Subsurface CO₂ dynamics in a temperate karst system reveal complex seasonal and spatial variations

Abstract. Understanding the carbon cycle of the terrestrial Critical Zone, extending from the tree canopy to the aquifer, is crucial for accurate quantification of its total carbon storage and for modelling terrestrial carbon stock responses to climate change. Caves and their catchments offer a natural framework to sample and analyse carbon in unsaturated zone reservoirs across various spatial and temporal scales. In this study, we analyse the concentration, stable carbon isotopic ratio (δ¹³C), and radiocarbon (¹⁴C) compositions of CO₂ from atmosphere, boreholes (0.5 to 5 m depth), and cave sampled every two months over two years at Milandre cave in northern Switzerland. High concentrations of up to 35’000 ppmV CO₂ are measured in the boreholes. The δ¹³C values of CO₂ in the boreholes reflect the δ¹³C of C3 plants (~ -26 ‰) which dominate the catchment ecosystem. Shallow meadow boreholes host older CO₂ in winter and modern CO₂ in summer, while forest ecosystems consistently export modern CO₂ (F¹⁴C = ~1) to the unsaturated zone. Cave CO₂ concentrations exceed atmospheric levels and are diluted by temperature-driven seasonal ventilation. Keeling plot intercepts indicate that the cave CO₂, which mixes with atmospheric CO₂, is younger in summer (F¹⁴C = 0.94) and older in winter (F¹⁴C = 0.88), with a δ¹³C consistent with the C3 plant dominated catchment. Mixing models utilising drip water dissolved inorganic carbon ¹⁴C suggest that varying carbonate dissolution and degassing dynamics do not explain the F¹⁴C variation and δ¹³C stability of the mixing endmember. Rather, contributions from deeper aged carbon in the epikarst are likely. This study provides valuable insights into CO₂ source dynamics and cycling within karstic Critical Zones, highlighting the impact of seasonal variations and ecological factors on downward carbon export from terrestrial ecosystems.