Institute for Geochemical Research, Hungarian Academy of Sciences, Budaörsi út 45, H-1112 Budapest, Hungary.
Rapid Commun Mass Spectrom. 2010 Dec 30;24(24):3521-6. doi: 10.1002/rcm.4799.
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.
尽管方解石-水氧同位素分馏的温度依赖性已通过大量经验、实验和理论研究得到充分证实，但仍在讨论中，尤其是由于需要提高古温度计算的准确性。目前已有实验确定的方程，并通过理论计算得到验证（被认为代表同位素平衡）；然而，许多自然形成的物质似乎并不遵循这些关系，这意味着现有的分馏方程应该进行修订，或者碳酸盐沉积物受到动力学和溶液化学效应或后期变化的严重影响。为了检验现有的分馏-温度关系是否可用于天然沉积物，我们通过稳定同位素质谱研究了在各种环境中沉淀的方解石：在开放或准封闭环境中从热水和温水沉淀的石灰华（淡水石灰岩），以及在封闭系统中形成的洞穴沉积物。所有研究地点都监测了物理和化学参数以及水的氧同位素组成。测量沉淀温度以及水和方解石的氧同位素组成，得出了经验性的特定环境分馏-温度方程：[1] 1000 · lnα=17599/T - 29.64 [适用于温度范围为 30 至 70°C 的石灰华] 和 [2] 1000 · lnα=17500/T - 29.89 [适用于 10 至 25°C 范围内的洞穴沉积物]。最后，根据文献数据和我们的结果进行比较，评估了使用不同的方解石-水氧同位素分馏关系和应用策略来获取最可靠的古气候信息的方法。
