Sulphate and sulphate reduction in early Precambrian oceans

Sulphate reduction generally causes isotopic fractionation of sulphur¹. Modern sedimentary sulphide is largely produced by biogenic reduction of sulphate and is typically enriched in ³²S (ref. 2). This is balanced by excess ³⁴S in the oceanic sulphate reservoir and evaporites³. High-temperature, inorganic reduction of sulphate may also cause fractionation^4,5. Since the work of Ault and Kulp⁶, there has been interest in finding the beginnings of sulphate reduction in the sedimentary record. This is important for several reasons. First, sulphate-respiring bacteria are a milestone of evolution^7,8. Second, it established the exogenic sulphur cycle in an essentially modern form. This, with the interconnected oxygen and carbon cycles, regulates the composition of atmosphere and oceans^9-11. Third, widespread evidence of sulphate reduction in rocks of a given age and younger indicates that sulphate was established as a major constituent of seawater. In addition to identifying a stage in the evolution of an oxygenated environment¹⁰, this has important metallogenic implications. Schidlowski⁸ has recently concluded that dissimilatory reduction commenced at 2,800-3,100 Myr in an Archaean ocean that had relatively high concentrations of sulphate. I review here the published data and present additional sulphur isotope analyses obtained from the early Precambrian of South Africa. These results indicate that sulphate was a minor component of Archaean and early Proterozoic ocean water, probably <0.001 mol l^-1. The concentration had increased by ~2,350 Myr to levels allowing significant biogenic and inorganic fractionation and the partitioning of ³²S/³⁴S in the exogene cycle.