Washington, March 25 : A new study has shown that poisonous, ozone-destroying gases bubbling out of the oceans may not have triggered the world's greatest mass extinction, which had happened about 251 million years ago.
According to a report in National Geographic News, the mass extinction took place about 251 million years ago, at the end of the Permian period, when the world lost about 90 percent of its ocean species and 70 percent of its land species.
Scientists had suspected that the cause was high levels of hydrogen sulfide and methane in the atmosphere, which poisoned creatures and caused a collapse of the protective ozone layer.
"Toward the end of the Permian, we had a warming climate with much more carbon dioxide than today, ocean circulation was extremely sluggish, and the oceans became anoxic-essentially deprived of oxygen," explained geobiologist and study co-author David Beerling from the University of Sheffield in England.
Under these conditions, ocean microbes metabolize sulfur to produce hydrogen sulfide, which could have built up in the ocean and then welled up into the atmosphere.
"There is evidence for massive methane release at the end Permian as well, either from warming oceans or from coal deposits heated by extreme volcanic activity at around the same time," said Beerling.
But the discovery that the chemicals were unlikely to build up enough to destroy ozone leaves scientists hunting for another answer to the mystery of what caused such a biological catastrophe.
Beerling and his colleagues set up computer simulations of the Permian oceans and atmosphere to predict what might have happened when different amounts of hydrogen sulfide and methane were added to the mix.
"We found some interesting things going on with ozone chemistry, but we didn't find any evidence that hydrogen sulfide and methane triggered a collapse of the ozone layer," said Beerling.
"Previous models also used figures averaged for the globe-examining only altitude and not latitude-and thus overlooked the effects of hydroxyl radicals," he added.
According to Beerling, hydroxyl radicals are chemicals produced mainly at the tropics that oxidize and thus neutralize ozone-destroying pollutants.
Even when extremely high levels of hydrogen sulfide were added to the two-dimensional models, hydroxyl radicals mopped them up and prevented ozone collapse.
Scientists also believe that the ozone layer still suffered some sort of collapse during the Permian-but that another set of chemicals was responsible.
"This new study shows quite nicely that the collapse of the ozone layer may have required other circumstances than simply a large increase in hydrogen sulfide flux into the atmosphere," said Lee Kump, a geochemist from Pennsylvania State University in University Park.