Investigation of Small-Scale Age Inversions in Stalagmites Using in Situ 230Th/U-Dating By Laser Ablation-MC-ICP-MS

Abstract:

Post-depositional U loss or addition in stalagmites lead to increasing/decreasing (²³⁰Th/²³⁸U) activity ratios and thus to older/younger ²³⁰Th/U-ages, respectively. In order to determine potential small-scale age inversions, we determined ²³⁰Th, ²³⁴U, and ²³⁸U isotope abundances in a stalagmite from the Hüttenbläserschachthöhle, western Germany, applying a high-spatial-resolution in-situ LA-MC-ICP-MS technique. This sample has the advantage that – because of its aragonitic composition - the U content is high (several µg g^-1) and that large-scale age inversions have been previously detected by solution MC-ICP-MS. Due to the low intensity of ²³⁰Th (20 – 80 cps), we carefully optimized the operating parameters of the 213 nm Nd:YAG laser, such as scan speed (4 µm s^-1), ablation time (1000 s), spot size (110 µm), and pulse repetition rate (20 Hz). We obtained a repeatability (RSE) of about 0.6 % – 0.9 % for ²³⁰Th/²³⁸U. The isotope ratios were corrected for instrumental biases using an external carbonate reference material (i.e., a flowstone in secular equilibrium). Including all sources of analytical uncertainty, we obtain a total age error (2 SE) of ca. 8 ka for a 215 ka old sample. Repeated dating of the same layers of the stalagmite yields a reproducibility of ca. 4 %. The LA-MC-ICPMS ages agree with the solution MC-ICP-MS ages within the analytical uncertainty. The high spatial resolution enables to detect small, but significant age inversions, which could not be detected by solution MC-ICP-MS. These inversions can be explained by diagenesis of speleothem CaCO₃, which may play an important role for the alteration of speleothem ages.