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 the atmosphere, boreholes (0.5 to 5 m depth), and cave sampled every 2 months over 2 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 (1/4 -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 Combining double low line 1/4 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 Combining double low line 0.94) and older in winter (F14C Combining double low line 0.88), with a δ13C consistent with the C3-plant-dominated catchment. Mixing models utilizing drip water dissolved inorganic carbon 14C suggest that varying carbonate dissolution and degassing dynamics do not explain the F14C variation and concurrent δ13C stability of the mixing endmember. Rather, contributions from aged carbon reservoirs in the deeper unsaturated zone are likely. This study provides valuable insights into CO2 source dynamics and cycling within the karstic critical zone, highlighting the impact of seasonal variations and ecological factors on downward carbon export from terrestrial ecosystems.