Stratigraphy of Eastern Mediterranean Sapropel Sequences Recovered during DSDP Leg 42A and Their Paleoenvironmental Significance

Abstract

During Leg 42A of the Deep Sea Drilling Project, we recovered over 150 discrete layers of dark colored organic-rich sediment at five sites in the eastern Mediterranean. We consider about two- thirds of these to be sapropels, on the basis of a definition that recognizes a sapropel as "a discrete layer, greater than 1 cm in thickness, set in open marine pelagic sediment and containing greater than 2.0% organic carbon." This paper includes a catalogue of all the sapropels and sapropelic layers recovered, including analysis of their sedimentary structures, general composition, fossil content, and stratigraphic position. Sapropels are identified, dating back to the middle Miocene, and a single occurrence is recorded in the Pleistocene of the western Mediterranean. We recognize that the record of sapropel stratigraphy provided by Leg 42A and Leg 13 drilling is incomplete in both time and space. Despite this, we can make tentative basin-wide correlations between individual layers at least back to the early Pliocene, proving that these stagnation events cannot merely be linked to glacial phenomena. Their widespread distribution and the wide range of depths argue against the presence of an "oxygen-minimum layer." Stagnation of the Mediterranean basins resulted from a number of interacting factors which caused stratification within the water column. This was a transient situation that developed repeatedly since the mid-Miocene when the Mediterranean took on it present configuration. Glacial expansions were primary factors bringing about stratification in the Pleistocene; whereas, similar factors are as yet unidentified in the Pliocene and Miocene sediments.