Interstitial waters from six sites of the Western Mediterranean Basin (Sites 974-979) were analyzed for stable isotopes of dissolved sulfate (34S, 18O), water (18O), and dissolved inorganic carbon (13C), in addition to major and minor ions.
Sulfate reduction rates (as determined by modeling sulfate profiles) are positively related to bulk sedimentation rates, which indicates a higher burial of metabolizable organic matter with increasing sedimentation rate. Bacterial sulfate reduction in the deeper samples from Sites 974 and 978 is overprinted by a sulfate input from saline brines located at depth. Dissolution of gypsum within the section cored was found at Site 975. The concentration and sulfur isotopic composition of pore-water sulfate (34S values up to +89 relative to the Vienna-Canyon Diablo troilite standard) are dominated by microbial organic matter oxidation with associated sulfate reduction. Therefore, most interstitial sulfate is enriched in 34S with respect to modern Mediterranean seawater (34S = +20.7; Site 973 surface seawater). Dissolved sulfate at Sites 974, 975, 977, and 979 is also enriched in 18O with respect to Mediterranean seawater (18O[SO42-] = +9.4 relative to Standard Mean Ocean Water). The sulfur and oxygen isotopic compositions of dissolved residual sulfate are positively correlated to each other. Microbiologically mediated oxygen isotope exchange reactions lead to isotope shifts towards equilibrium between residual sulfate and interstitial H2O with increasing degree of sulfate reduction. The results support the previous suggestion that 18O-34S relations of residual sulfate directly reflect sulfate reduction rates in marine sediments.
The depth profiles of the carbon isotopic composition of dissolved inorganic carbonate species (13C values between -0.1 and -22.6 relative to the Vienna Peedee Belemnite standard) reflect the in situ degradation of organic matter via sulfate reduction, followed by the formation of methane via in situ fermentation of organic matter, and probably carbonate dissolution and precipitation.
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2Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University, P.O. Box 2503, D-26111 Oldenburg, Federal Republic of Germany.
3Geologisches Institut, ETH-Zentrum, CH-8092 Zürich, Switzerland.
4Present address: Department of Biogeochemistry, Max-Planck-Institute for Marine Microbiology, Celsiusstr.1, D-28359 Bremen, Federal Republic of Germany. email@example.com
Date of initial receipt: 9 May 1997
Date of acceptance: 20 January 1998
Reproduced online: 29 January 2004