Habitat mapping of cold seep associated with authigenic mineralization (Paola Ridge southern Tyrrhenian Sea): combining seafloor backscatter with biogeochemistry signals

Mineralogical, petrographic and stable isotopes analyses were carried out on sediments and rock samples collected in a deep-sea cold seep province in the Paola Ridge (southeastern Tyrrhenian Sea). The results coupled with the interpretation of the seafloor backscatter constrained both the biogeochemical zonation and the spatial distribution of the cold seep habitats in the area. These have proved to change vertically in a few meters and laterally within narrow zones. The mud volcanoes, characterized by a high backscatter signature, are the site of vigorous gas venting and show a rapid transition from the oxic toward the methane-sulfate transition zone. Intermediate backscatter typify areas where free venting is hampered by the presence of mud flows at the seafloor. These conditions favor the oxidation of sulfides near the seafloor, the precipitation of siderites a few meters below the seafloor and sulfides deeper in the sub-seafloor. Faults are likelycandidates to act as conduits for sulfate, nitrate and metal oxides that juxtapose different redox environments. Siderites precipitated in the fast and low venting sites show enrichment in d13C and d18O, that are compatible with precipitation in the methanogenic zone. The heavy-oxygen isotopic composition of the siderites may be due to dissociation of gas hydrates, which have not been mapped by seismic data so far in the study area. The mud diapirs are characterized by low backscatter seafloor, large fields of pockmarks and are dissected by normal faults. In coincidence with the normal faults, authigenic calcites and aragonites 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 youngest being dated 640-440 BP. This suggests that the seepage activity in the mud diapirs have been clogged by the carbonates or by the activity of the faults only very recently