Temporal And Spatial Variations Of Cave Dripwater Geochemistry And Its Soil Processes In Shihua Cave, Beijing, China

In recent years there has been increasing interest in the study of cave stalagmites because of its some unique advantages of being archives of palaeoclimate and palaeoenvionment.In this study,parameters of drip water geochemistry(including temperature,electrical conductance(EC),pH,main ions and dissolved organic carbon (DOC)),soil environment overlying cave(including temperature,humidity and CO₂ concentration of soil air) and cave air(including temperature,relative humidity and CO₂ concentration) have been monitored over the past 3 years to understand the mechanisms of formation of stalagmites and the relationship of stalagmites growth with soil processes in Shihua Cave.A number of conclusions have been achieved as shown below.1) The temperature,humidity and CO₂ concentration of soil air are highest in July or August(rainy season) and lowest in December or January(dry season).It has been observed that CO₂ concentration is exponentially correlated with temperature and linearly correlated with humidity.With the increase of the depth of soil section, temperature of soil air decreases,but humidity increases.The CO₂ concentration of cultivated soil in valley is higher than shrub-covered soil on the slope of hill.2) The highest value of soil DOC occurred at the beginning of rainy season. Afterwards,it shows a decresing trend due to the rain leaching.The higher value of DOC is distributed in the soil profile between 10cm and 30cm from the surface associated with the microbial activities and plant roots.The valley has the maximal concentration of soil DOC,followed by shrub-covered soil,and the discontinuous soil between bedrocks has the lowest one. 3) Dripwater in Shihua Cave comes from surface flow.Drip discharges show three types of response to surface flow variations:1) rapid response,2) time-lag response and 3) stable response,which are related to different flow routes of recharge.For rapid reponse,the drip discharge is recharged through the flow routes with intensive fractures and interconnectivities;for time-lag response,the drip discharge is recharged by double-porosity system composed of a high conductivity,low storage capability conduit network and a low-conductivity high-storage capability rock matrix under variable boundary conditions;for stable response,the drip discharge is mainly recharged by seepage flow and base flow.4) Dripwater in all studied sites shows clearly seasonal variations in hydrochemical parameters.During the rainy period,pH is lower because the rain water dissolves more CO₂ and SO₄^2- than dry period.On the contrary,concentrations of Ca²⁺,Mg²⁺, Si and EC increase in rainy season and the more rapid response of the drip water to the recharge,the higher degree of the increase.In addition,Mg/Ca and Si/Ca in drip sites are higher during dry period than rainy season.The trends of Mg/Ca and Si/Ca with Ca²⁺ concentration are consistent with the results of the prior calcite precipitation model,suggesting that there is a process of prior calcite precipitation in the recharge of drip sites during dry period.5) The saturation index of cacite(Sic) of dripwater in all drip sites shows some degree of seasonality.Drip water is more saturated with respect to Ca during the dry period and lower during the rainy period,in agreement with the results of situ calcite growth experiments.It is negatively exponential correlated between the value of SIc and drip rate.In addition,variation in temperature,humidity and CO₂ concentration of interior air within Shihua Cave could affect the calcite growth.6) The content of DOC in drip waters varies inter- and intra-annually and has a significant correlation with discharge of dripwater for the rapid response sites.High DOC was detected in July or August in the three observed years.The flushing of soil organic matter is dominated by the intensity of rain events.The content of DOC was lower and less variable during the dry period than the rainy period.The shape of DOC peak also varies annually as it is influenced by the intensity and frequency of rainfall. Different drip sites show marked differences in DOC response that are dominated by hydrological behaviour linked to the recharge of the soil and karst micro-fissure/porosity network.It has been demonstrated in this study that the sharp peak of DOC in drip water during the rain period corresponds with the formation of the thin and dark layers of the stalagmites,while high Sic in the dripwater during the dry period corresponds with the formation of calcite layer.The variation of DOC content in drip waters,in the mean time,reflects the changes of the origination and transport of DOC in soil.