The Variation And Controlling Factors Of CO₂ Concentration In A Karst Cave

The running of the karst dynamic system is accompanied by the migration of carbon,the migration among each phase is accompanied by variation of concentration and isotopic fractionation of carbon species.After the integration of previous research data in Xueyu Cave,build high resolution and high precision monitoring network of atmosphere-soil-cave,from the variation pattern and controlling factors of four cave carbon sources?soil,atmosphere,underground river anddirp water?to analysis the variationand controlling factors of cave CO₂ on annual,seasonal,daily and rainy scales.In this study,the monitoring which lasted from October 2014 to February2017,contained automatic monitor of atmospheric CO₂ and meteorology,soil CO₂,cave air CO₂,physico-chemical parameters and pCO₂ of groundwater.The DIC concentration and ?¹³CDIC,DIC concentration and ?¹³CDIC,ions of groundwater in the river were sampled and titrated once a month from the beginning of July 2015.In order to study the interaction between underground river and cave air CO₂,laboratory simulation was carried out in July 2015.The variation patterns of pCO₂ and DIC in the underground river were analyzed through comparison with solution which physico-chemical and biological process dominated degassing,respectively.All samples were prepared in duplicate and one set was agitated to simulate mixing in surface waters.In order to study the response of cave CO₂ to rainy events,three monitors on November 2014,March 2016,June 2016,different rainfall intensities during different seasons were conducted respectively during the study period,the isotopic samples of soil CO₂,cave air CO₂,and DIC concentration of drip water,underground river.The study found:?1?Xueyu cave-soil-cave system showed significant seasonal variation,the variations of soil CO₂,cave air CO₂ and pCO₂ of underground river,can be considered as the pattern of springelevation,summer peak,autumn decrease and winter low values.The variation pattern means cave air CO₂ may be controlled by soil CO₂ and underground river pCO₂ on seasonal scale.?2?Through the three monitors of November 2014,March 2016 and June 2016,the results showed that the underground river had different variation patterns to respond different rainfall intensities.The variation of cave CO₂ increased at first,then decreased,and varied with the variation of conductivity and flow rate.The statistical analysis of all the rainfall events during the rainy season of 2015 showed that the greater the rainfall intensities were,the larger the amplitudes of cave CO₂ were,and the shorter response and equilibrium time reached.The interval between rainfalls and outside temperature also affect the response process.?3?The cave air CO₂ of Xueyu Cve will suddenly increase and decrease during season alternations,and the variation ranges can reach 16000 ppm.It is found that the CO₂ mutation period has a good correspondence with the abrupt change of outside air temperature.During the rising phase of CO₂ mutation in the cave,the pCO₂ of the underground river increased firstly,and showed a significant daily increase by turn.The conductivity of underground river increased with the rising of pCO₂,while pH decreased,and vice versa.The day when the outside temperature was higher than cave air temperature,underground river pCO₂ rise;when the night temperature was lower than the inside temperature,the underground river p CO₂ decreased,but the decline is relatively small,so that the cave CO₂ cumulated totally.This may be the CO₂ which dissolved in the vadose zone and then eroded into underground river,enhancing the degassing of underground river to lead an overall elevation of cave CO₂ concentration.During decline stage of the cave mutation,the gas exchange between the cave and atmosphere leads to the rapid decrease of cave CO₂.?4?The laboratory simulations showed that during the interation of the underground river and atmosphere,the p CO₂ variation ofreactors which biological process dominated weredifferent from the reactors which physical and chemical process dominated.The reactors which dominated by biological process,showed obvious diurnal variations,and the pCO₂ equilibrium timings were24h?still water?,24h?agitated water?.However,there was no significant diurnal variation in the reators dominated by the physical chemical degassing,and the equilibrium timings were 68h?still water?,331h?agitated water?.There was no obvious diurnal variation in the interaction between pCO₂ of the underground river and cave air CO₂,which indicated that physical and chemical degassings were dominant.When the degassing rates during the response of the underground river to ranfall were larger than that of the equilibrium,so the equilibrium time which cave CO₂ took was shorter.?5?The isotopic characteristics analysis results of the atmosphere-soil-cave system showed,?¹³CCO₂ values of soil and cave,?¹³CDIC values of drip water and underground water havesignificant seasonal variation,which were lighter during rainy seasons,heavier during dry seansons.The response processes of rainfall can be divided into rising and falling phases.During the rising stages of November 2014 and June 2016 rainfall monitors,through estimation for the diffusion amount of CO₂ from soil into cave and soil ?¹³CCO₂,found that the soil CO₂ accounted 91%?November?,96%?June?for cave air CO₂ variation,which dominates the cave CO₂.