A REACTIVE TRANSPORT APPROACH TO MODELING CAVE SEEPAGE WATER CHEMISTRY (Invited Presentation)

Weathering of carbonate dominated lithology results in residual mineral deposits, such as speleothems, which provide a long-term archive for Critical Zone processes, including changes in Earth’s climate from seasonal to millennial timescales. However, the majority of Critical Zone research has emphasized silicate lithologies, which are typified by relatively slower rates of reactivity and less congruent weathering. Thus, interpretation of the unique time-averaged records accumulated through the evolution of carbonate landscapes requires integration of observational data and simulations specific to these geologic, hydrologic and environmental conditions. Such model-data integration provides a temporal bridge between the rapid drainage of reactive fluids through karstic lithology and the long-term variations in isotopic and geochemical tracers recorded in speleothems. Here, we present a forward and process-based reactive transport simulation based on a multi-year monitoring study of the Blue Spring Cave in Tennessee, USA. The simulations describe the fluid-driven weathering of limestone including explicit tracking of stable carbon and radiocarbon isotope ratios and trace elements Sr, Mg and Ba. We find that the drip waters currently emerging in the Blue Spring Cave are only moderately influenced by degassing and prior carbonate precipitation. Radiocarbon isotope ratios are strongly influenced by the combination of fluid flow rate and soil CO₂ content, while stable carbon isotopes are largely dictated by the temperature dependent equilibrium fractionation among contemporaneous species, with some modification by degassing. Drip water Sr/Ca and Mg/Ca are sensitive to both prior calcite precipitation and heterogeneously distributed diagenetic dolomite and celestite in the limestone. These simulations are extended to a wide range of parameter space to demonstrate the factors that these isotopic and elemental proxies record.