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

Speleothem delta O-18 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 delta O-18 data. In this study, delta O-18 values were measured for calcite deposited on artificial substrates at four sites in two central Texas caves monitored for over 10 years. The results are used to evaluate the equilibrium isotopic fractionation factor and the impact of climatic and hydrologic conditions on kinetic isotopic fractionation. The delta O-18 values of calcite from the four sites ranged from 24.8 parts per thousand. to 26.7 parts per thousand (V-SMOW), and associated drip water values ranged from -4.6 parts per thousand to -3.9 parts per thousand. A comparison of predicted equilibrium calcite delta O-18 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 Delta O-18(cc-e)) ranges from -0.7 parts per thousand to 1.4 parts per thousand. Three of the four drip sites yield similar linear relationships between Delta O-18(cc-e), and water temperature (t(w)), and can be collectively expressed as: Delta O-18(cc-e) = 0.3t(w) - 4.7: r(2) = 0.56 (n = 93). Therefore, calcite deposited during time periods of lower t(w) lower cave-air CO2 concentration and faster calcite deposition rates, have delta O-18 values closer to equilibrium. This contradicts conceptual models, which predict that a faster calcite deposition rate leads to a larger departure of calcite delta O-18 from equilibrium. If slower calcite deposition indeed facilitates equilibrium fractionation (i.e., Delta O-18(cc-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 Delta O-18(cc-e) values of up to -2.4 parts per thousand.. 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 delta O-18 value than that of calcite in isotopic equilibrium with ambient water. The relationship between deposition rates and Delta O-18(cc-e) for this study is consistent with the same relationship using data for synthetic calcite from the literature. This relationship indicates a -0.8 parts per thousand shift of calcite delta O-18 for every ten-fold increase in deposition rate at 5 to 25 degrees C and a pH of -8.3 parts per thousand. The significant kinetic fractionation observed in this study warrant consideration in applying measured speleothem calcite delta O-18 values to interpret past climate conditions. (C) 2012 Elsevier B.V. All rights reserved.