Formation of iron sulfide at faecal pellets and other microniches within suboxic surface sediment

Faecal pellet deposition and bioturbation may lead to heterogeneously distributed particles of localized highly reactive organic matter (microniches) being present below the oxygen penetration depth. Where O₂, NO₃^-, and Fe/Mn oxyhydroxides become depleted within these microniches or where they exist in zones of sulfate reduction, significant localized peaks in sulfide concentration can occur. These discrete zones of sulfide evolution can cause formation of iron sulfides that would not be predicted by analysis of the 'bulk' sediment. Using a reaction-transport model developed specifically for investigating spherical microniches, and incorporating 3D diffusion, we investigated how the rate constants of organic matter (OM) degradation, particle porosity and niche lifetime, affect dissolved sulfide and iron concentrations, and formation of iron sulfide at such niches. For all of the modelled scenarios the saturation index for iron sulfide is positive, indicating favourable conditions for FeS precipitation in all niches. Those simulations within the microniche lifetime range of 2.5-5days gave comparable concentration ratios of sulfide to iron in solution within the niche to experimentally observed values. Our model results provide insight into the mechanisms of preservation of OM, including soft tissue, in the paleo record, by predicting conditions that result in preferential deposition of precipitates at the edge of microniches. Decreases in porosity, shorter niche lifetimes and increases in OM degradation rate constants, all tend to increase the likelihood that FeS precipitation will preferentially occur at the edges of a niche, rather than uniformly throughout the niche volume.