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

The relation of two well-known ancient carbonate deposits to hydrocarbon se epage was confirmed by this study. Archaea are found to be associated with the formation of Oxfordian seep carbonates from Beauvoisin and with a Mioce ne limestone from Marmorito ("tube-worm limestone"). Carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative delta(13)C(carbonate) values, respectively. Highly positive values (+15 par ts per thousand) reflect the use of C-13-enriched CO2 produced by methanoge nesis. Low delta(13)C values of the Marmorito carbonates (-30 parts per tho usand) indicate the oxidation of seepage-derived hydrocarbons. Likewise, th e delta(13)C content of specific tail-to-tail linked isoprenoids, biomarker s for archaea, was found to be strikingly depleted in these samples (as low as -115 parts per thousand). The isotopic signatures corroborate that arch aea were involved in the cycling of seepage-derived organic carbon at the a ncient localities, Another Miocene limestone ("Marmorito limestone") shows a strong imprint of methanotrophic bacteria as indicated by delta(13)C valu es of carbonate as low as -40 parts per thousand and biomarker evidence. Ep ifluorescence microscopy and field-emission scanning electron microscopy re vealed that bacterial biofilms were involved in carbonate aggregation. In a ddition to lucinid bivalves previously reported from both localities, we in fer that sponges from Beauvoisin and tube worms from Marmorito depended on chemosynthesis as well. Low delta(13)C values of nodules related to sponge taphonomy (-27 parts per thousand) indicate that sponges might have been li nked to an enhanced hydrocarbon oxidation. Tube worm fossils from Marmorito closely resemble chemosynthetic pogonophoran tube worms from Recent cold s eeps and are embedded in isotopically light carbonate (delta(13)C -30 parts per thousand).