Constraining the soil carbon source to cave-air CO<sub>2</sub>: evidence from the high-time resolution monitoring soil CO<sub>2</sub>, cave-air CO<sub>2</sub> and its δ&am

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 δ13Csoil, δ13CDIC in June were −23.9 ‰ and −13.4 ‰, respectively; δ13CCO2 of atmospheric air was −10.0 ‰ and δ13CCO2 of cave air was −23.3 ‰. The average values of δ13Csoil, δ13CDIC in November were −18.0 ‰ and −12.2 ‰, respectively; δ13CCO2 of atmospheric air was −9.6 ‰ and δ13CCO2 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 δ13CCO2. 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.