Methane-Derived Authigenic Carbonates (MDAC) in northern-central Adriatic Sea: Relationships between reservoir and methane seepages

In the Adriatic basin, gas venting characterized by pockmarks, gas bubbling and other fluid escape related fea- tures, is diffused and has been linked or to shallow gases trapped in the Late Pleistocene and below the Ho- locene maximum flooding surface or to leakage from more deeply buried Pliocene to Pleistocene foredeep succession. Some seepages are associated with carbonate crusts, but a possible correlation between methane leakage and carbonate precipitation in this basin never reached a clear definition. Carbonate concretions re- covered by different areas of the Adriatic Sea (northern, central and southern) at different depths (from ca 30 to ca 300 m) have been analysed. Different fabrics, petrography and mineralogy can be evidenced for the dif- ferent sets of samples; all of them show a carbonate content mainly represented by high-Mg calcite: up to 66% in weight of total carbonates for the samples from central Adriatic (close to Bonaccia gas field, offshore Ancona), and 79 to 96% of the total carbonate weight for the other samples that are mainly biogenic. These differences are also reflected by the δ13C values, ranging between −19‰ and −29‰ VPDB, for the samples collected in the Bonaccia area and moderately positive values for all the other samples. Bonaccia samples contain carbonate cements that record the contribution of methane-derived carbon and have been interpreted as formed below the sea floor, in correspondence of the Sulphate-Methane Transition Zone. Carbonates recovered from the southern and northern Adriatic basin, do not show clear evidence of methane-derived carbon and are mainly made up by organisms typically recovered in the Adriatic cor- alligenous assemblage. Nonetheless micro-morphological evidences suggest influence of bacterial sulphate reduction and methanogenesis in precipitation of some of these carbonates.