Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Abstract: Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sedi-ments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly b porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH < 8, porewater Fe 2+ = 10-6 g cm-3) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100-1000 lag cm-2 a-k The addition of reactive iron to deep-basin sediments i determined by the magnitude of this diffusive flux, the export efficiency (e) of recycled iron from the shelf, the ratio of source area (S) to basin sink area (B) and the trapping of reactive iron in the deep basin. Values of e are poorly constrained but modern enclosed or semi-enclosed sedimentary basins how a wide vari-ation in S/B ratios (0.25-13) where the shelf source area is defined as sediments at less tha