The second-most severe mass extinction in Earth鈥檚 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鈥檚 temperature.
鈥淚t completes the jigsaw puzzle in many ways,鈥� says Andrew Kerr at Cardiff University, UK. Geologists recognise five points in time when huge numbers of species were wiped out, although recent research suggests at least one of these might have been too slow to be a mass extinction.
But 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.
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In each case, the volcanic activity 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 it wasn鈥檛 so different after all. 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.
鈥淟arge volcanic eruptions put anomalously high levels of mercury into the atmosphere,鈥� says Bond. There seems to have been large-scale volcanic activity during this period after all.
鈥淚t鈥檚 a great boon to the mass extinction story, which now links all past mass extinctions to large igneous province volcanism,鈥� 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 at the time too. Bond says it looks as if this only began after the volcanism and global warming had triggered the mass extinction. 鈥淓verything lines up nicely,鈥� he says, although 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. 鈥淵ou need a way to end the glaciation, and global warming from a large igneous province could do that,鈥� he says.
Kerr 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.
Geology
