PLANTS, H2S, CO2, O2 AND THE PERMO-TRIASSIC EXTINCTION

It has been suggested (Kump et al, 2005) that the emission of H2S to the atmosphere from a Late-Permian to Early Triassic euxinic (anoxic subsurface waters) ocean helped to bring about the massive extinction of organisms. This was due to both H2S poisoning and the destruction of atmospheric ozone leading to increased UV-B radiation reaching the earth surface. During the same period, atmospheric O2 dropped (Berner, 2005) due to a decrease in the rate of burial of terrestrially-derived organic matter and there was an increase in volcanic degassing of CO2 to the atmosphere. This combination should have led to two positive feedback loops 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 systems analysis. The first loop starts with the eruption of the Siberian plateau basalts which should have led to an increase in atmospheric CO2 and global greenhouse warming, including warming of the ocean. A coupled ocean-atmosphere general circulation model (Kiehl and Shields, 2005) has shown that high CO2 and ocean warming at this time should have led to the development of an euxinic ocean with increased areas of bacterial sulfate reduction to H2S..As the zone of H2S production got close enough to the ocean surface, as indicated by fossil biomarkers, the H2S, could have escaped into the atmosphere and brought about the killing of plants. Since terrestrial plant debris was a major source of global organic burial, decreased primary production over time led to decreased burial and decreased removal of CO2 from the atmosphere. This led in turn to CO2 buildup and global warming, reinforcing the warming due to volcanic–derived CO2, and resulting ultimately in an increase in euxiniciity and H2S release thereby completing a positive feedback loop.

The other loop involves atmospheric O2. Less terrestrially derived organic burial. should have led to less O2 production and a lowering of atmospheric O2. Less atmospheric O2, upon gas exchange between the atmosphere and ocean, would then have led to lower oceanic O2 and the enhancement of sulfate reduction Thus, increased sulfate reduction, by way of the poisoning of plants and lowering of atmospheric O2, led to a further increase in sulfate reduction and H2S emission to the atmosphere.