EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Andrea Schröder-Ritzrau1, Jens Fohlmeister2,3, Martin Trüssel4, Julius Förstel1, Norbert Frank1, Christoph Spötl5, and Marc Luetscher6
Andrea Schröder-Ritzrau et al.
  • 1Institute of Environmental Physics, University of Heidelberg, 69120 Heidelberg, Germany
  • 2Potsdam Institute for Climate Impact Research, Telegrafenberg, 14473 Potsdam, Germany
  • 3GFZ German Research Centre for Geosciences, Climate Dynamics and Landscape Development, Telegrafenberg, D-14473 Potsdam, Germany
  • 4Stiftung Naturerbe Karst und Höhlen Obwalden (NeKO), 6065 Alpnach, Switzerland
  • 5Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria
  • 6Swiss Institute for Speleology and Karst Studies (SISKA), 2301 La Chaux-de-Fonds, Switzerland

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 SO42- 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.

How to cite: Schröder-Ritzrau, A., Fohlmeister, J., Trüssel, M., Förstel, J., Frank, N., Spötl, C., and Luetscher, M.: Uranium isotope systematics in the Melchsee-Frutt cave region (Central Swiss Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6972,, 2020