环境地球化学国家重点实验室知识库

Conveyance and modification of carbon-isotope signals within the karst system remain difficult to constrain, due to the complexity of interactions between multiple components, including precipitation, bedrock, soil, atmosphere, and biota. Cave monitoring is thus critical to understanding both their transport in the karst system and dependence on local hydroclimatic conditions. Jiguan Cave, located in Funiu Mountain in central China, is representative of karst tourist caves with relatively thin epikarst zone. We conducted a comprehensive monitoring program of cave climate from 2018 to 2021 and measured δ¹³C during 2021 in monthly and heavy-rainfall samples of soil CO₂, cave CO₂, cave water (drip water and underground river), and underground river outlet. Our results demonstrate synchronous variations between CO₂ concentration and δ¹³C_CO2 in both soil and cave air on seasonal time scales. Cave pCO₂ and carbon-isotope composition further exhibited a high sensitivity to human respiration with fluctuations of ~2000–3000 ppm within 4 days during the cave closure period in July 2021 without tourists. ¹³C-depleted isotopic signal of cave air in summer is the mixture of human respiration and soil CO₂ which varies as a function of regional hydrological conditions of the summer monsoon during the rainy season with high temperatures and humidity. However, respired CO₂ from the overlying soil was expected to be the only principal source of the cave CO₂ when the anthropogenic CO₂ source was removed. The high seasonal amplitude of cave air δ¹³C_CO2 reflects ventilation dynamics, which leads to a prominent contribution from the external atmosphere during winter. Intriguingly, although the δ¹³C signal reflects complex vertical processes in the vertical karst profile, a heavy summer rainfall event was related to anomalously high δ¹³C values of cave water that can be utilized to interpret rainfall intensity and regional hydroclimate.