Sixth extinction, rivaling that of the dinosaurs, should join the big five, scientists say

Earth has seen its share of catastrophes, the worst being the “big five” mass extinctions scientists traditionally talk about. Now, paleontologists are arguing that a sixth extinction, 260 million years ago, at the end of a geological age called the Capitanian, deserves to be a member of the exclusive club. In a new study, they offer evidence for a massive die-off in shallow, cool waters in what is now Norway. That finding, combined with previous evidence of extinctions in tropical waters, means that the Capitanian was a global catastrophe.
“It’s the first time we can say this is a true global extinction,” says David Bond, a paleontologist at the University of Hull in the United Kingdom. Bond led a study that was published online this week in the Geological Society of America Bulletin. He adds that in magnitude, the Capitanian event was on par with the dinosaur-killing extinction 66 million year ago. “I’d put this up there with it, albeit with slightly less attractive victims,” Bond says.
Interest in the Capitanian began in the early 1990s, when paleontologists found evidence for fossil extinctions in rock formations in China. The rocks had originally formed on the floor of a shallow tropical sea. Most foraminifera—tiny, shelled protozoans—were wiped out, along with many species of clamlike brachiopods. There was also a possible trigger to blame: a set of ancient volcanic outbursts in China that solidified into rocks called the Emeishan Traps. The hot flood basalts would have released huge amounts of sulfur and carbon dioxide, potentially causing a quick global chill followed by a longer period of global warming. The gases could have also driven acidification and oxygen depletion in the oceans. Many scientists think that a similar massive burst of volcanic activity in Siberia touched off the biggest extinction of all time, just 8 million years later, at the end of the Permian period.
But the older, less studied Capitanian extinction has been dogged by criticism that it may have been a regional event, or just part of a gradual trend en route to the larger Permian extinction. Some of those criticisms may be quelled by the new evidence, which comes from Spitsbergen, the largest island in the Svalbard archipelago off the coast of Norway in the Arctic Ocean. There, Bond and his colleagues examined chert rocks—silica formations, created by the skeletons of dead sponges, that also contain many species of brachiopods. At the time, the rocks would have been forming in tens of meters of cooler water at midlatitudes. But at a stark point in the rock record, the fossils disappeared.
“They all drop out,” says study co-author Paul Wignall, a paleontologist at the University of Leeds in the United Kingdom. “It’s like a blackout zone and there’s nothing around.” A little further in the rock record, a few brachiopod species recover, Wignall says, and then mollusks take over en masse, before the devastation of the Permian extinction, 8 million years later.
The research team had a hard time tying the new record to the same moment in fossil records in China. Isotopic dating systems are too uncertain to provide a helpful absolute date. Another standard biostratigraphic method—linking the timing of different rock layers by the comings and goings of fossilized teeth of tiny eellike creatures called conodonts—also couldn’t be used, because the same species didn’t live in cool and tropical waters. Instead, the team points out that similar swings in different isotopes’ levels, occurring in both parts of the world, suggest that the two regions were experiencing the same changes in ocean chemistry at the same time.
That’s part of the problem, says Matthew Clapham, a paleontologist at the University of California, Santa Cruz. He thinks the study team has dated something a bit younger—maybe 255 million years old. “They’ve definitely identified a real event, which is really interesting,” he says. “Their age model is less convincing.” He also says that recent work in China on the extent of the Capitanian extinction across different species shows it may not have been quite as bad as originally thought. Clapham thinks the Capitanian is probably 30th or 40th in the hierarchy of extinctions, not sixth.
But Bond is still convinced that the Capitanian will go down in the history books as one of the world’s worst. “You have to change a lot of people’s minds,” he says. He is now studying fossil records in Russia and Greenland that could further buttress his arguments for a global disaster. Clapham, too, wants to see more work done on this enigmatic stretch of Earth history. “It’s a very mysterious event—it’s an interesting thing to study,” he says.

resource : http://www.sciencemag.org/news/2015/04/sixth-extinction-rivaling-dinosaurs-should-join-big-five-scientists-say

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