The impact of bioturbation on pyrite sulfur isotope ratios: A numerical experiment

The biogeochemical cycling of sulfur stands as a cornerstone in the regulation of the Earth's surface redox state, acting as a primary buffer for atmospheric oxygen and a critical player in the burial of organic matter. The formation and subsequent preservation of sedimentary pyrite represents the dominant sink of reduced sulfur from the global ocean. For decades, the sulfur-isotopic composition of pyrite has been utilized by geochemists as a proxy to reconstruct the chemical evolution of earth's oceans and atmosphere. However, the reliability of this isotopic archive is linked to the physical and chemical state of the sediment-water interface, a boundary layer that was radically transformed by the evolution and intensification of bioturbation - the mixing and ventilation of sediments by burrowing animals. While it is often assumed that the onset of benthic faunal activity had a profound effect on the preserved S-isotope ratio (e.g., (Canfield and Farquhar 2009), actual studies exploring the impact of bioturbation are scarce. (Riemer et al. 2023) show experimental data that suggests that bioturbation shifts the isotope ratio of dissolved H₂S towards more negative values. This is in contrast to numerical studies that suggest that bioturbation has no effect on the pyrite S-isotope ratio (Mertens, Paradis, and Hemingway 2025). Here we extend the iron-redox shuttle model of (Van de Velde and Meysman 2016) to include iron-monosulfide and pyrite precipitation, dissolution and oxidation reactions. Our model explicitly tracks, O₂, SO₄, OM, S0, S^2-, Fe^3+, Fe²⁺ in liquid and sorbed state, FeS and FeS₂ and their respective S-isotope ratios. Model results suggest that bioturbation has either no, or only a very small, impact. However the current model does not yet include sulfur disproportionation and organic sulfur, so these findings are preliminary.

References

Canfield, Donald E., and James Farquhar. 2009. “Animal Evolution, Bioturbation, and the Sulfate Concentration of the Oceans.” Proceedings of the National Academy of Sciences 106 (20): 8123–27. doi:10.1073/pnas.0902037106.

Mertens, Cornelia, Sarah Paradis, and Jordon D. Hemingway. 2025. “Sedimentary Conditions Drive Modern Pyrite Burial Flux to Exceed Oxidation.” Nature Geoscience. doi:10.1038/s41561-025-01855-5.

Riemer, Sydney, Alexandra V. Turchyn, André Pellerin, and Gilad Antler. 2023. “Digging Deeper: Bioturbation Increases the Preserved Sulfur Isotope Fractionation.” Frontiers in Marine Science 9 : 1039193. doi:10.3389/fmars.2022.1039193.

Velde, Sebastiaan van de, and Filip J. R. Meysman. 2016. “The Influence of Bioturbation on Iron and Sulphur Cycling in Marine Sediments: A Model Analysis.” Aquatic Geochemistry 22 (5-6): 469–504. doi:10.1007/s10498-016-9301-7.