Methane-related carbonate cementation of marine sediments and related macroalgal coralligenous assemblages in the Northern Adriatic.

Abstract This multidisciplinary study presents the major characteristics of a set of submarine rock outcrops in the Northern Adriatic Sea: the rock occurrence, embedded down to about 1 m from the seafloor in the bottom sediments; numerous gas accumulations in the proximity of the outcrops (e.g., bubbling gas from the sediment–water interface); small-scale mud volcanoes; and microbial mats around the seepage site. Geochemical analysis revealed that these features are related to the seepage of CH4-rich fluids, which causes the precipitation of methane-derived calcium carbonate as a cement (δ13C = -49.80‰ V-PDB). 14C data gave radiocarbon ages of about 21,700 ± 2,265 years bp for two cement samples. From the biological point of view, a total of 112 macroalgal taxa were recorded. The different number of taxa recorded at the San Pietro and Bardelli sampling sites has been related to the different distance from the coast and to water depth. Some species that characterize these outcrops are acknowledged as important bioconstructors distinctive of the Mediterranean area.