Widespread volcanic eruptions that led to global warming were likely the cause of massive extinctions 250 million years ago, not the impact of an asteroid or comet, according to two studies published in the online edition of the journal Science.
The catastrophe called the "Great Dying" wiped out 90 percent of the planet's marine life and 70 per cent of all plant and animal life. The most recent popular scientific belief about what caused it was the impact of a large object from space, but the examination of sediment and fossils deposited at the time has led two groups of scientists to rethink those beliefs.
"Animals and plants both on land and in the sea were dying at the same time and apparently from the same causes -- too much heat and too little oxygen," said Peter Ward, a University of Washington paleontologist and lead author of one of the studies.
In the research published in Science Express, the online version of the journal Science, and slated to be published in Science's print edition, the groups found increased levels of sulphur and depleted oxygen in the ancient rock.
Ward and his team of researchers from the US and South Africa, surmised that the sulphur came from continuous volcanic eruptions in an area known as the Siberian Traps. The eruptions warmed the Earth, trapping sunlight and depleting oxygen in the air, he said.
At the same time, the Earth's shifting tectonic plates lowered the levels of the ocean, exposing seabeds and releasing methane trapped in the sediment there, further stoking the global warming, the research indicated.
"I think temperatures rose to a critical point," Ward said. "It got hotter and hotter until it reached a critical point and everything died."
"It was a double-whammy of warmer temperatures and low oxygen, and most life couldn't deal with it," he added.
He said temperatures around the world rose 80C, killing off plants, which served as food for many animals, starving them.
In addition, he said, oxygen levels dropped to about 16 percent of the atmosphere when normal levels today are at 21 percent. Such a drop would be equivalent to trying to breathe on top of a 4,250m mountain, he said.
"High and intermediate elevations would have become uninhabitable," he said. "More than half the world would have been unlivable. Life could only exist at the lowest elevations."
The Great Dying occurred between the Permian and Triassic periods, when all of today's continents were concentrated in one great land mass called Pangea.
Ward and his team studied 300m-thick sediment from Pangea, today's Karoo Basin in South Africa and off the coast of China, while a second team, led by Kliti Grice, a paleontologist with Curtin University of Technology in Perth, Australia, looked at sentiment from the Perth Basin.
Ward said the findings of the two teams were "eerily similar."
Grice's researchers -- from Australia, China, the US, Germany and Britain -- found that the ocean was low in oxygen and teeming with sulphur-loving bacteria while Ward's team found that life had been dying off slowly for about 10 million years before a rapid dying rate marked the next 5 million years.
The scientists said they found no evidence of an impact by an asteroid or comet. They said that if an impact had occurred, it would have been a minor element in the mass extinction.
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