Carbon cycling in the mantled karst of the Ozark Plateaus, central United States

The nature of carbon (C) cycling in the unsaturated zone where groundwater is in contact with abundant gas-filled voids is poorly understood. The objective of this study was to trace inorganic-C cycling in a karst landscape using stable-C isotopes, with emphasis on a shallow groundwater flow path through the soil, to an underlying cave, and to the spring outlet of a cave stream in the Ozark Plateaus of northwestern Arkansas. Carbon dioxide (CO₂) concentration and isotopic composition (δ¹³C-CO₂) in gas and dissolved inorganic carbon (DIC) concentration and isotopic composition (δ¹³C-DIC) in water were measured in samples collected from two suction-cup soil samplers above the cave, three sites in the cave, and at the spring outlet of the cave stream. Soil-gas CO₂ concentration (median 2,578 ppm) and δ¹³C-CO₂ (median − 21.5‰) were seasonally variable, reflecting the effects of surface temperature changes on soil-CO₂ production via respiration and organic-matter decomposition. Cave-air CO₂ (median 1,026 ppm) was sourced from the soil zone and the surface atmosphere, with seasonally changing proportions of each source controlled by surface temperature-driven air density gradients. Soil-DIC concentration (median 1.7 mg L^− 1) was lower and soil-δ¹³C-DIC (median − 19.5‰) was lighter compared to the cave (median 23.3 mg L^− 1 and − 14.3‰, respectively) because carbonate-bedrock dissolution provided an inorganic source of C to the cave. Carbon species in the soil had a unique, light stable-C isotopic signature compared to the cave. Discrimination of soil-C sources to karst groundwater was achieved, which is critical for developing hydrologic budgets using environmental tracers such as C.