Methane-derived authigenic carbonates (mdac) in northern-central Adriatic Sea.

In the Adriatic Sea, carbonate patches are known since about two centuries and, during the last two decades, many samples have been recovered and studied. In some cases, observation suggested that the occurrence of these carbonates was in the proximity of gas emission in the seafloor. Notwithstanding some descriptions of the biogenic calcareous components, the possible correlation between methane leakage and carbonate precipitation in the Adriatic Sea, never reached a clear definition. The aim of this study is, then, to describe and characterize the occurrence of Methane-Derived Authigenic Carbonates (MDAC) in the Bonaccia site, located in the Northern-Central Adriatic Sea, by means of the investigation of their relationship with different gas seeps and gas-related structures. In order to achieve our goal, we firstly analysed the texture of different samples of the concretions, to define the spatial relationships between clastic components and authigenic carbonate cements. We have carried out thin sections observation and SEM-EDAX microscopy analysis in order to describe structures and textures of the samples, highlighting spatial relationships between clasts (mainly rapresented by quartz, plagioclase and foraminiferal skeletons) and authigenic carbonate cements, represented by euhedral to sub-euhedral crystals of dolomite, calcite and aragonite. This analysis has been coupled with the study of the mineralogical composition, performed with X-Ray diffractometry, which allowed to determine the different carbonate phases (aragonite, calcite, protodolomite, dolomite). Furthermore, bulk sample stable isotopic composition of carbon yielded δ13C values as low as -29,5 ‰ VPDB, suggesting methane as the main carbon source. SEM microscopy analysis also revealed typical microbial fabrics, such as “peloids”, “pyrite”, “microbial filaments” and structures referable to bacterial cells; besides “nanograins”, we have observed cauliflower structures and aragonite batons, all with dimension ranging from 300 nm to 2µm, in tight spatial connection with cements. Mineralogy, isotopic composition and microscopy evidences suggest MDAC cements, surrounding clasts, and microbially-mediated precipitation of carbonate mineralogical phases. We infer that this precipitation can be ascribed to both Anaerobic Oxidation of Metane (AOM)/Sulphate Reduction (SR) processes in the Sulphate-Methane Transition Zone (SMTZ), operated by a consortium of Archea and Sulphate Reducing Bacteria (SRB) at the seafloor, or in the first few tens of cm of modern sediments. In anoxic/disoxic condition, these processes produce bicarbonate and an increase of pore water alkalinity, promoting the precipitation of carbonates. We analysed Chirp sub bottom and multichannel seismic profiles in order to ascertain if the occurrence of MDAC were in correspondence of different gas-seeps and gas-related morphologies. Gas-related structures (i.e. pockmarks, mounds and depressions) were largely described in this area of the Adriatic Sea. The interpretation of the seismic profiles allowed to locate these structures in correspondence of gas leakage from pools of the Bonaccia gas field. It can be documented that methane in shallow sedimentary layers, of Pleistocene age, is likely supplied from a deeper source, deformed by compressive structure and reverse faults.