HYDROGEN SULFIDE AS A LINK BETWEEN MARINE AND TERRESTRIAL EXTINCTION

Oceanic anoxia is a compelling explanation for the marine component of the end-Permian mass extinction, but it fails to explain why there was a concomitant biotic disaster on land. Previously we proposed that release of hydrogen sulfide (H₂S) from euxinic surface waters might have created toxic atmospheric conditions that, together with intense warming, led to terrestrial extinction. Subsequent numerical modeling supports this proposal, but important modifications to the original simple scheme are emerging. An Earth system model of intermediate complexity reveals that the flux of hydrogen sulfide to the atmosphere is restricted to regions of strong winds, whereas high surface-water H₂S concentrations develop in regions where winds are weak (see the companion poster abstract by Meyer et al.). The geographic extent of carbon dioxide-rich waters that might have led to hypercapnia in sessile calcifying organisms is greater than that of H₂S, because H₂S reacts with oxygen in surface waters. Significant H₂S fluxes to the atmosphere begin once the phosphate content of the deep ocean exceeds about 3 times the modern value, causing modest accumulations of H₂S in the atmosphere. The steady-state atmospheric H₂S concentration likely increases proportionately to the phosphate content of seawater up to a threshold beyond which the atmosphere loses its ability to oxidize the H₂S and ozone depletion begins. The extent to which nitrogen may limit the development of H₂S rich deepwaters is also evaluated quantitatively, with implications for the evolution of the marine biosphere during the Paleoproterozoic.