WHERE DOES CAVE-AIR CO2 ORIGINATE? EVIDENCE FROM STABLE CARBON ISOTOPE RATIOS

The use of speleothems as paleoclimate proxies has grown in recent years. Although speleothems give precise paleoclimate records, their growth has been found to vary seasonally. The seasonal CO₂ flux into and out of caves drives the growth of speleothem calcite deposits. The CO₂ flux out, through ventilation of the cave atmosphere, has been studied, but the CO₂ flux in has not yet been thoroughly analyzed. Therefore in order to better describe the CO₂ cycle within caves, and ultimately improve speleothem paleoclimate proxies, it is important to determine the source of CO₂ flux into caves on seasonal and diurnal timescales.

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 (δ¹³C_r) which removes the effects of diffusion and contamination by atmospheric CO₂. The δ¹³C_r values for all three of the caves overlap with the C₃ soil δ¹³C_r 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 δ¹³C_r values were measured in one of the caves every two hours for a 24 hour period. CO₂ (δ¹³C_r) 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 δ¹³C_r. The regions closer to the entrance and deeper within the cave have δ¹³C_r values of -20‰ and -22‰, respectively. The correlation between CO₂ and δ¹³C_r 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.