Characterisation of an extremely organic sulfur-rich, 150 Ma old carbonaceous rock: Palaeoenvironmental implications

Abstract

An extremely organic sulphur-rich coccolithic limestone of the Upper Jurassic Calcaires en plaquettes Formation (southern Jura, France) was analysed for its molecular composition using chemical and instrumental methods. The bitumen accounts for 19% of the total organic carbon, and consists predominantly of high molecular weight sulphur-bound constituents in the asphaltene and polar fractions. n-Alkane and hopanoid carbon skeletons, together accounting for the majority of compounds in the polar and apolar fractions, occur mainly in a sulphur-bound form as thiophenes, dithiophenes, cyclic sulphides and compounds that are intermolecularly sulphur-bound. A series of C18-C32 9-methylalkanes, components of as yet unknown origin, is present exclusively in a sulphur-bound form. Sulphur incorporation into the aromatic carotenoid isorenieratene provided many thiophenic and sulphidic aryl and diaryl isoprenoids. Although steroids are only minor constituents of the bitumen, it is noteworthy that 24-methyl-5,14,17(H)-cholestanes are more abundant than their 14,17(H)-counterparts, and that a series of C27-C29 steradienes was detected with unknown positions of the double bonds. Like the bitumen, the kerogen and asphaltene pyrolysates are dominated by organic sulphur compounds. In addition to thiophenes and benzothiophenes (common in pyrolysates of organic sulphur-rich macromolecules), several novel homologous series of sulphur-containing products were found. Based on mass spectra and desulphurisation products, these were identified tentatively as alkylbithiophenes, dialkylbithiophenes, alkylphenylthiophenes, alkyl(methylphenyl)thiophenes and components possessing both a thiophene and a benzothiophene moiety. The abundant sulphurisation of organic matter and the presence of isorenieratene derivatives indicate that the depositional environment was euxinic most of the time. Since the hopanoid biomarkers far outnumber the steroids, and since the contribution of algal biopolymers to the kerogen is low, it is believed that the water column-dwelling cyanobacteria were important contributors of organic matter.