Oxidation of Reduced Sulfur Compounds: A Triple-oxygen-isotope Perspective

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

Extreme enrichment in atmospheric 15N15N.

Molecular nitrogen (N2) comprises three-quarters of Earth's atmosphere and significant portions of other planetary atmospheres. We report a 19 per mil (‰) excess of 15N15N in air relative to a random distribution of nitrogen isotopes, an enrichment that is 10 times larger than what isotopic equilibration in the atmosphere allows. Biological experiments show that the main sources and sinks of N2 yield much smaller proportions of 15N15N in N2. Electrical discharge experiments, however, establish 15N15N excesses of up to +23‰. We argue that 15N15N accumulates in the atmosphere because of gas-phase chemistry in the thermosphere (>100 km altitude) on time scales comparable to those of biological cycling. The atmospheric 15N15N excess therefore reflects a planetary-scale balance of biogeochemical and atmospheric nitrogen chemistry, one that may also exist on other planets

Early formation of the Moon 4.51 billion years ago

Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system

No oxygen isotope exchange between water and APS-sulfate at surface temperature: Evidence from quantum chemical modeling and triple-oxygen isotope experiments

In both laboratory experiments and natural environments where microbial dissimilatory sulfate reduction (MDSR) occurs in a closed system, the δ34SSO4 (( 34S/ 32S) sample/( 34S/ 32S) standard-1) for dissolved SO 42- has been found to follow a typical Rayleigh-Distillation path. In contrast, the corresponding δ18OSO4 (( 18O/ 16O) sample/( 18O/ 16O) standard)-1) is seen to plateau with an apparent enrichment of between 23‰ and 29‰ relative to that of ambient water under surface conditions. This apparent steady-state in the observed difference between δ18OSO4 and δ18OH2O can be attributed to any of these three steps: (1) the formation of adenosine-5\u27-phosphosulfate (APS) from ATP and SO 42-, (2) oxygen exchange between sulfite (or other downstream sulfoxy-anions) and water later in the MDSR reaction chain and its back reaction to APS and sulfate, and (3) the re-oxidation of produced H 2S or precursor sulfoxy-anions to sulfate in environments containing Fe(III) or O 2. This study examines the first step as a potential pathway for water oxygen incorporation into sulfate. We examined the structures and process of APS formation using B3LYP/6-31G(d,p) hybrid density functional theory, implemented in the Gaussian-03 program suite, to predict the potential for oxygen exchange. We conducted a set of in vitro, enzyme-catalyzed, APS formation experiments (with no further reduction to sulfite) to determine the degree of oxygen isotope exchange between the APS-sulfate and water. Triple-oxygen-isotope labeled water was used in the reactor solutions to monitor oxygen isotope exchange between water and APS sulfate. The formation and hydrolysis of APS were identified as potential steps for oxygen exchange with water to occur. Quantum chemical modeling indicates that the combination of sulfate with ATP has effects on bond strength and symmetry of the sulfate. However, these small effects impart little influence on the integrity of the SO 42- tetrahedron due to the high activation energy required for hydrolysis of SO 42- (48.94kcal/mol). Modeling also indicates that APS dissociation via hydrolysis is achieved through cleavage of the P-O bond instead of S-O bond, further supporting the lack of APS-H 2O-oxygen exchange. The formation of APS in our in vitro experiments was verified by HPLC fluorescence spectroscopy, and triple-oxygen isotope data of the APS-sulfate indicate no oxygen isotope exchange occurred between APS-sulfate and water at 30°C for an experimental duration ranging from 2 to 120h. The study excludes APS formation as one of the causes for sulfate-oxygen isotope exchange with water during MDSR. © 2012 Elsevier Ltd

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Early formation of the Moon 4.51 billion years ago.

Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system

Methane sources and sinks in continental sedimentary systems: New insights from paired clumped isotopologues 13CH3D and 12CH2D2

Stable isotope compositions of methane (δ13C and δD) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane 'clumped' isotopologues Δ13CH3D and Δ12CH2D2 are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis. In this study, we report the first systematic use of methane Δ13CH3D and Δ12CH2D2 in the context of continental sedimentary basins. We investigated sedimentary formations from the Southwest Ontario and Michigan Basins, where the presence of both microbial and thermogenic methane was previously proposed. Methane from the Silurian strata coexist with highly saline brines, and clumped isotopologues exhibit large offsets from thermodynamic equilibrium, with Δ12CH2D2 values as low as -23‰. Together with conventional δ13C and δD values, the variability in Δ13CH3D and Δ12CH2D2 to first order reflects a mixing relationship between near-equilibrated thermogenic methane similar to gases from deeper Cambrian and Middle Ordovician units, and a source characterized by a substantial departure from equilibrium that could be associated with microbial methanogenesis. In contrast, methane from the Devonian-age Antrim Shale, associated with less saline porewaters, reveals Δ13CH3D and Δ12CH2D2 values that are approaching low temperature thermodynamic equilibrium. While microbial methanogenesis remains an important contributor to the methane budget in the Antrim Shale, it is suggested that Anaerobic Oxidation of Methane (AOM) could contribute to reprocessing methane isotopologues, yielding Δ13CH3D and Δ12CH2D2 signatures approaching thermodynamic equilibrium