The Migration Characteristics Of Carbon Isotopes And Its Ecological Environment Significance In Karst Critical Zone In Central China

The monitoring of cave systems is helpful to study the physical,chemical,and biological processes of climate signal transmission in the karst critical zone,revealing the relationship between regional climate change and the ecological environment.It is of great significance to systematically study the transport process of carbon isotope in the karst critical zone,understand the response of cave system to environmental changes,and deeply reveal the influencing factors of carbon isotope in the karst system,as well as improve the understanding of carbon source,carbon sink and the carbon cycle in karst critical zone.However,due to the complexity of multi-interface in the karst critical zone（precipitation-bedrock-soil-atmosphere-biology）,the characteristics and transformation mechanism of carbon isotope transport are still unclear.In this study,the Jiguan Cave in the west of Henan and the northern foot of Funiu Mountain in the North-south intersection zone of the Qinling Mountains were monitored for 12 consecutive years（2010-2021）to analyze the carbon isotope composition of the cave system and explore the impact of climate change on carbon isotope signals at different time scales.To refine the carbon transport characteristics of each component and expand the research scope to the karst critical zone,a high-resolution monitoring device was added in Jiguan Cave from May 2019 to December 2021 to monitor all parts of the outlet of atmosphere-vegetation-soil-cave water（drip water,pool water and underground river）-outlet of underground river.In this paper,the variation characteristics and transport mechanism of carbon isotope in the karst critical zone and its response to the regional environment are discussed on seasonal scale,interannual scale,and high-resolution heavy precipitation scale.The results show that:（1）Dissolved inorganic carbon（DIC）and its stable carbon isotopes in Jiguandong cave water are sensitive to regional precipitation and hydroclimate changes and have obvious annual changes.In the annual time scale,the negative/positive value of cave waterδ¹³C_DIC can respond to the changes of El Ni（?）o Southern Oscillation（ENSO）and is affected by the increase/decrease of precipitation in summer（June to August）.Due to the short lag time of atmospheric precipitation through the epikarst into the cave,the cave water DIC and itsδ¹³C can quickly respond to regional extreme climate events.The changes of the dry and wet conditions control theδ¹³C_DIC values of cave water by regulating the soil and biological processes as well as the degree of water-rock interaction in the epikarst zone.The cave waterδ¹³C_DIC values reached the most negative values in the two extreme precipitation periods with-13.91‰in 2010 and-12.98‰in 2021,and the most positive value with-2.52‰in the extreme drought year in 2013.On interannual or even longer time scales,caveδ¹³C may be an effective indicator of regional extreme climate events,East Asian monsoon summer precipitation,and ENSO changes.（2）In the soil-cave system,soil air and cave air CO₂ andδ¹³C_CO2 have the same seasonal variation characteristics.The concentration of CO₂ in soil and cave air is high in summer and low in winter,andδ¹³C_CO2 is negative in summer and positive in winter.Cave air CO₂ is mainly derived from two sources,which is overlying soil and outside air.In rainy summer,the overlying soil air was the main source of the cave CO₂,contributing63%CO₂ to the cave.The cave airδ¹³C_DIC inherits the lighter soil carbon source with the increase of CO₂ concentration in the cave,which results in the negativeδ¹³C_DIC value of drip water due to the CO₂ degassing.Under the influence of the strong ventilation effect,the external atmosphere is the main source of CO₂in the cave in winter,with a contribution ratio as high as 88%.Theδ¹³C_DIC values of cave air and drip water are positive due to the dilution of cave air CO₂ by the external surface atmosphere.（3）At the spatial scale,δ¹³C is enriched gradually in the process of carbon isotope migration in plants-soil-cave drip water-modern sediment-underground water.Under the scale of short-term heavy rainfall,rainfall intensity has a significant influence on the transport and variation characteristics of carbon isotope in space（soil-drip-underground river-outlet of the underground river）.Continuous precipitation with low-intensity results in negativeδ¹³C values of all components except drip water from top to bottom of the karst critical zone.If the rainfall amount brought by short-time heavy precipitation reached the threshold of direct runoff,soil CO₂ and fast-passing rainfall did not reach the isotope equilibrium fractionation,and theδ¹³C value of drip,underground river,and outlet of the underground river showed positive characteristics.（4）Human tourism activities have a significant impact on CO₂ flux in Jiguan Cave,which is a tourist cave.On the monthly and diurnal scales,when the number of tourists reaches a certain threshold,the CO₂ generated by human activities will become an important contributor to the cave CO₂,superimposing the effect of soil CO₂ to influence the dynamic change of cave CO₂ andδ¹³C_DIC of drip water by controlling the degassing of CO₂.Theδ¹³C_DIC of air and water in the cave is negative when the number of tourists increases,and vice visa.But its effect on the seasonal scale is negligible.Therefore,when conducting high-resolution carbon flux studies on natural caves and tourist caves in the future,it is necessary to clearly distinguish the overlapping effects of human tourism activities.