Uranium isotope systematics in the Melchsee-Frutt cave region (Central Swiss Alps)

We investigated the uranium isotope systematics in an alpine cave environment (central Switzerland). We measured the U concentration and the ²³⁴U/²³⁸U activity ratio of the two main host rock formations in which the cave developed and the ²³⁴U/²³⁸U activity ratio of drip water. We further investigated the U characteristics of young carbonate precipitates (< 500 a old) at these drip sites. In addition, we analysed several speleothems between 1 and ~200 ka old. We observe variable U concentrations (between 0.1 – 5 ppm) and a significant spread in the initial activity ratio of ²³⁴U/²³⁸U (between ~1 and 5) between individual drips and individual stalagmites. In general, high U concentrations are accompanied by low initial ²³⁴U/²³⁸U ratios and vice versa. However, these data do not follow a binary mixing line between the two host rock endmembers (both show low U concentrations of about 1 ppm). Instead, we argue that redox processes within the karst might govern the U systematics of cave drip water and speleothems, as high and variable SO4^2- concentrations in drip water are observed, which point to at least locally constrained anoxic conditions in the host rock. The dependence of the initial ²³⁴U/²³⁸U activity ratio and U concentration of the stalagmites from this cave in concert with the partly large absolute value in their initial activity ratio open the perspective for dating speleothems from this karst region back to ~1 Ma.