Constraining the soil carbon source to cave-air CO₂: evidence from the high-time resolution monitoring soil CO₂, cave-air CO₂ and its δ¹³C in Xueyudong, Southwest China

Abstract. Cave CO₂ plays an important role in carbon cycle in a karst system, which also largely influences the formation of speleothems in caves. The partial pressure of CO₂ (pCO₂) of the cave air and cave water (cave stream and drip water) in Xueyu Cave was monitored from 2015 to 2016. The pCO₂ for cave air and stream over two years showed very similar variations in seasonal patterns, with fluctuated high CO₂ concentrations in the wet season and steady low CO₂ concentrations in the dry season. Soil CO₂ which is largely controlled by soil temperature and soil water content as well as stream degassing are main origins for the Xueyu cave air pCO₂. The average values of δ¹³C_soil, δ¹³C_DIC in June were −23.9 ‰ and −13.4 ‰, respectively; δ¹³C_CO2 of atmospheric air was −10.0 ‰ and δ¹³C_CO2 of cave air was −23.3 ‰. The average values of δ¹³C_soil, δ¹³C_DIC in November were −18.0 ‰ and −12.2 ‰, respectively; δ¹³C_CO2 of atmospheric air was −9.6 ‰ and δ¹³C_CO2 of cave air was −18.8 ‰. Moreover, the contribution from soil CO₂ is higher in June (78.8 %) than in November (67.1 %) based on the model of carbon stable isotopes. The contribution of C from the soil was larger in summer than in winter. The very similar (negative) values of carbon isotopes between soil and cave air CO₂ suggests that there were no potential geological/deeper sources with more positive δ¹³C_CO2. Stream pCO₂ degases from upper stream to downstream in the cave, resulting in slightly decreased pCO₂ but increased carbon isotope values in the downstream. The influence of these regional controls on stalagmite records requires a better understanding of modern interaction between cave CO₂ sources, transport paths and mechanisms.