Publication | Open Access
Sources of iron for pyrite formation in marine sediments
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1998
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Organic GeochemistryBiogeochemistryEnvironmental ChemistryIron FractionsEngineeringDeep SeaEnvironmental GeochemistryEnvironmental EngineeringMarine PollutionOrganic-rich Sedimentary RockMarine ChemistryMicrobial EcologyAerobic Continental MarginGeochemistrySedimentary PetrologyPyrite FormationSedimentology
The authors examined over 200 continental‑margin and deep‑sea sediments, characterizing operationally defined iron fractions, and found that pyrite‑rich layers arise from the decay of a large biogenous, organic‑C‑rich fraction via sulfate reduction in iron‑rich water columns, with only a minor lithogenous contribution. They observed that aerobic and dysaerobic sediments have comparable reactive iron pools, whereas euxinic sites (Black Sea, Cariaco Basin, Framvaren) are enriched in highly reactive iron, and that degrees of pyritization in most samples match ancient analogues, indicating that pyrite formation is dominated by the highly reactive iron fraction.
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 HC1 less that soluble in dithionite), and unreactive iron (total iron less that soluble in HC1). 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 large 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.