A SUBSOIL CARBON SOURCE TO CAVE-AIR CO2 AND SPELEOTHEM CALCITE IN CENTRAL TEXAS (Invited Presentation)

The canonical model for speleothem formation invokes soil respiration as the dominant carbon source. However, a number of lines of evidence point to a deeper, older carbon source to cave air CO₂ in many karst regions. Here we use the oxidative ratio (OR, the number of moles of O₂ consumed per mole of CO₂ produced during respiration) to distinguish between soil and subsoil carbon sources. Because O₂ and CO₂ have different diffusion coefficients in air, measured ‘apparent’ OR values are controlled by diffusion in addition to the actual OR during respiration. We compared apparent OR values in caves and soils at two localities in central Texas: Natural Bridge Caverns (NB) and Inner Space Cavern (IS). At NB, soil gas samples follow the trend expected for respiration + diffusion (OR = 0.67 ± 0.05) whereas cave air samples follow a trend expected for respiration without subsequent diffusion (OR = 1.09 ± 0.06). At IS, most soil gas samples follow the respiration + diffusion trend, but some samples likely record substantial CO₂ solution into water in a partly closed system resulting in a higher average OR value (OR = 1.01 ± 0.08). Cave air samples at IS also follow a trend expected for respiration followed by CO₂ solution into water in a partly closed system (OR = 2.40 ± 0.14). These observations indicate that soil respiration is not the dominant carbon source to cave air CO₂ at NB. In addition, these observations indicate that the majority of CO₂ is advected into these caves as a gas, as opposed to diffusing into the caves or being transported by water. We suggest that 1) microbial oxidation of aged organic carbon in the bedrock fracture network is the dominant carbon source to cave air and speleothems and 2) bedrock fractures ventilate, advecting CO₂ into caves. Radiocarbon activities of CO₂ in cave air and soil gas support these interpretations and the chemistry of drip waters suggest that these interpretations also apply to the carbon source for speleothems.