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

摘要

Speleothem δ ~(18)O 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 δ ~(18)O data. In this study, δ ~(18)O 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 δ ~(18)O 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 δ ~(18)O 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 δ ~(18)O _(cc-e)) ranges from -0.7‰ to 1.4‰. Three of the four drip sites yield similar linear relationships between δ ~(18)O _(cc-e) and water temperature (t _w), and can be collectively expressed as: δ ~(18)O _(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 CO _2 concentration and faster calcite deposition rates, have δ ~(18)O values closer to equilibrium. This contradicts conceptual models, which predict that a faster calcite deposition rate leads to a larger departure of calcite δ ~(18)O from equilibrium. If slower calcite deposition indeed facilitates equilibrium fractionation (i.e., δ ~(18)O _(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 δ ~(18)O _(cc-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 δ ~(18)O value than that of calcite in isotopic equilibrium with ambient water. The relationship between deposition rates and δ ~(18)O _(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‰ shift of calcite δ ~(18)O 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 δ ~(18)O values to interpret past climate conditions.