Fluid emission affecting lowstand shelf deposits on the flank of a volcanic island (Zannone Island, Tyrrhenian Sea, Italy).

Several Gas Related Features (GRFs) have been detected offshore the Western Pontine Archipelago by means of very high resolution multibeam bathymetry, high resolution seismic profiles, ROV video observations, water and sediment sampling. Importance of GRFs is related to their association with hydrocarbon occurrence in the subsurface, settlement of endemic ecosystems and the possible direct or indirect linkage with marine geohazards (submarine slides, earthquakes, damage to seafloor infrastructures). In particular, at a broader spatial and temporal scale, geological emissions of methane may be take into consideration as geological factors controlling Quaternary atmospheric and climate changes. Pontine Archipelago is located 30 kilometers from the Italian peninsula (Eastern Tyrrhenian Sea) and is composed of five Plio-Pleistocene volcanic islands: Ponza, Palmarola, Zannone (western sector) and Ventotene and S. Stefano (eastern sector). The research is focused on specific GRFs located offshore the eastern sector of Zannone Island, at water depth ranging between 105-130 m (outer continental shelf). GRFs comprise: giant pockmark, several pockmarks and dome topographic features. The giant pockmark has elongated shape and is 900 m long and 500 m wide; pockmarks are characterized by circular, sub-circular and elongated shapes with dimensions ranging between 2-80 m; whereas dome topographic features are mostly cone-shaped structures with dimensions between 8-36 m. ROV observations have revealed the occurrence of active fluid emissions escaping from the seafloor, characterized by different discharge modalities (continuous and intermittent) and presence of widespread bacterial mats, possible chemosynthetic bivalve aggregations, small scales cone structures and several burrows linked to bioturbation and/or fluid escaped. Moreover, water column backscatter data acquired by multibeam have revealed the occurrence of plumes extended to at least 70 m into the water column. Analysis of very high seismic profiles show the occurrence of several flares at the same location of plumes individuated by water column backscatter data and the occurrence of lowstand prograding deposits, covered by a few meters thick of Holocene deposits. Lowstand prograding deposits have a max thickness of about 35 m and across the giant pockmark show a chaotic seismic facies, indicating intense deformation. To date, rare cases of active shallow-water cold seeps have been described in the Mediterranean Sea. Analysis of the morphological, sedimentological and stratigraphic characteristics of the study area provide the first evidence of an active shallow-water cold emission site in the Eastern Tyrrhenian Sea.