Refinement to the extraction of in-situ cosmogenic C-14
Dr Reka-Hajnalka Fulop1,3, Dr Smith Andrew1, Dr Bin Yang1, Dr Duanne White2, Dr Alexandru T. Codilean3
1Ansto, Menai, Australia, 2University of Canberra, Canberra, Australia, 3University of Wollongong, Wollongong, Australia
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.
Reka-H Fulop is an Earth Scientist who set up the in-situ cosmogenic C-14 extraction system at ANSTO in 2017. In the last few years as a side hustle she investigated the dispersion behind the intercomparison materials. Smith and Yang joined her on measuring duplicates/triplicates while White supplying samples and Codilean facilitating sample cleaning.