Shallow gas and flood deposition on the Po Delta

Abstract

Sediment cores acquired on the Po delta, northwestern Adriatic Sea, in water depths of 10 to 25 m yielded anomalous concentrations of methane up to 41,300 ppm. Of the 19 unique sites analyzed 5 sites (9 cores) had more than 90 ppm of CH 4 and of those, 3 sites (4 cores) had more than 13,800 ppm methane. Compositional and isotopic analyses of the gas support a bacterial origin. Anomalous methane concentrations were found in kasten, gravity, and box cores; where more than one core type was acquired at a single location, all cores contained anomalous methane. Despite the regional high productivity in the Adriatic, all of the highest gas concentrations were found in the region associated with the thickest accumulation of recent flood deposits off the most active channel of the Po delta (Po di Pila). Cores acquired in this region contain primary sedimentary structures that indicate rapid burial of thick (> 10 cm) flood deposits and a relative lack of bioturbation down to the base of the flood layer. We propose that in the Po delta, flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried on the prodelta, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. In areas unaffected by this high flux of organic matter and rapid/thick flood deposition, or in between flood events, our data indicate that the conditions for methanogenesis and gas accumulation have not been met. We suggest that in these areas, the physical and biological reworking of the surficial sediment between flood events may effectively oxidize and mineralize organic matter (derived from both marine and terrestrial sources) and limit bacterial methanogenesis in the subsurface.