The Studies On Cave Dripwater And The Feature Of Its Geochemical Parameters In Guilin Area

Based on the studies of 2 cave drip water monitoring spots at Great Cave and Seven star Cave respectively in Guilin area from November, 2004 to December,2005, the author carried out a 1-year cave modern drip water/present carbonate sediment researching project. Collecting about 12 month water geochemistry and solid stable isotope field and laboratory data, including O,C,D isotopic data of water and that of sediments at the surface of glass light bulb set up inside the researching cave corresponding to monitoring drip water, cave temperature, humidity, drip rate, pH, electrical conductivity, concentration of Ca2+,Mg2+,SO42-, HCO3-, partial pressure of CO2 both inside and outside cave , as well as it under surrounding soil (30 centimeters below surface). Exceptional concern has be shown on regional precipitation amount per month and its D,O stable isotope.From these field and laboratory data, the paper suggested some new conclusion in explaining the dynamic and Rayleigh effective on isotope equilibrium under carbonate precipitate in cave environment. Rainfall amount had been considered as the main factor on variation of cave drip water rate, and thus control the drip rate changes. The hydrogen and oxygen isotope of drip water being well fit for district water line established from 20 years study on regional precipitation in Guilin, which implied that cave drip water resourced from rain, and capable for representing precipitation in this site. The regularity of drip rate changing model showed us that timing lag behind synchronic rain, this time of drip rate can be reckon on within 10 days affected by piston effects without consideration of seasonal exchanges, but 1~2 month lag time refers to oxygen isotope because of recharging feature. Although not same to the models haven been appeared at Shihua cave studied in Beijing area, both propelled by upper vadose condition within rock and soil that covering top of the cave, the differences between this 2 cave is that the distinguished seepage or/and conduit texture inside epikarst system by which is the drip recharge source. Variation of O isotope of drip water assumed us seasonal difference at monitoring spot 1, not strikingly appeared at spot 2 that can be explained as influenced by buffering effect respect to thick stratification and soil above cave roof .Oxygen isotope equilibrium between drip water and carbonate deposits supplied us a good climatic proxy for reflecting surface temperature and meteorological precipitation, the negative correlation betweenδ18O of drip water and summer rainfall amount in southwest china has be re-identified. But forδ13C, half of eight sediment sample being away fromδ13C equilibrium up to 2~3‰, owning to the dynamic fractionation induced by slow drip rate, resulting from more CO2 degassing while water fall into cave ground. The paper attempt to evaluated the atmospheric CO2 concentration throughδ13C isotopic composition of current calcite deposit, which demonstrated clearly that it is feasible to fulfill such target. Bothδ18Oandδ13C reflecting rainfall amount at reverse order, even not in isotope equilibrium. The thickness or quantities of carbonate sediments accounting for rainfall amount yet in terms of SIc (saturate index of calcite).