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Evolution of Atmospheric O2, Climate, and the Terrestrial Biosphere 9dm)

This report covers work done in association with the rise of atmospheric O2. We have studied the amount of greenhouse warming expected from CH4 during the Archean Era. The thesis is that atmospheric CH4 levels should have been high in a low-O2 environment. This idea is supported by thermodynamic calculations that predict that methanogenic bacteria should have converted most of the available atmospheric H2 into CH4. We also predict that the transition from an O2-poor to an O2-rich atmosphere around 2.3 Ga could have triggered the Huronian glaciation by destroying the methane component of the atmospheric greenhouse.

We have described a possible mechanism for delaying the rise of atmospheric O2. The problem is that O2 concentrations did not increase substantially until ~2.3 Ga, whereas cyanobacteria were evidently producing O2 at least 400 million years earlier, and possibly well before that. We suggest that oxidized lithospheric slabs subducted deep into the mantle were brought back to the upper mantle by convection at 2.3 Ga, and that this caused a change in upper mantle redox state that, in turn, caused volcanic gases to become more oxidized.

We also have described quantitative modeling of an organic haze layer produced by photochemical reactions in a CH4-rich Archean atmosphere. We suggest that the isotopically light (i.e., low d13C) kerogens seen in Late Archean sediments are, in fact, the remnants of such an organic haze layer. This work is an outgrowth of work that we have been performing under separate funding from NASA’s Exobiology program.