Analysis of the stable sulfur isotopes in sulfate and sedimentary sulfide can identify the activity of sulfate-reducing bacteria in marine sediments through Earth history. Sulfate-reducing bacteria prefer to reduce sulfate containing ³²S compared to sulfate containing ³⁴S, and therefore sedimentary sulfides depleted in ³²S were likely formed by sulfate-reducing bacteria. In modern sediments and in sediments deposited through the Phanerozoic, sulfides are however, generally depleted in ³²S by more than can be explained by sulfate reduction alone. Additional fractionation to that of sulfate reduction must occur in these sediments, probably during the reoxidation of sulfide to sulfate. The oxidative part of the sulfur cycle is very complex and generally no isotope fractionations are associated with the direct biological and chemical oxidation of sulfide to sulfate. The only processes known to deplete sedimentary sulfide in ³²S during the oxidative part of the sulfur cycle are the microbial disproportionation of elemental sulfur, thiosulfate and sulfite. The disproportionation processes might, therefore, be important in Phanerozoic sediments.

In the Precambrian sedimentary sulfides are not depleted by more than about 45‰, which is also the maximum fractionation yet observed during the microbial reduction of sulfate. Additional isotope fractionation during the oxidative part of the sulfur cycle was probably not important in the Precambrian. I will discuss which processes within the sulfur cycle might dominate in Phanerozoic sediments and in the Precambrian. Sedimentary sulfides depleted in ³²S by more than 45‰ are first encountered in sediments around 0.8 billion years old. This increase in fractionation might correlate with an increase in the O₂ concentration in the atmosphere at this time.