Iron and sulfur reactions in the ocean (summary)

　If anoxic conditions exist near the sediment surface, the redox potential drops to near 0 (V). This causes a rapid dissolution of iron hydroxide (Fe(OH)₃) precipitated on the sediment surface, and the concentration of Fe²⁺ reaches a dissolution equilibrium of several tens of µmol/L. This is a thousand times higher than the dissolved iron concentration in the seawater immediately above. In deeper sediments, hydrogen sulfide is produced by the action of sulfate-reducing bacteria when the concentration drops to -0.1 (V).

　Where hydrogen sulfide ions occur, the presence of previously leached Fe²⁺ causes a reaction between the two, resulting in the black precipitation of iron sulfide FeS. FeS (which eventually becomes FeS₂) does not dissolve easily, so it will be buried deep beneath the seafloor, creating iron deposits of pyrite (FeS₂) in a few tens of millions of years . Thus, in sedimentary layers that have progressed to sulfate reduction, the concentration of dissolved Fe²⁺ ions in the pore water will be significantly lower. HS^- ions that occur relatively deep in the sediment and escape reaction with Fe²⁺ are transported by molecular diffusion to the upper part of the sediment. HS^- ions are oxidized when they encounter oxygen, so HS^- ions are affected only up to a redox potential near zero (anoxic layer). If the water directly above the marine sediments is anoxic for some reason, HS^- ions may be affected up to near the surface of the sediments. Of course, if the bottom waters of the ocean are anoxic and the redox potential of seawater falls below -0.1 (V), sulfate reduction occurs in the seawater and HS^- ions are generated. This can happen when bottom water stagnates in a closed system inner bay.