Their studies indicate that enough H₂S was produced by such ocean upwellings at the end of the Permian to cause extinctions both on land and in the sea. And this strangling gas would not have been the only killer. Models by Alexander Pavlov of the University of Arizona show that the H₂S would also have attacked the planet's ozone shield, an atmospheric layer that protects life from the sun's ultraviolet (UV) radiation. Evidence that such a disruption of the ozone layer did happen at the end of the Permian exists in fossil spores from Greenland, which display deformities known to result from extended exposure to high UV levels. Today we can also see that underneath "holes" in the ozone shield, especially in the Antarctic, the biomass of phytoplankton rapidly decreases. And if the base of the food chain is destroyed, it is not long until the organisms higher up are in desperate straits as well.

   
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ABOUT THE AUTHOR(S)
PETER D. WARD is a professor in the University of Washington's biology department and its earth and space sciences division, where he investigates both realms. His terrestrial research centers on ancient mass extinction events as well as the evolution and ultimate extinction of the nautiluslike marine animals known as ammonites, which he described in his first article for Scientific American in October 1983. Ward also applies principles gleaned from studying the earth's earliest life-forms to research for the NASA Astrobiology Institute into potential habitats for life elsewhere. He discussed those environments in an October 2001 Scientific American article, "Refuges for Life in a Hostile Universe," written with Guillermo Gonzalez and Donald Brownlee, as well as in a popular book co-authored with Brownlee, Rare Earth: Why Complex Life Is So Uncommon in the Universe (Springer, 2000).