Although there are 25 known isotopes of sulfur, only four are stable and of geochemical importance.  Of those four, two (S, light and S, heavy) comprise (99.22%) of sulfur on Earth.  The vast majority (95.02%) of sulfur occurs as S with only 4.21% in S.  The ratio of these two isotopes is fixed in the Solar System and has been since its formation.  The bulk Earth sulfur isotopic ratio is thought to be the same as the ratio of 22.22 measured from the Canyon Diablo troilite (CDT), a meteorite.  That ratio is accepted as the international standard and is therefore set at δ = 0.00.  Deviation from 0.00 is expressed as the δS which is a ratio in per mill (‰).  Positive values correlate to increased levels of S, whereas negative values correlate with greater S in a sample.  
Formation of sulfur minerals through non-biogenic processes does not substantially differentiate between the light and heavy isotopes, therefore sulfur isotope ratios in gypsum or barite should be the same as the overall isotope ratio in the water column at their time of precipitation.  Sulfate reduction through biologic activity strongly differentiates between the two isotopes because of the more rapid enzymic reaction with S. Average present day seawater values of δS are on the order of +21‰.  
Prior to 2010s, it was thought that sulfate reduction could fractionate sulfur isotopes up to 46 permil and fractionation larger than 46 permil recorded in sediments must be due to disproportionation of sulfur intermediates in the sediment. This view has changed since the 2010s that sulfate reduction can fractionate to 66 permil. As substrates for disproportionation are limited by the product of sulfate reduction, the isotopic effect of disproportionation should be less than 16 permil in most sedimentary settings.  
Throughout geologic history the sulfur cycle and the isotopic ratios have coevolved with the biosphere becoming overall more negative with the increases in biologically driven sulfate reduction, but also show substantial positive excursion. In general positive excursions in the sulfur isotopes mean that there is an excess of pyrite deposition rather than oxidation of sulfide minerals exposed on land.