Quantifying carbon isotope disequilibrium during in-cave evolution of drip water along discreet flow paths

Abstract

Paleoclimate reconstructions that use speleothem proxy data have increased our understanding of terrestrial climate change, but gaps remain in our understanding of in-cave processes that influence speleothem chemistry. The δ13C values of speleothem calcite are typically influenced by kinetic isotope effects that operate during CO2 degassing and calcite precipitation. Therefore the identification and quantification of these isotopic effects is important in interpreting speleothem stable isotope records. Here we studied the change in water chemistry and δ13C values of dissolved inorganic carbon (DIC) along discreet flow paths at multiple drip sites in Inner Space Cavern, central Texas. We quantified the extent to which the water remains in C isotopic equilibrium during flow along speleothem surfaces as CO2 degasses and calcite precipitates.Two locations in the study cave that have long in-cave flow paths were examined to determine the geochemical evolution and its driving processes along these paths. At each location cave water was sampled at two points 1–2 meters apart along each flow path. Among the key spatial changes observed is a <∼1‰ to ∼4‰ increase in δ13C values of DIC along the flow paths. The magnitude of the increase in δ13C values is controlled by the extent of DIC loss to CO2 degassing. The extent of DIC loss and CO2 degassing is controlled by the pCO2 gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ13C value of the DIC reservoir can be accounted for by a Rayleigh distillation model with equilibrium C-isotope fractionation factors for (CO2(g)-HCO3−(aq)) and (CaCO3-HCO3−(aq)). As the depletion of the DIC reservoir exceeds 15% the DIC δ13C values become progressively higher such that the (HCO3−(aq)-CO2(g)) fractionation values needed to explain the observations change from equilibrium values of ∼8‰ to non-equilibrium values of up to ∼25‰. This variance in magnitude of C-isotope fractionation during CO2 degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss. Such kinetic effects have significant implications for speleothem C-isotope proxy interpretations, as these kinetic isotope effects are of a similar magnitude as those used to infer past changes in drought and vegetation.