Anomalous enrichments of iron monosulfide in euxinic marine sediments and the role of H2S in iron sulfide transformations:: Examples from Effingham Inlet, Orca Basin, and the Black Sea

It is well documented that sedimentary pyrite formation proceeds through an iron monosulfide (FeS) precursor. Laboratory studies have traditionally indicated that intermediate S species such as elemental sulfur (SO) or polysulfides (S-x(2-)) are responsible for the transformation of FeS to pyrite (FeS2). Recent experimental work however, has suggested that H2S may also be responsible for the transformation. The present study extrapolates reaction pathways responsible for pyrite formation in the laboratory to two fundamentally different modern anoxic marine systems. We hypothesize that on decadal timescales, H2S (or HS-) is responsible for transformations of FeS into FeS2 in natural systems where intermediate S species are isolated from FeS production. The possibility of prolonged coexistence of high levels of H2S and FeS, however, challenges recent experimental predictions of extremely rapid transformations (that is, timescales of hours or less) of FeS to FeS2 via the H2S pathway. Sediments of Effingham Met (a fjord on Vancouver Island) and the Orca Basin tan intraslope brine pool in the northern Gulf of Mexico) are both characterized by atypically high concentrations of FeS but contrasting levels of H2S and FeS2 production. In both systems, FeS formation is spatially decoupled from intermediate S species as a consequence of either rapid deposition/burial or extreme water-column stratification. Within settings that promote this separation, H2S may be the principal species responsible for pyrite formation. FeS to FeS2 transformations are favored by the high concentrations of H2S in sediments of Effingham Inlet. Additional results from the margin of the anoxic Black Sea corroborate the Effingham model for iron sulfide transformation. H2S concentrations are controlled by the amount of bacterial sulfate reduction and the availability of reactive Fe, H2S concentrations will be buffered to low levels via the production of FeS in systems with appreciable amounts of reactive Fe and therefore be unavailable to transform FeS to FeS2. Under conditions where H2S is the only species available for FeS to FeS2 transformations, some degree of Fe Limitation may promote pyrite formation by allowing H2S to accumulate in the pore waters. Ultimately, this balance between H2S production and reactive Fe availability may strongly influence the amount of pyrite formed in anoxic systems.