The sources and sinks of CO ₂ in caves under mixed woodland and grassland vegetation

We measured concentrations and stable carbon isotope compositions of carbon dioxide in the atmospheres of three caves in central Texas and one cave in southern Arizona in order to identify CO ₂ sources and sinks. The vegetation above the caves studied is either savannah (two caves, above which vegetation has been minimally disturbed) or discrete patches of grassland and woodland (two caves, above which vegetation has been highly disturbed). We tested two hypotheses concerning CO ₂ in the cave atmospheres: (1) cave ventilation by tropospheric air is the primary sink for CO ₂ and (2) CO ₂ is primarily derived from the deepest rooting plants growing above the caves. Within caves, we monitored CO ₂ at individual locations on monthly and daily time-scales and measured CO ₂ along transects with increasing distance from the cave entrances. We also measured CO ₂ in the pore spaces of soils under grasses and trees above each of the caves. We calculated δ ¹³C values of respired CO ₂ (δ ¹³C _r) for all gas samples using measured δ ¹³C values and CO ₂ concentrations. We then identified the sources of cave CO ₂ by comparing cave-air and soil CO ₂ δ ¹³C _r values. At all locations in each Texas cave, CO ₂ concentrations were highest (lowest) and δ ¹³C values were lowest (highest) during the summer (winter). Cave-air CO ₂ concentrations consistently increased and δ ¹³C values consistently decreased with distance from the cave entrances. Similar but smaller magnitude seasonal variations in CO ₂ concentrations occurred in the Arizona cave and no seasonal or spatial variation in the δ ¹³C of cave-air CO ₂ was observed. The mean δ ¹³C _r values of CO ₂ in soils under grass were 3.5-4.5‰ higher than the δ ¹³C _r values of CO ₂ in soils under trees. In the caves under savannah, mean δ ¹³C _r values of cave-air CO ₂ (-24‰ in both caves) were within 1‰ of the mean δ ¹³C _r values of CO ₂ in soils under trees. In caves covered by large, contiguous areas of grassland, the δ ¹³C _r values of cave-air CO ₂ were similar to grassland soil values during the summer and were intermediate between grassland and woodland soil values during the winter. The observed spatial and temporal variations in cave-air CO ₂ are consistent with density-driven ventilation controlled by seasonal surface temperature changes as the primary sink for CO ₂ in the Texas caves. The consistent agreement between soil and cave δ ¹³C _r values indicate that the same mixing and diffusion equations that are used to calculate δ ¹³C _r values of soil CO ₂ also apply to cave-air CO ₂. Our results suggest that the majority of CO ₂ advects or diffuses into these caves from soils as a gas rather than being transported in aqueous solution. Measured δ ¹³C _r values and numerical production-diffusion modeling supports our hypothesis that the majority of gaseous CO ₂ in these caves is derived from deeply rooted vegetation. The carbon isotope composition of groundwater and speleothem calcite used for paleoclimate records are therefore likely biased toward deeply rooted plants, even if sparsely present.