THE ARCHEAN ATMOSPHERE PRIOR TO THE RISE OF OXYGEN

Most (but not all) researchers accept that atmospheric O2 experienced a large increase near 2.3 Ga. Little agreement has been reached, however, concerning the nature of the atmosphere prior to that time. Evidence from uranium deposits and paleosols indicates that pO2 was less than ~10-3 PAL (times the Present Atmospheric Level) during the Archean, but theoretical considerations suggest that pO2 was actually much lower. As pointed out originally by J.C.G. Walker (Evolution of the Atmosphere, 1977), the initial rise of O2 should have occurred when the net source of O2 from photosynthesis followed by burial of organic carbon exceeded the volcanic outgassing rate of reduced gases. Photochemical models based on this presumption predict that pO2 would have been <10-7 PAL even in the presence of a significant photosynthetic O2 flux. Under these low-O2 conditions, CH4 produced by methanogenic bacteria could have reached ~1000 ppmv, or 600 times higher than today's value. At this concentration, CH4 would have made a significant contribution to the atmospheric greenhouse effect. Thus, the faint young Sun problem can be resolved without exceeding upper limits on atmospheric CO2 inferred from paleosols. Polymerization of this CH4 by UV photolysis may have produced hydrocarbon haze similar to that observed today on Saturn's moon, Titan. The haze could not have been too optically thick in the visible, however, or the surface would have been strongly cooled by the anti-greenhouse effect. Modeling suggests that UV shielding by the haze would have been minimal, so that early organisms would have had to contend with a nearly unattenuated solar UV flux. Accumulation of haze particles in sediments may account for the extremely 13C-depleted kerogens found in Late Archean sediments. Finally, evidence for mass-independent fractionation of sulfur isotopes provides strong support for a very low-O2 Archean atmosphere.