The most likely culprit for the gradual bryozoan die-offs was the upwelling of toxic water unleashed from the deep ocean by rising global temperatures.
At the time, "there was massive volcanism in Siberia, which threw up lots of carbon dioxide into the atmosphere—leading to global warming," Powers said. The warming of surface water decreased the oceans' ability to absorb oxygen, she said.
This caused the upwelling of toxic hydrogen sulfide, which is produced by deep-ocean bacteria that don't use oxygen to survive.
With a mix of low oxygen, high carbon dioxide, and high hydrogen sulfide, the ocean was "a bit of a witches' brew coming out of deeper water," said study co-author David Bottjer of the University of Southern California.
Co-author Powers said the atmosphere and oceans are always interacting.
"So it is likely that toxic fumes of hydrogen sulfide diffused out of the oceans, [killing] land-based animals, which plummeted by 70 percent in the same mass extinction event."
Margaret Fraiser, a paleoecologist from the University of Wisconsin at Milwaukee, was not involved in the study.
"Until now we've [only] had geochemical and sedimentological evidence for these environmental perturbations towards the end of the Permian," she said.
"Now we have this biological information on how these organisms responded to that stress."
Other theories behind the Permian extinction involve volcanic eruptions and meteors.
(Related news: "Did Million-Year-Long Eruptions Cause Mass Extinction?" [May 2, 2006].)
"A lot of extinction mechanisms recently proposed for the end of the Permian have all focused on short, sharp, sudden catastrophic things, like very intense volcanism and meteorite impacts," said Paul Wignall, a paleoecologist at the University of Leeds in the United Kingdom.
Even if such cataclysmic impacts did take place, "they were superimposed on a background of pretty stressful conditions anyway," he said.
This study, Wignall commented, is "making the pendulum swing back the other way, away from the idea that everything happened in an instant."
The study, which appears in this month's issue of the journal Geology, raises the issue of how modern-day global warming will affect life in the oceans and on land.
Predicting whether the oceans will once again become stagnant due to modern global warming is not possible, study co-author Powers said.
"At the end of the Permian, the continents were not in the same place they are today," she said.
"Instead they formed a single supercontinent, which means that ocean circulation would have been completely different."
Even so, ancient and current global warming patterns are related phenomena, said co-author Bottjer.
"Whether modern-day global warming will get to the level where it creates such a crisis as we have been working on, we can't say," he said.
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