Theoretical estimates of sulfoxyanion triple-oxygen equilibrium isotope effects and their implications

Abstract

Triple-oxygen isotope (d 18 O and D 0 17 O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which may rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors ( 18 a and 17 a) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18 a data and no 17 a estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18 a and 17 a for a suite of sulfoxyanions-including several sulfate, sulfite, sulfoxylate, and thiosulfate species-using quantum computational chemistry. We determine fractionation factors for sulfoxyanion ''water droplets" using the B3LYP/6-31G+(d,p) method; we additionally calculate higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) scaling factors, and we qualitatively estimate the importance of anharmonic zero-point energy (ZPE) corrections using a suite of gaseous sulfoxy compounds. Methodological scaling factors greatly impact 18 a predictions, whereas ZPE corrections are likely small (i.e., 6 1‰) at Earth-surface temperatures; existing experimental data best agree with 18 a predictions when including redox state-specific CCSD/aug-cc-pVTZ scaling factors. Theoretical pH-and temperature-specific bulk-solution (i.e., abundance-weighted average of all species) 18 a values yield root-mean-square errors for sulfate/water, sulfite/water, and thiosulfate/water equilibrium of 4.5‰ (n ¼ 18 experimental conditions), 3.7‰ (n ¼ 27), and 2.2‰ (n ¼ 3), respectively. However, sulfate-and sulfite-system agreement improves considerably when comparing experimental results only to SO 3 (OH) À /H 2 O (RMSE = 1.6‰) and SO 2 (OH) À /H 2 O (RMSE = 2.2‰) predictions, rather than bulk solutions. This is particularly true for the sulfite system at high and low pH, when SO 2 (OH) À is not the dominant species. We discuss potential experimental and theoretical biases that may lead to this apparent improvement. By combining 18 a and 17 a predictions, we additionally estimate that sulfate, sulfite, sulfoxylate, and thiosulfate species can exhibit D 0 17 O values as much as 0.199‰, 0.205‰, 0.101‰, and 0.186‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.

The triple oxygen isotope composition of marine sulfate and 130 million years of microbial control

“…These isotope effects are mass dependent, meaning they lead to much smaller Δ’ 17 O effects than that generated in atmospheric O 2 . As quantifications of kinetic effects in this system are lacking, we use, instead, theoretical predictions of equilibrium isotope effects to approximate the influence of microbial sulfur cycling ( 38 , 39 ). Here we find the inclusion of mass-dependent fractionation crucial to the interpretation of the smaller-magnitude Δ’ 17 O changes in marine sulfate from the Cretaceous to Cenozoic.…”

“…These could carry slight differences in θ (see ref. 39 ). Conversely, if the composition of sulfate does, indeed, carry a memory of atmospheric O 2 incorporation, it is possible to deviate from these relationships.…”

“…If microbial metabolism (MSR and otherwise) catalyzes an isotopic equilibrium with water as predicted by O studies ( 33 ), then theoretical equilibrium fractionation predictions can help refine a mechanistic understanding of the observed isotopic relationships. Quantum mechanical modeling, specifically, density functional theory, allows for calculations of specific triple oxygen isotope offsets tethered to local waters (meteoric or seawater itself) ( 39 ). The most parsimonious explanation of the Cretaceous–Cenozoic record is then contributions from both riverine sulfate and an MSR-catalyzed equilibrium with seawater.…”