In this paper, the authors present evidence of fine-grained cryogenic cave minerals (CCMs) found in Cove Cave in northeast Greenland. They provide evidence for their ability to date these fine-grained CCMs, unlike previous studies, and ultimately demonstrate that these CCMs likely formed during a short, but extreme, melting event on the Greenland Ice Sheet in 1889, which would have provided sufficient liquid water in the cave to form these minerals.

I’m not an expert in geochemistry, or dating, so my review is general and focuses mainly on the plausibility of the 1889 melting event on the Greenland Ice Sheet providing sufficient meltwater to the Cove Cave system.

General Comments:

1. The authors describe how CCMs are a paleo-permafrost proxy and their importance to mid- and lower-latitude sites. What do the presence of CCMs in this northern-most location tell us about permafrost in this high-latitude site? It’s clearly interesting that these CCMs appear to have formed during a short-term extreme event on the ice sheet in 1889. A little more context of why this is an important finding in the Discussion and Conclusion sections would be helpful for the broad audience of this journal.

1. The authors present 5 potential scenarios that could have produced the CCMs found at Cove Cave and walk the reader through the logic that eliminates 4 of those scenarios. They then cite publications (Clausen et al., 1988; Fischer et al., 1998; Neff et al., 2014; Keegan et al., 2014), which all describe a widespread melting event that occurred in the dry snow zone of the Greenland Ice Sheet in 1889. They hypothesize that this widespread melting event provided liquid water to the Cove Cave system, which allowed the formation of the CCMs found. Indeed, the studies cited identify a widespread, extreme melting event on the ice sheet that created surface meltwater, but it’s initially concerning that the study sites in these references are pretty far from Cove Cave. Looking at available records from closer ice core sites B19 (Hatvani et al., 2022) and Tunu (Grieman et al., 2018), it doesn’t look like it was particularly warm in that region of the ice sheet in 1889 and there wasn’t a lot of deposition of vanillic acid (VA; an indicator of forest fire activity) which would suggest a decrease in albedo due to black carbon. The closest ice core record would be from Flade Isblink, and the authors report that this record does not suggest excess melting in 1889, but a recent publication shows that there was a high concentration of black carbon in 1889 at Flade Isblink (Eckhardt et al., 2023). Understandably, the perfect ice core record does not exist to prove the theory that these CCMs were formed due to excess meltwater from the 1889 widespread melting event that occurred at higher elevations on the Greenland Ice Sheet. With the increased concentrations of black carbon at the Flade Isblink site so close to Cove Cave, it does appear that the melting event could have happed in this region too. I suggest including the Flade Isblink black carbon record to bolster this hypothesis.

Specific comments:

Line 21: add ‘Cove Cave’ before ‘CCMs’ here to let the reader know you’re talking specifically about the Cove Cave CCMs again here

Line 25: why is ‘weather’ in parentheses here? I’d suggest removing the parentheses

Lines 53-56: for ease of reading, I suggest breaking this up into two sentences like ‘…in northeast Greenland. Cove Cave is currently…’

Line 68: remove ‘therefore’

Line 258: ‘sublime’ here should be ‘sublimated’

Line 294: a comma is needed after ‘than’

Line 297: ‘were’ should be ‘where’ here

Figure 1: are the blue lines indicating ice margins referring to present-day ice margins, or ice margins from the former presence of additional ice caps that are mentioned in the last sentence of the figure caption?

References:

Eckhardt, S., Pisso, I., Evangeliou, N., Zwaaftink, C. G., Plach, A., McConnell, J. R., ... & Stohl, A. (2023). Revised historical Northern Hemisphere black carbon emissions based on inverse modeling of ice core records. Nature Communications, 14(1), 271.

Grieman, M. M., Aydin, M., McConnell, J. R., & Saltzman, E. S. (2018). Burning-derived vanillic acid in an Arctic ice core from Tunu, northeastern Greenland. Climate of the Past, 14(11), 1625-1637.

Hatvani, I. G., Topál, D., Ruggieri, E., & Kern, Z. (2022). Concurrent Changepoints in Greenland Ice Core δ18O Records and the North Atlantic Oscillation over the Past Millennium. Atmosphere, 13(1), 93.

