Sensitivity Of Karst Dynamics System To Temperature,Precipitation And Soil CO₂ Changes

Process of weathering of carbonate minerals in karst region which absorb a certain amount of CO₂ from the atmosphere,is a key link in the global carbon cycle.Existing research suggests that half of HCO3-which occurring in the process of carbonate rock weathering from soil CO₂ and atmosphere CO₂,and the other half came from carbonate rock itself.Long-term karstification continuously consumes CO₂ in the air,and transform it into dissolved HCO3-,then make it with the runoff into the sea.At last,the karstification has established close ties with the global carbon cycle.At the same time,the karstification have a rapid and sensitive response to the external environment,including climate,rainfall,these changes could influence the carbon flux and the karst geological carbon cycle by changing the soil CO₂ concentration.So the study of the karst geological effect of environmental factors such as temperature,precipitation,soil CO₂ concentration has very important scientific significance to improve the global carbon cycle model,solve the problem of missing link and master the interaction between layers of earth and the global environmental changes.In addition,by studying the carbon flux change of the karst watershed under the influence of climate,we can discover the response mechanism of the karst geological carbon cycle to climate change,help to accurately estimate the karstification of atmospheric CO₂ emission reduction effect,then find a new ways of karst region CO₂ emission reduction.Xueyu Cave in Chongqing was selected as the study area.We collected the climate and hydrological data of the study area from 2009 to 2015,then analyzed the relationship between theclimate,the carbon flux change and the carbonate weathering process.We consider the interaction between the water-rock-earth-air – biology system,and using high precision automation equipment online monitoring the study area,the index including precipitation,air temperature,soil temperature,soil humidity,soil CO₂ concentration,groundwater water temperature,p H,conductivity,discharge and the CO₂ partial pressure.We test and analysis the chemical index of groundwater?including Ca2+,Mg2+,K+,Na+,Cl-,SO42-,NO3-,and HCO3-?,and the ?13C of the vegetation,soil and groundwater DIC.Then we could discover the source of the DIC and exogenous acid in underground river,and estimate the carbon flux by using the method of hydrochemistryrunoff.On the basis of above research,we can study how the carbonate weathering process and the carbon flux response to the climate change in basin scale.The results show that the variation range of hydrogeochemical indicators of Xueyu Cave underground river shows different variation rules with the change of environmental factors on different time scales.The variation amplitudes of these hydrochemical parameters tends to be in the order of storm-scale > seasonal > interannual >diurnal.Temperature,precipitation and soil CO₂ concentration are the main factors affecting hydrochemical.The dominant factors of groundwater hydrochemical indicators?especially p CO₂?at diurnal scale are temperature and soil CO₂ concentration.The fluctuation range of temperature in different seasons leads to the difference of diurnal hydrochemical variations in different seasons.Although the temperature has a large variation amplitude in diurnal scale,the variation of soil temperature,which is directly related to the soil CO₂ concentration,has a small variation amplitude,and the fluctuation range of hydrochenical parameters of underground water is the smallest in each time scale.On the storm scale,the dominant factor of groundwater hydrochemical is precipitation,and the difference of hydrochemical change process under different rainfall conditions is due to the difference rainfall intensities.In the process of heavy rain,the change of precipitation is very large,and the change range of hydrochemical parameters is also the largest in each scale.In seasonal scale,the dominant factors of groundwater hydrogeochemical changes are temperature,precipitation and soil CO₂ concentration.The temperature fluctuation in this time scale is the largest in each scale,the precipitation and soil CO₂ concentration in seasonal scale also have a large fluctuation range,but because of the large time span,the change degree of underground river hydrochemical parameters are smaller than that in storm scale.At the interannual scale,the dominant factors of groundwater hydrochemical varations are temperature and precipitation.Although the variation amplitude of the temperature is the smallest in each time scale,the variation amplitude of hydrochemical is between the diurnal scale and the seasonal scale because of the great change of rainfall.According to the results of the isotope test of water samples which collected from July to June2016,the ?13C fluctuates from-15.23 ‰ to-9.51 ‰ in one year.And the ?13C in April,May,June,August,September were close to the CO₂ ?13C in soil water,which indicating that the CO₂ consumed in the dissolution of the carbonatite in Xueyu Cave karst power system was mainly from the soil.In addition,the average proportion of HCO3-produced in non-exogenous acid dissolution was 88%based on the amount concentrations which measured per month.Based on our results,DIC fluxes were estimated from January 2009 to December 2015 in the Xueyu Cave underground river basin,and the relationship between them and other factors was analyzed.The results show that the average carbon flux is about 1.00 g / s during the study period,and the average annual carbon sink is about 31.54 t,with an average of about 2.63 t per month.The highest monthly carbon flux is happened in May,June and September,the largest of which occurs in June,up to 2.40 g/s;January,February and December have less carbon flux,which the minimum appears in the February,only 0.30 g/s.So the annual maximum monthly carbon flux is the minimum monthly average carbon sink flux of 8 times.There is a good correlation between monthly mean carbon flux and monthly mean precipitation and monthly mean discharge,and both appear the maximum in June and the minimum in February.The maximum value of monthly carbon flux and the maximum value of [HCO3-] are very different in time from the combined effect of temperature,precipitation and soil CO₂ concentration in Xueyu Cave underground river,which caused by precipitation Changes caused by changes in runoff caused by the dilution caused.The carbon fluxes of Xueyu Cave underground river are mainly determined by the discharge of the river,and is proportional to discharge.The discharge of the underground river is mainly affected by the precipitation,so the precipitation is the dominant factor which controlling the change of the carbon sink flux in the underground river.