EVALUATION OF ISOTOPIC EQUILIBRIUM DURING PRECIPITATION OF MODERN SPELEOTHEM CALCITE IN A TROPICAL SETTING

Applications of the carbon and oxygen isotopic composition of speleothem calcite to paleoclimate reconstructions can be limited by a lack of information on the extent to which isotopic equilibrium is attained during calcite growth. We tested for isotopic equilibrium during calcite precipitation by analyzing modern cave calcites and their corresponding drip waters at multiple sites in Harrison’s Cave on Barbados.

The O isotopic composition of drip waters average -3.2 ± 0.14‰. This falls within the narrow range of groundwater compositions, measured throughout the aquifer, which is controlled by wet season recharge. A larger range of O isotopic compositions is observed in modern speleothem calcite. These calcites have been collected from the tips of actively forming speleothems and from substrates placed under drips. Only the slowest of five drip waters precipitates calcite in O isotopic equilibrium, where as the other drip waters are precipitating calcite up to 2‰ heavier than predicted on the basis of water temperature and published fractionation factors. The C isotopic composition of dissolved inorganic carbon (DIC) in drip water was compared to the C isotopic composition of the calcite precipitating from that water. Although the C isotopic composition of calcite forming in the cave varies by 9‰ between sites, results from four out of five sites are consistent with calcite precipitating in isotopic equilibrium with DIC. In summary, calcite precipitating from the majority of drips in Harrison’s Cave is not in isotopic equilibrium with respect to O isotopes but is with respect to C isotopes.

Several forcing mechanisms are likely responsible for the isotopic variability observed in modern speleothem calcite. These mechanisms include 1) kinetic isotope effects caused by evaporation and/or high precipitation and degassing rates, 2) variability in the openness of the CO2 (g)-host limestone system, and 3) variability in flow routes through the vadose zone.