Comment on tc-2021-142 Anonymous Referee # 1 Referee comment on " Hyper-accumulation of legacy fallout radionuclides in cryoconite on Isfallsglaciären ( Arctic Sweden ) and their downstream distribution

This manuscript provides an original dataset including gamma-emitting fallout radionuclides activities, geochemical oxide composition/trace element concentrations, and C/N compositions obtained on cryoconite samples (n=14), sediment sources (n=23) and a 38-cm long lacustrine sediment core collected in a 1-km2 glaciated catchment of Sweden. The authors discuss the spatial and temporal distributions of these properties in response to glacial processes and sediment transport. They also underline the potential environmental and health deleterious impacts of the redistribution of these potentially harmful substances (e.g. in response to the ongoing global warming and snowmelt processes). Overall, the manuscript is well written, the data is well described and the results are well discussed at the light of those previously published in the literature.

59 please remove the . before the ( L.60 (and elsewhere in the text): Is 'prevalence' the right term to use here? Wouldn't the terms 'persistence' or simply 'occurrence' be more appropriate?
L.62 'environmental fate of the radioactivity' >> fate of the radionuclides? 63 unclear what you mean with 'in addition to any potential socio-economic impacts' here? Could you be more specific?
LL.64-65 same remark with 'subsequent dilution in the hydrological system', could you specify what you mean?
LL.67-76; I had the impression that this paragraph could be rewritten to better outline scientific questions instead of listing the analyses made and 'arguing' that FRN activities are higher than those found in other environments?

Study site
In my opinion, this section was pretty short and focused on glacial geomorphology. What about rainfall/snowfall in this catchment? What about the bedrock lithology, the soil characteristics (if this is relevant in this catchment?), etc.

Methods
Overall, in this section, the sampling period should be better defined (in the introduction, the period of 'August 2017' is mentioned, then nothing else is added… How high was the precipitation during the months before sampling? How was it distributed with time? I guess that this is a crucial aspect for supporting the interpretation of the results (e.g. mainly for Be-7 and Pb-210 data).
L.98 the authors refer to 'sources' here, although the reference to sources is no longer used in the interpretation… Were all the potential sources covered by this sampling (or maybe this wasn't the purpose…)?
L.102 could you be more specific on what you mean when referring to the 'proglacial outlets'? L.106 At 100°C? Isn't it too high for the organic matter composition analyses? L.107 'due to the limited amount of cryoconite available' >> what did it represent in grams of material (to have an idea of the difficulties encountered)?
L.107 why using a < 75µm sieving threshold (compared to the classical 63 µm threshold for instance?) L.109 was there a specific preparation protocol implemented before conducting the particle size analyses?
L.115 the article cited here (Wynants et al., 2020) refers to a study conducted in the African Rift Region with limited additional details on the gamma spectrometry analyses: I am not sure this has an added value here, or did I miss something?
L.130 I guess that you used two certified material samples to account for the matrix composition differences between cryoconites (more similar to moss soil?) and other sediment materials (more similar to the 'soil' TEL-2012-03); is it so?
LL.134-147: the section on WD-XRF analyses is well described, except maybe the calibration/validation issue: was it exclusively based on comparing the results obtained with WD-XRF and ICP-OES on a selection of samples, or were certified materials also used for this crucial step? L.212-230: to better support the interpretation of Be-7 results here, additional information should be provided on precipitation (snow/rainfall + snowmelt) during the months before the sampling campaign was conducted here. Furthermore, given the short half-life of Be-7 (~53 days), information on the analysis period after sampling is also crucial (as Be-7 could be <MDA in some samples just because some samples were analysed too late, and Be-7 initially present had just decayed to undetectable levels?) L.220 maybe the following ESSD manuscript (on Be-7 and Pb-210 levels across the globe) could be of interest to the authors here: https://essd.copernicus.org/preprints/essd-2021-35/, similar work must have been published on Cs-137 as well. LL.384-386 'It has been suggested that areas with previous 137Cs contamination may augment 137Cs transfer following future contamination events due to fixation in soils' >> unclear what you mean here, could you please clarify this statement?

Conclusions
Based on the changes made when revising the manuscript, some conclusions could be revised here (e.g. interpretations related to the organic matter content in cryoconites vs. other samples; interpretations related to Be-7 activities…) L.412: nuclear incidents >> nuclear accidents? Importantly, the thermonuclear bomb testing supplied most of these FRN (and I am not sure that they can be considered as nuclear incidents or accidents?) L.434 see the previous comment regarding the use of the 'prevalence' term Figures Figure 1: this catchment map actually does not provide catchment delineation (to the best of my understanding of this map); could this be improved? Furthermore, there is no North arrow/ scale on the inset map of Sweden (I guess this is the map of Sweden?); it is hard to see the river network and the sample symbols are not so easy to see/understand on these maps… This is nice to see the glacier lobes on the image but the other features of interest (sampling locations and types, catchment delineation, river network…) could be presented in a much clearer way in my opinion… Figure 2: it is not easy to read/infer the FRN activities based on this map (as single values are attributed to the circles), the exact values could maybe be added near the circles on the map in red/blue? Figure 5: I don't understand why the core samples were split into 4 sections; in my opinion, the core samples should be merged and compared to the potential sources (maybe the continuously supplied FRN such as Pb-210 and Be-7 as they decay with time/with depth in the core and they are therefore not fully relevant for this comparison but some of the geochemical elements/ organic matter properties might be?); I would consider adding the cryoconite properties to the graphs (after normalization to the particle size/organic matter properties?) Figure 6 is not very easy to read neither, at least the river network should be clearly added to the map? Figure 7: figure resolution seems not to be optimal, the years attributed to the peaks could be added on the Cs-137 part of the graph? Appendices Table A.1 what was the decay-correction date? Not sure that you should provide 2 decimal digits for the Am-241 activities? Table A.2 I would remain consistent with the number of decimal digits provided in this table… Do the values after +/-refer to the SD? For the 'central forefield' samples (n=2!), this is meaningless to provide a mean/SD and I would provide the range of values instead…?
Adding the particle size/organic matter data to this summary table would be very useful… Regarding the footnote on Be-7: information on the time between sampling and analysis would be particularly meaningful here…