Primary Research On The Geochemistry Of Cave Water And The Characteristics Of Active Carbonate Deposits In Furong Cave

High-resolution records from stalagmite have become the focus of paleo-climate research. Various proxies can be extracted from stalagmite, such asδ¹⁸O andδ¹³C, trace elements, growth rate, and so on.The palaeo-climatic researches via stalagmite focuses on the usage of oxygen and carbon stable isotopes to reconstruct paleo-climate under high-precise U/ Th dating on a series of comparison study of climate events. However, the deposition mechanisms of speleothems and its responsibility to the changes of external environment should be studied deeply. The cave water is not only the cave sedimentary material transmission medium, but also the carrier of climatic indicator, so long-term monitoring in cave should be beneficial in understanding the hydro-chemical characteristics of cave water and how the characteristics of various indicators changed when them saved into speleothems.Furong Cave, Chongqing, China, is an important component of the "South China Karst" which involved in the World Natural Heritage in 2007. Four sites had been selected for a long term monitoring in Furong Cave from Oct. 2005, including two drip water sites(1#, 3#) and two pool sites (2#, 4#) respectively. The pH, temperature and concentration of HCO₃^- in water samples had been tested on the spot. Then the water sample were analyzed for the concentration of other main ions in the laboratory (e.g. SO₄^2-,Cl^-,Ca²⁺,Mg²⁺, K⁺, Na⁺, etc ), and the value of Mg/Ca, the Pco₂ (Pressure of CO₂) and the SIc(Saturation Index of calcite) were calculated. After the preliminary study, we can get some cognition which were shown in the following text:(1) During 2006-11 to 2008-10, in the all four sites, the pH of cave water fluctuate largely and reduce obviously lag 1 to 2 months to large amount of precipitation in the year, this shows the significant dilution effect in the karst aquifer which caused by precipitation. In a year, the geochemistry features of two dripping water changed slightly, and it can be defined as the HCO₃^--Ca²⁺-Mg²⁺ water during most of the time. Following the rain season, this can turn to HCO₃^--Mg²⁺-Ca²⁺ type; whereas the geochemistry characteristics of the two pools water are more stable and can be termed as HCO₃^--Mg²⁺-Ca²⁺ water.The relative proportion of the ions in cave water of the four sites has a slight variation during the monitoring period. The content of HCO₃^- in the anion occupies a very high proportion, four sites all exceed 85%, in the cation, the sum content of Ca²⁺ and Mg²⁺ is exceed 95%.(2) The concentration of Ca²⁺ and Mg²⁺ has a cyclic change in a year timescale, which is much higher from July to December than that from January to June. While the concentration of Mg²⁺ shows fluctuations too, both of them are dominated by the seasonal temperature and precipitation change. Temperature and precipitation are the major factors which control the changes of solute and solvent in karst water, same as determining the ions concentration. There is higher ions concentration in cave water when the conditions out of cave are in high temperature and less precipitation. The increase of ions concentration is propitious to the deposition of carbonates.Both of the concentration of Ca²⁺ and Mg²⁺ became higher in September 2006, which showed that the cave water in Furong cave responded very sensitively to the dry weather, and this dry weather period results in higher concentration of the ions accelerating the deposition of carbonates. The dilution effect of the rainstorm during the summer, 2007, had changed the concentration of ions in the cave water. But the lag time is still dubious in different sites in Furong cave.(3) The value of Mg/Ca of the four cave water can be divided into two phases in a year. In general, the value of Mg/Ca shows decreasing from winter to summer, and increasing from summer to winter.(4) In most months from 2006-11 to 2008-10, the water of the four sites is oversaturated to calcite, which shows the active deposition of carbonate. Furthermore, the SIc of drip water was usually higher than that of pool water in the same season. This should attribute to the depisition of carbonate in pool water which do not occur in drip water because of the "dripping".In about 1-2 months after the abundant precipitation, Pco₂ of the four cave water increased, because of the dilution effect, while pH and SIc reduced. So the effect of precipitation dilution dominated the deposition when temperature is in order. Rainfall maybe is the key factor for the deposition and lead to the alternation between saturation and unsaturation, which eventually lead to the seasonal deposition of carbonates. Heavy precipitation is adverse to carbonate deposition.