Abstract:
Recurrent organic-rich layers (sapropels) in eastern Mediterranean sediments are enriched in Corg, Fe, and S. Sulphur and Fe are enriched in a zone immediately below the sapropels, whereas Corg is not. δ34S values of bulk sediments and simple mass-balance calculations indicate that SO2-4 reduction has taken place in an open system, with all HS- formed at, or close to, the sediment surface. Formation of pyrite in the sapropel was Fe-limited and consequently, excess HS- was able to migrate downwards (downward sulphidisation). This resulted in the formation of pyrite below the sapropel by reaction of this HS- with solid-phase ferric iron and Fe2+ diffusing upwards from underlying sediments. The Fe2+ source probably includes Fe (hydr)oxide layers formed at former oxidation fronts above previously deposited and buried sapropels. This downward sulphidisation mechanism allows accumulation of twice as much S in alternating organic-rich-anoxic/organic-poor-suboxic sediments compared to what is preserved in organic-rich anoxic sediments.