USING STABLE CARBON ISOTOPES TO CHARACTERIZE CARBON AND NUTRIENT DYNAMICS IN A NORTHWESTERN ARKANSAS CAVE

Exchange of carbon between dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and gaseous CO₂ pools is poorly characterized for karst hydrologic systems. Heterogeneity inherent in conduit- and fracture-dominated flow paths characteristic of karst introduces challenges that have caused a lag in our understanding of these systems as compared with the status of knowledge for granular-media aquifers. But, understanding these systems is important for numerous issues, such as managing nutrients in vulnerable karst watersheds and effectively using speleothems as records of paleoclimate change.

Carbon and nutrient dynamics were investigated in a cave in northwestern Arkansas using geochemical characterization and stable isotopic analyses (δ¹³C). Combined continuous and periodic monitoring of carbon species and water geochemistry along karst flow paths allowed for delineation of carbon and nutrient fluxes.

The distributions and stable isotopic compositions of dissolved and gaseous phases of inorganic and organic carbon were found to be very heterogeneous in the soil; illustrating the importance of the epikarst zone in controlling the composition of recharge water. Seasonal changes in surface temperature and soil productivity were found to affect the concentration of CO₂ in the cave atmosphere, which varied by an order of magnitude between winter and summer. The δ¹³C values of CO₂ and DIC in the cave also showed seasonal changes and were inversely correlated to cave-CO₂ concentration, suggesting that changes in surface temperature and soil productivity also affected isotopic compositions of inorganic carbon pools in the cave.

During the winter when cave-CO₂ concentration was steady, DIC- and CO₂-δ¹³C values in the cave were relatively low, indicating input of organically-derived carbon into the inorganic carbon pool. After January, the concentration and isotopic composition of DOC in the cave were inversely correlated; decreased DOC concentrations associated with isotopically heavier organic matter provided further evidence of biological processing along the cave flow path. Seasonal changes in DOC were additionally over-printed by precipitation events, which flushed DOC into the cave, providing organic substrate for biological processing such as denitrification.