Exploring CO2 Cycling in Karst Critical Zones: Lessons from Milandre Cave

We know that the concentrations of CO2 and DIC in the subsurface are often magnitudes higher than in the soil zone, and therefore we need to understand this reservoir and its vulnerability to change. Understanding the critical zone in the context of CO2 input, cycling dynamics, and export is essential as this carbon is particularly vulnerable to changes in water table rise which may result in rapid release of CO2 into the atmosphere. Cave environments provide an accessible natural window into the critical zone as they connect meteoric water, soils, the unsaturated vadose zone, and saturated zone. We conducted a two year monitoring campaign at Milandre cave in northern Switzerland, analyzing pCO2, d13CO2, and 14CO2 at various environmental interfaces, including the soil zone, within the epikarst, and in the cave itself. Forest soils maintained stable, modern 14C signatures and low d13C indicating year-round contribution of CO2 from C3 tree and plant root respiration. Conversely, meadow soils exhibited notable seasonality in F14C, suggesting a dominance of respiration from older soil pools in the winter months. Distinct variations in CO2 concentrations were observed within the cave, influenced by temperature driven ventilation dynamics. Keeling plot analysis revealed a consistent contributing endmember of C3 vegetation. However, similarities between the F14C of the meadow soils and cave CO2 suggests a significant contribution of meadow soil CO2 into the cave. These findings offer vital insights into the nuanced dynamics of CO2 sources and cycling processes within the critical zone of Milandre Cave, shedding light on the influences of seasonal variation and ecological influences of critical zone carbon and the export of carbon from terrestrial ecosystems.