The world’s worst mass extinction has been a great whodunit for decades. Some 252 million years ago, 75 percent of land species and 90 percent of those in the oceans disappeared. But what caused trilobites, Eurypterid “sea scorpions” and all those other species to go extinct?
Scientists had long suspected that massive releases of magma from the Siberian Traps played a key role, and now they have the best evidence yet that the ancient volcanic activity most likely triggered the Great Dying.
The key to solving this puzzle was figuring out the timing of the two events. Early studies had estimated that the mass extinction and the Siberian Traps eruptions happened within a few million years of each other. But there was so much uncertainty in the dates for the two events that no one could say for certain which happened first.
“For magmatism to be a plausible trigger, we must be able to assert outside the uncertainty on the dates that it preceded [the] mass extinction,” says Seth Burgess, a post-doc at the U.S. Geological Survey who completed this research while he was a graduate student at MIT. “Simply put, if magmatism began after the onset of mass extinction, then magmatism isn’t the cause.”
Burgess and his colleagues nailed down the timing of the mass extinction last year by determining the ages of rocks in China that had been laid down before and after the epic die-off. They found that the event occurred within a 60,000-year period 252 million years ago.
The new study, published today in Science Advances, focused on the other half of the equation, the Siberian Traps. This eruptive event brought some 700,000 cubic miles of rock and lava to the surface, the remains of which cover an area of Siberia equivalent to all of Western Europe.
Burgess and Samuel Bowring of MIT used uranium-lead dating—the same technique employed in their previous study—to provide a timeline of the eruptions. They calculate that magmatism began about 300,000 years before the mass extinction and continued for some 500,000 years after.
“We show that magmatism is a plausible trigger” for the Great Dying, Burgess says. A big question, though, is why the die-off didn’t start until hundreds of thousands of years after the eruptions began. It could be that the planet reached a tipping point only after a critical volume of magma had erupted, Burgess says. Or only small amounts of magma erupted until right before the mass extinction began.
Answering this question may be tied to figuring out exactly how the magmatism caused such devastation to the planet’s life.
“We’ve now got a pretty good handle on the ‘when,’ but details of the ‘how’ are still uncertain,” Burgess says. For the oceans, at least, scientists have a good working theory: In addition to rock and lava, the Siberian Traps released huge amounts of carbon dioxide into the atmosphere. Earlier this year, researchers presented evidence that this caused an abrupt increase in ocean acidification that would have driven many species out of existence.
What caused the terrestrial creatures to go extinct, though, is more of an enigma. “There are quite a few theories,” Burgess notes, such as hot atmospheric temperatures, huge fires and rain as acidic as lemon juice.