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.