Study On Vertical Migration Of CO₂ In Karst Cave System

In recent years,due to global warming and frequent extreme weather,climate issues have attracted extensive attention.The global carbon cycle is considered to be an important driving force to control global climate change and has become one of the core issues in the current research on climate change.In the global carbon cycle,the whereabouts of 1 billion tons of carbon are still unknown.Karst areas are widely distributed in the world,and karstification is considered to have a significant carbon sink effect.As an important driving force of karstification,CO₂ plays a key role in karstification.The research shows that karstification,as an important part of the global carbon cycle,accounts for 1/3 of the global missing carbon sink.Due to the existence of a missing carbon sink,it is difficult to accurately predict the future CO₂ change,thus interfering with climate regulation.Unique binary three-dimensional structure in karst area.The underground cave system is relatively developed,which creates conditions for Earth atmosphere exchange.As the window of karst underground space,CO₂ in the air forms carbonic acid with precipitation,and a series of hydrogeochemical processes occur with the bedrock,releasing the old carbon in the bedrock into the cave for degassing,which makes a large amount of CO₂ accumulate in the cave.Because the complex three-dimensional structure and overlying process of karst caves are unknown,the migration and transformation law of elements between phases in the cave system is not well understood,and it is still unclear whether a new carbon sink is formed to cause "carbon loss sink".In the generalized surface karst zone,the soil,vadose zone,pores,and fractures are usually rich in CO₂,which migrates and transports in each layer,accompanied by a series of material and energy transmissions.As a unique window of underground space,there is a large amount of CO₂ in the cave.Affected by the coupling effect of the three-phase carbon transfer process of the soil cave system and karst hydrogeochemical process,CO₂ in the cave system participates in the process of the atmospheric carbon cycle and becomes an important relative carbon source area in the karst area.In this process,there is also a series of material cycles and energy exchange,which further affects cave sediments.Therefore,as an important part of the karst carbon cycle,the study of carbon migration in the karst cave system is not only conducive to the study of the karst cave carbon cycle mechanism but also conducive to the understanding of the three-phase process of the karst dynamic system,which is of positive significance to find "missing carbon sink".To further explore the seasonal variation of CO₂ in each karst layer and reveal the characteristics of CO₂ migration.From August 2018 to July 2019,CO₂ in the overlying soil,cave drip,and cave air of Mahuang Cave in Southwest China were monitored,and the vertical migration characteristics of CO₂ were systematically analyzed by mathematical-statistical analysis method based on carbonic acid balance system theory and stable isotope technology.The following conclusions are drawn:1.PCO₂ in each layer of Mahuang Cave shows the interannual characteristics of high in the rainy season and low in the dry season,while ?13C in each layer shows an opposite trend.In addition,different land types showed woodland > shrub > bare land.The PCO₂(W)of each water point showed a general change characteristic of seepage > drippwater.PCO₂(a)at different monitoring points in the cave showed a trend of gradual temperature change with increasing distance from the cave entrance,while the environmental parameters(temperature,humidity,and air pressure)in the cave also showed the same characteristics.2.Based on correlation analysis,it can be seen that PCO₂ changes in each layer of the cave system are driven by different factors.The change of PCO₂(S)is mainly affected by biological effects and thus is mainly controlled by temperature and humidity.As a link of water-rock interaction,the change of PCO₂(W)is affected by supply sources,and its change is directly driven by water-rock interaction and soil CO₂.The PCO₂(a)inside the cave is affected by the change of recharge from the surface karst zone(soil and bedrock)as well as the ventilation effect driven by the temperature difference between inside and outside the cave.3.Through the calculation of relevant models,it can be seen that there are three different ventilation modes in Mahuang Cave due to different air density differences at different times.The restricted ventilation model is formed in rainy season,when the CO₂ in the cave reaches the annual maximum value.In the dry season,the positive ventilation pattern is presented.During the dry season and rainy season,there is a transition stage,during which the air flow exchange between inside and outside the cave is low.4.Based on Iso Source model calculation,it can be seen that there are three main replenishment sources of CO₂ in Mahuang Cave: soil CO₂,external atmospheric CO₂,and old carbon in bedrock.Soil CO₂ mainly has two replenishment modes: direct diffusion and water degassing.The external atmosphere enters the cave by exchanging air flows inside and outside the cave through the ventilation effect,while the old carbon in the bedrock enters the cave mainly by water degassing.The whole cave system is mainly replenishment by soil CO₂ rather than the "vadose zone".In the rainy season,soil CO₂ is the main replenishment mode,and in the dry season,the external atmosphere and bedrock carbon is the main replenishment mode.5.In the process of CO₂ migration and transformation in each layer of the Mahuang Cave system,each water point is mainly deposited throughout the year,and this process is also controlled by changes in external conditions and drip types.In the dry season,it is mainly characterized by "water-rock interaction" and "prior degassing",while in the rainy season,it is mainly characterized by "water-rock interaction" and "dilution effect".Different season and different hydrothermal conditions directly affect the percolation migration process that affects water supply way of caves in the migration process under the conditions of different humidity "CO₂ effect" and "dilution effect" alternate control,and the complexity of karst conduit make different drop point migration path differences,affect the drop point supply source,and control of cave secondary sediment formation.