Oxygen isotopic fractionation between drip water and speleothem calcite: A 10-year monitoring study, central Texas, USA

Abstract

Speleothem δ18O values can serve as a paleoclimate proxy, yielding information about past temperature and rainfall. An accurate determination of the equilibrium calcite–water oxygen isotopic fractionation factor and an assessment of kinetic isotopic fractionation are required for interpretation of speleothem δ18O data. In this study, δ18O values were measured for calcite deposited on artificial substrates at four sites in two central Texas caves monitored for over 10years. The results are used to evaluate the equilibrium isotopic fractionation factor and the impact of climatic and hydrologic conditions on kinetic isotopic fractionation.The δ18O values of calcite from the four sites ranged from 24.8‰ to 26.7‰ (V-SMOW), and associated drip water values ranged from −4.6‰ to −3.9‰. A comparison of predicted equilibrium calcite δ18O values, calculated using a commonly-used isotopic fractionation factor, with measured values indicates that 94% of the calcite samples are not in oxygen isotopic equilibrium with respect to their associated drip water. The departure from oxygen isotopic equilibrium (expressed as Δ18Occ-e) ranges from −0.7‰ to 1.4‰. Three of the four drip sites yield similar linear relationships between Δ18Occ-e and water temperature (tw), and can be collectively expressed as: Δ18Occ-e=0.3tw−4.7; r2=0.56 (n=93). Therefore, calcite deposited during time periods of lower tw, lower cave-air CO2 concentration and faster calcite deposition rates, have δ18O values closer to equilibrium. This contradicts conceptual models, which predict that a faster calcite deposition rate leads to a larger departure of calcite δ18O from equilibrium.If slower calcite deposition indeed facilitates equilibrium fractionation (i.e., Δ18Occ-e→0), then the results of this study support a larger than commonly accepted value for the equilibrium calcite–water oxygen isotopic fractionation factor. Adopting a larger published value for the fractionation factor yields negative Δ18Occ-e values of up to −2.4‰. These negative values cannot be explained by existing kinetic fractionation models. Alternatively, they may reflect the trapping of a calcite “surface layer” with a lower δ18O value than that of calcite in isotopic equilibrium with ambient water. The relationship between deposition rates and Δ18Occ-e for this study is consistent with the same relationship using data for synthetic calcite from the literature. This relationship indicates a −0.8‰ shift of calcite δ18O for every ten-fold increase in deposition rate at 5 to 25°C and a pH of 8.3. The significant kinetic fractionation observed in this study warrant consideration in applying measured speleothem calcite δ18O values to interpret past climate conditions.