Large kinetic isotope effects in modern speleothems

The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions reflect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m2 area in Harrison's Cave, Barbados, West Indies. some of the plate δ13C values and none of the plate δ18O values correspond to equilibrium values, indicating significant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects. On each plate, speleothem calcite was deposited with distinct δ18O and δ13C compositions that increase progressively away from the growth axis, with up to 6.6‰ 13C and 1.7‰ 18O enrichments. The positive δ13C versus δ18O trends are likely a result of 18O and 13C Rayleigh-distillation enrichment in the HCO3− reservoir owing to progressive CO2 degassing and CaCO3 precipitation. The magnitude of the δ13C versus δ18O slope is likely controlled by the extent to which CO2 hydration-hydroxylation reactions buffer the oxygen isotope composition of the HCO3− reservoir during calcite precipitation. Complete oxygen isotopic buffering of the HCO3− reservoir by CO2 hydration-hydroxylation reactions will produce a vertical δ13C versus δ18O slope in calcite sampled along a growth layer. As oxygen isotope buffering of the HCO3− reservoir decreases to no buffering, the δ13C versus δ18O slope in calcite sampled along a growth layer will decrease from vertical to 0.52 at the cave temperature. In this study, modern speleothem calcite sampled along the growth layer produced a δ13C versus δ18O slope of 3.9, indicating incomplete oxygen isotope buffering of the HCO3− reservoir during calcite precipitatio