Geologists at the Massachusetts Institute of Technology say they've uncovered surprisingly high amounts of the elements fluorine and chlorine in Siberian lavas dating back 250 million years, a time when roughly 90 percent of marine species and 70 percent of land species went extinct, ScienceNews.org reported Wednesday.

MIT graduate student Benjamin Black and colleagues described their findings in a presentation at a meeting of the American Geophysical Union in San Francisco.

Researchers have long sought reasons for the "Great Dying" that occurred 250 million years ago at the end of the Permian period. Some have proposed a cataclysmic event such as an asteroid impact like the one thought to have wiped out the dinosaurs and other species 65 million years ago. Volcanoes might be another suspect, other researchers say.

In Siberia, around 250 million years ago, a series of massive volcanic eruptions spewed out lava over more than 750,000 square miles. Studying rocks in Siberia, Black and others say the amount of chemicals in the rocks could translate to 9 trillion tons of sulfur, 8.5 trillion tons of fluorine and 5 trillion tons of chlorine spewing into the atmosphere during the eruptions.

If the eruptions were violent enough to send the chemicals high into the atmosphere, the researchers theorize, the chemicals could have damaged the ozone layer much as chlorofluorocarbons do today, helping cause or at least worsen the mass extinction.

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