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Interstitial waters from eleven sites of the Eastern Mediterranean Basin (Sites 963-973) were analyzed for stable isotopes of dissolved sulfate (δ 34 S, δ 18 O) and major and minor ions. Sulfate reduction rates are positively related to bulk sedimentation rates, which indicates a higher burial of metabolizable organic matter with increasing sedimentation rates. Bacterial sulfate reduction in the deeper samples from most of the sites is superimposed by a sulfate input from Messinian evaporites or late-stage evaporite brines that are located at depth; dissolution of gypsum within the cored section was found at Sites 967 and 968. Authigenic gypsum precipitation was identified at Site 973 below 100 mbsf. Intense sulfate reduction is also indicated for the pore waters from the mud volcanoes of Sites 970 and 971. In addition to high concentrations of hydrocarbons (mainly CH 4), coexisting H 2 S and SO 4 2were also present, indicating that methane percolates through the sediment from greater depths. The observed variabilities in sulfate concentrations between different holes of Sites 970 and 971 are caused mainly by locally varying-upward fluxes of methane. Extremely high alkalinity values in the pore waters of Sites 970 and 971 are the result of microbial CH 4 oxidation. The concentration and sulfur isotopic composition of pore-water sulfate (δ 34 S values up to +112 ‰ vs. the Vienna-Canyon Diablo troilite standard) are dominated by microbial organic matter degradation with associated sulfate reduction. Therefore, most interstitial fluids are enriched in 34 S with respect to modern Mediterranean seawater (δ 34 S = +20.7‰; Site 973 surface seawater). δ 18 O(SO 4 2-) values at Site 963 and 964 are also enriched in 18 O with respect to Mediterranean seawater (δ 18 O[SO 4 2-] = +9.4‰). δ 34 S and δ 18 O values of dissolved residual sulfate are positively related to each other. An initial kinetic isotope effect is superimposed by oxygen isotope exchange reactions leading to an increased equilibration between residual sulfate and pore water with increasing degree of sulfate reduction. It is suggested that δ 18 O−δ 34 S relations of residual sulfate directly reflect sulfate reduction rates in marine sediments.

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We present a barium (Ba) isotope fractionation study of marine biogenic carbonates (aragonitic corals). The major aim is to provide first constraints on the Ba isotope fractionation between modern surface seawater and coral skeleton. Mediterranean surface seawater was found to be enriched in the heavy Ba isotopes compared to previously reported values for marine open ocean authigenic and terrestrial minerals. In aquarium experiments with a continuous supply of Mediterranean surface water, the Ba isotopic composition of the bulk sample originating from cultured, aragonitic scleractinian corals (d 137/134 Ba between +0Á16 AE 0Á12& and +0Á41 AE 0Á12&) were isotopically identical or lighter than that of the ambient Mediterranean surface seawater (d 137/134 Ba = +0Á42 AE 0Á07&, 2SD), which corresponds to an empirical maximum value of Ba isotope fractionation of D 137/134Ba coral-seawater = À0Á26 AE 0Á14& at 25°C. This maximum Ba isotope fractionation is close and identical in direction to previous results from inorganic precipitation experiments with aragonitestructured pure BaCO 3 (witherite). The variability in measured Ba concentrations of the cultured corals is at odds with a uniform distribution coefficient, D (Ba/Ca) , thus indicating stronger vital effects on isotope than element discrimination. This observation supports the hypothesis that the Ba isotopic compositions of these corals do not result from simple equilibrium between the skeleton and the bulk seawater. Complementary coral samples from natural settings (tropical shallowwater corals from the Bahamas and Florida and cold-water corals from the Norwegian continental shelf) show an even wider range in d 137/134 Ba values (+0Á14 AE 0Á08 to +0Á77 AE 0Á11&), most probably due to additional spatial and/ or temporal seawater heterogeneity, as indicated by recent publications.

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