UNUSUAL MICROBIAL-DERIVED STRUCTURES IN TERTIARY COLD SEEP CARBONATES FROM NORTHERN APENNINE, ITALY

A number of hydrocarbon/hydrogen sulfide seep-carbonate masses are embedded in Upper Tertiary, siliciclastic units along the Apennine range and Sicily. Megafaunal assemblages, adapted to bacterial chemosymbiosis, and negative stable isotopic signatures (d¹³C as low as -55‰ PDB) allow a ready recognition of these seep occurrences.

The role played by microbial consortia in the chemical reactions developed in these fossil seep ecosystems is potentially high, both as endosymbiotic communities hosted in soft tissues of invertebrates and as mat-forming communities of sulfur-oxidizing chemotrophs. Microbial processes have also a direct role as geological agents by promoting carbonate precipitation. Homogeneous micrite and microsparite, with planktic foraminiferal tests as common biotic component, make up most of the carbonate groundmass of the Apennine occurrences. Microbial-inferred features, such as laminations and peloidal fabrics, certain mineral phases, and corrosion horizons are comparatively rare and point to their limited fossilization potential.

In two Miocene and Pliocene seep-carbonate masses of the northern Apennine, we describe unique and well-preserved structures suggesting a direct intervention of microbial consortia. Some of these structures are interpreted as the direct mineralization of microbial frames and include complex alveolar textures and biofilms that line and fill microconduits, pores and other voids, similarly to living bacterial aggregations described from modern seep-carbonates. Mineral textures, such as calcite-dolomite spherulites and rhombs with concentric fabrics, abound as cement components and suggest a nucleation via bacterial oxidation of hydrocarbons. The same origin is also assumed for the acicular aragonite bundles that in the Miocene seep-carbonate mass enclose extremely abundant clusters of millimeter-size, tubular morphologies, which are interpreted as fecal pellets by polychaete worms. In at least one modern sulfide/methane seep site, polychaetes are known as bacteria-symbiotic animals.