Diffuse Soil CO2 Degassing at Solfatara of Pozzuoli (Campi Flegrei, Italy): 1998-2015, Sixteen Years of Flux Measurement.

Solfatara of Pozzuoli is one of the largest studied volcanic-hydrothermal system of the world releasing a large amount of deeply derived fluids. Since 1998, extensive soil CO2 flux surveys where performed using the accumulation chamber method over a large area (1.45 km2). The statistical analysis of CO2 flux, coupled with the investigation of the CO2 efflux isotopic composition, allowed to characterize the different CO2 sources and to investigate their temporal variability. The geostatistical elaboration of CO2 fluxes allowed to define the spatial structure of the degassing area, as well as the total amount of released CO2, pointing out the presence of a well defined diffuse degassing structure interested by the release of deeply derived CO2 (Solfatara DDS). The extension of the DDS experienced relevant variations with two major enlargements, the first consisted in its doubling in 2003-2004 and the second in further enlargement of about 30% in 2011-2012. These variations mainly occurred external to the crater area in correspondence of a NE-SW fault system (Pisciarelli area). The first event was previously correlated with the occurrence in 2000 of a relatively deep seismic swarm, which was interpreted as the indicator of the opening of an easy-ascent pathway for the transfer of magmatic fluids towards the shallower portion of the hydrothermal system; the second enlargement well correlates with the recent unrest phase of the system, characterized by an acceleration of the ground uplift. The amount of released CO2 has been estimated ranging between about 700 t/d and about 1500 t/d (with errors between 9 and 15 %) until the January 2015 when there was an increase up to 2800 t/d. The CO2 variations in the last two years seems to follow the trend depicted by ground deformations, with increases of fluxes during the uplift accelerations and decreases of fluxes during the phases of relative "no-uplift". The comparison of the CO2 flux data with the chemical composition of the main fumaroles suggests that the variation of in the DDS extension is correlated to processes of condensation of the vapor plume feeding the Solfatara manifestation accompanied by an overall increase of the temperatures, caused by the arrival of increasing amounts of deep fluids, possibly magmatic.