Long‐term variations of the Campi Flegrei, Italy, volcanic system as revealed by the monitoring of hydrothermal activity

Long‐duration time series of the chemical composition of fumaroles and of soil CO 2 flux reveal that important variations in the activity of the Solfatara fumarolic field, the most important hydrothermal site of Campi Flegrei, occurred in the 2000–2008 period. A continuous increase of the CO 2 concentrations and a general decrease of the CH 4 concentrations are interpreted to be the consequence of the increment of the relative amount of magmatic fluids, rich in CO 2 and poor in CH 4 , hosted by the hydrothermal system. Contemporaneously, the H 2 O‐CO 2 ‐He‐N 2 gas system shows remarkable compositional variations in the samples collected after July 2000 with respect to the previous ones, indicating the progressive arrival at the surface of a magmatic component different from that involved in the 1983–1984 episode of volcanic unrest (1983–1984 bradyseism). The change starts in 2000, concurrently with the occurrence of relatively deep, long‐period seismic events which were the indicator of the opening of an easy ascent pathway for the transfer of magmatic fluids toward the shallower, brittle domain hosting the hydrothermal system. Since 2000, this magmatic gas source is active and causes ground deformations and seismicity as well as the expansion of the area affected by soil degassing of deeply derived CO 2 . Even though the activity will most probably be limited to the expulsion of large amounts of gases and thermal energy, as observed in other volcanoes and in the past activity of Campi Flegrei, the behavior of the system in the future is, at the moment, unpredictable.