Systematic investigation of stable isotope fractionation in speleothems with novel laboratory experiments

尽管近年来进行了大量的研究，但洞穴藻类生长过程中影响稳定同位素信号(d13C和d18O)的基本过程尚未被详细了解。特别是，动力学同位素分馏的影响很难量化。目前，这项提案的两个主要调查者开发了两种不同的模型。然而，主要的不确定性来自模型中使用的相对不确定的同位素分馏系数。我们开发了一套新的实验装置来研究从水薄膜中析出CaCO3的基本过程及其对稳定同位素分馏的影响。实验中精确模拟了天然洞穴系统的条件，并可对水膜厚度、溶液pH、溶液中钙离子含量等参数进行精确调控。我们的方法允许研究溶解的HCO3-和沉淀的CaCO3的d18O和d13C值的演化。与以前的实验相比，这些都是重要的发展。此外，我们的实验装置不需要进一步修改也可以用来研究簇状同位素的分馏和微量元素在溶液和沉淀CaCO3之间的分配系数。在我们的模型的基础上，我们将使用我们的装置进行几个实验，以提高对洞穴沉积物中稳定同位素分馏的基本理解。我们将定量确定相应的同位素分馏系数和地球化学(同位素交换)过程的时间常数。此外，我们还将进行对比实验，以研究成团同位素的分馏，并确定微量元素(例如，镁、锶、钡、磷、铀)在溶液和沉淀碳酸钙之间的分配系数。

Despite of intensive research in recent years, the basic processes during speleothem growth influencing the stable isotope signals (d13C and d18O) have not been understood in detail yet. In particular, the influence of kinetic isotope fractionation is difficult to quantify. Currently, two different models developed by the two principle investigators of this proposal are available. The major uncertainties, however, arise from the relatively uncertain isotope fractionation factors used in the models. Kinetic isotope fractionation factors for the system Ca2+-CO2-H2O have not been determined yet.We have developed a novel experimental setup in order to investigate the basic processes during precipitation of CaCO3 from thin films of water and the resulting effect on stable isotope fractionation. The conditions of natural cave systems are precisely simulated, and several parameters, such as the thickness of the water film, pH and the Ca2+ content of the solution, can be precisely adjusted and controlled during the experiment. Our approach allows studying the evolution of the d18O and d13C values of both the dissolved HCO3- and the precipitated CaCO3. These are important developments in comparison to previous experiments. Furthermore, our experimental setup can also be used without further modifications to study the fractionation of clumped isotopes and the distribution coefficients of trace elements between the solution and the precipitated CaCO3.On the basis of our models, we will use our setup in order to perform several experiments in order to improve the basic understanding of stable isotope fractionation in speleothems. We will quantitatively determine the corresponding isotope fractionation factors and the time constants of the geochemical (isotope exchange) processes. The results will be used to validate and, if necessary, improve the available models.Furthermore, we will perform comparable experiments to investigate the fractionation of clumped isotopes and to determine the distribution coefficients of trace elements (e.g., Mg, Sr, Ba, P, U) between the solution and the precipitated CaCO3.