Early diagenesis in a sulphate-reducing environment: an isotopic study in the Lower Jurassic of the Paris Basin

Lower Jurasssic argilo-limestones from a continental platform environment have been drilled at Montcornet in the Paris Basin (Andra, 1989). The isotopic analysis of calcite yield delta(13)C values between -3 and +1 parts per thousand, and delta(18)O between +21 and +27 parts per thousand. Depletion in C-13, is caused by secondary crystallisation of calcite in a sulphate-reducing environment, where isotopically light dissolved inorganic carbon, resulting from organic matter mineralization, is added into the pore fluids. The depletion in delta(18)O results from the isotopic equilibrium between pore water and calcite, which is controlled by temperature, fluid/rock ratio and the delta(18)O Of intersititial fluid. This study clearly shows that stable isotopes are a powerful tool for the study of local diagenetic mechanisms, knowledge of which is a key to the understanding of the sedimentary record from geochemical and geophysical points of view. ((C) Academie des sciences / Elsevier, Paris.).