text: Two mechanisms of fractionation occur that alter sulfur stable isotope ratios: kinetic effects, especially due to the metabolism of sulfate-reducing bacteria, and isotope exchange reactions that occur between sulfide phases based on temperature. With VCDT as the reference standard, natural δS value variations have been recorded between -72‰ and +147‰.
  The presence of sulfate-reducing bacteria, which reduce sulfate () to hydrogen sulfide (HS), has played a significant role in the oceanic δS value throughout the earth's history. Sulfate-reducing bacteria metabolize S more readily than S, resulting in an increase in the value of the δS in the remaining sulfate in the seawater. Archean pyrite found in barite in the Warrawoona Group, Western Australia, with sulfur fractionations as great as 21.1‰ hint at the presence of sulfate-reducers as early as .
  It is now better known that the degree of isotope fractionation during microbial sulfate reduction depends on the cell-specific sulfate reduction rate of the sulfate-reducing microorganism. The relative extent of sulfur isotope fractionating activities, including sulfate reduction, sulfide reoxidation and disproportionation, determines the isotopic compositions of the minerals or fluid measured. Other than microbial activities and environmental conditions, isotopic compositions also change due to diffusion, accumulation and mixing after burial.
  The δS value, recorded by sulfate in marine evaporites, can be used to chart the sulfur cycle throughout earths history. The Great Oxygenation Event around  altered the sulfur cycle radically, as increased atmospheric oxygen permitted an increase in the mechanisms that could fractionate sulfur isotopes, leading to an increase in the δS value from ~0‰ pre-oxygenation. Approximately , the δS values in seawater sulfates began to vary more and those in sedimentary sulfates grew more negative. Researchers have interpreted this excursion as indicative of an increase in water column oxygenation with continued periods of anoxia in the deepest waters. Modern seawater sulfate δS values are consistently 21.0 ± 0.2‰ across the worlds oceans, while sedimentary sulfides vary widely. Seawater sulfate δS and δO values exhibit similar trends not seen in sedimentary sulfide minerals.