Normal faults control fluid flow structures and cold seep habitats at the rear of the Calabrian Arc (Paola Ridge, southeastern Tyrrhenian Sea). Geophysical Research

Cold seeps were discovered at 500-1000 m water depth along the Paola Ridge, on the continental slope of the NW Calabrian margin (southeastern Tyrrhenian Sea). When the fluid flow is vigorous, it produces mud volcanoes, gas plumes in the water column and precipitates oxy-hydroxides, sulfides and siderites in the sub-seafloor. Conversely, when the flux is very low or even absent, calcite and aragonite, associated with chemosymbiontic fauna, are formed near the seabed on pockmarks-punctuated mud diapirs and are exposed by normal faults. Seismic data show that NW-SE and NNW-SSE-oriented normal faults are the most probable mechanism for the emplacement of these structures. According to the seismic data, diapirism is no longer active, but the normal faults are still exploited as primary conduits for upward fluid migration and control the seepage distribution and activity. The faults can be regarded as the marine prolongation of the fault zones that dissect the Calabrian Arc from SE to NW. The mud volcanoes, characterized by a high backscatter signature, show a rapid transition from the oxic toward the methane-sulfate transition zone. Intermediate backscatter typifies areas where free venting is hampered by the presence of mudflows at the seafloor. These conditions favor: i) the oxidation of sulfides near the seafloor, ii) the precipitation of siderites a few meters below the seafloor and iii) the formation of sulfides deeper in the sub-seafloor. Normal faults, probably rooted in the Messinian evaporites, are likely candidates to act as conduits for sulfate, nitrate and metal oxides that juxtapose different redox environments. Siderites precipitated in the fast and low venting sites showed enrichment in 13C and 18O, which are compatible with their precipitation in the methanogenic zone. The heavy-oxygen isotopic compositions of the siderites are possibly related to the dissociation of gas hydrates, which have not been mapped so far by the available seismic data. The mud diapirs are, on the contrary, characterized by low seafloor backscatter and are dissected by normal faults. In coincidence with these faults, calcite and aragonite are present at or very close to the seafloor. They have the typical isotopic signature indicating formation during sulfate-dependent microbially-mediated anaerobic oxidation of methane. They are associated with Lucinoma borealis, the oldest being dated Cal BP 13710 to 13340 and the youngest Cal BP 640 to 440 and with Acharax sp. the youngest dated Cal BP 24240 to 23700. This is the first documented occurrence of Acharax sp. in the Quaternary of the Mediterranean region and probably shows the replacement of the Acharax sp. with Lucina sp. in the cold seep habitat during the last sea level rise and general warmer conditions.