The conditions controlling the formation of sedimentary dolomite are still poorly understood despite decades of research. Reconstructing formation temperatures and δ18O of fluids from which dolomite has precipitated is fundamental to constrain dolomitization models. Carbonate clumped isotopes are a very reliable technique to acquire such information if the original composition at the time of precipitation is preserved. Sedimentary dolomite first mostly forms as a poorly-ordered metastable phase (protodolomite) and subsequently transform to the more stable ordered phase. Due to this conversion its important to determine if the original clumped isotope composition of the disordered phase is preserved during diagenetic conversion to ordered dolomite, and how resistant clumped isotope signatures are against bond reordering at elevated temperatures during burial diagenesis. Here, we present a series of heating experiments at temperatures between 360 and 480 °C with durations between 0.125 and 426 hours. We uses fine-grained sedimentary dolomites to test the influence of grains size, and cation ordering on bond reordering kinetics. We analyzed a lacustrine dolomite with poor cation ordering and well ordered a replacement dolomite, both being almost stoichiometric. The poorly ordered dolomite shows a very rapid alteration of its bulk isotope composition and higher susceptibility to solid state bond reordering, whereas the well-ordered dolomite behaves like a previously studied coarse-grained hydrothermal dolomite. We derive dolomite-specific reordering kinetic parameters for ordered dolomitea and show that ∆47 reordering in dolomite is material specific. Our results call for further temperature-time series experiments to constrain dolomite ∆47 reordering over geologic timescales.

Triple-oxygen isotope (δ18O and Δ17O) 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 can rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate isomers—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets”; using the B3LYP/6-31G+(d,p) method; we additionally determine higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) and anharmonic zero-point energy (ZPE) scaling factors using a suite of gaseous sulfoxy compounds and test their impact on resulting sulfoxyanion fractionation-factor estimates. When including redox state-specific CCSD/aug-cc-pVTZ and anharmonic ZPE scaling factors, our theoretical 18α predictions for protonated isomers closely agree with all existing experimental data, yielding root-mean-square errors of 1.8 ‰ for SO3(OH)-/H2O equilibrium (n = 18 experimental conditions), 2.2 ‰ for SO2(OH)-/H2O (n = 27), and 3.9 ‰ for S2O2(OH)-/H2O (n = 3). This result supports the idea that oxygen exchange occurs via isomers containing oxygen-bound protons. By combining 18α and 17α predictions, we additionally estimate that SO3(OH)-, SO2(OH)-, SO(OH)-, and S2O2(OH) exhibit Δ17O values as much as 0.167 ‰, 0.097 ‰, 0.049 ‰, and 0.153 ‰ 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.