Refinement to the extraction of in-situ cosmogenic C-14

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Date
2021-11-17
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Publisher
Australian and New Zealand Institutes of Physics
Abstract
In situ C-14 is an important addition to the cosmogenic nuclide toolkit. Its relatively short half-life - 5730 years - as compared to the longer-lived cosmogenic nuclides, means that it is substantially more sensitive to short term variations in process rates or more suitable at investigating recent exposure events. In-situ C-14 used in combination with Al-26 and Be-10 is also particularly well suited to studying the relatively short timescales that characterize fluvial sediment transfer and storage. Despite the above, the extraction of in-situ C-14 from geological samples is still problematic, with recent laboratory intercomparison studies showing considerable overdispersion in both intra and inter laboratory comparisons of standard materials. The discrepancies between laboratories have been attributed to several factors, including the quality of some intercomparison materials, however, clear consensus on the matter is yet to be reached. Here we present results of in-situ cosmogenic C-14 analyses in the Cronus-A and Cronus-R laboratory intercomparison materials and various samples obtained using the ANSTO/UOW in-situ C-14 extraction system, that suggest the presence of carbon containing minerals within these materials. Our results indicate that quartz separates need a different purity measurement that what would normally be acceptable for cosmogenic Be-10 and Al-26 analyses. Further, we do not observe a bias in results on those samples that underwent froth-floatation to remove feldspars, however applying a final 50% HF etch will result in removal of unwanted minerals. Based on analyses on fluid inclusions we adopted cycled in-vacuo 600oC pre-cleaning for samples and observe improvements in reproducibility for material in the 212-500 micron grainsize range.
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Keywords
Extraction, Carbon 14, Half-life, Isotopes, Aluminium 26, Beryllium 10, Laboratories, Sediments, Materials, ANSTO
Citation
Fulop, R.-H., Smith, A., Yang, B., White, D., & Codilean, A. T. (2021). Refinement to the extraction of in-situ cosmogenic C-14. Oral presentation to the 15th International Conference on Accelerator Mass Spectrometry. ANSTO Sydney, Australia. November 15th – 19th, 2021. (pp. 263). Retrieved from: https://ams15sydney.com/wp-content/uploads/2021/11/AMS-15-Full-Program-and-Abstract-Book-R-1.pdf
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