A model for iron deposition to euxinic Black Sea sediments

High degrees of Fe mineral sulfidation are found in the euxinic sediments of the Black Sea. With the short time scale of diagenesis (2000-3000 yr or less) and the slow reaction kinetics between sulfide and Fe-containlng silicates, extensive sulfidation of silicate Fe cannot account for these results. Also, the extent of Fe sulfidation in Black Sea sediments is much higher than sulfidic normal marine sediments encountering similar exposure times to sulfide. Sediment trap results indicate that sulfidized Pe is delivered to the sediment with depositing organic matter and independently of lithogenic particles containing unreactive Fe silicate phases. We combine these observations and propose that Fe mineral sulfidation in the euxinic Black Sea is driven by the decomposition, by sulfate reduction, of particulate organic matter settling through the anoxic water-column as marine snow aggregates. Locally rapid rates of sulfate reduction at the site of organic decomposition produce elevated sulfide concentrations, encouraging dissolved Fe2+ precipitation as Fe sulfide minerals in the upper Fe-rich portion of the anoxic zone. The high deposition rate of this sulfidized Fe, compared to lithogenic particulates, generates the high extents of Fe sulfidation. Sediments from some regions in the euxinic Black Sea have low extents of Fe sulfidation, similar to those from sulfidic normal marine environments. In some instances these low extents of sulfidation are associated with turbidites originating from the basin margins. In other instances sedimentology Is poorly constrained so that an association between turbidite deposition and extents of Fe mineral sulfidation cannot be certain. We note a seasonality in the extent of Fe mineral sulfidation from sediment trap results such that in the summer months highly sulfidized Fe deposits,whereas in the winter, the depositing Fe ess sulfidized. Thus, variability in the proportion of winter-like and summer-like particle deposition can provide a mechanism, in addition to turbidites, by which variable extents of Fe mineral sulfidation can be generated in euxinic basin sediments.