Within the epicratonic Selwyn Basin, at least three cycles can be recognized for Paleozoic time when the water column alternated from open and ventilated to closed and stratified conditions. These cycles are recorded by δ34S values in pyrite that exceed those for coeval seawater during periods of stagnation and decrease markedly during periods of greater circulation. The marked increase in δ34S values for pyrite formed in stratified seas is controlled by the high percentage of sulfate bacterially reduced to sulfide, coupled with the removal of isotopically lighter sulfur from a closed system during pyrite sedimentation. The δ34S curve for barite during this time has a shape similar to the mean evaporite curve except that it is displaced positively, particularly in Frasnian time. This pronounced increase in δ34S values for barite of Frasnian age coincides with the mixing of isotopically heavier sulfur, accumulated earlier in a stratified water column, with surface waters during a ventilation event.