Carbon, sulfur, and oxygen isotope geochemistry of interstitial waters from the western Mediterranean

Abstract

Interstitial waters from six sites of the Western Mediterranean Basin (Sites 974‐979) were analyzed for stable isotopes of dissolved sulfate (δ34S, δ18O), water (δ18O), 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 th e deeper samples from Sites 974 and 978 is overprinted by a sulfate input from saline brines located at depth. Dissolution of gyp sum within the section cored was found at Site 975. The concentration and sulfur isotopic composition of pore-water sulfate (δ34S 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 (δ34S = +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 (δ18O[SO 4 2‐] = +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 oxy gen isotope exchange reactions lead to isotope shifts towards equilibrium between residual sulfate and interstitial H 2O with increasing degree of sulfate reduction. The results support the previous suggestion that δ18O‐δ34S 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 ( δ13C 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.