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Speleothems have been shown to provide important information about the climate of the past. Their probably greatest advantage as a climate archive is that they can be dated precisely by U-series disequilibrium methods. Due to their protected location in caves, they are normally not affected by post-depositional alteration of the U-series decay system. However, some samples apparently violate the constraint that stalagmites must become progressively older from top to bottom and, thus, represent age inversions. This may be particularly the case for aragonitic samples, which may be affected by recrystallization to calcite resulting in U-series open-system behavior and apparently wrong ages.
Here we present U-series and trace element data from stalagmite HBSH-1, which was collected in western Germany (Hüttenbläserschachthöhle). HBSH-1 shows apparent age inversions in the bottom section, which grew between 230 and 180 ka according to preliminary U-series dating (Fig. 1). In order to investigate the processes leading to the abnormal dating results, high-resolution LA-ICP-MS was applied to determine the trace element composition of the bottom section. The concentration of 34 trace elements has been analyzed. The variability of Sr, Ba, Mg, U, Th and P along the growth axis of the speleothem is shown in Fig. 1. High Mg concentration coinciding with low Sr, U and Ba indicates at least one calcite layer at ca. 455 mm distance from top (Fig. 1). However, this short interval is probably not the reason for the observed age inversions. Since high concentrations of P in stalagmite samples usually correspond to high soil productivity in the sampling area due to wet climate conditions, the bottom of our stalagmite sample can be divided into five periods, two of which may be related to wet climate conditions, whereas the other three may represent relatively dry periods. The fluctuations of Mg and Ba are similar to those of P, which could be caused by the same past climatic process. |
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