Review of the manuscript “NEEM to EastGRIP Traverse – spatial variability, seasonality, extreme events and trends in common ice core proxies over the past decades”, by Kjær et al. cp-2021-99

General comments

The manuscript is focused on the analysis of the chemical records obtained by CFA from 6 shallow firn cores retrieved along the NEEM - EastGRIP Scientific Traverse. The Authors present a study of both spatial variability along the 6 sites spanning West to East Greenland and temporal variability, after yielding an ice core chronology, basing on annual layer counting. As regarding the latter, dust concentration of size-sorted particles was used to spot possible local dust sources, free acidity and conductivity were employed to detect volcanic eruptions and a stacked ammonium record was found as a valuable proxy of forest fires in Northern America.

The paper presents an ample set of new data which can be useful to a broad community of scientists involved in recent climate reconstructions from ice core records and I find it apt to be published on Climate of the Past eventually.

However, I find that the manuscript should go through some consistent revisions.

Some parts of the text, e.g. ice core chronology (see Specific Comments) should be better detailed and deserve a further short discussion.

A general English revision is also suggested: the text is usually easy to read but sometimes sentences look as broken or dashed off hurriedly and should be rephrased.

Furthermore, there are many basic format and punctuation issues which can be easily fixed.

Here below I am listing some specific remarks to help in this process.

Specific comments

Abstract

Line 18 page 1 (and related Table 2). “Annual mean and quartiles of the…”: the sentence is not immediately clear upon reading if one has not gone through the text and Table 2 could be accompanied by a figure showing the overlap of data distributions to better appreciate it. For instance, a box and whiskers plot could be helpful, but any other solution is welcome. 

2. Materials and methods

Lines 14-16 page 4. Melting a firn core is always a critical issue and certainly deserves some more precautions with respect to ice core sections. A melt rate of 4 cm/min sounds fine but probably even a higher rate would work. The addition of a metal coin is interesting, and I guess it is to separate the melting section from the head so that the produced water stays in contact with the firn section as little as possible, but the authors are invited to add some details about the metal coin addition. It could be better shown also in Figure S1 (here metal coin is not visible).

Section 2.1. Core chronology. As a general remark on the section, I would invite the Authors to complete it because it lacks some details in my view. In particular, the Authors should find a way to better show the seasonal pattern of the chosen marker, maybe making lines thinner in Figure 2 and possibly adding a figure with a close-up on a few years. It would be also interesting to read a brief discussion on the stability/loss of H₂O₂ seasonality as depth increases. It cannot be appreciated from Figure 2.

Moreover, the Authors are invited to briefly mention the reasons why they have chosen to use only annual layer counting for the dating without using volcanic signatures of acidity and conductivity, since they have used them to study the spatial variability of volcanic eruptions in section 6.1.

3. Spatial variability

Figure 2 page 6. As mentioned above, Figure 2 is very relevant and necessary to the manuscript but the concentration profiles from all the cores cannot be well appreciated. A simple way to make it all clearer without redrawing completely the figure is to use slightly thinner lines or maybe dashed or dotted lines for one or two cores. Any idea from the Authors in order to make it more readable is welcome.

Lines 13-14 page 7. Is 5 ppb a mean or median or which other reference value? Anyway, one only value as a term of comparison is not sufficient to state that “…no significant recent increase” is observed with respect to the rest of the Holocene. Please, provide a better support to this statement.

Lines 2-4 page 8. More than relative variability (which is lower in the NorthWest than Central and NorthEast – 15% vs. 25%, respectively), absolute values are higher, accordingly with post-depositional processes Authors mention.

Lines 5-6 page 8. Are 2 mS and 5 mS average values? Which is the associated variability? This can be important to know to evaluate if the two values are significantly different.

4. Seasonal cycles

As a general remark for this section and for Figure 3, I don’t find text and figure consistent: Figure 3 displays “formal season” instead of “formal month”. Besides, seasons are reported from the right to the left (if I well interpreted) while it would be easier if they were shown in the opposite direction. I can understand that ice core records go backwards in time but in this case I find it confusing.

