Faunal and oxygen isotopic evidence for surface water salinity changes during sapropel formation in the eastern Mediterranean

Abstract

The discovery of widespread anaerobic deposits (sapropels) in late Cenozoic sediments of the eastern Mediterrenean has prompted many workers to propose the periodic occurrence of extremely low surface salinities in the Mediterranean. Oxygen isotopic determinations and total faunal analyses were made at 1000-year intervals across two equivalent sapropels in two piston cores from the Levatine Basin. The sapropel layers were deposited approximately 9000 y.B.P. (Sapropel A) and 80,000 y.B.P. (Sapropel B). Significant isotopic anomalies were recorded by the foraminiferal species within Sapropels A and B in both cores. The surface dwelling species record a larger 18O depletion than the mesopelagic species suggesting that surface salinities were reduced by 2–3% during sapropel formation. The faunal changes associated with the sapropels also indicate that the oceanographic conditions which lead to anoxic conditions in the eastern Mediterranean involve the formation of a low salinity surface layer. The source of the low salinity water might be meltwater produced by disintegration of the Fennoscandian Ice Sheet which drained into the Black Sea, into the Aegean Sea and finally into the eastern Mediterranean.