Doctor of Philosophy (PhD)
Geology and Geophysics
The Earth’s geochemical evolution is recorded in the rocks that compose its lithosphere. Specifically, sulfate minerals have been identified as being repositories of information concerning the past hydrosphere, atmosphere and biosphere. This is due to the non-labile nature of SO42- and its ability to store a record of the oxidative reactions and oxygen sources involved in its formation. Microbial dissimilatory sulfate reduction (MDSR) and sulfide oxidation cause oxygen from H2O and O2 to be trapped to varying degrees in ambient, dissolved SO42-. In order to better interpret the H2O and O2 signals in SO42-, we must deepen our understanding of how sulfur redox processes incorporate and preserve O2 and H2O oxygen signals in SO42-. I attack this problem through 3 main questions. 1) Does the SO42- contained in the MDSR-intermediate, adenosine-5’-phosphosulfate (APS) exchange oxygen with water? 2) Can we predict the oxygen source ratios (O2:H2O) in SO42- produced from aerated pyrite oxidation, variable pH (2-11)and variable [Fe3+]? 3) How does the pH dependent competition between sulfite-water-oxygen exchange and sulfite oxidation effect the source ratios (O2:H2O) in produced SO42-? Each question constitutes an individual chapter in my dissertation. I show that APS-sulfate and water-oxygen do not exchange. The sulfite (SO32-)-H2O-oxygen exchange processes, in competition with SO32- oxidation, was determined to control the O2: H2O oxygen source ratio for SO42- formed during the oxidation of pyrite, resulting in a consistent O2-oxygen% in SO42- (25 ± 4%) produced from pyrite oxidation between pH 2-11. Slight differences in the oxygen source ratios found in these experiments point to the pH dependent rate competition between SO32--H2O-oxygen exchange and SO32- production vs. SO32- to SO42- oxidation. SO32- oxidation was found to be more sensitive to pH than exchange, which results in less H2O-oxygen being incorporated in precipitated SO42- produced from pyrite and SO32 at lower pH. This was assisted by a unique oxygen isotope parameter used in my experiments, the 17O-label. This study should provide a template for future use of 17O-labeled solutions in determining the role of H2O, O2, or O3 in the formation of other oxyanions.
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Kohl, Issaku E., "Oxidation of Reduced Sulfur Compounds: A Triple-oxygen-isotope Perspective" (2010). LSU Doctoral Dissertations. 3207.