Pyrite within and below sapropels in the eastern Mediterranean is a result of microbial SO42− reduction within the sapropel, and the subsequent reaction of sulfide (HS) with detrital Fe and Fe2+ diffusing upward from underlying sediments. Below the youngest Mediterranean sapropel, S1, pyrite (as much as 281 µmol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 µmol pyritic S/g) has been detected. Framboidal microtextures indicate pyrite formation at the site of HS production within the sapropel. Euhedral pyrite, below the sapropel, forms when sulfate reduction in the sapropel outcompetes iron liberation and supply, and HS diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (−37.3‰ to −38.2‰) as well as below the sapropel (−45.6‰ and −49.6‰), indicating that HS has formed in a system with abundant SO42− and in the presence of oxidants.

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