The second most severe mass extinction in Earth’s history may have been triggered by global warming. The discovery means that, for the first time, all of the largest known extinctions can be linked to a rapid rise in the planet’s temperature.
Geologists recognise five times in the geological record when huge numbers of species were wiped out, although recent research suggests that the decline in species numbers during at least one of these (in the late Devonian) might have been too slow to be classed as a mass extinction. However, the second most severe of these five extinctions, the late Ordovician event about 445 million years ago, has always seemed different. The others coincided with epic volcanic activity that smothered millions of square kilometres with lava to create what is called a large igneous province. In each case, the volcanic activity released enormous amounts of carbon dioxide which triggered global warming that is likely to have contributed to extinction. In contrast, the consensus had been that the late Ordovician extinction was prompted in part by global cooling.
David Bond at the University of Hull, UK, thinks that this extinction wasn’t so different. With his colleague Stephen Grasby at the Geological Survey of Canada, Bond took samples from a site in Scotland where rocks that formed on the late Ordovician sea floor are well-preserved. They found a spike in the level of mercury in rocks that formed just before and during the extinction.
“Large volcanic eruptions put anomalously high levels of mercury into the atmosphere,” says Bond. There seems to have been large-scale volcanic activity during this period after all.
“It’s a great boon to the mass extinction story, which now links all past mass extinctions to large igneous province volcanism,” says Gerta Keller at Princeton University.
Bond thinks this might have led to global warming that heated the oceans, reducing their ability to hold dissolved oxygen and suffocating marine life. This would explain why the Scottish rocks also contained high levels of uranium, as this element precipitates out of seawater and accumulates on the sea floor when oceans lose their oxygen.
Confusingly, there was global cooling around this time too. Bond says it looks as if this only began after the volcanism and global warming had triggered the mass extinction. However, he accepts that the new extinction scenario will be controversial.
Charles Mitchell of the University at Buffalo, New York, remains to be convinced. He says the global cooling and a severe glaciation may well have begun before, and contributed to, the extinction. But he says volcanic activity and global warming could have played a part in the latter stages of the extinction. “You need a way to end the glaciation, and global warming from a large igneous province could do that,” he says.
Andrew Kerr at Cardiff University, UK is more enthusiastic. He has long argued that volcanic activity and associated effects such as global warming are the key drivers of mass extinctions.
Keller says the finding means there might be a new odd one out among mass extinctions: the one we may currently be in is also due to warming, but the carbon dioxide responsible was produced by us, not volcanoes.