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The precise determination of the stable C and O isotope fractionation between water and calcite (CC) and water and aragonite (AR) is of special interest for climate reconstructions, e.g. paleotemperatures. Previous studies reported results from both laboratory and field experiments, but their results are only partly consistent. Here we present C and O isotope data of a stalagmite from the Swiss Alps, which shows CC-AR transitions along individual growth 2 layers. Using detailed analyses both laterally and perpendicular to such layers we examined the difference in the C and O isotope fractionation factor of the HCO 3-CC and the HCO 3-AR system. For O this difference is similar to the water-CC and water-AR offset provided in experimental studies. The O isotope fractionation difference in the water-CC and water-AR system is comparable to those determined in laboratory studies but shows a statistically significant correlation with the CaCO 3 precipitation rate. For C we found a fractionation difference, which is independent of CaCO 3 precipitation rate and with slightly smaller values for the fractionation offset between HCO 3-CC and HCO 3-AR compared to literature values. However, we also found an unexpected decrease in  13 C along growth layers, which contradicts the widely used concept of Rayleigh fractionation during CO 2 degassing and CaCO 3 precipitation. The results of this study can be used e.g., to correct stable isotope time series of stalagmites showing CC-AR transitions along their growth axes.

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