Variations and significance of Mg/Sr and 87Sr/86Sr in a karst cave system in southwest China

Abstract The geochemical compositions of cave drip water and speleothems such as Mg, Sr, Mg/Ca, Sr/Ca, and 87Sr/86Sr are considered to be responsive to changes in the local climate and hydrological conditions. Systematic monitoring was performed on the Mg and Sr contents, Mg/Sr ratio and 87Sr/86Sr 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 87Sr/86Sr ratios to explore the main sources and controlling factors of Sr and other trace elements in drip water. (1) Due to the decrease in winter and spring rainfall, the residence time of water in the soil was prolonged, which resulted in increasing of Mg and Sr concentrations and 87Sr/86Sr ratios in soil water. 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 the soil water, as well as the 87Sr/86Sr ratios of the cave drip-water was closer to that of the bedrock, which indicated that the overlying bedrock was the main source of the trace elements in the drip water and the speleothems in Furong Cave. (3) Mg contents and Mg/Sr ratios in drip water and AS showed decreasing trend, which may be affected by the shorter water-rock contact time due to the increasing annual rainfall in the monitoring period. (4) The Sr contents in AS might be affected by the growth rate of AS because of the similar increasing trend. (5) The Mg and Sr contents and the Mg/Sr ratios of the 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. In short, this study highlights the responses of the changes of Mg, Sr contents and Mg/Sr ratios of drip water and AS 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.