230Th/U Isochron Dating of Cryogenic Cave Carbonates
- 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
- 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.
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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
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General comments
One of assumptions on which is based this study is formation of all CCC particles in one patch within one water freezing event. The reasoning for this assumption is based on the occurrence of typical sequence of CCC morphological types, each characterized by carbon and oxygen stable isotope data in an explainable logical sequence, which is in accord with known principles of stable isotope fractionation under water freezing conditions. This assumption is most probably valid, nevertheless, certain uncertainty remains. Under oscillating climate of the Last Glacial similar climatic conditions could occur repeatedly and all other factors are constant or highly similar – cavity morphology, its depth below the surface, lithology and chemistry of the limestone, characteristics of the epikarst zone, etc. We can therefore speculate that repeated freezing conditions with similar morphology of the ice fill the cave and similar chemistry of water which freezes could produce repeatedly the same sequence of morphological types with similar evolution of C and O stable isotope data in the carbonate. After ice melting the products of two or more freezing events could have been deposited together in one patch. An approach which can possibly shed some light on this can be detailed mapping of size distribution and distribution of individual CCC morphological types within each of studied patches together with detailed mapping of patches (giving their dimensions and shapes).
Since the study site is presented as a CCC locality for the first time in the international literature, the position of the studied sites within the cave should be presented in more detail, either in a cave map (can be included in an electronic supplement) or by coordinates and/or description in the text. I also consider as important to specify in the discussion from which direction the water producing the CCCs most probably entered the cave. Was it dripping water or flowing water from some kind of periodic water stream? This has some consequences regarding the transport of clay particles within the cave and thus for the interpretation of Th contamination of the samples. It would be useful to compare the levels of clastic Th contamination of the studied samples with 230Th/232Th ratios of other studied and published localities.
Detailed comments
The studied site
Please give an information here about the cave chimneys directed toward the surface. Are they all sediment or limestone boulder blocked? The cave entrance was artificially excavated? What have possibly been the circumstances of cave connection toward the surface in the Last Glacial?
Results
How do you know it is only the calcite? Based on crystal morphology only? There is no information about XRD mineral identification in the Methods.
References
The list of references is complete and there is no extra reference. The guide for authors is not followed completely. The journal names should be abbreviated and the order of references should follow the guide for authors (copied from this source):
- Single author papers: chronologically, beginning with the oldest. If there is more than one paper in the same year, a letter (a, b, c) is added to the year, both in the in-text citation as well as in the reference list.
- Co-author papers: first alphabetically according to the second author's last name, and then chronologically within each set of co-authors. If there is more than one paper in the same year per set of co-authors, a letter (a, b, c) is added to the year both in the in-text citation as well as in the reference list.
- Team papers: first chronologically (beginning with the oldest), independent of the team author names, then alphabetically within each year according to the second (third, etc.) author. If there is more than one paper in the same year for a first author (independent of the team), a letter (a, b, c) is added to the year both in the in-text citation as well as in the reference list.
Paul Töchterle et al.
Paul Töchterle et al.
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