|This manuscript presents new constraints on production of in situ cosmogenic 14C (in situ 14C by both capture of slow negative muons and high energy muon interactions within ancient ice in the ablation zone at Taylor Glacier, Antarctica. The authors measure the partitioning of in situ 14C between CO2, CO, and CH4 and compare their results to both earlier work in ice in Antarctica and Greenland as well as to measurements of in situ 14C in a deep bedrock core. Results of this work indicate significant discrepancies with fast muon and slow negative muon production rates that are predicted by the commonly cited target irradiation studies of Heisinger et al. (2002a, 2002b), and the bedrock core mentioned above. The authors’ measurements appear to be very well-characterized, and they consider a wide range of possible explanations for the discrepancy. In the end, the source(s) of the discrepancy are not identified, and it remains as an interesting and important target of future research.|
I found this manuscript to be well written and thoroughly argued, providing a valuable and interesting dataset that points to a need for additional work to resolve the observed discrepancy in predicted muogenic production rates. That said, I think that there some points in their analysis that need addressing before the paper would be ready for publication.
First, all isotopic results should be re-calculated using the methods of Hippe and Lifton (2014), that are specific to the in situ 14C AMS analyses as opposed to traditional radiocarbon measurements. The authors use measured pMC as the basis of their calculated 14C concentrations, which relies on a number of assumptions for organic radiocarbon that don’t apply to in situ 14C systematics. Hippe and Lifton (2014) provides recipes for converting the pMC values to the raw measured isotopic ratios – the differences are often small but can be significant depending on the measurement details. Any older in situ 14C data to which these are being compared should also be recast appropriately per Hippe and Lifton (2014). This includes the Young et al. (2014) spallation production rate dataset and derived reference production rate.
Second, the authors should clarify whether production rate scaling is done using the LSDn nuclide-specific model from Lifton et al. (2014). If scaling does not use LSDn, then the authors should recalculate scaling factors using that model, as it accounts for the effects of both the cosmic ray flux and excitation functions for production of in situ 14C.
I also think the authors should also include the formulations of Balco (2017) in their discussions (given that it’s in the reference list already) – a study that uses a different approach to fit the muon production proportions from the in situ 14C depth profile data of Lupker et al. (2015).
Finally, there are several points in the manuscript where the authors assert and/or imply that the Heisinger et al. (2002a, 2002b) muogenic production predictions are incorrect (as well as those from Lupker et al., 2015, and Balco, 2017, which support them) – Abstract, conclusions, and section 4.2 around Line 470, for example. I think it would be more correct to frame the comparison of this study’s findings with those other predictions as a discrepancy that merits additional investigation, particularly with regard to comparisons of ice vs. rock measurement techniques. To their credit the authors do mention the need for future investigation to resolve this discrepancy, but I would just ask that they soften the assertions of correctness.
Balco, G. (2017). Production rate calculations for cosmic-ray-muon-produced 10Be and 26Al benchmarked against geological calibration data. Quaternary Geochronology, 39, 150–173. https://doi.org/10.1016/j.quageo.2017.02.001
Heisinger, B., Lal, D., Jull, A. J. T., Kubik, P., Ivy-Ochs, S., Neumaier, S., et al. (2002a). Production of selected cosmogenic radionuclides by muons 1. Fast muons. Earth and Planetary Science Letters, 200(3–4), 345–355.
Heisinger, B., Lal, D., Jull, A. J. T., Kubik, P., Ivy-Ochs, S., Knie, K., & Nolte, E. (2002b). Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons. Earth and Planetary Science Letters, 200(3–4), 357–369.
Hippe, K., & Lifton, N. A. (2014). Calculating Isotope Ratios and Nuclide Concentrations for In Situ Cosmogenic 14C Analyses. Radiocarbon, 56(3), 1167–1174. https://doi.org/10.2458/56.17917
Lifton, N., Sato, T., & Dunai, T. J. (2014). Scaling in situ cosmogenic nuclide production rates using analytical approximations to atmospheric cosmic-ray fluxes. Earth and Planetary Science Letters, 386, 149–160. https://doi.org/10.1016/j.epsl.2013.10.052
Lupker, M., Hippe, K., Wacker, L., Kober, F., Maden, C., Braucher, R., et al. (2015). Depth-dependence of the production rate of in situ 14C in quartz from the Leymon High core, Spain. Quaternary Geochronology, 28(c), 80–87. https://doi.org/10.1016/j.quageo.2015.04.004