The Variability Characteristics And Environmental Effects Of Trace Elements In The Cave System In Central Guizhou, China

Controversy about global climate change is everywhere and it has captured worldwide attention in today’s society, due to it is closely related to human survival environment.Karstification can reflect the change of global change and a large number of information, especially, the environment and climate change is recorded in karst areas.Due to its high resolution, continuous recording, and high precision, the cave sediments have become one of the most important geological carriers in paleoclimate and paleoenvironment studies, with numerous proxies.At present,δ18O has been widely applied and gradually mature,while appling trace elements are limted due to its diverse sources and complex factors.But trace elements has recorded that cave water can reflect the external environment as well as the migration process in karst aquifer.Researching on the migration of trace elements in the cave system can provide a scientific basis for environmental information,which recored incave sediment.For the research of continuous Monitoring work was carried out in Shijiangjun and Benzhai cave,where locted in Anshun city, Guizhou province.The main work of the research are as follows:long-term monitoring of atmospheric temperature, precipitation, CO2concentration.etc.and collecting the cave drip water, analysising the concentration of Ca, Mg, Ba and Sr as well as Mg/Ca, Sr/Ca, Ba/Ca and Mg/Sr ratio,and other response geochemical indicators to environmental change.Emphasising onthe effects of atmospheric environment, soil chemical composition and vegetationon dripwater,analysis of the variation characteristics of trace elements from cave drip water and modern sediments in Shijiangjun and Benzhai cave was used to response the freezing rain events at the beginning of2011and the spring drought and summer floods climate events in2012.The following conclusions are drawn:(1) The trace elements and their ratio change from cave drip water controlled by the overlying soil,bedrock thickness,the migration way-. path and distance factors of the groundwater in the bedrock, with different response of the extreme droughts and heavy rainfall weather.It is obvious that Ca,Mg concentration of drip water response fast to the extreme dry weather an d freezing rain in January2011in Shijiangjun Cave.The range of decreases is lager than the stage tha t after April in2011. As temperatures rase, with the increased precipitation,the dissolution of ground water is more powerful that leads to Ca concentration increased slightly, Mg concentration increased as the solubility increased because of the higher temperature. Ca, Mg and Sr of drip water in Shijiangjun cave responsing to the drought in spring and flood in summer in2012was obvious. From April2012to July2012, the concentration of Ca, Mg, Sr, Ba increased, but the increase of Ca was larger than Mg, Sr, Ba. The change of Mg/Ca and Sr/Ca ratio was not obviously. Ba/Ca ratio increased at begin and then decreased, which also induced by enhance of dissolution of bedrock with increased precipitation. In July2012, concentration of Ca, Ba and Sr decreased while the concentration of Mg increased. The concentration of Ca had the most obvious decreased trend. The reason could be that precipitation had increased groundwater water since May and enhanced dilution effect. Therefore, the concentration of Ca was diluted. It indicated that concentration of Ca of drip water in shijiangjun cave was sensitive to the flood in2012summer.The Ca,Mg,Sr of dripping water in Benzhai cave was apparenting in response to the events of drought in spring and flood in summer.The trend of Sr concentration was consistent with Ca concentration in Benzhai cave during April to December of2011.Although Sr is not affected by temperature, as the temperature rising,the precipitation increasing the effect of dilution increased.But due to the presence of overlying soil and seasonal vegetation, then providing the source.There may be a mixing of different karst water added.In2012, from April to July, the concentration of Ca increased,although the concentration of Mg was not increasing obviously,it was still increasing slowly,the concentration of Sr and Ca was similarly.The reason was the preferential dissolution of CaCO3in the bedrock.After July in2012,the decreased variation of concentration and ratio of Ca, Sr, Ba were consistent with that in Shijiangjun cave, indicating that the concentration of Ca, Sr, Ba was also responsing to the event of heavy rain,because the precipitation increased, dilution strengthened,so the concentration of Ca, Sr decreased consistently,but Mg was affected by temperature, so the rate of dissolution increased then causing the concentration increased slowly.(2) Trace metal concentrations in solutions and interaction of karst groundwater with limestone host rocks as well as prior calcite precipitation (PCP) are suggested to be the dominant controllers of Mg/Ca and Sr/Ca ratios. High concentration of Ca and steady-state concentration of Mg in Limestone area lead to low Mg/Ca and Sr/Ca ratios in SJJ-W1#, SJJ-W2#、SJJ-W6#and SJJ-W7#while light concentration of Ca and Mg in dolomite area of SJJ-W3#、SJJ-W5#and SJJ-W8#may be the primary factor that cased the different changing trends between Mg/Ca and Sr/Ca ratios from Jan.2011to Dec.2012in Shijiangjun Cave. The analysis of the correlation showed that concetration of Mg was the major factor contribution to the Mg/Ca ration of drip water, and the concetration of Sr had more contribution to Sr/Ca ratio in Benzhai cave from April2011to December2012.(3) When the stalagmites appear hiatus, we deduce that may be affected by extreme droughts and heavy rainfall weather. By comparing the deposit amount in Shijiangjun cave and Benzhai cave, they have sediments from November2010to March2010.In March2012to December2012,only SJJ-W1#and SJJ-W2#with sediment in Shijiangjun cave,but in March2012to December2012all drip with sediment Benzhai cave. Cave overlying environments and different soil has influence to the deposition rate, according to the existing modern sediment deposition volume and combining the caves monitoring data shows the appearance of the extreme droughts and heavy rainfall weather cave sediments without deposit, so we concluded that when the stalagmites appear hiatus, may be affected by such extreme droughts and heavy rainfall weather.(4) Modern sediments collected from the two caveshas show that elements and its ratio of extreme climate events record has a bigger difference.In Shijiangjun cave,freezing rain events at the beginning of2011was recorded by Sr/Ca ratioin SJJ-W6#and SJJ-W7#in modern sediments which consistent with drip water. In Benzhai cave spring drought summer floods in2012was recorded by Mg/Ca and Sr/Ca ratioin BZ-W3#in modern sediments which consistent with drip water, while it was not obvious in Shijiangjun cave.It showed that different overlying soil environment making the cave sediments have different environmental effects on change of trace element. Meanwhile, it was important to the high frequency sampling and sample lab saving.