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. Only 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 precipitation.

Both modern and Holocene speleothem calcite from Barbados, sampled temporally along the growth axis, shows similar positive δ¹³C versus δ¹⁸O slopes. These results, along with the spatial variations in glass plate calcite carbon and oxygen isotope compositions, suggest that the isotopic composition of the Holocene speleothems is in part controlled by non-equilibrium isotope effects. In addition, there is a correlation between stalactite length and oxygen and carbon isotope ratios of calcite precipitated on the corresponding stalagmite and glass plate, which may be due to ¹³C and ¹⁸ O enrichment of the HCO₃⁻ reservoir during CO₂ degassing–calcite precipitation along the overhanging stalactite.

We compiled 165 published speleothem stable isotope records with a global distribution and found that most of these records show a positive covariation between δ¹³C and δ¹⁸O values. Speleothem stable isotope records may be influenced by kinetic isotope effects such that temperature-controlled equilibrium fractionation models alone cannot directly explain the significance of the variations in these records. Advancing the interpretation of these records requires the calibration of cave environmental conditions with the non-equilibrium isotope effects that cause δ¹³C and δ¹⁸O covariations in speleothems.