A number of microbial metabolisms fractionate sulfur isotopes in distinctive ways, and the sulfur isotopic fingerprints of these metabolisms can be preserved in minerals and ancient organic matter. By measuring the sulfur isotopic composition of these preserved materials, scientists can reconstruct ancient biological processes and the environments where they occurred. δS values in the geologic record have been inferred to reveal the history of microbial sulfate reduction and sulfide oxidation. Paired δS and ΔS records have also been used to show ancient microbial sulfur disproportionation.
Microbial dissimilatory sulfate reduction (MSR), an energy-yielding metabolism performed by bacteria in anoxic environments, is associated with an especially large fractionation factor. The observed ε values range from 0 to −65.6‰. Many factors influence the size of this fractionation, including sulfate reduction rate, sulfate concentration and transport, availability of electron donors and other nutrients, and physiological differences like protein expression. Sulfide produced through MSR may then go on to form the mineral pyrite, preserving the S-depleted fingerprint of MSR in sedimentary rocks. Many studies have investigated the δS values of ancient pyrite in order to understand past biological and environmental conditions. For example, pyrite δS records have been used to reconstruct shifts in primary productivity levels, changing ocean oxygen content, and glacial-interglacial changes in sea level and weathering. Some studies compare sulfur isotopes in pyrite to a second sulfur-containing material, like dissolved sulfate or preserved organic matter. Comparing pyrite to another material gives a fuller picture of how sulfur moved through ancient environments: it provides clues about the size of ancient ε values and the environmental conditions controlling MSR fractionation of sulfur isotopes.