Friday, January 21 2005 16:42 Hrs (IST) - World Time -

New Delhi: The biggest mass extinction in the history of earth some 250 million years ago was caused by global warming and not by the impact of asteroid or comet as earlier believed, new evidence has indicated.

In a paper published by 'Science Express', the online version of the journal Science, yesterday (Jan 20, 2005) researchers headed by University of Washington scientist Peter Ward said they have found no evidence for an impact at the time of "the Great Dying" 250 million years ago.

Instead, their research indicates the culprit might have been atmospheric warming because of greenhouse gases triggered by erupting volcanoes.

The extinction occurred at the boundary between the Permian and Triassic periods at a time when all land was concentrated in a super-continent called Pangea.

The Great Dying is considered the biggest catastrophe in the history of life on Earth, with 90 per cent of all marine life and nearly three-quarters of land-based plant and animal life going extinct.

"The marine extinction and the land extinction appear to be simultaneous, based on the geo-chemical evidence we found," their paper said.

"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."

In their work, the researchers were able to use chemical, biological and magnetic evidence to correlate sedimentary layers in the Karoo Basin of South Africa to similar layers in China that previous research has tied to the marine extinction at the end of the Permian period.

Over seven years, they collected 126 reptiles or amphibian skulls from a nearly 1,000-foot thick section of exposed Karoo sediment deposit from the time of the extinction.

The scientists said they found nothing in the Karoo that would indicate a body such as an asteroid hit around the time of the extinction, though they looked specifically for impact clays or material ejected from a crater left by such an impact.

Evidence from the Karoo, they said, is consistent with a mass extinction resulting from catastrophic ecosystem changes over a long time scale, not sudden changes associated with an impact.

In this case, they claim to have found ample evidence that the world got much warmer over a long period because of continuous volcanic eruptions in an area known as the Siberian Traps.

As volcanism warmed the planet, large stores of methane gas frozen on the ocean floor might have been released to trigger runaway greenhouse warming, the researchers said.

The species began dying out gradually as the planet warmed until conditions reached a critical threshold beyond which most species could not survive.