| As papers about stalagmite δ18O series published by China are widely cited in the field of paleoclimatology, cave stalagmite has become an important carrier focused by global change researches, and δ18O, undoubtedly, becomes the most widely used indicator in stalagmite of climate change. Past interpretations of stalagmite δ18O signals were based on the assumption that δ18O in cave drip water inherited the δ18O in atmospheric precipitation. Due to the differences in local climate conditions, cave overlay strata thickness, fracture development, temperature and humidity variation and air movement in cave, the δD and δ18O in deposition in each cave are likely to have their own uniqueness. Therefore, conducting modern monitoring of the δ18O in precipitation, soil water and cave drip water in the cave system and analyzing the migration process of δD and δ18O in the system is not only helpful for a better understanding of the forming mechanism of cave deposition, but also significant to the interpretation of the stalagmite δ18O signals in East Asian monsoon region.Through two hydrological years of field monitoring of Naduo Cave in Anshun, Guizhou, including observation of the temperature, humidity, air CO2 concentration, water drip rate, drip water pH and drip water conductivity among other physical and chemical indicators in the cave, and systematic sampling of the stable δD and δ18O in atmospheric precipitation, epikarst spring water, cave drip water and pool water in the cave system, and after discussion of their migration characteristics, it was concluded as follows:(1) Analysis of the internal and external environment characteristics of Naduo Cave indicated that Naduo Cave was a nicely enclosed one, so the inside of the cave saw more stable temperature and humidity and smaller inter-annual variation than the outside; correlation analysis of the drip rate and precipitation showed that the drip rate basically responded to the seasonal changes of precipitation, however, being affected by factors such as the overlay strata conduit development at each drip point as well as the overlay thickness, karst water retention time, migration pathway before the drip forming and the distance between each drip point and the cave entrance, etc., variation was shown among drip points in responding to precipitation.(2) Based on the monitoring data of stable δD and δ18O in atmospheric precipitation outside Naduo Cave, this thesis established the LMWL formula of the research area: δD=8.77δ18O+19.06(R2=0.99), which reflected the humid and rainy climate of the research area; the δ18O and δD in epikarst spring water, soil water, drip water and karst pool water were distributed near the LMWL, indicating that the karst water in cave system mainly came from atmospheric precipitation which went through no measurable evaporation from hitting the ground until forming water drips in the cave, so drip water basically inherited the variation characteristics of the stable δD and δ18O in local atmospheric precipitation.(3) Large variation of the δ18O value of various karst waters was seen in the cave system, and the variation of the δ18O value of soil water, atmospheric precipitation, epikarst spring water, cave pool water and drip water within a year decreased in turn significantly. The migmatization of different periods of precipitation in overlay soil layer and bedrock resulted in varied concentrations of δD and δ18O in different karst waters, but overall, various types of karst waters in Naduo Cave inherited the basic characteristics of δD and δ18O in local atmospheric precipitation, and the arithmetic mean of the δ18O in drip water, epikarst spring water and pool water was close to the mean local atmospheric precipitation.(4) Stable isotope values in the atmospheric precipitation of Anshun showed measurable variation with the season: higher δ18O and δD values in winter and spring precipitation and lower δ18O and δD values in summer and autumn precipitation; the variation of δ18O in precipitation showed no notable precipitation and temperature effects, indicating that the isotopic composition in precipitation was mainly influenced by the vapor source of the precipitation: the vapor of summer precipitation was transported by tropical marine air while that of spring and winter precipitation was mainly influenced by the continental air carried by the westerlies; compared with the seasonal change of the δ18O and δD in precipitation, drip water and pool water showed a lag effect which was weaker in rainy season and stronger in dry season, proving that Naduo Cave could be used for palaeoclimatic reconstruction.(5) Using HYSPLIT, validation and analysis of the vapor transportation paths of the precipitation in Anshun showed that the main vapor sources of the research area were three vapor channels: the vapor of winter and spring precipitation, rich in heavy isotopes, mainly came from the continental air transported by the westerlies; the vapor of summer precipitation, showing isotopic depletion, was sourced from the marine air carried by southwest monsoon and southeast monsoon; the composition of δ18O in precipitation shared a similar cycle with El Nino in terms of inter-annual variation and strong signals of atmospheric circulation largely influenced the composition of the stable δD and δ18O in precipitation. |
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