An eruption-fueled extinction?


Around 250 million years ago, the most devastating mass extinction in Earth’s history marked a definitive end to the Permian geologic period. The global event extinguished more than 90 percent of the planet’s marine species and 70 percent of its terrestrial species. Exactly what caused the collapse has been an ongoing puzzle for scientists: Their theories have included massive volcanic eruptions, an asteroid impact and the formation of the supercontinent Pangaea.

Now researchers at MIT and elsewhere have found fresh evidence that the mass extinction may have been triggered by enormous volcanic eruptions that gave rise to the Siberian Traps, a wide expanse of volcanic rock in present-day Russia. The researchers discovered that these eruptions spewed vast amounts of gases into the atmosphere, possibly setting off a cascade of environmental effects that led to the end-Permian collapse. The team published their findings this week in the online edition of Earth and Planetary Science Letters.

Lead author Benjamin Black, a PhD student in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says emissions of sulfur, chlorine and fluorine from the Siberian Traps could have been up to one million times the amount released from all of today’s volcanoes in a typical year. While the volcanoes that generated the Siberian Traps likely erupted over an extended period, the total amount of gases released provides scientists tangible evidence for a potential cause of the end-Permian extinction.

“We have concrete numbers that we can put on these gases that would have been erupting about 250 million years ago,” Black says. “These numbers give us a much better chance of being able to evaluate whether the Siberian Traps caused the extinction.”

Black worked with former MIT professor Linda Elkins-Tanton, director of the Department of Terrestrial Magnetism at the Carnegie Institution for Science in Washington, D.C., to measure the amount of gas trapped in volcanic rock obtained from the Siberian Traps. Three years ago, the team trekked to central Siberia, a region composed of flood basalts — remnants from immense lava eruptions hundreds of millions of years ago that blanketed the area and hardened into rock formations.

“When you go in by helicopter, you see trees until the horizon, but then there’s a river, and suddenly you see these immense cliffs of black basalts,” Black says. “In some places, if you’re very lucky, you can see them sitting directly on the Permian sedimentary rocks.”

The team rode rafts down the river and stopped at points to scale the cliffs, hammering out sections of rock that might hold remnants of Permian gases.

Back in the lab, Black broke the basalt samples apart to expose tiny crystals. He then polished away the crystals to reveal miniature frozen droplets, called melt inclusions. These droplets are preserved remnants of liquid magma, which, as the volcanoes erupted, was trapped within crystals. Over hundreds of millions of years, the droplets hardened into glass, preserving a record of the gases released at the end-Permian.

The team analyzed melt inclusions from multiple rock samples, measuring the percentage of sulfur, chlorine and fluorine — typical volcanic gases — in each droplet. The researchers then calculated the total amount of gas released into the atmosphere, based on the volume of the Siberian Traps, and found that the eruptions likely emitted up to 7,800 gigatons of sulfur, 8,700 gigatons of chlorine and 13,700 gigatons of fluorine. These immense amounts, Black says, are substantial enough to potentially contribute to a mass extinction.

Thorvaldur Thordarson, a reader in volcanology at the University of Edinburgh in Scotland, says the methods by which Black and his team gained their results are a notable achievement.

“The rocks are old enough to make the hunt for measurable melt inclusions extremely challenging,” says Thordarson, who was not involved in the research. “A number of scientists held the view that this is an impossible task. [Black] proved them wrong. The data generated … will provide valuable input for future research.”

The group is now working to model the end-Permian climate to understand what effects such gas concentrations may have had on the atmosphere. Black speculates that effects may have included acid rain, ozone depletion or global climate change.

“There’s evidence that this was a relatively warm, high-carbon-dioxide world,” Black says. “In some ways the end-Permian environment could have been very similar to the kind of environment that we’re creating through global warming.”

The paper’s other authors are Michael Rowe, a research associate at Washington State University, and Ingrid Ukstins Peate, an assistant professor of geoscience at the University of Iowa.