Mass-independent sulfur isotope fractionation during photochemistry of sulfur dioxide

Mass-independent sulfur isotope signatures are observed in Archean and early Paleoproterozoic sedimentary sulfate and sulfide minerals, and provide the most robust constraints on early atmospheric oxygen levels. Smaller mass-independent sulfur isotope anomalies are observed in ice cores and interpreted as a tracer of stratospheric volcanic loading. Photochemistry of sulfur dioxide (SO 2) has been implicated as a possible source of the mass-independent sulfur isotope signatures in both the modern stratosphere and on the early earth. However, the mechanisms responsible for the production of mass-independent sulfur isotope fractionation remain poorly constrained. This thesis investigates the multiple sulfur isotope systematics during photochemical reactions of sulfur dioxide as a function of a variety of experimental conditions. Two absorption regions of SO 2 are tested photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. Experimental conditions modified include temperature, SO 2 pressure, bath gas pressure, and addition of reactive gases (C2 H2, 02 and CH 4). Results of photochemical experiments are compared with isotope systematics predicted from isotopologue-specific absorption cross-sections to identify potential mechanisms for the production of mass-independent fractionation during photochemical reactions. Strong similarity between the isotope systematics of SO 2 photolysis and ice core data suggest that SO 2 photolysis is responsible for the production of mass-independent sulfur isotope effects in the modern stratosphere. In contrast, significant discrepancies between the isotope signatures from SO 2 photochemistry and those in the Archean record suggest that, although SO 2 photolysis was likely an important process in the Archean atmosphere, an additional reaction likely contributes to the mass-independent sulfur isotope signatures preserved in the Archean rock record. Thesis Supervisor: Shuhei Ono Title: Kerr-McGee Career Development Professor of Biogeochemistry