General Comments

This paper covers three separate (but interlinked) concepts, it identifies the mineralogy of cryogenic cave minerals (CCM) found in Cove Cave in Greenland, demonstrates that dating information can be extracted from fine grained cryogenic cave carbonates (CCC_fine, which has proven difficult in other studies) and uses those dating results and other information to infer the circumstances that led to the formation of the CCM in Cove Cave.

Overall this paper seems coherent and well written, I would recommend this paper be accepted pending minor revisions. Detailed comments are provided below.

1. Does the paper address relevant scientific questions within the scope of CP? – Yes it looks at reconstructions of the past by looking at the CCC in a cave in Greenland and also shows how you can get age data from CCC_fine.
2. Does the paper present novel concepts, ideas, tools, or data? – Yes, age dating of CCC_fine is a novel tool.
3. Are substantial conclusions reached? – Yes, it identifies CCM in cove cave and determines their age and formation circumstances.
4. Are the scientific methods and assumptions valid and clearly outlined? - Yes
5. Are the results sufficient to support the interpretations and conclusions? - Yes
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? – Needs improvement. The paper would be improved if they showed more of their mineralogy data (such as XRD diffractograms), and their methods could be more clearly written to allow fellow scientists to use the methods that they outline (eg they need to more explicitly how they are making their mixtures).
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution? – Yes they seem to be citing other relevant work, but they could cite more studies related to the CCM presence/absence of other Greenland caves.
8. Does the title clearly reflect the contents of the paper? - Yes
9. Does the abstract provide a concise and complete summary? - yes
10. Is the overall presentation well structured and clear? -yes
11. Is the language fluent and precise? - yes
12. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? - yes
13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? - No
14. Are the number and quality of references appropriate? - Yes
15. Is the amount and quality of supplementary material appropriate? – No, we could do with more of the mineralogy either in the text or in the supplementary information.

Specific Comments

37 – It would be good to explicitly state the criteria for differentiating CCC_fine from CCC_coarse, presumably a grainsize limit.

70 – It would be better to report an approximate distance from Cove Cave to the weather stations rather than just saying ‘closest’.

83 – A quick definition for what ‘inactive’ means in this context might be helpful, presumably it is common in speleothem geology but I am unfamiliar with it.

91 – Should probably change “rather low” to something less casual.

105 – Were there any obvious visual differences (color, texture etc) between the samples during collection?

110 – Clarity could be improved here, are you taking sample KC19CCC-4, splitting it into different mineral fractions and then mixing the relative amounts? Or are you mixing KC19CCC-4 with another phase?

116 – More analytical details for the mineralogy and crystal morphology analysis would be good. For example, with the XRD what was your scan range and analysis time?

166 – Why could you not ID the very fine brownish crystals? Even if it was too fine to manually separate under a microscope for analysis you had XRD data and could identify the other crystals in the sample so I would have thought it should be possible by process of elimination.

171 – An XRD figure either here or in the appendices showing the results from the 4 samples would be very useful as it would allow for easy comparison between the mineralogy of the four samples, rather than just having it written out.

249 – SEM may provide useful insights here, were any textures observed that could only be explained by synchronous formation (crystals intergrown etc)

250 – Any idea where does the quartz, dolomite and potassium feldspar in KC19CCC-1 come from? Country rock?

280 – No theory as to why your difference between δ¹⁸O values of your CCC_fine vs. common speleothems is different to that seen in other studies?

314 – Would be good to have a reference supporting your claim that there are not CCMs overserved in other caves in the area.

This is a very interesting paper that uses unique geologic samples – a cryogenic cave minerals – that grew in a northeast cave of Greenland to reconstruct past climate history. The primary geochemical analyses are mineralogical inspection, stable isotope analyses, and U-Th dating. From these analyses (primarily from the dating results) they deduce that CCMs formed during the Little Ice Age during a period of anomalously high temperatures that occurred over a few days in the summer of 1889 CE. These extreme warm conditions led to widespread melting over the Greenland Ice Sheet.

