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Project Progress

Progress has been made in three different areas. First, Alex Pavlov and I have completed a set of calculations and a paper on the topic of mass-independent fractionation (MIF) of sulfur isotopes in Archean rocks. We have shown that the observation of MIF in rocks older than 2.3 Ga implies that atmospheric O₂ levels were < 10^-5 times present levels at that time. In combination with the measurements obtained by James Farquhar and others, this essentially resolves the ongoing debate about when atmospheric O₂ first rose.

Second, Alex and I have also published two papers on methane photochemistry in anoxic atmospheres. One is a detailed model of the photochemical reactions. We showed that a hydrocarbon haze may have formed, but that it would not have been optically thick enough to provide an effective solar ultraviolet (UV) screen. The second paper argued that the isotopically light (in ¹³C) kerogens in 2.7-b.y. old Archean sediments may represent the remnants of that haze.

Finally, we have also taken initial steps in modeling Archean ecosystems. Janet Siefert, Pushker Kharecha, and I have been creating a numerical model of an anaerobic, methanogen-based Archean ecosystem. We hope to use this model to predict the methane concentration in the Archean atmosphere both before and after the invention of oxygenic photosynthesis.