Multiple sulfur isotope constraints on microbial sulfate reduction below an Archean seafloor hydrothermal system.

Microbial sulfate reduction is among the most ubiquitous metabolic processes on earth. The oldest evidence of microbial sulfate reduction appears in the ca. 3.5 Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia. That evidence was found through analysis of quadruple sulfur isotopes of sulfate and sulfide minerals deposited on the seafloor. However, the activity of microbial sulfate reduction below the Archean seafloor remains poorly understood. Here, we report the quadruple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and less altered basaltic komatiite collected from the North Pole Dome area. The Δ33 S values of the sulfide minerals were nonzero negative, suggesting that sulfate reduction occurred below the Archean seafloor. To constrain the substrate sulfate sources and sulfate reduction processes, we constructed a numerical model. Comparing the modeled and observed sulfur isotopes, we show that the substrate sulfate comprises seawater sulfate with a negative Δ33 S anomaly and 34 S-enriched sulfate with no anomalous Δ33 S. The latter component probably represents sulfate produced by local hydrothermal processes. The maximum sulfur isotopic fractionation between the putative substrate sulfate and the observed sulfide minerals within the altered basalt and basaltic komatiite is 35‰, which is consistent with a microbial origin. Alternatively, thermochemical sulfate reduction may also produce sulfide. However, considering the hydrothermal temperature inferred from the metamorphic grade of the altered basalt, the sulfur isotopic fractionation produced by inorganic sulfate reduction is probably below 20‰. Collectively, larger fractionations imply the involvement of biological sulfate reduction processes, both in the hydrothermal system below the seafloor and in less altered subsurface settings.