Chemosynthetic communities as fingerprints of submarine sliding-linked hydrocarbon seepage, Miocene deep-sea strata of the Tuscan-Romagna Apennines, Italy

A specialised low-diversity bivalve assemblage dominated by chemosymbiotic vesicomyid bivalves (?Calyptogena sp.) is associated with a large submarine landslide (Casaglia-M. della Colonna slide) within the turbiditic Miocene Marnoso-arenacea Formation in the northern Apennines. This macrofossiliferous horizon is located just at the top of the slide close to the boundary between intrabasinal deformed sediments and extrabasinal sediments. There is a causal relationship between the emplacement of this large submarine slide and fluid expulsion promoting the localised and ephemeral establishment of specialised communities. The occurrence of exclusively sulphur-based biota and the concomitant absence of 13C-depleted authigenic carbonates from the host sediment are likely an indication that hydrocarbons within subsurface fluids were consumed through sulphate reduction processes. As a result, high levels of shallow-seated H2S were produced and then exploited by chemosymbiotic bivalves.