INVESTIGATING CAVE VENTILATION IN CENTRAL TEXAS USING SEASONAL VARIATIONS IN THE δ13C VALUE AND CONCENTRATION OF CAVE-AIR CO2

Speleothem calcite growth rate is a common proxy for paleoclimate. Seasonal variations in cave CO₂ are known to control speleothem growth. Thus, it is important to understand the mechanisms of CO₂ input and removal from cave systems in order to determine the factors that control speleothem calcite growth rate. To this end, spatial and temporal variations in the atmosphere of several central Texas caves were studied over the course of a year. Endmembers that contribute to seasonal variations in cave-air CO₂ concentrations were determined through analyzing δ¹³C values of air CO₂ samples collected along spatial transects within the caves.

CO₂ concentrations (pCO₂) consistently increase and δ¹³C values consistently decrease with distance from the cave entrance. The values along these transects shift seasonally towards higher δ¹³C values and lower pCO₂ in the winter months (October-March). A linear relationship between δ¹³C and 1/CO₂ suggests that air in the cave is a mixture between atmospheric air (~380 ppm CO₂, -8‰ δ¹³C, PDB) and a cave-air endmember with high pCO₂ and low δ¹³C. The cave-air endmember is calculated to find the δ¹³C value of respired CO₂ by correcting for atmospheric mixing and diffusion in the soil. The data are consistent with dominantly C₃ vs. C₄ vegetation above the caves, with δ¹³C values closer to -27‰ than to -13‰ δ¹³C. The δ¹³C value of respired CO₂ is generally higher in the summer and lower in the winter, which is consistent with seasonal changes in the relative productivity of C₃ vs. C₄ plants. The seasonal- and regionally-consistent isotopic and pCO₂ variations support density-driven ventilation, controlled by seasonal temperature changes, as the major influence on cave-air CO­_2.