oa Lipid Biomarkers as Proxies for Microbially-Mediated Authigenic Carbonate Formation

Late Jurassic organic-rich deposits of the Argiles de Châtillon formation (Boulonnais area, northern France) are characterized by the presence of authigenic carbonate beds predominantly induced by bacterially-mediated organoclastic sulfate-reduction. Among them, cone-in-cone (CIC)-bearing nodules are sedimentary structures that may have originated during early diagenesis as a result of sulfate-dependent anaerobic oxidation of methane (AOM). However, the mode of formation of CIC structures and the possible contribution of AOM remain a matter of debate. By performing GC-MS lipid biomarker analysis of a representative set of carbonates beds, we identified that CIC-bearing nodules show a molecular signature that differs from the other carbonate beds with a significantly higher abundance of isoprenoids attributed to archaeal biphytanyl compounds. Archaea-derived lipids and light isotopic signature point to methanotrophic activity associated with sulfate-dependent AOM during the precipitation of the CIC-bearing nodules. The marked difference in the abundance of isoprenoids allows us to confirm that the formation of CIC-bearing nodules occurred under different biogeochemical conditions, i.e. in the AOM zone, compared to the other authigenic carbonates that resulted mostly from organoclastic sulfate reduction.