Multiple-Isotope Insights into the Earth's Early Sulfur Cycle

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FarquharEAR-0003419Preliminary sulfur multiple-isotope data point to a fundamental change in the processes controlling the sulfur cycle during the Paleoproterozoic (2.0 to 2.5 Ga). These data suggest that before this time, the sulfur cycle was strongly influenced by atmospheric sulfur chemistry. After this time, the sulfur cycle was more strongly influenced by oxidative weathering and microbial sulfate reduction, and is inferred to be similar to the modern, pre-anthropogenic sulfur cycle. To extend these preliminary conclusions and to understand the role of oxidative weathering and microbial reduction on the Precambrian sulfur cycle, a three part strategy is needed: (1) Additional documentation of the sulfur multiple-isotope geochemistry in sedimentary, metasedimentary and igneous rocks will provide a record of the role of atmospheric sulfur chemistry in the sulfur cycle over time; (2) A deeper examination of the Paleoproterozoic transition period that is evidenced by D33S shifts in our preliminary data, will add new insights into the changes that occurred at 2.0-2.5 Ga and explore possible links to the intensity of oxidatitve weathering and the atmospheric oxidation state; (3) further investigation of the Archean sulfur cycle will constrain the roles of atmospheric reactions, oxidative weathering, and microbial activity in the Earth's earliest sulfur cycle.