World's Greatest Extinction Not Caused By Toxic Gases

Helen Scales
for National Geographic News
March 24, 2008

Poisonous, ozone-destroying gases bubbling out of the oceans may not have triggered the world's greatest mass extinction after all, a new study shows.

The "Great Dying" 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," Beerling said.

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.

Self-Cleaning System

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," Beerling said.

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.

"These are chemicals produced mainly at the tropics that oxidize [and thus neutralize] ozone-destroying pollutants," Beerling said.

Continued on Next Page >>


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