Abstract
Recurrent organic-rich layers (sapropels) in eastern Mediterranean sediments are enriched in C org, Fe, and S. Sulphur and Fe are enriched in a zone immediately below the sapropels, whereas C org is not. δ 34S values of bulk sediments and simple mass-balance calculations indicate that SO 4 2− 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 Fe 2+ diffusing upwards from underlying sediments. The Fe 2+ 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.
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