The Cretaceous period ended 65 million years ago with a bang—an asteroid impact—that did in the dinosaurs. But what about the other mass extinctions? Now researchers teasing apart the record of mass extinction at the dawn of the dinosaur age have come up with a possible volcanic killer: a burst of methane gas triggered by volcanic greenhouse gases.
Linking eruptions to evolution has taken some detective work. The mass extinction 201 million years ago at the end of the Triassic period (known as the end-Triassic extinction) wiped out half the known species on land and in the sea, paleontologists found, which triggered the rise of the dinosaurs. When the extinction nearly wiped out the early relatives of the crocodiles, it removed the biggest competitors of early dinosaurs and let them rise to dominance.
More recently, researchers dating both that extinction and massive volcanic eruptions around then have found that the two major evolutionary and geologic events occurred at the same time. The massive outpourings of lava now strewn along the edges of the North Atlantic Ocean were laid down at the very start of the mass extinction, geochronologists have found. The coincidence is so close—a geologic moment of a few tens of thousands of years or so—that the eruptions and the mass extinction appear to have been connected.
To find out how eruptions—however massive—could cause global extinctions, paleoecologist Micha Ruhl of Utrecht University in the Netherlands and his colleagues went to the fossil record—the very small-scale fossil record. As they report online today in Science, they extracted distinctive organic molecules from marine sediments in Austria. The molecular chains, each 23 to 35 carbon atoms long, had been part of plant waxes washed off the land. The proportion of two isotopes of the carbon in the former plant waxes changed dramatically right at the end of the Triassic. That ratio shift depended on the amount of carbon dioxide used by the plants from different sources: carbon dioxide from the eruptions or methane (later converted to carbon dioxide) locked in ice in the sea floor. The group found that at least 12 trillion tons of carbon in the form of carbon dioxide or methane—two greenhouse gases—had gushed into the atmosphere during just 10,000 to 20,000 years.
That is almost twice the amount of carbon dioxide or methane previously suggested, released over a shorter interval. And the new isotopic result suggests that much of the released gas was methane, a far stronger greenhouse gas than carbon dioxide. Ruhl and his colleagues think methane may have been released from methane-laced ice in the sea floor when greenhouse warming melted the ice. The warming in turn could have been brought on by carbon dioxide spewed by the eruptions.
The new isotopic record clearly reveals "a big signal," says paleoceanographer James Zachos of the University of California, Santa Cruz. "There's compelling evidence for a connection between a major mass extinction" and the release of carbon gases. But exactly how the gas burst did in so many species remains unclear, notes paleontologist Paul Olsen of Lamont-Doherty Earth Observatory in Palisades, New York. "I believe [the volcanism] actually did cause the extinction, one way or another," he says.
The greenhouse gases are now the most obvious candidate for linking eruptions and the extinction. On land, the rapidly strengthening greenhouse might have pushed life to the edge by rapidly warming the world. In the ocean, the link could have been warming, or carbon dioxide could have turned the water acidic, as is happening in today's ocean. Sorting out just how the killing happened, Olsen says, may take a lot more detective work on how the various victims succumbed.