When SO42− is assimilated by organisms, it is reduced and converted to organic sulfur, which is an essential component of proteins. However, the biosphere does not act as a major sink for sulfur, instead the majority of sulfur is found in seawater or sedimentary rocks especially pyrite rich shales and evaporite rocks (anhydrite and baryte). The amount of sulfate in the oceans is controlled by three major processes
1. input from rivers
2. sulfate reduction and sulfide reoxidation on continental shelves and slopes
3. burial of anhydrite and pyrite in the oceanic crust.