Evaluating the role of microbial sulfate reduction in the early Archean using quadruple isotope systematics

Microscopic pyrites with low 34S/32S ratios in ~ 3.47-Gyr-old sedimentary barites from North Pole, Australia have been interpreted as evidence for microbial sulfate reduction and/or sulfur disproportionation in the early Archean. We show that these microscopic sulfides have similar to slightly less negative Δ33S and slightly more negative Δ36S values compared to the enclosing sulfate crystals. This finding is consistent with a primary mass-independent signature overprinted by biological sulfate reduction, as calibrated by previous experimental laboratory culture studies. However, it is inconsistent with an overprint by abiological sulfate reduction or sulfur disproportionation, as predicted by isotope exchange theory and laboratory culture studies. Thus, our multiple sulfur isotope measurements support the contention that sulfate-reducing microbes had evolved by ~ 3.47 billion years ago.