Deep-sea fluid expulsion and related products in the Miocene foredeep and satellite basins of the northern Apennines, Italy.

The last two decades have convincingly documented that both hot and cold fluid expulsion on the deep seafloor is a widespread, almost cosmopolitan phenomenon whose hydrological, geological and biological implications are multifold. Defluidization has been shown to be conducive to: (i) precipitating a variety of authigenic minerale, primarily carbonates (calcite, aragonite, dolomite), sulfides and sulphates; (ii) imposing distinct chemical signatures in sediments, rocks, biota and water; (iii) sustaining peculiar microbial consortia and macrofaunal communities. Beginning with the mid-80’s submarine investigations revealed the presence of localized deep-sea seeping of light hydrocabon-rich fluids (cold seeps) in many different geodynamic settings of the World’s oceans (e.g., Paull et al., 1984, 1992; Kulm et al., 1986; Aharon and Sen Gupta, 1994; Aharon, 2000; Bohrmann et al., 1998, 2002). As observed earlier at hydrotermal vent sites, modern loci of hydrocarbons-rich fluid seepage are characterized by a significantly high benthic biomass represented by microbial consortia and chemosynthetic macro-assemblages typically dominated by endosymbiont-bearing bivalves including clams (lucinaceans, Solemya, vesicomyids etc.), modiolid mussels (e.g., Bathymodiolus, Modiolus) as well as gastropods (e.g., Thalassonerita, Provanna), and tube worms (Pogonophora). These communities, in analogy with those found around hydrothermal vents, are basically sustained by a chemosynthetic food chain exploiting the bacterial oxidation of methane and/or hydrogen sulfide (Van Dover, 2000, with references therein.). Cold seepage may promote the formation of authigenic carbonates which can be recognized in the geological legacy of the Earth since the Phanerozoic and possibly earlier (Barbieri et al., 2001; Campbell et al., 2002).