云南白水台泉水地球化学特征及其气候环境指示意义OA北大核心CSTPCD

Understanding the geochemical characteristics and dynamic changes of groundwater(e.g.,springs,etc.)is an important scientific reference for accurately interpreting the paleoclimatic and environmental information on karst deposits(e.g.,stalagmites,travertine,etc.).In this study,we continuously collected local rainwater samples and S3 spring water in Baishuitai area of Yunnan Province on a monthly frequency for nearly 5 years(January 2018 to October 2022).We analyzed the temporal variations of stable isotopes of hydrogen and oxygen(δD and δ18O)compositions and trace element contents in the two types of water bodies to identify the recharge source and hydrogeochemical evolution process of Baishuitai spring water,and to reveal the source differences of trace elements in spring water and their indicative significance on climate and environment. The results showed as follows.(1)Values of stable isotope composition of rainwater in Baishuitai region of Yunnan had obvious seasonal variation characteristics—high in the dry season and low in the rainy season.This result was mainly affected by water vapor source and evaporation conditions.During the dry season,water vapor mainly came from the stable continental air mass,and the evaporation effect was strong,which resulted in the enrichment of D and 18O in the remaining water vapor mass.During the rainy season,multiple fractionation and condensation processes occurred when water vapor migrated from the sea to the land,due to the influence of the southwest monsoon and the southeast monsoon.D and 18O were severely scoured,thus making δD and δ18O light in rain during the precipitation process.(2)The stable isotopic compositions of hydrogen and oxygen in Baishuitai spring water were on or near the atmospheric precipitation line,indicating that the spring water was mainly supplied by atmospheric precipitation.During the infiltration of atmospheric precipitation into the zone of shallow circulation runoff,new and old water from different channels and fissure networks was continuously mixed to regulate and store hydrogen and oxygen isotopes in groundwater,resulting a much smaller variation amplitude of δD and δ18O in spring water than in rainfall.During the rainy season in 2018,2019 and 2022,δD and δ18O in spring water witnessed a similar trend to δD and δ18O in rainwater with a lag of about one month,indicating that δD and δ18O in water evolved to some extent in underground runoff,so that δD and δ18O in spring water correctly inherited some of the characteristics of δD and δ18O in rainwater.The δD-δ18O scatter points of spring water in Baishuitai region all shifted to the area near the left side of the atmospheric precipitation line.This means that δ18O in spring water became lighter under the exchange of lighter oxygen isotopes with CO2 gas from deep sources,while δD basically did not change,indicating that atmospheric precipitation was fully mixed with deep source CO2 during the infiltration process.The smoothing effect of isotope regulation and storage generated by the mixing of old water,as well as the exchange of oxygen isotope with CO2 from deep sources,indicated that the Baishuitai region contains a wide spring area,numerous underground passageways and fissures,and intricate karst features.(3)The trace elements in Baishuitai spring water show two types of dynamic change characteristics.Elements such as Ca,Mg,Sr,Ba,and Si constitute one category,mainly originating from the dissolution of surrounding rocks under karst water erosion.After entering the rainy season,the water-rock interaction was weakened by the dilution of rainfall.Concentrations of these elements decreased to varying degrees in the early rainy season,and increased in the late rainy season to the transition from the rainy season to the rainy and dry season,while these concentrations stayed stable or slightly fluctuated in the dry season.The decrease in precipitation promoted the precipitation of calcium carbonate caused by degassing,which increased the values of Mg/Ca,Sr/Ca and Ba/Ca.At the same time,water within the fractures between soil and bedrock was less disturbed by fresh water inputs.This reduction in the rate of water migration enhanced the interaction between water and rock,resulting in a preferential leaching of elements such as Sr and Ba.The consequent differential changes in the concentrations of these elements suggest that variations in their levels could be indicative of shifts in precipitation patterns.Elements like Fe,Al,and Mn constitute another category,primarily originating from the leaching effects of atmospheric precipitation on the overlying soil.The occurrence of peaks in these elements during the rainy season corresponds to periods of heavy annual precipitation,while peaks observed in the dry season may be associated with the pulse-like action triggered by a single instance of heavy precipitation.