Physical and geochemical response in cave drip waters to recent drought, central Texas, USA: Implications for drought reconstruction using speleothems

Abstract:

Present-day variations in climate, such as drought, influence the physical and geochemical characteristics of cave drip waters. These changes in water chemistry, water flux, and climate through time may be preserved in cave calcite formations (speleothems). In 2011, a record drought began in central Texas, presenting an opportunity to assess the physical and geochemical response of cave drip waters. As part of a long-term cave monitoring study, drip water samples from eight sites in a cave on the Edwards Plateau were collected every 3 to 6 weeks before, during, and after the peak of the 2011 drought and analyzed for major and trace element concentrations. Physical parameters such as calcite growth rate, drip rate, and cave-air CO₂ were also monitored.

We compare time series of the Palmer Drought Severity Index (PDSI) with time series of drip rate, calcite growth rate, and water composition. The drip sites show a range of response to drought by these parameters and only some sites record drought via elemental ratios and growth rate. Both drip rate and drip water elemental ratios (Mg/Ca and Sr/Ca) provide insight into water residence time and flow paths within the vadose zone. All sites show some correspondence between drip rates and PDSI and three of the eight have a statistically significant relationship (r²=0.29-0.58, p<0.001). Mg/Ca and Sr/Ca ratios for four out of eight drip sites were higher during drought than during wet periods, consistent with longer residence times and, as a result, more water-rock interaction during droughts. Calcite growth rates of fast dripping sites (mean drip rate greater than 4.5 mL/min) demonstrate a long-term decrease in response to drought. The complexity of drip site response is likely a consequence of the combined effects of prior calcite precipitation (PCP), water-rock interaction, seasonal ventilation of cave-air CO₂, and water flux in the vadose zone. The results of this study improve our understanding of variations in the modern cave system due to drought and have implications for selecting speleothems as paleo-drought proxies.