Geological CO₂ quantified by high-temporal resolution stable isotope monitoring in a salt mine

The relevance of CO₂ emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized. Although geogenic gas migration along faults and in volcanic zones is generally well studied, short-term dynamics of diffusive geogenic CO₂ emissions are mostly unknown. While geogenic CO₂ is considered a challenging threat for underground mining operations, mines provide an extraordinary opportunity to observe geogenic degassing and dynamics close to its source. Stable carbon isotope monitoring of CO₂ allows partitioning geogenic from anthropogenic contributions. High temporal-resolution enables the recognition of temporal and interdependent dynamics, easily missed by discrete sampling. Here, data is presented from an active underground salt mine in central Germany, collected on-site utilizing a field-deployed laser isotope spectrometer. Throughout the 34-day measurement period, total CO₂ concentrations varied between 805 ppmV (5th percentile) and 1370 ppmV (95th percentile). With a 400-ppm atmospheric background concentration, an isotope mixing model allows the separation of geogenic (16-27%) from highly dynamic anthropogenic combustion-related contributions (21-54%). The geogenic fraction is inversely correlated to established CO₂ concentrations that were driven by anthropogenic CO₂ emissions within the mine. The described approach is applicable to other environments, including different types of underground mines, natural caves, and soils.