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Oxygen self-diffusion in calcite and many other minerals is considerably faster under wet conditions relative to dry conditions. Here we investigate whether this "water effect" also holds true for solid-state isotope exchange reactions that alter the abundance of carbonate groups with multiple rare isotopes ('clumped' isotope groups) via the process of solid-state bond reordering. We present clumped-isotope reordering rates for optical calcite heated under wet, high-pressure (100 MPa) conditions. We observe only modest increases in reordering rates under such conditions compared with rates for the same material reacted in dry CO2 under low-pressure conditions. Activation energies under wet, high-pressure conditions are indistinguishable from those for dry, low-pressure conditions, while rate constants are resolvably higher (up to ∼3 times) for wet, high-pressure relative to dry, low-pressure conditions in most of our interpretations of experimental results. This contrasts with the water effect for oxygen self-diffusion in calcite, which is associated with lower activation energies, and diffusion coefficients that are ≥10 3 times higher compared with dry (pure CO2) conditions in the temperature range of this study (385-450 °C). The water effect for clumped-isotopes leads to calculated apparent equilibrium temperatures ("blocking temperatures") for typical geological cooling rates that are only a few degrees higher than those for dry conditions, while O self-diffusion blocking temperatures in calcite grains are ∼150-200 °C lower in wet conditions compared with dry conditions. Since clumped-isotope reordering is a distributed process that occurs throughout the mineral volume, our clumped-isotope results support the suggestion of Labotka et al. (2011) that the water effect in calcite does not involve major changes in bulk (volume) diffusivity, but rather is primarily a surface phenomenon that facilitates oxygen exchange between the calcite surface and external fluids. We explore the mechanism(s) by which clumped isotope reordering rates may be modestly increased under wet, high-pressure conditions, including changes in defect concentrations in the near surface environment due to reactions at the water-mineral interface, and lattice deformation resulting from pressurization of samples. 

Sulfur-cycling microorganisms are fundamental agents in sedimentary pyrite formation. The compounds that form pyrite include iron monosulfide (FeS), hydrogen sulfide (H 2 S), and polysulfide (S n 2−) (Butler et al., 2004), which are formed and consumed by microbially catalyzed reactions (sulfate reduction, sulfide oxidation, and elemental sulfur disproportionation) along with abiotic reactions. However, the role of microbes in pyrite precipitation also likely extends beyond simply generating the necessary reactant compounds (Picard et al., 2016;Wacey et al., 2015). For example, pyrite formation proceeds in some cases via the activity of microbial consortia (Thiel Abstract Sedimentary pyrite records are essential for reconstructing paleoenvironmental conditions, but these records may be affected by seasonal fluctuations in oxygen concentration and temperature, which can impact bioturbation, sulfide fluxes, and distributions of sulfide oxidizing microbes (SOMs). To investigate how seasonal oxygen stress influences surficial (<2 cm) pyrite formation, we measured time-series concentrations and sulfur isotope (δ 34 S) compositions of pyrite sulfur along with those of potential precursor compounds at a bioturbated shoal site and an oxygen-deficient channel site in Chesapeake Bay. We also measured radioisotope depth profiles to estimate sedimentation rates and bioturbation intensities. Results show that net pyrite precipitation was restricted to summer and early autumn at both sites. Pyrite concentration was higher and apparently more responsive to precursor compound concentration at the mildly bioturbated site than at the non-bioturbated site. This disparity may be driven by differences in the dominant SOM communities between the two sites. Despite this, the sites' similar pyrite δ 34 S values imply that changes in SOM communities have limited effects on surficial pyrite δ 34 S values here. However, we found that pyrite δ 34 S values are consistently and anomalously lower than coeval precursor compounds at both sites. A steady-state model demonstrates that equilibrium position-specific isotope fractionation (PSIF) effects in the S 8 -polysulfide pool can create a 4.3-7.3‰ gap between δ 34 S values of pyrite and zero-valent sulfur. This study suggests that SOM communities may have distinct effects on pyrite accumulation in seasonally dynamic systems, and that PSIF in the polysulfide pool may leave an imprint in pyrite isotope records.Plain Language Summary Formation of the iron sulfide mineral pyrite in sediments has contributed to long-term oxygen accumulation on Earth, and pyrite's sulfur isotope composition in ancient sediments offers a lens onto low-oxygen intervals in Earth's past. However, seasonal environmental changes in low-oxygen waters can lead to complex relationships between animal burrowing activity, microbial processing of sulfur compounds, and the formation of pyrite. To understand controls on pyrite accumulation and sulfur isotope composition under seasonally dynamic, low-oxygen waters, we analyzed shal...

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ∼60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process.

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