Multiple layers of dark-colored sediments rich in organic carbon have been recognized as common features of the post-Messinian sedimentary record of the Mediterranean Sea in both uplifted exposures on land and in deep-water settings (e.g., Cita and Grignani, 1982; Thunell et al., 1984; Rossignol-Strick, 1985; Fontugne and Calvert, 1992). These sequences, called rhythmites, laminites, or sapropels depending on their settings, are noteworthy because the expected fate of most organic matter in the oceans is oxidation and destruction, rather than accumulation in the seafloor. These unusual accumulations required special depositional conditions that involved both enhanced delivery of organic matter to the seabottom and improved preservation of the organic matter during and after burial.
The term "sapropel" is strictly applied to unconsolidated oozes or sludges that accumulate in shallow anaerobic environments (Bates and Jackson, 1987), although the term is commonly applied to organic-carbon-rich sedimentary layers that may have resulted from such or similar settings. We will use the "sapropel" in the more general context of a discrete layer of dark-colored, fine-grained, deep-sea sediment in which the organic-carbon concentration exceeds 0.5%. For those dark-colored intervals that are stratigraphically less well defined than sapropel layers, but in which organic-carbon concentrations still exceed 0.5%, we will use the term "sapropel-like sediments." In this overview of the organic matter contents of sapropels and sapropel-like sediments at locations cored during Leg 161, we also consider the significance of these organic-carbon-rich deposits to paleoceanographic and paleoclimatic reconstructions of the Mediterranean Sea and its surrounding land areas.