Comparison of Spring and Cave Drip Water in Westcave Preserve, Central Texas May Reveal Epikarst CO2 Degassing

The cave at Westcave Preserve, in central Texas, is a unique location to study karst processes due to its low, nearly atmospheric cave-air CO₂ levels and seasonally variable temperature. The source of water that drips into the cave, however, has not been constrained, limiting interpretation of climate proxies in the cave. It is possible that a nearby spring and the cave drip-waters share a common source. Alternatively, the drip-waters could represent precipitation that has infiltrated the host rock. These hypotheses should be tested using Sr isotope ratios and/or other tracers. If they do share a common source, analysis of dissolved inorganic carbon (DIC) concentration , δ¹³C_DIC, and cation concentrations of the two waters could provide insight into epikarst processes such as CO₂ degassing and prior calcite precipitation (PCP) that are otherwise difficult to constrain. Westcave Preserve includes outcrops of the Hensell Sand, the Cow Creek Limestone, and the Hammett Shale, with a small cave at the contact between the Cow Creek and Hammett formations. The overlying Hensell Sand contains water that emerges at the surface as a spring near the cave. Water also drips directly into the cave, forming speleothems. Previous research has established that although δ¹⁸O values of rainfall in the area vary seasonally, between -10.5 and 1.1‰ with a weighted mean of -6.5‰ (VSMOW), the drip-water varies only between -4.7 and -4.3‰ with a weighted mean of -4.5‰ (Feng et al., in review). This suggests a large well-mixed reservoir above the cave. The soils above the cave have high CO₂ of up to 17,500 ppmv, but because the cave is shallow with multiple large openings, cave CO₂ levels are near-atmospheric (Casteel and Banner, in review). This creates a steep CO₂ gradient between the soil and the cave air. The spring water DIC is nearly in carbon-isotope equilibrium with the soil CO₂, suggesting that soil respiration, here controlled by C₃ plants, is the primary source of CO₂ for this reservoir. The drip water δ¹³C_DIC is higher than the spring water (-10.3‰ versus -13.0‰). Although the spring water has higher DIC concentration than the drip water, with mean values of 128 mg/L C versus 113 mg/L C, respectively, preliminary data suggest that for some drips, the drip water DIC concentrations and δ¹³C_DIC may vary with spring DIC values. We propose that if the spring and the drip water prove to be derived from the same source, the differences in DIC and δ¹³C_DIC between spring and drip water are due to epikarst CO₂ degassing as the water percolates down the CO₂ gradient toward the cave ceiling. If the spring represents the source of the drip water, the calculated δ¹³ value of degassed CO₂ is -33.3‰, assuming no PCP. PCP may occur, leading to a δ¹³C of degassed CO₂ lower than calculated, but would result in a decrease or no change in δ¹³C_DIC and therefore does not explain the observed difference between spring water and drip water.