Constraining the subsoil carbon source to cave-air CO 2 and speleothem calcite in central Texas

Canonical models for speleothem formation and the subsurface carbon cycle invoke soil respiration as the dominant carbon source. However, evidence from some karst regions suggests that belowground CO 2 originates from a deeper, older source. We therefore investigated the carbon sources to central Texas caves. Drip-water chemistry of two caves in central Texas implies equilibration with calcite at CO 2 concentrations (P CO2_sat ) higher than the maximum CO 2 concentrations observed in overlying soils. This observation suggests that CO 2 is added to waters after they percolate through the soils, which requires a subsoil carbon source. We directly evaluate the carbon isotope composition of the subsoil carbon source using δ 13 C measurements on cave-air CO 2 , which we independently demonstrate has little to no contribution from host rock carbon. We do so using the oxidative ratio, OR, defined as the number of moles of O 2 consumed per mole of CO 2 produced during respiration. However, additional belowground processes that affect O 2 and CO 2 concentrations, such as gas-water exchange and/or diffusion, may also influence the measured oxidative ratio, yielding an apparent OR (OR apparent ). Cave air in Natural Bridge South Cavern has OR apparent values (1.09 ± 0.06) indistinguishable from those expected for respiration alone (1.08 ± 0.06). Pore space gases from soils above the cave have lower values (OR apparent  = 0.67 ± 0.05) consistent with respiration and gas transport by diffusion. The simplest explanation for these observations is that cave air in NB South is influenced by respiration in open-system bedrock fractures such that neither diffusion nor exchange with water influence the composition of the cave air. The radiocarbon activities of NB South cave-air CO 2 suggest the subsoil carbon source is hundreds of years old. The calculated δ 13 C values of the subsoil carbon source are consistent with tree-sourced carbon (perhaps decomposing root matter), the δ 13 C values of which have shifted during industrialization due to changes in the δ 13 C values and concentrations of atmospheric CO 2 . Seasonal variations in P CO2_sat in most of the drip waters suggest that these waters exchange with ventilated bedrock fractures in the epikarst, implying that the subsoil CO 2 source contributes carbon to speleothems.