Seasonal and interannual variations of hydrochemical characteristics and stable isotopic compositions of drip waters in Furong Cave, southwest China based on 12 years’ monitoring

Abstract

Cave monitoring is crucial for the interpretation of climatic and environmental significances of various geological proxies in speleothem. Therefore, the hydrochemical and stable isotopic compositions (δ18O, δD, Mg/Ca, Sr/Ca, and Ba/Ca) of karst cave waters during 2005–2016 CE were constantly monitored in Furong Cave, Chongqing City, Southwest China. A comparison with local hydrological conditions led to 4 main conclusions as follows: (1) the Mg/Ca ratio is significantly responsive to the changes in drought/wet conditions outside the cave, which increased in drought years and decreased in wet years, respectively. Seasonal variation of Sr/Ca is more significant than those of Mg/Ca and Ba/Ca ratios. (2) Prior calcite precipitation (PCP), incongruent calcite dissolution (ICD), water–rock interaction (WRI), and pCO2 of soil and cave air, may account for the changes in trace element ratios in the epikarst, which resulted in a complex variation of element ratios in the cave drip water. In general, WRI in drought years is stronger than that in wet years, and that in low discharge sites is stronger than in high discharge sites. Seasonal variation of Ca2+ concentration, induced by PCP, exerts significant impact on the evolution of Sr/Ca ratio in drip water. (3) δ18O and δD of drip water are influenced by the “mixing effect”, leading to the result that their seasonal variations are less significant than that of precipitation. (4) At least in the study period, the ratios of trace elements in the drip water in Furong Cave mainly reflected the variations of local hydrological conditions (drought or wet) dominated by precipitation. Because of the “mixing effect” of groundwater, high-resolution δ18O record (e.g., seasonal and annual) of speleothem in Furong Cave may not be recommendable. However, the δ18O is potentially a reliable proxy in speleothems to record the change of rainfall on decadal and longer timescales.