Study On The Variation Of Air CO₂Concentration And Water-Gas Carbon Conversion In Xueyu Cave, Chongqing

Caves, one of the most important forms of karst topography, are very important tourism resources and are of great attraction for visitors by its deep, unreal, mysterious environment and rich speleothems formed by deposition and many dissolution at features. Compared with the external atmospheric environment, the environment inside a cave is relatively closed and stable. In a cave, the temperature is relative stable which is closed to the local average annual temperature, and the humidity is generally higher, especially in the caves where underground river or drip waters developed. Cave air CO2concentration is the most important indicator of the cave environment, which has direct impacts on the chemical deposition balance, the dissolution of rock and the comfortable feelings of tourists. Therefore, the study on the variation of CO2concentration and water-gas carbon conversion is very significant not only for the protection of cave landscapes but also for revealing the carbon conversion in a karst dynamic system.Xueyu Cave in Chongqing was selected as the study area. On the basis of previous studies, more frequently monitoring on the cave air CO2concentration were carried out and new indexes were added including wind speed and pressure etc. in Nov2011and Nov2012, respectively. By monitoring data, the characteristics of the cave air CO2concentration distribution in time and space during the monitoring period were figured out; the reasons of the cave CO2dump were analysed from three aspects including tourism activities, airflow factors and the underground river affect; and finally, the amount of CO2dump was estimated and the water-gas carbon conversion of karst dynamic system were revealed. Conclusions were drawn as followed:(1)The variation of CO2concentration in Xueyu Cave repeated well in seasonal scale featured by a high value in summer and low in winter. The concentration difference is up to an order of magnitude or more between summer and winter. There is a sudden drop in November every year. In the suddenly dropping period, tourism activities can have a significant effect on cave air CO2drop in a short period of time (hour) but little effect on the longer time scales (day), and it cannot prevent the drop. Meanwhile, the distributions of cave air CO2concentration in space are quite different because of the cave morphology. In rainy season, the spatial distribution of cave air CO2is insignificant because the CO2concentration is mainly caused by overburden microbial activities in the soil outside the cave and the airflow is from the cave to the outside. In contrast, the cave air CO2concentration in spatial distribution has a larger difference in dry season.(2) Tourism activities are not the primary causes for the changing of air CO2concentration in the Xueyu Cave. The drop rate of air CO2can be mitigated when there are many visitors but the decline of air CO2concentration can not be prevented. The airflow difference caused by the temperature difference is an important factor of cave air CO2concentration drop, which lead to air exchange between outside and inside environment of the cave. Due to the cave structure, the numerical of the cave air wind speed and air pressure are featured as:lower level cave>the middle level of the cave> upper level of the cave.(3) In a seasonal scale, the trend of CO2partial pressure of the underground river and that of the cave air is consistent. The degassing and suction of the underground river is an important factor for seasonal changes of cave air CO2concentration, which is the same with the previous studies. It is found by monitoring the CO2concentration of the overburden soil of the Xueyu cave that:the trend of seasonal variation of soil CO2and cave air CO2concentration is in consistency. It was also found by monitoring the underground river that the content of the HCO3-is increasing when the stream flows to the outside of the cave. The water-gas carbon conversion in Xueyu Cave is an important reason for cave air CO2drop.(4)Through monitoring the number of visitors and the content of HCO3-in the underground river in the dropping period, combined with the cave volume (1.804×104m3), the amount of CO2dump was estimated and the water-gas carbon conversion of karst dynamic system were revealed. It was found by calculating that the contribution of visitors to the cave air CO2amount is between55.02mol～207.74mol (2.42kg～9.14kg) per day during monitoring, and its value is determined by the number of visitors. The air carbon conversion in the underground river and cave is between186.00mol～356.00mol (8.18kg～15.66kg) per day with a large daily variation, which may be related to the changes in the external environment and the river flow. The cave air CO2reduce the amount between252.00mol～656.61mol (11.09kg～28.89kg) per day with a large daily variation, which may be related to the changes in the atmospheric environment in the outside. In summary, although the daily variation of cave air CO2concentration reduction and water-gas carbon conversion are largely different, the water-gas carbon conversion contribute rate stabilize, remaining between40%～50%during the monitoring period. In the cave karst dynamic system, water-gas carbon conversion is the important factor of the cave air CO2concentration decrease.