DEATH BY HYDROGEN SULFIDE: A KILL MECHANISM FOR THE END-PERMIAN MASS EXTINCTION

In the absence of compelling evidence for extraterrestrial causes of the end-Permian mass extinction, most proposed explanations center on internal disruptions to the biosphere, occurring at a time when it was particularly sensitive to such disturbances. Currently favored mechanisms center on the presence of widespread oceanic anoxia, perhaps triggered by greenhouse warming induced by Siberian trap volcanism. The spread of anoxic waters onto continental shelves restricted the available habitat for aerobic organisms, and upwelling of sulfidic and CO₂-rich deepwaters may have induced hypercapnia in planktonic organisms. However, we find mass extinction on land to be an unlikely consequence of carbon dioxide levels equilibrated with the ocean at 2000ppmv, only 7 times the preindustrial level.

We thus propose a modification of the anoxia mechanism, one that considers the possibility that the chemocline separating anoxic, sulfidic deep waters from the atmosphere, originally at the base of the wind-mixed layer of the ocean, collapsed catastrophically to the surface as sulfide levels built up in the deep ocean, especially if Late Permian oxygen levels were significantly lower than today’s. We calculate the conditions under which such a collapse would occur, and the consequences for the resultant flux of H₂S to the atmosphere. Widespread outcropping of sulfidic waters creates a flux to the atmosphere that quickly depletes the troposphere of hydroxyl radical, thereby allowing a hydrogen-sulfide rich plume to waft across the continents. In the Permian, such a cloud would poison terrestrial organisms in the way that aqueous H₂S had already caused the demise of marine organisms. Biomarker data from Permian deep-sea sediments from Japan are evaluated in light of the predicted burst of productivity of photoautotrophic sulfur bacteria.