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 (S⁰) 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 Inlet (a fjord on Vancouver Island) and the Orca Basin (an 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.