Trace metal signatures of eastern Mediterranean sapropels

Sapropel samples of Pliocene to Pleistocene age recovered during ODP Leg 16 0 from Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 966, 967 (top a nd northern slope of the Eratosthenes Seamount, south of Cyprus) and 968 (C yprus margin) were analysed for major and trace elements. Eastern Mediterra nean sapropels are characterized by high organic carbon and sulphur content s 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). Enric hment factors relative to 'average shale' are comparable to those found in CTBE black shales. The Re content is extremely high in the sapropels studie d with maximum values up to 1000 ng/g. Re/Mo ratios averaging 2.7 x 10(-3) are close to the seawater value. High trace metal enrichments and Re/Mo rat ios close to the seawater value point to an anoxic water column during sapr opel formation. Additionally, enhanced Ba concentrations in the sapropels s upport the assumption that bioproductivity was also high during these episo des. Trace metal signatures of sapropels originating from different drill s ites on a transect across the eastern Mediterranean are not only influenced by water depth but also by their location in the eastern Mediterranean. Me tal enrichments seem to be higher at the western sites compared with the ea stern sites. This probably reflects a change in the circulation pattern dur ing periods of sapropel formation. Pliocene sapropels exhibit elevated trac e metal contents in comparison with Pleistocene examples. Therefore: differ ent palaeoceanographic conditions may have developed during the Pleistocene compared with the Pliocene. Altering palaeoceanography was probably contro lled by climatic change, which may be related to the onset of glaciation of the northern Hemisphere. Decreasing summer precipitation throughout the la te Pliocene and the Pleistocene may also have influenced sapropel depositio n. (C) 2000 Elsevier Science B.V. All rights reserved.