Sources of iron for pyrite formation in marine sediments

More than two hundred aerobic continental margin, aerobic deep sea, dysaerobic, and anaerobic/euxinic sediments have been examined for their variations in different operationally defined iron fractions, each of which represents a different reactivity toward dissolved sulfide. Aerobic continental margin, deep sea, and dysaerobic sediments contain similar contents of highly reactive iron (dithionite-soluble iron plus pyrite iron), poorly reactive iron (iron soluble in HCl less that soluble in dithionite), and unreactive iron (total iron less that soluble in HCl). By contrast non-turbidite euxinic samples from the Black Sea, as well as euxinic samples from the Cariaco Basin and Framvaren are enriched in highly reactive iron. These sediments contain a small lithogenous fraction and a Targe biogenous, organic C-rich fraction, which decays by sulfate reduction in an iron-rich water column to form pyrite-rich sediment. Other anaerobic/euxinic samples from the Black Sea, Orca Basin, and Kau Bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron. Degrees of Pyritization (DOP) for all the aerobic, dysaerobic, and anaerobic/euxinic samples (except those low in biogenous material) are consistent with analogous ancient sediments and indicate that most pyrite formation occurs from the highly reactive iron fraction.