230 Th / U isochron dating of cryogenic cave carbonates

. Cryogenic cave carbonates (CCCs) are a type of speleothem, typically dated with 230 Th / 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 floors of cave chambers, so they lack a frame-work that would allow ages to be validated by stratigraphic order. Correction factors for the initial 230 Th ( 230 Th 0 ) are of-ten based on the bulk-earth-derived initial 230 Th / 232 Th activity ratio (( 230 Th / 232 Th) 0 ), which is a well-established approach when 230 Th 0 is moderately low. For samples with elevated levels of 230 Th 0 , however, accuracy can be improved by constraining ( 230 Th / 232 Th) 0 independently. Here, we combine detailed morphological observations from three CCC patches found in Water Icicle Close Cavern in the Peak District (UK) with 230 Th / 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 230 Th / U dating indicate that samples with a large surface area relative to the sample volume are systematically more affected by contamination with 230 Th 0 . By fitting isochrons to these results, we test whether the CCCs in a patch formed during the same freezing event, and demonstrate that ( 230 Th / 232 Th) 0 can deviate substantially from the bulk-earth-derived value and can also vary between the different CCC patches. Where CCCs display elevated 230 Th 0 , isochrons are a useful tool to constrain ( 230 Th / 232 Th) 0 and obtain ages with improved accuracy. Detritus absorbed to the crystal surface is shown to be the most likely source of 230 Th 0 . Our results suggest that some previously published CCC ages may merit reassessment, and we provide suggestions on how to approach future dating efforts.