Trace Element And Stable Carbon Isotope Variations In Karst Cave Dripwater And Hydroclimatic Significance

Stalagmites are considered natural archives of climate proxies.However,under the combined effects of atmospheric circulation patterns,precipitation,and karst environments,drip hydrogeochemical processes can be coupled and linked to each other to control cave sediment record information.Therefore,the evolution of chemistry and factors controlling the isotopic composition of the dripwater during regional precipitation migration from the surface to caves need to be evaluated.Through field monitoring and indoor experiments on the external temperature and precipitation of Mahuang Cave,CO₂ concentration of the overlying soil,CO₂ concentration of the cave air,physicochemical indicators of cave dripping water and stable carbon isotopes of dripping water,the study uses relevant mathematical and statistical methods to analyse their spatial and temporal distribution patterns,identify the main hydrogeochemical processes controlling theδ¹³C_DIC values and trace element ratios of dripping water,and investigate the response between different The response of different hydrological types of dripping water to water chemistry is explored,and the control mechanisms are then analysed in relation to regional atmospheric circulation and groundwater hydrochemical characteristics to investigate the transfer of climatic and environmental information in the whole cave system（atmospheric precipitation-cave dripwater）.This will provide an important basis for the correct interpretation of the formation mechanism of stalagmites and the paleoclimatic information they record.In this study,hydrogeochemical characteristics and the isotopic composition of the dripwater in the Mahuang Cave in Guizhou Province,including stable isotope(δ¹³C_DIC)and trace element ratios,were monitored from August2018 to December 2021.（1）Analysis of the external climatic environment and the overlying soil CO₂ concentration in Mahuang Cave revealed that on a seasonal scale the soil CO₂ concentration showed a change pattern of high in summer and low in winter,with soil respiration and microbial activity weakening under the control of precipitation and temperature,and soil CO₂ concentration decreasing significantly in August and September2019 and August 2020,with this change transmitted to the interior of the cave.The monitored cave air CO₂ concentrations are consistent with the changes in soil CO₂ concentrations,showing a trend of high summer and low winter,with similar decreasing changes in August and September2019 and August 2020,indicating that soil CO₂ are the main source of recharge for cave air CO₂.（2）During the monitoring period,the results demonstrated the significant influences of the recharge process from precipitation,soil CO₂ concentrations,and CO₂ outgassing caused by cave ventilation on the changes in dripwater chemistry andδ¹³C_DIC values.Theδ¹³C_DIC,Mg/Ca,and Sr/Ca ratios showed seasonal changes.The highest and lowest values were observed in the dry and wet seasons,respectively,highlighting the influence of precipitation changes and water-gas-rock reactions,such as soil CO₂ concentrations and carbonate dissolution in the dry and wet seasons.Therefore,soil CO₂ concentrations and the non-homogeneous aquifer media demonstrated an important role in changing the physicochemical parameters of the dripwater.（3）The five dripwater points in Mahuang Cave consist of slow seepage and fast seepage,the high-flow dripwater points were more responsive to external precipitation.Indeed,the results revealed strong influences of water-rock interaction/PCP in the wet/dry seasons,respectively.Moreover,the karst aquifer recharge process is complex,the results indicated spatial differences in the water chemistry indicators between the dripwater points in the Mahuang Cave,the hydrogeochemical processes control the hydrological signals of dripwaterδ¹³C_DIC,Mg/Ca,and Sr/Ca ratios under different hydrological flow paths,which generally reflect the changes of precipitation and are of great significance for hydroclimatic studies in monsoon regions using cave dripwaterδ¹³C_DIC and trace elements.（4）The dripwaterδ¹³C_DIC,Mg/Ca,and Sr/Ca ratios were used to determine the precipitation anomalies in the summer of 2020,especiallyδ¹³C_DIC responds rapidly to extreme precipitation and drought events.This shows thatδ¹³C_DICvalues were sensitive to changes in local hydrological conditions,which may also reflect the regional precipitation changes caused by ENSO on the interannual scale.Therefore,δ¹³C_DICcan be used as an indicator to record regional environmental changes,especially precipitation changes.