Incorporation and elimination of molecular oxygen signal in sulfate: Experiments and modeling

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The stable isotope composition of atmospheric Oxygen in the geological past conveys rich information on the history of Earth system. Few compounds, however, are known to record oxygen isotope composition directly and reliably. Sulfate, being a non-labile oxyanion and capable of forming weakly soluble minerals, is thus far the only viable carrier from which direct atmospheric oxygen signals in the distant past can be retrieved. Nonetheless, how the oxygen signal is initially incorporated, later removed, or finally retained in sulfate is not clear. Earlier studies and the investigator?s initial results indicate that both water and oxygen contribute to the oxygen in intermediate and final sulfoxyanions generated during pyrite oxidation to sulfate. These earlier results also revealed that the current mechanistic understanding of the sulfide oxidation process is evidently inadequate. The full potential of this oxygen archive can only be attained through a study focusing on the kinetics of sulfur redox cycling. In this project, the investigator sets to develop and test, both theoretically and experimentally, a set of non-equilibrium, dynamic models for the oxygen signal incorporation during sulfide oxidation and for the oxygen signal elimination during dissimilatory sulfate reduction. It is expected that the study will 1) present a new mechanistic interpretation of sulfide to sulfate oxidation on mineral surface and in solution; 2) account for the intrinsic dynamics of competing oxidation and exchange pathways; and 3) unify the many chemical variables that appear to be responsible for the final oxygen apportionment in sulfate. At least five model predictions will be tested by quantifying rates of oxidation and exchange reactions as reflected in the oxygen isotope composition of sulfoxyanions. The experiments will utilize three-oxygen-isotope labeled water by which subtle changes in apportionment and in the magnitude of fractionation can both be measured. Although this study is initially driven by an interest in understanding a mysterious non-mass-dependent sulfate oxygen-17 depletion found in world-wide post-glacial deposits at about 635 million years ago, the result will offer an exemplary solution to a class of science problems that involve a set of dynamic isotope fractionation processes that rarely reach equilibrium. The project will be led by a postdoctoral researcher and a graduate student along with assisting undergraduate and high-school students of diverse ethnic and cultural backgrounds. Research results will be disseminated in introductory and upper-level undergraduate classes and in two local high schools. Two teaching modules will be developed to convey scientists? efforts in exploring mineral archives for ancient atmospheric signatures.