SAN FRANCISCO — The greatest extinction in the history of life may have been caused, in part, by ozone-depleting gases spewed in a massive volcanic eruption, a new study suggests. 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.
Benjamin Black, a graduate student at MIT, and his colleagues described their theory December 13 in a poster presentation at a meeting of the American Geophysical Union.
Researchers have long struggled to explain the “Great Dying” that occurred at the end of the Permian period. Some think that the extinction was a long, drawn-out affair caused by multiple factors — perhaps gradual changes in oceanic or atmospheric chemistry (SN: 5/28/05, p. 339). Others have blamed a single catastrophic event such as a belch of methane from the seafloor or an asteroid impact (SN: 2/24/01, p. 116) like the one thought to have wiped out the dinosaurs and other species 65 million years ago.
Volcanoes might be one of those calamities. In Siberia, around 250 million years ago, a series of massive volcanic eruptions spewed out lava over more than 2 million square kilometers. Some scientists have blamed these eruptions, known as the Siberian Traps, for climatic changes that contributed to the extinction.
Black and his MIT adviser, Lindy Elkins-Tanton, have been traveling to Russia for the past few summers to test such theories in the Siberian Traps.
The 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 those blobs, Black found surprisingly high levels of those elements — up to 0.75 percent chlorine and 1.95 percent fluorine, by weight, in one sample. That’s significantly more than the amounts found in similar 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 team proposed, but melted into the magma as it passed through salt-rich rock before erupting on the surface.
In all, the amount of chemicals in the Siberian 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. Such elements, when pumped out by power plants, can cause acid rain locally. If the eruptions were violent enough to lift the chemicals 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 main question is how long the chemicals would have stayed in the atmosphere. In a 2008 Science paper, Self and his colleagues reported finding high levels of sulfur and chlorine in the Deccan Trap lavas, though not as high as Black’s team reports.
“What happens to the chlorine is highly questionable,” Self said. It might, for instance, react with water droplets and rain out quickly.
Black’s team is now starting detailed calculations to see how high the chemicals would have gotten into the atmosphere. Regardless, the researchers reported, Siberia at least would have had one very bad time of it.
