Seasonal variation of carbon and nutrient transfer in a northwestern Arkansas cave

An understanding of carbon (C) cycling in karst is critical because the interplay between inorganic and organic C pools controls the dissolution and precipitation of calcium carbonate (CaCO3) minerals, and organic C availability is a controlling factor for subsurface biologic activity. Compared to granular-media aquifers, karst aquifers are considerably more heterogeneous and anisotropic because a substantial component of groundwater flow is focused along fractures and conduits. Understanding karst systems is important for a diversity of issues, such as managing nutrients in vulnerable karst watersheds and effectively using speleothems as records of paleoclimate change.;A cave in northern Madison County, Arkansas was monitored to constrain the transfer of C species along groundwater flow paths in a karst hydrologic system. The concentration and isotopic composition (delta13C) of C species were monitored at points representative of a flow path from soil, cave, and seeps for a year to quantify the seasonal changes in the transformations of C species. The study sought to identify the potential for and characterize the conditions of C and nutrient processing along groundwater flow paths.;The distributions and delta13C of dissolved and gaseous phases of inorganic and organic C were found to be very heterogeneous in the soil; illustrating the dynamic nature of biogeochemical processes and the importance of the soil zone in controlling the composition of recharge water in karst settings. Soil water showed a direct correlation between dissolved inorganic carbon (DIC) concentration and surface temperature; as rates of microbial respiration increased with increasing surface temperature, the soil-CO 2 exchanged with soil water and was measured as DIC. The seasonal signature from the soil zone was transmitted to the cave environment through both gaseous and dissolved C species. Cave-CO2 concentration was correlated to seasonal changes in surface temperature and cave ventilation. The delta 13C-DIC values in the cave waters were relatively heavier than soil-DIC isotopic compositions because of larger inorganic C inputs in the cave from carbonate bedrock. Although two seeps monitored during the study were not observed to directly discharge cave waters, the differences in the flow paths of the two seeps were reflected in both the geochemistry of the water and isotopic compositions of C species. This initial investigation of processing along groundwater flow paths in an Ozark cave has shown that monitoring natural levels of C species and nutrients can provide insight into how C is cycled in a cave setting.