Research On The Geochemical Characteristics,Relations And Environmental Significance Of Cave Water-modern Deposits In Furong Cave,Chongqing,China

With the great progress of the economy, global warming has become increasingly prominent; its relationship with the survival of the human environment safety and social economic development has now become a global concern environmental problem. The researches on the environmental changes had been widely carried out during the past years. Stalagmite has been one of the most important archives in the reconstruction of paleoclimatic change and paleoenvironmental changes. Mass of proxies from stalagmite can be used in the reconstruction of a series of paleoenvironmental information, such as temperature, precipitation, vegetation, hydrological condition and so on. But there still have much uncertainties and controversies for the suitable interpretation of these proxies. In order to improve the accuracy interpretation of the proxies, it is necessary to research the depositional mechanism of the stalagmite formation and to know the relationship among the cave environments, cave water and modern deposits.To understand cave environment, cave water and modern deposits characteristics of Furong cave, including their relations with outside environment, the continuous monitoring work was conducted in Furong cave, Chongqing, China form September 2009 to August 2011. Following were the primary conclusions:(1) The observations during the study period showed that the variation range of temperature and humidity in Furong cave was small during different seasons, but the cave air pCO2 was opposite. In winter, by the reason of the low temperature and less precipitation, the CO2 produced by the overlaying soil and vegetation breathing and biological activity was less, and at the same time, air flowed from outside into cave resulting in the decrease of CO2 concentration of cave air, so at this moment, the CO2 concentration of the cave air was minimal.(2) The characteristics of the cave water were as follows:the geochemical indices (pH value, HCO3-, EC, Saturation Index of Calcite (SIC)) showed obvious temporal and spatial variation characteristics. The most obvious change was the pH value among the hydrochemical indices, which was highest in winter months. The main reason for the change of the pH value was that less CO2 was dissolved into the Karst water in the overlying soil, combined with saturated zone of the overlying soil and bedrock did not have enough water, and it took long time to complete the migration of karst water, causing the CO2 of the water was easily degassed, while the pH values was highest in the winter. Furthermore, the differences of discharge and concentration of SO42- of drip water among the various sites, resulted in the slightly different pH values too. The pH values of the drip water were high, which with small discharge and low concentration of SO42-. Additionally, there were small seasonal changes between EC and HCO3-, only a slight increase during the rainy season. There was a similar change between the SIc and pH values for drip water. During the monitoring period, the SIc was controlled by the pH values.(3) Concentration of SO42- in drip water varied with drip sites and seasons, which was generated from the difference of biological processes in overlying soil and different bedrock through which the Karst water transferred. Additionally, concentration of Ca2+ and Mg2+ in drip water depended on the interaction of water-soil-bedrock. What’s more, water sources, piston effect and residence time of water in bedrock also influenced the concentration of Ca2+ and Mg2+. The Mg/Ca ratio of cave water was mainly controlled by the content of Ca2+, which could reflect the variation of temperature and precipitation outside the cave.(4) There is no significant correlation between the drip waterδ18O and rainfall or between the discharge of the drip water and rainfall. Theδ18O drip water was absent obvious seasonal variations, suggesting that the oxygen isotopic composition of the supplying water into the cave reflected the avarageδ18O of the annual rainwater. Hence the cave drip water oxygen isotopic composition is interpreted here as an averaged annual rainfallδ18O signal.(5) Modern secondary deposits were mainly composed of calcite in Furong cave. The depositional rate for active deposits under drip water sites in Furong cave was likely controlled by SIc, which had the highst values in winter months.(6) There was no obvious correlations among the Mg/Ca ratio of drip water, the temperature of cave air, the outside rainfall of the cave and the Mg/Ca ratio of modern secondary deposits. Partly, this should be attributed to the limited amplitude of the temperature variation of the cave air. In additionally, there was also no direct correlation between the discharge of drip water and the precipitation outside of the cave, because of the mixed effect of thick bedrock overlying the Furong cave. Additionally, mineral and crystallization also have certain influences on Mg/Ca ratio of modern secondary deposits in the process of water deposition. Hence, the Mg/Ca ratio of modern secondary can not indicate environmental change in a short time scales. While, the long-term environmental interpretation of the Mg/Ca ratio is still need to be researched further.(7) The equilibrium fractionation of oxygen isotope for the seceondary deposits in Furong cave indicated the signal of oxygen isotope of the annual rainfall.The multy proxies in the cave secondary deposits could imply the environmental condition, while because of the complexity of the influencing factors which makes external information need a long time and a complex process to reach the cave led to a serious influence to the proxies, makes the indicators can not indicate environmental change in a short time scales. So, more detailed and systemic long time monitoring works are needed.