Dual carbonate clumped isotope (A47-A48) measurements constrain different sources of kinetic isotope effects and quasi-equilibrium signatures in cave carbonates

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (A47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO2 degassing. More recently, studies have shown that mass-47 and mass-48 CO2 from acid digested carbonate minerals (A47 and A48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO2, temperature) for (dis)equilibrium using A47-A48 values, in concert with traditional stable carbon (813C) and oxygen (818O) isotope ratios. Data showed that 19 of 44 samples exhibited A47-A48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield A47-predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothem samples, we find A47-A48 values consistent with CO2 degassing effects, however, the majority of samples with KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopic buffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Delta 47 and Delta 48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Delta 47 and Delta 48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest average departures from equilibrium (Delta Delta 47 =-0.032 +/- 0.007, Delta Delta 48 =-0.104 +/- 0.035, where Delta Delta i is the measured value - the equilibrium value). This study shows how the Delta 47-Delta 48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alterna-tively, constraining the mechanisms of kinetic effects.