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.

Even when extremely high levels of hydrogen sulfide were added to the two-dimensional models, hydroxyl radicals mopped them up and prevented ozone collapse.

The study appears in the latest issue of the journal Nature Geoscience.

Not So Disastrous

Lee Kump, a geochemist from Pennsylvania State University in University Park, was involved in earlier studies that predicted catastrophically high hydrogen sulfide levels at the end of the Permian.

The conditions created by earlier models "should have wiped out all life on Earth and not allowed anything to survive," Kump said. "It would have been impossible to hide from."

Kump said he welcomes the new study since it mitigates the dire consequences of anoxic oceans and helps to explain how some life managed to hold on.

But he also warned that the chemicals could still have played a substantial role in the mass extinction.

"Hydrogen sulfide levels may not have been enough to trigger ozone collapse. Nevertheless, these new models still show substantial increases," Kump said.

"We don't know what the consequences of that would be for terrestrial life."

Co-author Beerling added: "Hydrogen sulfide poisoning in the ocean is still a possibility. Our calculations don't rule that out."

(Related: "Toxic Deep-Ocean Water Triggered 'Great Dying' [November 26, 2007].)

UV Damage

Scientists also believe that the ozone layer still suffered some sort of collapse during the Permian—but that another set of chemicals was responsible.

Researchers, for example, have discovered mutated plant pollen that supports the theory that a depleted ozone layer was allowing damaging levels of ultraviolet light to reach Earth's surface.

"There is a very high increase in the abundance of tetrads—weird, mutated spores—in end-Permian rocks from all around the world," Beerling said.

"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," Kump added.

One alternative theory is that a bout of massive volcanic activity known as the Siberian Traps released hydrochloric acid and organohalides into the air.

"Volcanic activity is an even more likely explanation for the extinctions now, because we have ruled out these other possible alternatives from the list," Beerling said.

Paul Wignall, a palaeobiologist from the University of Leeds in England, was not involved in the study.

"Now that this study has shown the hydrogen-sulfide-and-methane model is unlikely to work, we're back to square one and scratching around for an extinction mechanism," Wignall said.

He pointed out, however, that the volcanic theories are still pure speculation.

"They might be correct about organohalogens," he said, "but there is no supporting field evidence yet."

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