Processes controlling dripwater hydrochemistry variations in Xueyu Cave, SW China: implications for speleothem palaeoclimate signal interpretations

Cave dripwater hydrochemistry responds to environmental changes, both within and outside of the cave, and thereby conveys this information to any stalagmite fed by the drips. As stalagmites are important archives of climate proxy information, understanding how dripwater hydrochemistry responds to environmental forcing is critical. However, despite the large number of speleothems in SW China, the response of dripwater to regional climate variability is not yet adequately understood. A 3-year study of three drip sites in Xueyu Cave, Chongqing Municipality, SW China, revealed the most important mechanisms controlling dripwater chemical variability. The principal chemical indices (pH, specific conductivity, Ca2+, Mg2+, Sr2+ and ) in collected dripwaters and the local climate data were analysed in this study. The principal controls on the hydrochemistry were found to be the external climate and its changes, groundwater residence time, cave ventilation and prior calcite precipitation (PCP) processes. Dripwater hydrochemistry showed strongly coherent seasonal patterns despite the fact that all sites are Ca–HCO3 type waters and supersaturated with calcite. Seasonal changes in dripwater hydrochemistry were influenced by the soil and vadose zone CO2 content as well as groundwater residence time in the upper karst zone. Cave-air CO2 seasonal variations were consistent with changes in dripwater PCO2 and cave ventilation. Trace element ratios (Mg/Ca and Sr/Ca) of dripwater were controlled by PCP processes. Seasonal variations in dripwater Mg/Ca and Sr/Ca ratios in Xueyu Cave showed inverse changes with the Asia Monsoon Index during the monitoring period, reflecting the seasonal climate changes that may have been recorded in the speleothems. Based on a linear regression of PCO2 and the Ca2+ data in the cold–dry winter season, a 130-ppm shift in cave-air PCO2 results in a 1-ppm shift in dripwater Ca2+ concentration in Xueyu Cave. This study illustrates the importance of understanding factors controlling the changes in the composition of dripwater before it reaches the speleothem.