A Common Mantle Source for the Endogenous CO2 Degassed at the Eifel (Western Germany) and the Ardennes (Eastern Belgium) Regions

Tectonic CO2 Earth degassing is globally relevant and has probably controlled climate on a geological scale (Brune et al., 2019, https://doi.org/10.1038/s41561-017-0003-6). Endogenous CO2 outgassing from rifting areas remains poorly constrained, with most available data from the East African Rift. Here, we investigate the CO2 degassing in the European Cenozoic Rift System (ECRIS), focusing on the chemical and isotopic composition of 161 springs emerging from the Eifel-Ardennes-Rhenish Massif region (Eastern Belgium-Western Germany). Theoretical water-gas-rock interaction models based on the chemistry of gases dissolved in East Eifel groundwaters reveal CO2 efflux takes place in a P-T range between the aquifer depth (4 bar, 160°C) and spring emergence (1 bar, 9°C). He and C isotopes show that Ardennes, Volcanic Eifel and Rhenish Massif are all part of a unique degassing system. The average mantle CO2 flux emitted from the region is 5 ± 2 × 106 mol yr−1 km−2, corresponding to a total deeply derived CO2 emission rate of 7 ± 4 × 109 mol yr−1. These values are of the same magnitude as the global baseline defined for convective hydrothermal CO2 emitted from areas of high heat flow, demonstrating the relevance of passive rifts as CO2 emitters on a global scale.