Sulfur has four stable isotopes, S, S, S, and S, of which S is the most abundant by a large margin due to the fact it is created by the very common C in supernovas. Sulfur isotope ratios are almost always expressed as ratios relative to S due to this major relative abundance (95.0%). Sulfur isotope fractionations are usually measured in terms of δS due to its higher abundance (4.25%) compared to the other stable isotopes of sulfur, though δS is also sometimes measured. Differences in sulfur isotope ratios are thought to exist primarily due to kinetic fractionation during reactions and transformations.
Sulfur isotopes are generally measured against standards; prior to 1993, the Canyon Diablo troilite standard (abbreviated to CDT), which has a S:S equal to 22.220, was used as both a reference material and the zero point for the isotopic scale. Since 1993, the Vienna-CDT standard has been used as a zero point, and there are several materials used as reference materials for sulfur isotope measurements. Sulfur fractionations by natural processes measured against these standards have been shown to exist between -72‰ and +147‰, as calculated by the following equation:
As a very redox-active element, sulfur can be useful for recording major chemistry-altering events throughout Earths history, such as marine evaporites which reflect the change in the atmospheres redox state brought about by the Oxygen Crisis.