A MODEL FOR IRON DEPOSITION TO EUXINIC BLACK-SEA SEDIMENTS

High degrees of Fe mineral sulfidation are found in the euxinic sedime nts of the Black Sea. With the short time scale of diagenesis (2000-30 00 yr or less) and the slow reaction kinetics between sulfide and Fe-c ontainlng silicates, extensive sulfidation of silicate Fe cannot accou nt for these results. Also, the extent of Fe sulfidation in Black Sea sediments is much higher than sulfidic normal marine sediments encount ering similar exposure times to sulfide. Sediment trap results indicat e that sulfidized Pe is delivered to the sediment with depositing orga nic matter and independently of lithogenic particles containing unreac tive Fe silicate phases. We combine these observations and propose tha t Fe mineral sulfidation in the euxinic Black Sea is driven by the dec omposition, by sulfate reduction, of particulate organic matter settli ng 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+ pr ecipitation 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 sulfidat ion. Sediments from some regions in the euxinic Black Sea have low ext ents of Fe sulfidation, similar to those from sulfidic normal marine e nvironments. In some instances these low extents of sulfidation are as sociated with turbidites originating from the basin margins. In other instances sedimentology Is poorly constrained so that an association b etween turbidite deposition and extents of Fe mineral sulfidation cann ot be certain. We note a seasonality in the extent of Fe mineral sulfi dation from sediment trap results such that in the summer months highl y sulfidized Fe deposits,whereas in the winter, the depositing Fe ess sulfidized. Thus, variability in the proportion of winter-like and sum mer-like particle deposition can provide a mechanism, in addition to t urbidites, by which variable extents of Fe mineral sulfidation can be generated in euxinic basin sediments.