Using speleothem ⁸⁷Sr/⁸⁶Sr variations tracking the controls on trace element (Sr-Mg) compositions

Speleothem Sr/Ca and Mg/Ca ratios have emerged as important hydroclimate proxies due to their geochemical responses under uniform hydrological conditions. However, inconsistent Sr/Ca-Mg/Ca patterns in individual or coeval speleothems within cave systems complicate hydroclimate reconstructions, highlighting the complexities in the controls of Sr-Mg compositions. Recent advances in cave monitoring and multiproxy studies demonstrated the potential of ⁸⁷Sr/⁸⁶Sr in discriminating the sources and hydrological processes of Mg and Sr variations. Nevertheless, systematic applications of ⁸⁷Sr/⁸⁶Sr in both modern process studies and paleoclimate reconstructions are lacking. In this study, we presented two speleothems (HZZ41B-1 and HZZ11) with similar growing intervals and divergent Sr/Ca and Mg/Ca variations from Haozhu cave in central China. In order to decipher the underlying controls on trace element (Sr-Mg) compositions, we conducted a detailed investigation of ⁸⁷Sr/⁸⁶Sr and trace elements composition of the overlying soil, bedrock, cave sediment, drip water, and the two speleothems. The ⁸⁷Sr/⁸⁶Sr compositions in speleothems are closer to the bedrock end-member, suggesting a dominant bedrock contribution to speleothem Sr compositions. The ⁸⁷Sr/⁸⁶Sr values in speleothem HZZ41B-1 exhibit significant variations and positive correlation with Mg/Ca, which might be caused by the inconsistent dissolution of limestone and dolomite modulated by the varied water-rock interaction (WRI) time in the karst layer. In contrast, ⁸⁷Sr/⁸⁶Sr values in HZZ11 exhibit minimal variability, further supporting the prior calcite precipitation (PCP) control on the Sr-Mg compositions. In short summary, this study offers an approach to understanding the inconsistent variations in speleothem Sr/Ca and Mg/Ca by extra investigation of ⁸⁷Sr/⁸⁶Sr, further conducive to the recognition of the proper proxy for local hydroclimate.