Kump et al. (2005) have suggested that the emission of H2S to the atmosphere from a Late Permian to Early Triassic euxinic (anoxic deep waters) ocean helped to bring about the massive extinction of organisms. Berner (1989; 2005) has suggested that the rate of burial of terrestrially derived organic matter during this period decreased considerably. We demonstrate here how decreased terrestrial organic burial should have led to positive feedback and reinforcement of H2S release, due to both a drop in atmospheric O2 and a rise in atmospheric CO2.

Positive feedback is best illustrated in terms of a systems analysis diagram (Fig. 1). To start, consider the eruption of the Siberian plateau basalts (e.g., Benton and Twitchett, 2003; Grard et al., 2005). These authors suggest that this eruption should have led to an increase in atmospheric CO2 and global warming, including warming of the ocean, via the atmospheric greenhouse effect (Fig. 1, arrows a and b). A coupled ocean-atmosphere general circulation model (Kiehl and Shields, 2005) has shown that high CO2 and ocean warming at this time likely led to decreased circulation and the development of a stratified anoxic ocean (arrow c). Stratification then should have brought about increased oceanic areas of bacterial sulfate reduction, a process that takes place only at depth in the absence of dissolved oxygen (arrow d). Hydrogen sulfide is produced by bacterial sulfate reduction. If the zone of H2S production gets close enough to the ocean surface, the H2S may escape into the atmosphere and kill organisms (Kump et al., 2005). Thus, as sulfate reduction increased, occasional emissions of H2S into the air led to increased die-offs of land plants (arrows e and f).

Protracted lethal H2S episodes should have led to a decrease in the standing crop of land plants, resulting in less production and less burial over time of organic debris derived from plants (arrow g). During the Permian, land plants were a major contributor to global organic matter deposition and burial (Berner, 2005). For every mole of organic matter buried, one mole of CO2 is removed from the atmosphere via the overall reaction:  
therefore, decreased organic burial should have led to an increase in atmospheric CO2 (arrow h). (The production of dead debris following short-term sporadic lethal episodes would exert only a minor long-term effect on CO2). Because this cycle began with an increase in CO2 and ends with a further increase in CO2, the whole cycle is reinforced and should have led to further poisoning of plants by H2S.

There is another positive feedback cycle shown by the succession of arrows e-f-g-i-j-k. More sulfate reduction led to increased H2S emission to the atmosphere, increased die-off of plants, and less terrestrially derived organic burial. Less organic burial should have led to less O2 production (Eq. 1) and a reduction of atmospheric O2 (Berner, 2005) (arrow i). Less atmospheric O2, upon gas exchange between the atmosphere and ocean, would then have led to reduced oceanic O2 and the enhancement of sulfate reduction (arrows j and k). Thus, increased sulfate reduction, by way of the poisoning of plants and lowering of atmospheric O2, led to a further increase in sulfate reduction.

In summary, the two cycles (b-c-d-e-f-g-h and e-f-g-i-j-k) both lead to the increased die-off of plants (and animals) due to the emission of H2S to the atmosphere. If H2S poisoning was an important cause of extinction across the Permo-Triassic boundary, then the cycles shown here illustrate that this process was self-reinforcing.

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