Characteristics of strontium isotopes and their implications in the Qixing Cave of Guizhou,China

The strontium isotopic compositions (⁸⁷Sr/⁸⁶Sr) of samples including soils, bedrock, soil waters, drip waters and their corresponding speleothems in the Qixing Cave (QXC), Guizhou Province, China, were systemically measured and analyzed. The results indicate that there are significant Sr isotopic differences among samples. The mean ⁸⁷Sr/⁸⁶Sr ratios in drip water for the samples 1^#, 4^# and 9^# were 0.709568, 0.709139 and 0.708761, respectively, which possibly result from different flow paths, residence times, and other hydrogeological processes in the unsaturated zone overlying QXC. Meanwhile, levels of 40.8%, 57.6% and 72.4% of Sr in drip waters for 1^#, 4^# and 9^#, respectively, were derived from bedrock dissolution, which was calculated by the mixture model of the two end-members (soil and bedrock). There is, however, no positive correlation between the relative proportion from bedrock dissolution (δ ¹³C value is 1.8‰) and drip water δ ¹³C_DIC values. The mean drip water δ ¹³C_DIC value in 1^# is the heaviest (−4.5‰) with the lowest contribution rate of bedrock dissolution, whereas the value in 9^# is the lightest (−9.3‰) with the highest contribution rate of bedrock dissolution. The proportion from host rock dissolution in 4^# is higher than that in 1^# and lower than that in 9^#, while its mean drip water δ ¹³C_DIC value (−8.6‰) is higher than that of 9# and lower than that for 1^#. This suggests that the prior calcite precipitation (PCP) processes in the unsaturated zone overlying the cave are responsible for the δ¹³CDIC value differences between different drip waters, and not bedrock dissolution. Furthermore, this study also demonstrates that the ⁸⁷Sr/⁸⁶Sr ratios of speleothems in the 1^# and 4^# mainly reflect the variation in the relative proportions from the soil system (soil water) and bedrock dissolution overlying the cave. It is, therefore, feasible to use the strontium isotopic signals of speleothems as an indicator for soil chemical weathering intensity, and consequently as a monsoon proxy in the study area.