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.