Empirical equations for the temperature dependence of calcite-water oxygen isotope fractionation from 10 to 70°C.

Although the temperature dependence of calcite-water oxygen isotope fractionation seems to have been well established by numerous empirical, experimental and theoretical studies, it is still being discussed, especially due to the demand for increased accuracy of paleotemperature calculations. Experimentally determined equations are available and have been verified by theoretical calculations (considered as representative of isotopic equilibrium); however, many natural formations do not seem to follow these relationships implying either that existing fractionation equations should be revised, or that carbonate deposits are seriously affected by kinetic and solution chemistry effects, or late-stage alterations. In order to test if existing fractionation-temperature relationships can be used for natural deposits, we have studied calcite formations precipitated in various environments by means of stable isotope mass spectrometry: travertines (freshwater limestones) precipitating from hot and warm waters in open-air or quasi-closed environments, as well as cave deposits formed in closed systems. Physical and chemical parameters as well as oxygen isotope composition of water were monitored for all the investigated sites. Measuring precipitation temperatures along with oxygen isotope compositions of waters and calcites yielded empirical environment-specific fractionation-temperature equations: [1] 1000 · lnα = 17599/T - 29.64 [for travertines with a temperature range of 30 to 70°C] and [2] 1000 · lnα = 17500/T - 29.89 [for cave deposits for the range 10 to 25°C]. Finally, based on the comparison of literature data and our results, the use of distinct calcite-water oxygen isotopic fractionation relationships and application strategies to obtain the most reliable paleoclimate information are evaluated.