Multiple sulfur isotopes in Paleoarchean barites identify an important role for microbial sulfate reduction in the early marine environment

Bedded barites from the Barberton greenstone belt (South Africa and Swaziland) preserve a comprehensive record of atmospheric, oceanic and microbial processes involved in the formation and evolution of the Paleoarchean (3.6–3.2Ga) oceanic sulfate reservoir. Here, we report multiple sulfur isotopic compositions from four of these barite occurrences. Relatively constant mass-independent signatures (Δ36S/Δ33S=−1.0±0.2) within deposits support an important role for atmospheric photolysis in the production of oxidized sulfur, whereas 34S enrichments relative to the inferred composition of photolytic sulfate suggest drawdown of 34S by microbial sulfate reduction. Strong compositional overlap with barites from India and Western Australia indicates the presence of a large-scale and well-mixed marine sulfate pool. Covariation between δ34S and Δ33S within individual deposits also suggests a role for processes occurring in semi-closed basins fed by this global reservoir. Based on modeling results, we interpret variations in δ34S by local microbial sulfate reduction and correlations with Δ33S by weak inputs of sulfur from magmatic sources, microbial sulfide oxidation or sulfur disproportionation. This agrees with the early occurrence of sulfate reducers in the geological record as inferred from published microscopic pyrite data, and identifies their role as important in both global oceans and local basins in the Paleoarchean. ► S-isotope analyses of 3.55–3.23Ga barites from the Barberton greenstone belt. ► MIF-S suggests a dominant atmospheric sulfate source with Δ36S/Δ33S=−1. ► Global microbial sulfate reduction enriched the residual seawater pool in 34S. ► Oceanic mixing produced a marine sulfate reservoir with homogeneous δ34S. ► Distinct Δ33S/δ34S trends per deposit reflect basinal sulfate reduction and mixing.