Sapropels, unusually dark, organic-rich sediment layers whose origin is controversial, are widespread in the eastern Mediterranean. The upper surfaces of these layers appear sharp and show little evidence of bioturbation. It has been suggested that postdepositional oxidation may have removed the dark coloration from the upper part of the original layer, so that the top of the visible layer now lies somewhat below the original position of the sapropel top. Such a diagenetic alteration has considerable implications for the interpretation of the sedimentary record, where the tops of dark layers are usually correlated with the cessation of sapropel formation. Geochemical data demonstrate that all visual evidence of a thin example of the most recent eastern Mediterranean sapropel has been removed from the sedimentary record by oxidation processes. Such a process may have thinned all sapropel units by several centimetres. The most recent period of sapropel formation probably ended as recently as 5 ka (cf. cited values between 6.0 and 7.6 ka).