Trace metal signatures of eastern Mediterranean sapropels

Abstract

Sapropel samples of Pliocene to Pleistocene age recovered during ODP Leg 160 from Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 966, 967 (top and northern slope of the Eratosthenes Seamount, south of Cyprus) and 968 (Cyprus margin) were analysed for major and trace elements. Eastern Mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox-sensitive and/or sulphide-forming trace metals (Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V). Enrichment factors relative to ‘average shale’ are comparable to those found in CTBE black shales. The Re content is extremely high in the sapropels studied with maximum values up to 1000 ng/g. Re/Mo ratios averaging 2.7×10 −3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to an anoxic water column during sapropel formation. Additionally, enhanced Ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes. Trace metal signatures of sapropels originating from different drill sites on a transect across the eastern Mediterranean are not only influenced by water depth but also by their location in the eastern Mediterranean. Metal enrichments seem to be higher at the western sites compared with the eastern sites. This probably reflects a change in the circulation pattern during periods of sapropel formation. Pliocene sapropels exhibit elevated trace metal contents in comparison with Pleistocene examples. Therefore, different palaeoceanographic conditions may have developed during the Pleistocene compared with the Pliocene. Altering palaeoceanography was probably controlled by climatic change, which may be related to the onset of glaciation of the northern Hemisphere. Decreasing summer precipitation throughout the late Pliocene and the Pleistocene may also have influenced sapropel deposition.