Response of drip water Mg/Ca and Sr/Ca variations in ventilated caves to hydroclimate

Abstract

Mg/Ca and Sr/Ca in speleothems which record valuable information regarding past variations of precipitation and cave air pCO2 are promising proxies because the degrees of water-rock interaction (WRI) and prior calcite precipitation (PCP) are directly and indirectly related to these changes. However, the controls on Mg/Ca and Sr/Ca can be complex, and most studies ignored the combined effects of rainfall and cave air pCO2. Moreover, knowledge of the influence of seasonal rainfall and cave air pCO2 on seasonal fluctuations in drip water Mg/Ca and Sr/Ca are limited for caves with different regions and ventilation types. Drip water Mg/Ca and Sr/Ca were monitored for five years at Shawan Cave. The results indicate that the irregular seasonal oscillation in drip water Mg/Ca and Sr/Ca is controlled by inverse-phase seasonal changes between rainfall and cave air pCO2. The rainfall amount may be the primary controlling factor of the interannual variation in drip water Mg/Ca, whereas the interannual variation in drip water Sr/Ca is most likely controlled by cave air pCO2. Furthermore, we compared drip water Mg/Ca and Sr/Ca of caves in different regions to fully understand how drip water Mg/Ca and Sr/Ca respond to hydroclimate changes. The drip water element/Ca, for seasonal ventilation caves with a fairly narrow range of cave air pCO2 respond well to the local hydroclimate associated with rainfall variation. If the range of cave air pCO2 is considerably large, the element/Ca in seasonal ventilation caves of subtropical humid regions may not reflect hydroclimate and that of Mediterranean and semi-arid regions may be primarily controlled by cave air pCO2. The element/Ca in the low year-round pCO2 caves could reflect the hydroclimate associated with surface temperature. Therefore, observations of drip water monitoring and comparative analysis can provide a reference for the explanation of speleothems element/Ca ratios from seasonally ventilated caves worldwide.