The Character Of Carbon Variation And Source Of CO₂in Xueyu Cave

The variation and transfer of carbon accompanied with the function of karst dynamic system, soil CO2accelerates the karst process and it is an important source of carbon. Cave is an indispensable part of karst environment, and it is a perfect place to study the carbon variation. Cave air CO2impacts the environment of the cave, and it can reflect the behavior of the karst dynamic system. There are two main sources of the cave air CO2:the diffuseness of the soil CO2through crannies; the CO2degassing from drip water and underground stream. Drip water and underground stream are main carriers of the CO2when soil water gets into the caves, the degassing of water in cave is the key process that dominates the variation of the cave air CO2. Karst water is the main carrier in the cave system that controls the carbon transfering. Some studies have indicated that the air CO2concentration in Xueyu cave is higher than that in other caves in Chongqing, and the variation of CO2in the cave is notable, but the rise process of the CO2concentration has not been caught and the source has not been clearly traced.Based on the previous researches, we monitored the key process that the carbon transferred from the soil CO2to the cave air. The soil CO2concentration, soil water, epikarst spring, groundwater, cave air CO2concentration were studied and the characters of carbon variation were deliberated in the thesis. The quickly variation of the cave air CO2was monitored from October31to November17,2013and the rise process of the air CO2was caught. The hydrochemistry of the groundwater was monitored at the same time to estimate the amount of the CO? degassing from the groundwater, so we can make sure whether the degassing of the groundwater is capable of making the air CO? rise. The carbon stable isotope was also studied in the thesis, the source of the CO2in the cave was traced. The main results are as followed:(1) The carbon in Xueyu cave system varies seasonally, the soil CO2, cave air CO2and the CO2partial pressure in ground stream water varied correspondingly, indicating that the cave air CO2may be influenced by the dissolved soil CO2in groundwater. In the cave system, from soil water, epikarst spring to drip water, the DIC rises firstly and drops afterward, indicating that the karst process is promoted at the beginning and then becomes weak. The karst process is important for carbon transfer in the cave system. The stable carbon isotope shows the dissolved CO2in soil water is more than that in epikarst spring and groundwater, as more carbonate corroded the dissolved CO2was consumed and carbon isotope enriched in the water.(2) The continuous monitoring from October31to November17,2013was carried out. Cave air CO2concentration rose quickly from3965ppm to9792ppm between2and9November,2013, pH of the groundwater decreased and the CO2partial pressure rose. The CO2partial pressure fell downstream, the margin of the CO2partial pressure between Rl (upstream) and R3(downstream) got the largest, the cave air CO2may come from groundwater degassing. By calculating the amount of the CO2from degassing and the increased amount of CO2in cave air, we found the amount of CO2from degassing was larger than the increased air CO2, so the groundwater degassing had the ability to make the air CO2rise quickly. The stable carbon isotope indicates the degassing CO2from groundwater comes from the soil CO2,δ13CDIC of groundwater got enriched downstream, it can also show that the degassing of groundwater causes the rising of the air CO2.(3) During the quickly variation of the cave air CO2in Xueyu cave, the calcite saturation index became lowest when the cave air CO2concentration rose to the highest, and it was the same situation when the seasonal variation was studied. These shows the trend of the sedimentation gets weak in the circumstance with high CO2concentration. The same result was found by monitoring the weight of the sediment precipitated from the drip water, the largest weight of the sediment was in the winter with low cave air CO2concentration. So the sediment precipitation may not result from the high CO2concentration, the factors that affect the precipitation and whether the sediment precipitating is quickly should be researched further.