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

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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.