William F. Kenney

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.