Questions regarding Precambrian sulfur isotope fractionation.

The discovery by Farquhar et al. (1) of mass-independent isotope fractionation of S and S in rocks formed more than ;2 billion years ago (Ga), but not in younger rocks, has boosted the theory postulating a dramatic change from an anoxic to an oxygen-rich atmosphere about 2 Ga. That is because the only known natural process that may cause mass-independent fractionation of both S and S involves atmospheric photochemical reactions by ultraviolet light in the absence of an ozone shield (1). Here, we suggest the strong possibility that the fractionation observed by Farquhar et al. (1) may have been created by analytical rather than natural processes. The magnitude of mass-independent fractionation of S or S, expressed as DS or DS, is defined as the deviation of a measured dS or dS value from the dS or dS value expected from the mass-dependent fractionation relationships. That is,