CONCLUSIONS

The variation of concentration and stable isotopic compositions (³⁴S, ¹⁸O) of dissolved sulfate and ¹³C of inorganic carbonate species in interstitial waters from the western Mediterranean are dominated by the following processes:

1. Microbiological degradation of organic matter and related sulfate reduction in the upper sediment column;
2. Dissolution of gypsum (Site 975);
3. Dissolved sulfate derived from saline evaporite brines (Sites 974 and 978);
4. Generation of methane by in situ fermentation of organic matter (Sites 976-979);
5. Dissolution and precipitation of carbonate minerals; and
6. Oxidation of methane (Sites 977 and 979?).

Sulfur and oxygen isotope measurements on the dissolved sulfate of interstitial waters from the western Mediterranean are shown to be a powerful tool in the evaluation of sinks, sources, and microbiological transformation reactions during sediment diagenesis. A previous suggestion is confirmed that different sulfate reduction rates in marine sediments are directly reflected in ¹⁸O-³⁴S plots.