Numerous natural processes are capable of fractionating sulfur isotopes. Microbes are capable of a wide variety of sulfur metabolisms, including the oxidation, reduction, and disproportionation (or simultaneous oxidation and reduction) of sulfur compounds. The effect of these metabolisms on sulfur isotopic composition of the reactants and products is also highly variable, depending on the rate of relevant reactions, availability of nutrients, diagenesis, and other biological, physical and environmental parameters. As an example, the microbial reduction of sulfate to sulfide generally results in a S-depleted product, but the strength of this fractionation has been shown to range from 0 to 65.6‰ VCDT.
Many abiotic processes also fractionate sulfur isotopes. Small fractionations with ε values from 0–5‰ have been observed in the formation of the mineral gypsum, an evaporite mineral produced through the evaporation of seawater. Some sulfide minerals, including pyrite and galena, can form through thermochemical sulfate reduction, a process in which seawater sulfate trapped in seafloor rock is reduced to sulfide by geological heat as the rock is buried; this process generally fractionates sulfur more strongly than gypsum formation.
Prior to the rise of oxygen in Earth's atmosphere (referred to as the Great Oxidation Event), additional sulfur-fractionating processes referred to as mass-anomalous or mass-independent fractionation uniquely affected the abundance of S and S in the rock record. Mass-anomalous fractionations are rare, but they can occur through certain photochemical reactions of gases in the atmosphere. Studies have shown that photochemical reactions of atmospheric sulfur dioxide can cause substantial mass-anomalous fractionation of sulfur isotopes.