LAUSR

Abstract The existence of mass-independently fractionated sulfur in Archean rocks is almost universally accepted as evidence for low atmospheric O2 and O3 concentrations at that time. But the detailed patterns of the Δ 33 S values and of the ratios Δ 33 S / δ 34 S and Δ 36 S / Δ 33 S remain to be explained, and the mechanism for producing the mass-independent fractionation remains controversial. Here, we explore the hypothesis that the relatively low Δ 33 S values seen during the Mid-Archean, 2.7-3.5 Ga, were caused by the presence of organic haze produced from photolysis of methane. This haze helped shield SO2 from photolysis, while at the same time providing surfaces on which unfractionated short-chain sulfur species could condense. The evolution of oxygenic photosynthesis, and the concomitant disappearance of organic haze towards the end of the Archean allowed more negatively fractionated S4 and S8 to form, thereby generating large positive fractionations in other sulfur species, including sulfate and H2S. Reduction of this sulfate to H2S by bacteria, followed by incorporation of H2S into pyrite, produced the large positive Δ 33 S values observed in the Neoarchean rock record, 2.5-2.7 Ga.