KINETIC AND EQUILIBRIUM CONTROLS ON SPELEOTHEM ISOTOPIC VARIATIONS

The d¹⁸O and d¹³C values of cave calcite deposits (speleothems) are commonly used to construct paleoclimate records. For reliable use as a climatic proxy, speleothem calcite must be precipitated in isotopic equilibrium with its corresponding water. Modern calcite-water pairs from Harrison’s Cave, Barbados, were analyzed to determine the extent to which O and C isotopic equilibrium was achieved. The pairs constituted either the tips of actively growing speleothems or calcite grown on glass plates, and the corresponding drip waters.

The d¹³C values of the majority of the modern calcite samples fall within the range of values predicted from the combined use of (1) fractionation factors from the literature, (2) measured temperatures and (3) measured d¹³C values of dissolved-inorganic-carbon (DIC). Some samples range from ~2.0‰ higher to ~2.0‰ lower than predicted values. The modern calcite ¹³C depletions are likely caused by kinetically driven changes in the fractionation between HCO₃^- and CaCO₃ from equilibrium conditions (D¹³C _{CaCO3 - HCO3-} ~ 2.0‰) to non-equilibrium conditions (D¹³C _{CaCO3 - HCO3-} ~ 0‰), possibly caused by rapid growth of speleothem calcite. Modern calcite ¹³C enrichments may be caused by degassing of ¹³C-depleted CO₂. Through Rayleigh distillation, this degassing would increase the d¹³C value of the HCO₃^- reservoir, as well as the CaCO₃ precipitated from it.

The d¹⁸O values of modern calcite are not consistent with equilibrium precipitation (based on the combined use of fractionation factors, measured temperatures and measured d¹⁸O values of drip waters), and show 0.2‰ to 2.3‰ isotopic enrichments. Similar to the C-isotope system, the d¹⁸O value of modern calcite is likely controlled by kinetically-driven changes in the fractionation between HCO₃^- and CaCO₃ from inferred equilibrium conditions (D¹⁸O _{CaCO3 - HCO3-} ~ -6.3‰) to non-equilibrium conditions (D¹⁸O _{CaCO3 - HCO3-} ~ 0‰), possibly caused by rapid growth of speleothem calcite. In contrast to the C-isotope system, the d¹⁸O value of modern calcite may not be affected by Rayleigh distillation because the d¹⁸O value of HCO₃^- is theoretically calculated to lie half way between that of CO₂ and CaCO₃. Degassing of ¹⁸O-enriched CO₂ is thereby offset by precipitation of correspondingly ¹⁸O-depleted CaCO₃.