Ratios of Sulfur Isotopes (32S, 33S, 34S, and 36S) in Archean Rocks of Karelia: Evidence of Microbial Life and the Anoxic Atmosphere

--We have studied sulfur isotope ratios (S-32, S-33, S-34, and S-36) archived in the volcanic and ferruginous rocks of the Sumozero-Kenozero (ca. 2.9 Ga) and Kostomuksha (ca. 2.7 Ga) greenstone belts of the Fennoscandian Shield. The obtained data provide new arguments for developing ideas of the bacterial activity in the Archean ocean and the involvement of sulfur in the biogenic cycle and also indicate no free oxygen in the Archean Earth's atmosphere. The isotope data on S-34 and S-33 show a high activity of sulfate-reducing and sulfur-disproportionating bacteria in the surface environments of Archean sedimentary rocks. The discovery of S-33 and S-36 anomalies in the studied rocks is the main evidence for the anoxic atmosphere during the formation of Meso- and Neoarchean deposits in Karelia.