Unlocking Sulfur's Secrets

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Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 11
DOI: 10.1126/science.340.6128.11-b

The sulfur isotope signatures of sediments and aerosol particles provide a rich record of biological and chemical processes in modern environments and in the geologic record. For example, the ratio of two or more of sulfur's several stable isotopes can indicate whether it was enzymatically reduced by microorganisms or if it underwent photolysis in the atmosphere after exposure to ultraviolet (UV) radiation. There are myriad proposed mechanisms to explain the underlying reactions that induce isotopic fractionation; disentangling them from one another is critical for interpreting their preservation. In a series of photochemical experiments, Ono et al. demonstrate that the partial pressure of SO2 strongly influences the sulfur mass-independent fractionation, at least in part due to self-shielding from UV photolysis by other SO2 molecules with a different isotopic composition. The experimental isotopic patterns are consistent with sulfate aerosols generated in the stratosphere, where UV photolysis is mostly confined; however, around 3 billion years ago, Earth's atmosphere may have been much more transparent to UV radiation, creating a more extensive record of mass-independent fractionation. Farquhar et al. present an explanation for why the sulfur isotope signal generated by sulfate-reducing microorganisms—which were thought to have been present at that time—does not overprint mass-independent fractionation signals generated in Earth's early atmosphere. Spot analysis of sedimentary pyrites from Archean rocks suggests that there were two distinct sulfur pools contributing to most isotopic signatures: one of soluble oceanic sulfate and one of zero-valent sulfur derived from atmospheric deposition.

J. Geophys. Res. 118, 10.1002/jgrd.50183; Proc. Nat. Acad. Sci. U.S.A. 110, 10.1073/pnas.1218851110 (2013).

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