Chemical and isotopic signatures of fluids circulating in the Massif Central (France) and in the Volcanic Eifel (Germany): evidences of similar features and of an ongoing degassing process.

The French Massif Central (central-southern France) and the Eifel region (central-western Germany) are both young volcanic systems and considered dormant. They are part of the European Cenozoic Rift System (ECRIS) and show similar surficial manifestations of ongoing hydrothermal activity. For example, both areas exhibit numerous low flow rate CO₂-rich springs, mainly occurring in concomitance of faults and fractures inherited from the Variscan orogeny.

Here, the chemical and isotopic characterization of different fresh water bodies (springs, wells, rivers and volcanic lakes) has been provided. The composition of dissolved gases and the isotopic signatures of dissolved carbon indicate that meteoric water infiltrated and then interacted with a CO₂-rich, mantle-related, component. The majority of studied water samples exhibit pCO₂ between 0.3 and 1 bar and the total dissolved inorganic carbon (TDIC) is of the order of 0.01 mol/kg. At surface, most spring water samples are oversaturated with calcite, dolomite, chalcedony and quartz and are in equilibrium with amorphous silica. The correlation between the TDIC and its isotopic composition (δ¹³C_TDIC) suggests that part of the analysed water samples experienced a degassing process prior to or immediately after emergence. The computed CO₂ flux transported by groundwaters is of the same order of magnitude of the global baseline theorized for geothermal areas. This indicates that passive rifts systems contribute to the atmospheric CO₂ content and highlights the importance of taking into account each carbon source in the study of the global carbon cycle.