Volcanic CO2 degassing and microbial carbon fixation in a caldera offshore

Calderas are subsided volcanic terrains formed by the destructive power of some of the largest volcanic eruptions on Earth. Many such depressions globally are today submerged by crater lakes or seawater, rendering them less accessible to scientific scrutiny, and hence more complicated to monitor during unrest. One of such systems is the restless, partly submerged Campi Flegrei caldera (CFc) near Naples in Italy, where escalating hydrothermal activity during ongoing unrest demands for an improved understanding of the caldera offshore. Here, we present a high-resolution, system-wide mapping of CO2 degassing encompassing the caldera offshore and onshore sectors. Combining vertical seawater profiles with groundwater composition results, we obtain a refined delimitation of the anomalous CO2 degassing zones, finding an overall structural control on volatile transport and surface/seafloor discharge. Intense CO2 degassing on the seafloor also causes ocean acidification, sustains diverse chemolithotrophic communities near the hydrothermal emissions, and influences the near-vent environment, as demonstrated by taxonomic and functional diversity data. Submerged caldera sectors are extremely dynamic environments whose volcanological significance and biogeochemistry may have been overlooked.