Small but deadly

The biggest extinction in history was probably caused by a space rock that changed the climate

AS EVERY schoolchild knows, the dinosaurs were wiped out in an instant, when a rock from outer space hit what is now southern Mexico. That happened 66m years ago, at the end of the Cretaceous period. Well-informed schoolchildren also know that this mass extinction was neither unique nor the biggest. The geological record speaks of four others since animal life became complex at the beginning of the Cambrian period 541m years ago.
What neither these clever schoolchildren nor anyone else knows, however, is whether these extinctions had similar causes. But evidence is accumulating that the biggest extinction of all, 252.3m years back, at the end of the Permian period, was indeed also triggered by an impact. Nevertheless, though the trigger was the same, the details are significantly different, according to Eric Tohver of the University of Western Australia.

The idea that an impact caused the Permian extinction has been around for a while. As at the end of the Permian, as at the end of the Cretaceous, huge volcanic eruptions had been going on for hundreds of thousands of years. These may have weakened the world’s ecosystems, making them vulnerable to an external shock. But the abruptness of both extinctions indicates that the
 coup de grâce was administered by something else, and in the case of the Permian some fragments of meteorite of the correct age, found in rock in Antarctica, suggest that, as with the Cretaceous, that something was an asteroid or a comet. What was missing from the story, though, was a suitable crater.When the dinosaurs vanished they were accompanied by more than 70% of the other animal species on Earth. At the end of the Permian, the extinction figure was more than 80%. And just as the Cretaceous slate-clearing permitted the rise of a hitherto obscure group called the mammals (including, eventually, one now referred to by biologists as Homo sapiens), so the Permian clearance permitted the rise of the reptiles, one branch of which turned into Tyrannosaurus,Diplodocus and all the other names familiar from childhood.
Fracking hell!
Last year Dr Tohver and his colleagues thought they might have found it. They redated a hole that straddles the border of the states of Mato Grosso and Goiás in Brazil, called the Araguainha crater, to 254.7m years, with a margin of error of plus or minus 2.5m years. Previous estimates had suggested Araguainha was 10m years younger, but Dr Tohver has put it within geological spitting distance of the extinction date, which itself has a margin of error of plus or minus 200,000 years.
Which would all be fine and dandy, except most people think Araguainha is too small to be the culprit. It is a mere 40km (25 miles) across. The Chicxulub crater in Mexico, which did for the dinosaurs, is 180km in diameter, and it may have been paired with an even bigger impact in the Indian Ocean. (This could have happened if the incoming object was a comet that broke up in a close encounter with the sun.)
Dr Tohver, however, has an answer to this criticism. His latest paper, just published inPalaeogeography, Palaeoclimatology, Palaeoecology, describes the rock in the area in which Araguainha resides.
After an extensive geological survey, he and his team discovered that a sizeable amount of this rock is oil shale. Any hydrocarbons in the crater would certainly have been vaporised. More intriguingly, the researchers calculate that the impact would have generated thousands of earthquakes of up to magnitude 9.9 (significantly more powerful than the largest recorded by modern seismologists) for hundreds of kilometres around. In effect, it would have been the biggest fracking operation in history, releasing oil and gas from the shattered rock in prodigious quantities.
The upshot, Dr Tohver believes, would have been a huge burp of methane into the atmosphere. Since methane is a powerful greenhouse gas, that burp would have resulted in instant global warming, making things too hot for much of the planet’s animal life. Presto! The Permian mass extinction is explained.
Determining whether this was really what happened will take a lot more digging, of course. Even now, there are those who think the formation of the Chicxulub crater was a coincidence, and that what did for the dinosaurs was actually the volcanoes, so Dr Tohver will have to work hard to convince the sceptics. If he does, though, he will have proved himself a great geological detective, for he will have been responsible for solving one of the biggest puzzles in palaeontology.



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