Variations and Significance of Mg/Sr and 87Sr/86Sr of drip water and active speleothem in Furong Cave, Southwest China

<p>Cave monitoring is an important method to investigate the deposition mechanism and factor influencing the trace elements in drip water and speleothemsThe Mg and Sr concentrations and the Mg/Ca and Sr/Ca ratios and <sup>87</sup>Sr/<sup>86</sup>Sr in drip water and speleothems are often used to explore climate and environmental changes.Systematic monitoring was performed on the Mg and Sr contents, Mg/Sr ratio and <sup>87</sup>Sr/<sup>86</sup>Sr of soil, soil water, cave drip water, and the active speleothems (AS) in Furong Cave in Chongqing, southwest China, during 2009–2018 (A.D). The results were interpreted in conjunction with the changes in the <sup>87</sup>Sr/<sup>86</sup>Sr ratios to explore the main sources and controlling factors of Sr and other trace elements in drip water. (1) Mg and Sr concentrations and <sup>87</sup>Sr/<sup>86</sup>Sr ratios in soil water were increased which was related to the prolonged residence time of water in the soil in winter and spring because of less rainfall. It indicates that the trace element contents of soil water reflect seasonal changes of the rainfall. (2) The Mg and Sr contents were higher in drip water than in soil water, as well as the <sup>87</sup>Sr/<sup>86</sup>Sr ratios of drip water was closer to that of the bedrock, which indicates that the overlying bedrock was the main source of the trace elements in drip water and the speleothems in Furong Cave. (3) Mg contents and Mg/Sr ratios in drip water and AS showed decreasing trend corresponding to the increasing annual rainfall in the monitoring period which resulted in the shorter water-rock contact time. (4) The growth rate of AS may be an important factor to control the Sr contents in AS because of the similar increasing trend. (5) The Mg and Sr contents and the Mg/Sr ratios of drip water and AS did not exhibit seasonal variations due to the mixing of the fissure water and complex hydrology condition of the overlying bedrock, however, the geochemical indexes (Mg and Mg/Sr ratio) showed an opposite trend to the annual rainfall variation. This study suggests that the variations of Mg, Sr and Mg/Sr ratios of drip water and AS can response to the rainfall on the multi-year timescale, which contributes critical insights into the paleoclimate interpretation of proxies of speleothems in the cave with hundreds of meters thick bedrock. </p>