WHERE DOES CAVE-AIR CO2 ORIGINATE? EVIDENCE FROM STABLE CARBON ISOTOPE RATIOS
We compared the CO2 in the atmospheres of three caves in central Texas, USA with the CO2 in gas collected from the pore spaces of soils above the caves. Soil CO2 was collected from gas wells installed at different depths (5-35cm) within the soil in various locations where the vegetation was dominated by either C3 trees, C4 grasses, or CAM cacti. In order to compare cave-air CO2 with soil CO2 and vegetation above the cave, we calculated the δ13C value of respired CO2 (δ13Cr) which removes the effects of diffusion and contamination by atmospheric CO2. The δ13Cr values for all three of the caves overlap with the C3 soil δ13Cr values (-22 to -28‰) and not with the C4 soil values (-16 to -19‰). This indicates that cave CO2 originates mainly from the C3 soil respired CO2. C3 trees, when compared with the C4 grasses in the study area, may have a greater influence in cave-air CO2 because the trees root deeper in the soil. Cave-air CO2 concentrations and δ13Cr values were measured in one of the caves every two hours for a 24 hour period. CO2 (δ13Cr) reached a maximum (minimum) of 4400ppm (-22‰) at 1pm and a minimum (maximum) of 3100ppm (-20‰) at 7am. Transects in the cave show two distinct regions divided by an abrupt change in δ13Cr. The regions closer to the entrance and deeper within the cave have δ13Cr values of -20‰ and -22‰, respectively. The correlation between CO2 and δ13Cr is consistent with the observed spatial transects and a barometric control on movement of CO2 within the cave. The carbon isotope composition of cave-air CO2 may help interpret the δ13C value of speleothem calcite and by completing the cave CO2 cycle, we may improve the mechanistic understanding of speleothem growth.