Temporal Variability of Cave-Air CO2 in Central Texas

The growth rate and elemental and chemical composition of cave calcite deposits (speleothems) are important proxies for past environmental and hydrologic changes. Understanding the timing and driving mechanisms of calcite growth is therefore important for interpreting such proxies. The deposition of speleothem calcite in two central Texas show caves 130 km apart is known to vary seasonally in response to fluctuations in cave-air CO₂. Calcite deposition is highest in the late fall through spring when cave-air CO₂ is lowest, likely due to density driven ventilation. In the warmer summer months, when cave air becomes stagnant and high CO₂ concentrations develop, calcite deposition decreases to near zero. To determine if this phenomenon is unique to show caves, which, have been modified from their natural state (e.g., enlarged entrances, high numbers of visitors), CO₂ was measured along transects in three wild caves within the central Texas region.

Multiple transects of each cave showed that cave-air CO₂ varies seasonally in all undeveloped caves, with peak concentrations in late August to early October and lower concentrations in December through late March. The timing of seasonal CO₂ fluctuations is consistent in all caves studied. Cave-air CO₂ also shows significant diurnal fluctuations that are well correlated with changes in surface air temperature and barometric pressure. These results indicate that 1) the seasonality of calcite deposition observed in the show caves (as controlled by the concentration of cave-air CO₂) is not due to human alteration of the caves, 2) the concentration of cave-air CO₂ varies significantly on multiple timescales (i.e. daily to seasonally), 3) the seasonal fluctuation of cave-air CO₂ is regional in extent.