Micro-scale quadruple sulfur isotope analysis of pyrite from the ~3480 Ma Dresser Formation: New insights into sulfur cycling on the early Earth

We report in situ quadruple sulfur isotope analysis (³²S, ³³S, ³⁴S and ³⁶S) of a pyritized microbial mat from the ∼3480 Ma Dresser Formation, Pilbara Craton, Western Australia. These data yield positive δ³⁴S and Δ³³S, indicative of sulfur sourced from a pool with similar character as the putative atmospheric elemental sulfur channel of Pavlov and Kasting (2002). Contrary to previous data from the Dresser Formation, however, this pyrite is heavily depleted in ³⁶S with a Δ³⁶S/Δ³³S slope of c. −3.6, much steeper than slopes typically seen in other early Archean rock successions (Δ³⁶S/Δ³³S ≈ −1) which suggests either a different atmospheric signature for deposited S or a different pool altogether. Significant micro-scale isotopic heterogeneity is observed within the microbial mat (δ³⁴S = +1.6‰ to +6.7‰; Δ³³S = +0.4‰ to +2.6‰; Δ³⁶S = −3.1‰ to −8.1‰), implying a role for microbial S metabolism. While metabolic S cycling has been shown to shift Δ³⁶S to lower values, microbial metabolization of S does not appear sufficient to account for the full range of Δ³⁶S.We conclude that the isotopic composition of the pyrite was controlled by the relative proportions of mass independently fractionated (MIF) S⁰ and sulfate-derived sulfur incorporated into polysulfide pyrite precursors during reactions in the microbial mat. The dominance of the MIF-S⁰ isotopic signature (+δ³⁴S, +Δ³³S, −Δ³⁶S) indicates that contributions from the sulfate-derived sulfur pool were relatively small, consistent with low concentrations of sulfate in Archean seawater, and that contributions from a non-sulfate pool were significant. Micro-scale isotopic heterogeneity in the pyrite points to mixing between the two sulfur pools in selected micro-environments within the microbial mat. The particularly negative Δ³⁶S observed here reveals a 3480 Ma sulfur reservoir with novel Δ³⁶S/Δ³³S chemistry whose significance now requires further investigation.