Can organic haze and O2 plumes explain patterns of sulfur mass-independent fractionation during the Archean?
Peng Liu1,2, Chester E. Harman3,4,5, James F. Kasting2, Yongyun Hu1, and Jingxu Wang1
1 Department of Atmospheric and Oceanic Sciences, Peking University, Beijing 100871, China
2 Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA
3 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA
4 NASA Goddard Institute for Space Studies, New York, NY 10025, USA
5 NASA Astrobiology Institute–Virtual Planetary Laboratory, USA
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 33S values and of the ratios 33S/34S and 36S/33S remain to be explained, and the mechanism for producing the mass-independent fractionation remains controversial. Here, we explore the hypothesis that the relatively low 33S 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 33S values observed in the Neoarchean rock record, 2.5-2.7 Ga.
Full Text: https://doi.org/10.1016/j.epsl.2019.115767