The shallow water submarine hydrothermal field off Zannone Island (central Tyrrhenian Sea, Italy): the impact of venting activity on seafloor morphology and benthic community

The Zannone Hydrothermal Field (ZHF, 60 km2), located in the shallow-waters (<150 m wd) of the western Pontine Islands (Tyrrhenian Sea), represents a natural laboratory to study the mpacts of venting activity (mainly CO2) at the seabottom and to evaluate their effects. The ZHF is located on the Ponza Zannone morpho-structural high and was studied using a multi-disciplinary approach (integrated analysis of video observations, high resolution multibeam bathymetry and seismic profiles, water, gas, sediment and rock samples). Active venting was recognized in several locations by the observation of bubbles and gas plumes. The main seabed structures due to venting activity are five giant depressions located between -105 and -140 m that formed during the Holocene. Other morphologies related to vent activity are small pockmarks, authigenic mounds and sulphur crusts. Active vents are diffusely colonized by bacterial mats, whereas areas influenced by fluid emissions are characterized by specific foraminiferal assemblages and peculiar agglutinated species able to cope with acidic condition (i.e. Spiculosiphon oceana). The geochemical signature shows active, CO2-dominated degassing with a significant contribution of mantle volatiles (3He/4He values >3.0 Ra), similar to those recorded in other shallow water areas of the Tyrrhenian Sea (Stromboli and Panarea islands). Geochemical analyses suggest that the vented hydrothermal volatiles are probably associated with residual magma batches, chemically similar to the Pleistocene trachytes cropping out at Ponza Island, that still provide thermal energy to the submarine geothermal system. The vent activity at the ZHF seems to be controlled by the tectonic structures of the Ponza-Zannone structural high and fed by hot volatiles of magmatic origin. At present it is characterized by slow and diffuse degassing but in recent times it encompassed more explosive events. Finally, diffuse episodes of such activity have to be considered in terms of possible triggering mechanisms of shallow instability phenomena.