Gassy volcanoes tied to extinction: giant Siberian eruption may have destroyed ozone layer.
The greatest mass extinction in the history of life may have been caused, in part, by ozone-depleting gases spewed in a massive volcanic eruption. Geologists have found surprisingly high amounts of the elements fluorine and chlorine in Siberian lavas dating back 250 million years--when about 90 percent of marine species and 70 percent of terrestrial species went extinct.
MIT graduate student Benjamin Black and his colleagues described their theory December 13.
Researchers have long struggled to explain the "Great Dying" that occurred at the end of the Permian period. Some think the extinction was a drawn-out affair caused by multiple factors--perhaps gradual changes in oceanic or atmospheric chemistry (SN: 5/28/05, p. 339). Others blame a single catastrophic event such as a belch of methane from the seafloor, an asteroid impact (SN.. 2/24/01, p. 116) like the one thought to have wiped out the dinosaurs 65 million years ago--or a volcanic eruption.
In Siberia about 250 million years ago, huge lava flows spread across more than 2 million square kilometers. Some scientists have blamed these eruptions, known as the Siberian Traps, for climate changes that contributed to the extinctions.
The Siberian rocks contain tiny blobs of once-molten material, preserved like chemical time capsules from the earliest days of the eruption. Measuring the amounts of sulfur, chlorine and fluorine in the blobs, Black found surprisingly high levels of those elements--in one sample, up to 0.75 percent chlorine and 1.95 percent fluorine by weight. That's significantly more than the amounts found in similar lava deposits like the Deccan Traps in India and the Columbia River flood basalts in Washington and Oregon.
The chemicals probably weren't in the magma as it began traveling up from deep within the Earth, the researchers proposed, but melted into the molten rock as it passed through salt-rich deposits before erupting on the surface.
The concentrations in the Siberian rocks could translate to 9 trillion metric tons of sulfur, 8.5 trillion tons of fluorine and 5 trillion tons of chlorine spewing into the atmosphere during the eruptions. Such elements, when pumped out by power plants, can cause acid rain locally.
If the eruptions were violent enough to lift substances high into the atmosphere, the team proposed, the chemicals could have damaged the ozone layer just as chlorofluorocarbons do today--helping cause or at least exacerbate the mass extinction.
Stephen Self, a volcanologist at the U.S. Nuclear Regulatory Commission in Washington, D.C., said the big question is how long the chemicals would have stayed in the atmosphere. In 2008 Self and his colleagues reported finding high levels of sulfur and chlorine in the Deccan Trap lavas.
Black's team is now starting detailed calculations to see how high the chemicals would have gotten in the atmosphere. Regardless, the researchers said, Siberia at least would have had one very bad time of it.
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|Date:||Jan 15, 2011|
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