Characteristics And Environmental Significance Of ?¹⁸O Record From Drip Water And Modern Sediment In Naduo Cave,Guizhou

As an important carrier for climate evolution,Cave Secondary Sediment especially the cave stalagmite has made series of remarkable achievements.Modern carbonate sediment-stalagmite has many multiple proxies of paleo-climate,such as oxygen and carbon isotopes,rate of deposition,trace elements and their isotopes,stalagmite microlayer,etc.But the oxygen isotope as cave researchers are most concerned about at present,and the most widely used of many environmental indicators,which matured.Papers about stalagmite ?18O series published by China are also widely cited in the field of paleoclimatology.But before the formation of drip water and its modern carbon sediment,the atmospheric precipitation must passed the soil horizon and the Karst pipeline.This process will change the isotope composition of atmospheric precipitation because of the different climate,landform,cave overlay strata thickness,fracture development,and the different temperature and humidity variation,concentration of CO2 in cave.However,due to the diversity of influencing factors,the temporal and spatial differences,there is controversial explanation of the various indicators.Therefore,the modern monitoring of cave system is from outside to inside,and the differences of the climate and environmental records in the process of the oxygen isotope of the cave are analyzed.This study is not only helpful for a better analyzing of the theory of cave deposition,but also provide scientific reference for interpretation of stalagmite ?18O signals in East Asian monsoon region.In this paper,we select Naduo cave which locates in Guizhou province as the fixed monitoring points and take the oxygen isotope as the monitoring proxy.Through a comprehensive monitoring of three hydrological years of oxygen isotope compition of precipitation in Anshun area,soil water,epikarst water,drip water and modern carbon sediment through the research of field sampling,experimental analysis and combining with local climate data.The evolution and transmission process of the oxygen isotope in cave waters were also analyzed,and we also have a preliminary study of the theory of modern cave sediments by connect them with the temperature and precipitation outside the cave,and have the following preliminary conclusions:(1)Based on the monitoring data of stable ?D and ?18O in atmospheric precipitation outside Naduo cave,this thesis established the LMWL formula of the research area: ?D=9.24?18O+22.92(r=0.99,n=30,p<0.001),which has a bigger slope and intercept.When 2H is evaporated and enriched in unsaturated atmosphere,the fractionation effect is greater than 18 O,so it lead a relative enrichment of ?D in rainwater.(2)Because of the different sources of precipitation water vapor,the ?18O values of atmospheric precipitation has obvious seasonal characteristics of the rainy season is light,the emphasis of the dry season.The rainy season precipitation mainly from marine transportation of warm vapor;dry season precipitation mainly from the continental dry vapor and water vapor in the mainland recycling,strong evaporation process resulting in heavy isotope enrichment.Meanwhile,the ?18O values of atmospheric precipitation shows the high correlation between the local temperature and precipitation,and shows a significant "precipitation effect" and "anti-temperature effect".The seasonal characteristics of precipitation isotopes are also inherited by soil water.The seasonal characteristics of epikarst water isotopes are weak because of the significant homogenization.Overlying soil,epikarst retention and storage function cause the strong mixing process of the groundwater in vadose zone.(3)The six drip water have different degree of inheritance to the outside precipitation and even the sensitivity to the change of external environment is different,but the seasonal variation of the isotopic distribution of six drip water is consistent and shows the characteristics of the whole synchronization.In addition,the variation range of the ?18O values in drip water is smaller than them in precipitation means the source of drip water is meteoric water.The ?18O average values is closer to the period of the rainy season,so,it is inferred that the contribution of precipitation in rainy season to drip water is the main one.(4)The ?18O values of six drip water all shows a good positive correlation with monthly mean temperature outside the cave,because of the better ventilation condition and the thinner soil layer.The discontinuous soil layer and the thin vadose zone will make the temperature influenced easily by outside climate.So the groundwater is strongly evaporated under a high temperature in summer lead he relatively high ?18O values of six drip water.(5)Carbonate sediments in Naduo cave have different degrees of sedimentary discontinuity,and the equilibrium fractionation between cave sediments and cave water in Naduo cave is not good,the possible reason is the physical environmental properties of caves,such as bigger fluctuations of cave temperature,smaller humidity and so on.The ?18O values of carbonate sediments is relatively low because of the drip water main come from rainy season.The ?18O values of carbonate sediments D11 has the analogous seasonal characteristics of the ?18O values of drip water D11 shows us the fact that the ?18O values of carbonate sediments main come from drip water.