Cold seep deposits of Beauvoisin (Oxfordian southeastern France) and Marmorito (Miocene northern Italy) Microbially induced authigenic carbonates

Abstract

The relation of two well-known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study. Archaea are found to be associated with the formation of Oxfordian seep carbonates from Beauvoisin and with a Miocene limestone from Marmorito ("tube-worm limestone"). Carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative 813Ccarbonate values, respectively. Highly positive values (+ 15%0) reflect the use of 13C-enriched CO2 produced by methanogenesis. Low 813C values of the Marmorito carbonates (-30%0) indicate the oxidation of seepagederived hydrocarbons. Likewise, the 813C content of specific tail-to-tail linked isoprenoids, biomarkers for archaea, was found to be strikingly depleted in these sampies (as low as -115%0). The isotopic signatures corroborate that archaea were involved in the cycling of seepage-derived organic carbon at the ancient localities. Another Miocene limestone ("Marmorito li mestone") shows a strong imprint of methanotrophic bacteria as indicated by 813C va lues of carbonate as low J. Peckmann (IEI) . J. Reitner Institut und Museum für Geologie und Paläontologie, Georg-August-Universität. Goldschmidtstrasse 3, D-37077 Göttingen, Germany e-mail: jpeckma@gwdg.de. Fax: + 49-551-397918 V. Thiel, W. Michaelis Institut für Biogeochemie und Meereschemie, Universität Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany P. Clari, L. Martire Dipartimento di Scienze della Terra, via Accademia delle Scienze 5, 1-10123 Torino, Italy C. Gaillard UFR des Sciences de la Terre, UMR 5565 Centre de Paleontologie stratigraphique et Paleoecologie, Universite Claude Bemard, Lyon 1, 27-43 Boulevard du 11 Novembre, F-69622 Villeurbanne Cedex, France as -40%0 and biomarker evidence. Epifluorescence microscopy and field-emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation. In addition to lucinid bivalves previously reported from both localities, we infer that sponges from Beauvoisin and tube worms from Marmorito depended on chemosynthesis as weil. Low 813C values of nodules related to sponge taphonomy (-27%0) indicate that sponges might have been Iinked to an enhanced hydrocarbon oxidation. Tube worm fossils from Marmorito closely resemble chemosynthetic pogonophoran tube worms from Recent cold seeps and are embedded in isotopically light carbonate (813C -300/00).