The role of methane seepage in the formation of the Northern Adriatic Sea geosites

The northern Adriatic Sea hosts patches of rock formations that have given rise to large biodiversity not otherwise found on the predominantly sandy and flat seabed. Various hypotheses have been invoked concerning the origin of these features, which are widespread in the northern Adriatic Sea and have been intensively studied since the eighteenth century. Here, we provide a significant advancement of knowledge in the lithification process associated with these peculiar rock formations. This study is based on geological, geophysical and mineralogical analyses performed on 45 rock outcrops identified in the study area. A key result of our analysis is the reconciliation of the different models related to the genesis of the northern Adriatic Sea outcrops, i.e., Holocene beach rocks that typically formed in the intertidal zone; cementation of paleo-channel systems, implying a groundwater influence; and methane-derived carbonates. Regardless of the depositional environment, we highlight the relationship between sediments and the processes that led to their evolution into the sandstone formation. We suggest that a common cementation process occurred in the various sedimentary deposits that are representative of different environments of the northern Adriatic Sea and was related to the post depositional precipitation of methane-derived carbonates. Our findings highlight the potential key role of widespread methane seepage from very shallow marine environments into the water column and then, eventually, into the atmosphere as a potential source of an increasing greenhouse gas.