Researching On The Response Of Trace Elements Of Soil Infiltrating Water To The Surface Environment And Its Application In The Study Of Modern Speleothem

Speleothems are increasingly used to provide high resolution, accurately dated paleoclimate information to reconstruct paleoclimate and paleoenvironment. Speleothems contain a number of suitable proxies to record the environmental information. Although unlike oxygen isotopes, trace elements have been widely used for the variety of sources and complex factors, trace elements are used to reveal the groundwater runoff in the epikarst overlying the cave with the process and the surface environment. Study of the factors controlling the trace elements in water-soil-cave system is an important basis work to explain the environmental information recorded in speleothems. Currently, cave monitoring is a common method to study the source of trace elements and their controlling factors.We have installed equipment to monitor the rainfall and air temperature outside Furong Cave which is located at Wulong County in Chongqing. The content of CO2in the soil, temperature, humidity and concentration of CO2in Furong Cave were also monitored. The rainfall, soil, bedrocks, soil infiltrating water, cave water and modern speleothems were collected. In this paper, studying on the response of the concentration of trace elements and the ratios of Mg/Ca, Sr/Ca and Mg/Sr of the samples to the surface environment is the main objective. The other goal is to monitor geochemical properties of soil infiltrating water, cave water and modern speleothems reflecting the arid climate at southwestern region in winter and spring from2009to2010. The conclusions are as follows:1) The data of soil particle size showed that the diameter of main particles is<16μm which counts for63.2%in all the soil particles. The proportion of particles that have diameter of>63μm is9.07%in the depth of0-10cm while the proportion is5.14%in the depth below10cm indicating the proportion of clay (the particle diameter<4μm) increased with soil depth. The concertrations of Ca, Mg and Sr in the soil are also increased with soil depth. The Sr concentration is positively correlated to clay content. The difference of the element concentration in every soil profile was inferred that there was much unweathered carbonate debris in the soil. The chemical composition of soil was similar to the bedrocks that was the concentrations of Ca>Mg>Sr. However,the mobility of Ca was higher than Mg and Sr in the soil which resulted in the ratios of Mg/Ca and Sr/Ca in the soil were larger than the bedrocks. The higher concentration of Mg in the soil was attributed to the dolomite., More Mg was dissolved in water during the weathering of bedrock so that the ratios of Mg/Sr in the bedrocks was higher than the soil.2) The volume of soil infiltrating water varied seasonally and was corresponding to the precipitation and other factors, such as the intensity and the duration time of precipitation, the slope gradient and the soil particle composition.3) The concentrations of Ca, Mg, Sr of soil infiltrating water collected in April,2010, were higher than the samples collected in other months. This was explained that the soil infiltrating water was resident in the soil for longer time ascribing to the arid climate at southwestern region in winter and spring from2009to2010. The concentrations of Ca, Mg, Sr of soil infiltrating water were increased along with increasing soil depth. The results indicate that the concentrations of trace elements of soil infiltrating were controlled by the residence time and the distance flowing in the soil which resulted to the positive relationship of Ca, Mg, Sr with each other. The ratios of Mg/Ca and Sr/Ca were increased and the ratios of Mg/Ca were more than1in2011. It was argued that the prior calcite precipitation (PCP) was occurred in the soil because the higher temperature and more intense evaporation resulted in CO2degassing from the soil infiltrating water. The effect of PCP was to remove Ca from the water in the proportion. The maximum value of Mg/Sr was appeared in summer when temperature was highest and the precipitation was the most. In early spring and autumn, the peak value of Mg/Sr was behind the rainfall for the lower temperature. It was illustrated that the ratio of Mg/Sr of soil infiltrating water was controlled by rainfall and temperature. Higher temperature enhanced the sensitivity of Mg/Sr to rainfall.4) The content of CO2in cave was more in summer than in winter as same as in soil. The ratios of Mg/Ca and Sr/Ca of drip water were increased from May to August while decreased from September to November in2011. It was estimated that less water in the epikarst zone for the arid climate in winter and spring from2009to2010enhanced the ventilation in rock fracture and PCP was occurred as groundwater flowing in the rocks. The information of the arid climate was transferred by groundwater and responded to the geochemical character of drip water.For the drip water of all monitoring sites, the concentration of Ca,Mg, Sr and the Mg/Ca and Sr/Ca ratios presented similar variation in this study. This was attributed to the similar sources and the factors of influence of surface environment to groundwater. It was presumed that the difference of hydrogeology condition was the main factor to result in the different values of concentration and ratios of trace elements. The ratios of Mg/Ca of MP7drip water was more than I which was explained the groundwater flew across the dolomite. The dripping rate of MP5drip water was slower than MP4and the residence time of MP5was longer resulting in CO2degassing from drip water. PCP was occurred and the ratios of Mg/Ca and Sr/Ca of MP5were higher than MP4.5) The concentrations of Ca and Mg of the modern speleothems were small difference, while the maximum concentration of Sr was87.87μg/g and the minimum was5.37μg/g. The ratios of Mg/Ca and Sr/Ca of MP5modern speleothems were higher than MP4which was the same as drip water. It showed that the hydrogeology information could be transferred by drip water to the speleothems. Nevertheless, it was not found that the information about the arid climate at southwestern region in winter and spring from2009to2010was recorded in the modern speleothems.The partition coefficient of Mg (KMgl) of speleothems was0.0288±0.0038which was calculated by using the data of cave monitoring. The other partition coefficient of Mg (KMg2) was0.01856±0.01389by using the formula. The mean cave temperature was16.9±2.2℃(calculated by KMgl) and10.±8.2℃(calculated by KMg2), respectively. The average temperature of Furong Cave monitored by instrument was16-16.8℃. It showed that the monitoring data approached the actual data.