I am particularly intrigued by the interpretation of “a few days,” specifically at LINES 23-24: “We relate the CCM formation to a combination of black carbon deposition and anomalously high temperatures, which occurred over a few days, in the summer of 1889 CE.” The time constraint of “days” is an extraordinarily statement – the fact that a few days of extremely high temperatures caused widespread melting over northeast Greenland is an important finding, if it’s true. However, I find the author’s reasoning for relating CCM growth to this extreme climate event (a few days of warming) insufficient. There is not a thorough explanation for why authors jump to “days”? In the paper, the only citation is Neff et al. (2014). The authors need to add more explanation to this interpretation.

Overall, though, I find this an intriguing paper and I think the authors did a nice job thoroughly explaining their scientific methods and results. I do have a few clarifying points, though, that I think would make the paper stronger. Also, I feel some sections need added details. Most importantly, I find the authors reporting and explanation of the stable isotope data lacking. I explain this more below:

1. Does the paper present novel concepts, ideas, tools, or data? – Yes, the dating of CCC material is exceptionally novel, not to mention the location of this cave as the highest-latitude site with paleoclimate data is intriguing

2. Are the scientific methods and assumptions valid and clearly outlined? Yes, I think the authors do a nice job clearly stating their scientific processes and methods.

3. Are the results sufficient to support the interpretations and conclusions? Yes.

4. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? Yes.

5. Do the authors give proper credit to related work and clearly indicate their own new/original contribution? Yes.

6. Does the title clearly reflect the contents of the paper? Yes.

7. Does the abstract provide a concise and complete summary? Yes, except I recommend removing one part, given it is not relevant to main conclusions. 

8. Is the overall presentation well structured and clear? Mostly yes, though some sections need more explanation (see my line-by-line comments).

9. Is the language fluent and precise? Yes.

10. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? Yes.

11. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? No.

12. Are the number and quality of references appropriate? Yes, except more should be added in reference to the “few days” warm period in 1889 CE.

13. Is the amount and quality of supplementary material appropriate? Yes.

Line-by-line comments

Abstract: I am not sure why authors include the information about CCC_fine δ¹⁸O values in the abstract? It is my understanding that they do not use this data to make any interpretations?

Line 39: Though the authors link CCC formation as a “useful proxy for paleo-permafrost,” they do not state clearly whether the formation of CCC=permafrost is present? It may be worth stating this explicitly for readers who are unfamiliar with CCC.

Line 40: Please state the size difference between CCC_coarse and CCC_fine. Are CCC samples separated into "coarse” and “fine” categories by eye? By measurement?

Line 45: Is there a reason the authors report the CCM subtypes as CCC_coarse and CCC_fine versus CCM_coarse and CCM_fine? Lines 37-45 explain the difference between CCC and CCM, but then authors refer their CCM samples as CCC? Please clarify, because right now it seems these two are equivalent.

Line 48: “recently called into question.” How? Please briefly state this. Perhaps move line 50 up to follow this sentence, since I believe it’s because of detrital thorium contamination?

In Figure 2, it is interesting you only find fine-grained CCMs in the red shaded region, yet there are other below-0°C regions. Is there a hypothesis for why CCCs formed only in this location in the cave and not in others? This information may be helpful to scientists who want to go and try and find CCMs in other cave systems.

Figure 4: It is difficult to discern the difference between the light vs. darker gray shading colors. A suggestion to make one of the categories black?  

Line 125: Please specify where common speleothems were collected. Was it the same cave? Right now it is just reported as “in the study area,” which is not enough information. This could help shed light on the δ¹⁸O difference between common speleothem and CCCs?

Section 5.2: Are authors interpreting the low δ¹⁸O values as reflecting contribution of precipitation from the Arctic air mass? Doesn’t this location primarily receive precipitation from the Arctic air mass? Why is this significant? Also, the Greenland common speleothems have a higher δ¹⁸O value than CCM δ¹⁸O, but they were collected from the same cave? If they are from the same “northeast Greenland” cave, then they should receive precipitation from the same source, and therefore should have the same δ¹⁸O? I see at Lines 278-281 the authors address this difference, but do not provide a reason why? Please explain? Even if the authors are not sure why this is, that should be stated. As of now, it is unclear what the assumption of this is, and I find the discussion of the stable isotope data not sufficient.

Section 5.2 (continued): What is significant about the Greenland CCMs δ¹⁸O overlap with mid-latitude caves? This is not discussed, and I’m a bit confused why this is significant.