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Free keywords:
Mediterranean Sea; sapropel; trace element; sulfur; anoxic; pore water
Abstract:
A geochemical and stable isotope geochemical study was carried out on sediments and pore waters from the Strait of Sicily at the connection between the western and the eastern Mediterranean Sea (ODP Site 963). Organic-matter-rich layers (ORLs), which are proposed to be time-equivalent to eastern Mediterranean sapropels, only occur in sediments older than 550 kyr, most likely because of a strong dilution of late Pleistocene and Holocene sediments by terrigenous material. The ORLs have total organic carbon (TOC) contents of up to 1.6% and are enriched in pyrite and trace metals. TOC/pyrite ratios indicate the at least temporary presence of an anoxic water column during sediment formation, which is in agreement with the observed sulfur isotope discrimination. These conditions induced the enrichment in sulfide-forming and redox-sensitive trace elements. Trace metal enrichment is lower than at other sites in the Mediterranean Sea, which may be attributed to a less reducing or a less stable anoxic environment that was less favorable for the accumulation of redox-sensitive trace elements and organic matter. However, during sapropel events sedimentary conditions in the Strait of Sicily, based on manganese and barium geochemistry, must have been largely similar to those at other sites in the Mediterranean Sea. According to elemental, Rock-Eval, and stable carbon isotope analyses the organic matter in the ORLs is mainly of marine origin, which appears to have been overprinted significantly during diagenesis. A significant part of the organic matter in sediments from ODP Site 963 may also originate from uplifted land sections eroded during weathering and may primarily also be of marine origin. The results of pore water analyses illustrate the presence of an active deep biosphere in the sediments of Site 963, reflected for example by sulfate, methane and iodine concentrations. A brine, probably originating from underlying Messinian evaporites, was detected, but is only of minor influence.
