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

Abstract

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 O 2 , NO 3 -, 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 to 5 days 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 the 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.3

Denitrification and Nitrogen Fixation Dynamics in the Area Surrounding an Individual Ghost Shrimp (Neotrypaea californiensis) Burrow System

“…Furthermore, microniches can also form around decaying organisms (83), algal aggregates (81), other forms of organic matter (such as marine snow) that are deposited on the sediment surface and transported deeper into the sediment via bioturbation (72), and organic particles in seawater that are carried into the burrow via bioirrigation. As the sediment is bioturbated, these microniche-forming organic aggregates can be shifted, causing a complex and patchy distribution of microniches throughout the sediment column that can be seen by using whole-core resin embedding (80).…”

Effects of sediment geochemical properties on heavy metal bioavailability

“…These processes may also facilitate the metal mobilization from these organic phases and threaten to the growth and survival of organisms. But, some studies found that the organisms decomposed the availability of labile organic materials, especially sedimentary residues, decreased copper mobilization owing to the formation of copper-sulfide phases (Simpson and Batley, 2007;Simpson et al, 2012b;Stockdale et al, 2010). So the sediment factors influencing metal accumulation should be comprehensively considered when assessing toxicity to the benthic biota.…”

Geochemical behavior of metals–sulfide–phosphorus at SWI (sediment/water interface) assessed by DGT (Diffusive gradients in thin films) probes

“…Localized elevated sulfide formed during the oxidation of microniches of highly reactive organic matter, may provide discrete environments where appreciably more iron sulfide forms than indicated from bulk measurements (Stockdale et al, 2010). The removal of S(-II) and precipitation of FeS at the edge of niches also control sulfide microniche character.…”

“…Fecal pellet deposition and bioturbation may lead to heterogeneously distributed particles of localized highly reactive organic matter (microniches) being present below the oxygen penetration depth (Stockdale et al, 2010). Where O 2 , NO 3 − 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.…”

“…Where O 2 , NO 3 − 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 (Stockdale et al, 2010). Moreover, the simultaneous release of dissolved P and dissolved sulfide in sediments has been observed by Ding et al (2012Ding et al ( , 2013.…”