Sources and distribution of trace and minor elements in the western Dead Sea surface sediments

Abstract

Twenty Dead Sea surface sediment samples were analyzed for their major, minor and trace element compositions. The samples represent muddy sediments along the western parts of the lake, from water depths of 8–250 m. These sediments were deposited after 1983, under oxic conditions, following the overturn of the water column in 1979, which ended about 300 years of meromictic stratification with an anoxic lower water mass. The changes in their metal concentrations are discussed in view of the different brine oxidation state. The sediments consist of detrital minerals—carbonates, quartz and clays and authigenic minerals—aragonite, halite and traces of gypsum. Calculations indicate that all mud samples contain more than 3.6% authigenic aragonite, which was found to precipitate preferentially in near shore sediments. An increase in Ca and a decrease in Al concentrations with decreasing water depths and in a transect from N to S were observed. These are attributed to differential settling of detritus and authigenic carbonates close to the shore and fine Al-silicates in the deep waters, and to the southward decrease in the contribution of clay minerals, mostly derived from the Jordan river. Fe, Ce, Be and Eu were found to exhibit conservative behavior with respect to Al during the transition from stream sediments in the drainage basin to lake sediments. When compared to normal marine sediments, the Dead Sea sediments have similar Cu, Ni, Zn, Be, Ce and Eu concentrations, whereas Cd is enriched by nearly 1 order of magnitude. A good correlation exists between Cd and P, suggesting that the Cd enrichment arises from outcrops of Cd-rich phosphate rocks that are found in the Dead Sea basin. The somewhat depleted Pb concentrations in the lake muddy sediments might be explained by the somewhat high Pb concentrations (normalized to salinity) in the Dead Sea water column, as compared to seawater and by its removal, mainly through halite precipitation. The unusual distribution of Mn concentrations in the surface sediments and its association with authigenic aragonite imply, as has already been suggested, that Mn co-precipitates with aragonite.