²³⁰Th/U Isochron Dating of Cryogenic Cave Carbonates

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.