The organic component of the earliest sulfur cycling

The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the sulfate-poor ferruginous oceans of the Archean eon, organic sulfur must have played an important and previously unrecognized role in the formation of sulfides. In the anoxic ocean, mineralization of organic sulfur generated hydrogen sulfide, which provided a pathway to pyrite that bypassed the microbial reduction of sulfate, while organic-sourced sulfite could fuel microbial S reduction in the absence of ambient sulfate. Reaction transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. By offering an alternative explanation for the low range of δ34S in Archean sulfides, these effects alter the presently accepted picture of the Early Earth sulfur cycle, with a significant proportion of oceanic sulfur throttled through living cells. They also raise a possibility that sulfate scarcity in the anoxic mid-Archean oceans delayed the evolution of dissimilatory reduction of sulfate until the initial ocean oxygenation around 2.7 Ga.