors.

"The evidence is pretty compelling," says Gregory Retallack, a geologist at the University of Oregon, Eugene. Geophysicist Norman Sleep of Stanford University in Palo Alto, California, agrees. The team's findings "are an extremely major discovery," he says. As the ecological consequences piled up, he notes, the chain of events set in motion by the massive Siberian eruptions "went from something bad for life to a complete disaster." Finding cenospheres in late Permian rocks worldwide would bolster the notion that the tiny particles played a major role in causing the extinction, says Grasby.

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