Characterization of undocumented CO2 hydrothermal vents in the Mediterranean Sea: implications for ocean acidification studies

A previously undocumented shallow water hydrothermal field from Sicily (Southern Tyrrhenian Sea, Italy), is here described based on a multidisciplinary investigation. The field, covering an area of nearly 8000 m2 and ranging in depth from surface to -5 m, was explored in June 2021, to characterise the main physico-chemical features of the water column, describe bottom topography and features, and identify the main megabenthic and nektonic species. Twenty sites were investigated to characterize the carbonate system. Values of pH ranged between 7.84 and 8.04, {Omega}Ca between 3.68 and 5.24 and {Omega}Ar from 2.41 to 3.44. Geochemical analyses of hydrothermal fluids gases revealed a dominance of CO2 (98.1%) along with minor amounts of oxygen and reactive gases. Helium isotope ratios (R/Ra =2.51) and {delta}13CCO2 (3) support an inorganic origin of hydrothermal degassing of CO2 and the ascent of heat and deep-seated magmatic fluids to the surface. Visual census of fishes and megabenthos (mainly sessile organisms) allowed identification of 62 species, of which four are protected by the SPA/BIO Protocol and two by the International Union for Conservation of Nature. The macroalgae Halopteris scoparia and Jania rubens and the sponge Sarcotragus sp. were the dominant taxa in the area, while among fishes Coris julis and Chromis chromis were predominant. The preliminary description of this venting field indicates this site as an area of considerable interest and suitable for future experimental studies on ocean acidification.