Preprints
https://doi.org/10.5194/gchron-2022-10
https://doi.org/10.5194/gchron-2022-10
 
29 Mar 2022
29 Mar 2022
Status: this preprint is currently under review for the journal GChron.

230Th/U Isochron Dating of Cryogenic Cave Carbonates

Paul Töchterle1, Simon Steidle1, R. Lawrence Edwards2, Yuri Dublyansky1, Christoph Spötl1, Xianglei Li2, John Gunn3, and Gina E. Moseley1 Paul Töchterle et al.
  • 1Institute of Geology, University of Innsbruck, Innsbruck, 6020, Austria
  • 2Dept. Earth and Environmental Sciences, University of Minnesota, Minneapolis, 55455, MN/USA
  • 3School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, UK

Abstract. Cryogenic Cave Carbonates (CCCs) are a type of speleothem, typically dated with 230Th/U disequilibrium methods, that provide evidence of palaeo-permafrost conditions. In the field, CCCs occur as distinct patches of millimetre- to centimetre-sized loose crystals and crystal aggregates on the floor of cave chambers, lacking a framework to validate ages by stratigraphic order. Correction factors for initial 230Th (230Th0) are often based on the bulk-earth derived ratio of initial 230Th/232Th activity ((230Th/232Th)0), which is a well-established approach when 230Th0 is moderately low. For samples with elevated levels of 230Th0, however, accuracy can be improved by constraining (230Th/232Th)0 independently. Here, we combine detailed morphological observations from three CCC patches found in Water Icicle Close Cavern in the Peak District (UK) with 230Th/U analyses. We find that individual CCC crystals show a range of morphologies that arise from non-crystallographic branching in response to the chemical evolution of the freezing solution. Results of 230Th/U dating indicate that samples with a large surface area relative to the sample volume are systematically more affected by contamination with 230Th0. By fitting isochrons to these results, we test whether the CCCs in an individual patch formed during the same freezing event, and demonstrate that (230Th/232Th)0 can deviate substantially from the bulk-earth derived value and also vary between the different CCC patches. Where CCCs display elevated 230Th0, isochrons are a useful tool to constrain (230Th/232Th)0 and obtain ages with improved accuracy. Detritus absorbed to the crystal surface is shown to be the most likely source of 230Th0. Our results suggest that some previously published CCC ages may merit re-assessment and we provide suggestions on how to approach future dating efforts.

Paul Töchterle et al.

Status: open (until 02 Jun 2022)

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  • RC1: 'Comment on gchron-2022-10', Karel Zak, 09 May 2022 reply

Paul Töchterle et al.

Paul Töchterle et al.

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Short summary
Cryogenic cave carbonates (CCCs) provide a marker for past permafrost conditions. Their formation age is determined by 230Th/U dating. However, samples can be contaminated with small amounts of 230Th at formation, which can cause inaccurate ages and requires correction. We analysed multiple CCCs and found that varying degrees of contamination can cause an apparent spread of ages, when really all samples formed within a single freezing event. A statistical correction method is presented.