Acidifying oceans helped fuel mass extinction

The question of what killed most life on Earth 250 million years ago is a veritable Murder on the Orient Express, with multiple characters all dealing part of the deathblow. Now, scientists have learned how one of the assassins — acid — could have performed its part of the deed.

High levels of atmospheric carbon dioxide would have turned the oceans more than acidic enough to kill off marine critters, a computer simulation indicates.

“This would have been another stressor in the system that might have pushed things toward extinction,” says Alvaro Montenegro, a climate modeler at St. Francis Xavier University in Antigonish, Nova Scotia. He and his colleagues describe the finding in a paper published online August 2 in Paleoceanography.

At the end of the Permian period of geologic time, more than 90 percent of marine species and three-quarters of terrestrial species vanished. Leading suspects in the die-off include oxygen-starved oceans, a belch of hydrogen sulfide from the deep, a shutdown of great marine nutrient cycles, and massive volcanic eruptions.

Using a climate model developed at the University of Victoria, Montenegro and  colleagues set up nine hypothetical worlds — mixing and matching possible continental arrangements, seafloor topographies and levels of atmospheric carbon dioxide. Then the researchers fired up the model and watched how carbon flowed through the ocean and atmosphere.

At atmospheric carbon dioxide levels of 3,000 parts per million — roughly 10 times modern preindustrial levels — much of the gas dissolved in seawater, forming carbonic acid and releasing hydrogen ions. Acidity is measured on the pH scale; the lower the number, the more acidic the waters. Today’s oceans have a pH of around 8.1; those in the modeled end-Permian world dropped to around 7.3 near the equator and 7.1 near the poles. Such acidity would have made it hard for many marine organisms to use calcium carbonate to build protective shells, Montenegro says.

Today’s oceans also are growing more acidic because of carbon dioxide belched into the atmosphere by fossil fuel burning and other sources. Back then, most of the gas probably came from huge volcanic eruptions in Siberia.

How quickly carbon dioxide built up in the atmosphere would have affected how acidic the ocean got, says Jonathan Payne, a paleobiologist at Stanford University. If gas concentrations increased quickly, he says, “then this model may be a reasonable representation of how climate was changing at the time.” If gas built up slowly, the oceans may have been able to buffer the change in other ways.

But the model doesn’t include factors such as carbon weathering off land surfaces and into the oceans — an important player in the carbon cycle, says Lee Kump, a modeler at Pennsylvania State University in University Park. Including such effects, he says, could better show how life’s worst extinction came to pass.

Found in: Earth, Life and Paleobiology

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Gassy volcanoes tied to mass extinction

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.

Found in: Earth and Earth Science

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