Carbon Cycling in the Karst of Northwestern Arkansas: Linking the Soil and Cave Environments

Abstract:

Carbon is a ubiquitous element in karst systems. Stable isotope analysis of carbon reservoirs provides information about carbon cycling, allowing carbon to be tracked along groundwater flow paths from the surface atmosphere, through the soil and epikarst, and into caves and karst aquifers. In the Ozarks of northwestern Arkansas, karstified carbonates are covered with a mantle of cherty regolith, which can serve as either a sink or source of carbon. Samples for δ¹³C analysis of gaseous CO₂ and aqueous DIC were collected from two soil lysimeters installed in the soil zone above a cave, a drip-water pool in a side-passage of the cave, and three locations along the cave stream and spring outlet. A two-component, concentration-dependent mixing model of δ¹³C-CO₂ was used to calculate the proportions of cave-air CO₂ sourced from the surface atmosphere versus the soil zone. Median CO₂ concentrations were 2,578 ±1,504 ppm in the soil and 1,026 ±939 ppm in the cave, and CO₂ concentration increased with outside surface temperature. Median δ¹³C-CO₂ was -21.5 ±2.1 ‰ in the soil and -17.5 ±4.3 ‰ in the cave. Median DIC concentrations were 1.7 ±1.0 mg/L in the soil and 23.3 ±6.6 mg/L in the cave, and as discharge increased, DIC concentration decreased in the cave. Median δ¹³C-DIC was -19.5 ±2.5 ‰ in the soil and -14.3 ±1.6 ‰ in the cave. The concentration and isotopic composition of carbon species varied seasonally; for example, in the soil and cave, CO₂ was greatest in the summer and lowest in the winter. Additionally, the proportion of soil CO₂ contributing to the cave air was lowest during fall (8 %) and greatest during summer (71 %). At the spring outlet during storm events, DIC and calcite saturation decreased and δ¹³C-DIC became lighter, likely as pre-event water stored in the soil zone was displaced by storm-event water. Data indicate that, depending on the time of year, cave carbon species were sourced from the surface atmosphere, soil, and deeper groundwater flow paths.