Diffuse emission of CO2 and convective heat release at Nisyros caldera (Greece)

The diffuse emission of CO2 from the south east sector of Nisyros caldera (Lakki plain) has been measured during a detailed survey (~1400 soil CO2 flux measurements) performed in October 2018. The gas emissions are fed by hydrothermal sources and, in minor part, by the soil biogenic activity whose mean CO2 flux (4 g m−2 d−1) is here estimated for the first time. The total amount of hydrothermal CO2 reaches 92 ± 8 t/d, a value that is slightly higher than that estimated with the same method between 1999 and 2001 (74 ± 7 t/d). The gas is emitted by different diffuse degassing structures (DDSs), including volcanic-hydrothermal structures (craters and domes) and NE-SW and NW-SE-trending tectonic lineaments. Even if the total CO2 emission is not particularly high at Nisyros (close to the median of CO2 emissions measured in volcanoes worldwide), the process is very energetic. The thermal energy associated with the shallow condensation of the steam in the DDSs reaches ~60 MW, while we estimate at 134–270 MW the total amount of thermal energy involved in the convective rising of the deep geothermal liquids that transport the gas from the depth toward the surface. This large flux of energy could dramatically increase during future earthquakes by addition of heat and mass from a deep hydrothermal reservoir, potentially triggering hydrothermal explosions, as it happened several times in the past few centuries.