Sources of cave CO2 at Milandre cave, Switzerland constrained through multipool analysis of 14C and δ13C.

The cycling of subsurface karst CO₂ is not well constrained in terms of its source and transportation pathway. The classical model suggests that cave CO₂ is produced by the respiration of soils and vegetation in the catchment. In contrast, several new studies have proposed that the dominant source of CO₂ is from the respiration of older organic matter situated deeper within the karst, or from the degassing of supersaturated drip water.

We present over a year of monitoring data from Milandre cave, northern Switzerland, whereby we evaluated the ¹⁴CO₂ and δ¹³CO₂ composition of the atmosphere in the cave catchment, catchment soil gas, well gas, and cave air. Drip waters located throughout the cave also underwent various analysis. The cave ¹⁴CO₂ is more depleted compared to the soil and gas samples. The Keeling plot intercept of atmospheric and cave δ¹³CO₂ is ~-26‰, indicating a dominant contribution from biological respiration. The dissolved inorganic carbon from various cave drips have an F¹⁴C from ~ 0.84 to 0.96 and δ¹³C from ~ -16‰ to -11‰. F¹⁴C and δ¹³C are inversely correlated.  Considering both the ¹⁴C and δ ¹³C results, this suggests either a source of CO₂ from an aged reservoir of respiring organic matter contributing to the cave gas or substantial influence from degassing of ¹⁴C fossil carbonate CO₂ from drip water. These results have implications for the understanding of the subterranean carbon cycle and the interpretation of speleothem carbon isotope records for paleoclimate studies.