17/12/201017/12/2010 16/12/2010 1 Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian-Oxfordian clayey formation of Bure (France). Lerouge C. a* , Grangeon S. b , Gaucher E.C. a , Tournassat C. a , Agrinier P. c , Guerrot C. a , Widory D. a , Fléhoc C. a , Ramboz C. d , Vinsot A. e , Buschaert S. b a BRGM, BP36009, 45060 Orléans cédex 2, France b ANDRA, 1-7 rue Jean-Monnet, 92298 Châtenay-Malabry cedex, France c Laboratoire de Géochimie des Isotopes stables, IPGP, Place Jussieu, case 89, 75252 Paris cedex 05 d Institut des Sciences de la Terre d’Orléans (ISTO) – CNRS, 1A rue de la Ferollerie, 45071 ORLEANS Cedex 2, France e ANDRA, Meuse/Haute-Marne Underground research Laboratory (URL), RD 960, 55290 Bure, France * Corresponding author. E-mail address: c.lerouge@brgm.fr (C. Lerouge) Abstract The Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: 1) framboidal pyrite and micritic calcite, 2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite 3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, 4) chalcedony 5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ 34 S (-38 to +34.5 ‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (-38 to -22 ‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ 34 S values (-14 up to +34.5 ‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ 34 S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87 Sr/ 86 Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only insu-00587526, version 1 - 1 Feb 2012 Author manuscript, published in "Geochimica et Cosmochimica Acta Acta / Geochimica et Cosmochimica Acta Acta; Geochimica et Cosmochimica Acta Acta (England); Geochimica et Cosmochimica Acta Acta 75, 10 (2011) 2633-2663" DOI : 10.1016/j.gca.2011.02.025