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. delta(34)S values of bulk sediments and simple mass-balance calculations indicate that SO42- 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. [KEYWORDS: Deep-sea sediments; metal-rich layers; quaternary depositional sequences; semi-euxinic sediments; pyrite formation; marine-sediments; organic-carbon; black-sea; sulfate reduction; sulfur speciation] ; 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. delta(34)S values of bulk sediments and simple mass-balance calculations indicate that SO42- 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. [KEYWORDS: Deep-sea sediments; metal-rich layers; quaternary depositional sequences; semi-euxinic sediments; pyrite formation; marine-sediments; organic-carbon; black-sea; sulfate reduction; sulfur speciation]