Also, I would replace the term “Excess” in Figure 3 with “anomaly” or, at least, would explain it well also in the caption.

A higher definition would be helpful for Figure 3.

Line 30 page 10. It is not clear if the Authors refer to reproducibility here, how it is calculated and how “site specific noise” was evaluated. The issue of “noise” is recurring through the text, rightly so, and it deserves a more detailed discussion.

5. Temporal trends

Line 14 page 11. Again, the reference to “noise” should be made clearer. Do the Authors refer to the whole core or just to the most recent part? Even though median and topical quantiles are reported in Table 2, the calculation of trends and related significance would be important, in my opinion. The possible existence of trend cannot be read immediately from the Table.

6. Extreme events

I would add a mention in the section (for instance after Line 3 page 15) to the fact that other markers different from the ones analysed here can be more specific for detection and assessment of impact of volcanic eruptions (for instance, non-sea salt sulphate) as well for annual layer counting. The Authors could refer to some topical papers in the field, such as Sigl et al. (2016, CP) and Severi et al. (2012, CP).

Line 32 page 18 – lines 1-2 page 19. Since the Authors state (lines 9-11 page 5) that only hydrogen peroxide (with a supportive contribution of calcium) was used for dating, cannot understand now if the dating of A2 and A4 cores was tuned by using ammonium record, in the end, in order to achieve a definitive ice core chronology. It could be reasonable but it deserves a brief discussion since the time scale is basic to go on with further data interpretation. 

Supplementary Material

Figure S1. As mentioned above, please add the detail of the metal coin to the figure, since I have gathered that it is relevant to prevent the by-side effect to “backward sucking” and cannot be appreciated from the figure.

Besides, a slightly higher definition for the figure would be welcome.

Technical corrections 

Abstract

Line 23 page 1. I would replace “contribute” with “ascribe”

Line 29 page 1. English check suggested: “peak ammonium” and “peak volcanic layers” should be corrected.

1. Introduction

Line 8 page 2. English correction: “ammonium peak concentration” should probably be “ammonium concentration maxima” or similar.

Line 12 page 2. Add full stop and the end of the sentence (similar missing punctuation issues all through the text).

Line 15 page 2. English change suggested: maybe “has facilitated” could be replaced by something more apt, such as “allowed obtaining”.

2. Methods

Lines 26-27 page 2. Please check the format of NEEM and EastGRIP site coordinates.

Lines 5 and 6 page 2. Check punctuation: remove an “and” and insert semicolon.

Figure 1 page 3. The labels of the red circles indicating the drill sites overlap one with the other and cannot be read easily.

Table 1 caption, line 7 page 3. The reference is written in a different format from the rest of the text.

Line 6 page 4. In my opinion, “acid” is too vague and not corresponding to what is measured. It should be replaced by another expression, such as “acidic content”, “free acidity” or just “H+” or any other apt wording. This remark holds for the whole paper (e.g. already a few lines later, line 8, again “acid”).

Line 10 page 4. I guess the Authors refer to 8 pieces, each 55 cm long, please correct the expression in brackets.

Line 17 page 4. Please correct ammonium formula using superscript. Check carefully these format issues all through the text.

Line 20 page 4. I would replace “in sufficient resolution” with “with sufficient resolution”.

Line 22 page 4. I would write “it is produced” adding a verb. Otherwise, please rephrase.

Line 27 page 4. “Sufficiently high enough” contains a repetition, I find.

Line 3 page 5. Please use the same shortened name for the same core (e.g. 2015T-A6 or T2015-A6).

Lines 6-11 page 5. There is probably an issue with tense of verbs; please choose past tense (as mostly used in the rest of the text) or present.

Table 2 caption page 7. It is quite peculiar that you use 15^th and 85^th percentile here while you use 16^th and 84^th percentile in Figure 3; I don’t think it changes the result, of course, am just curious to know.

3. Spatial variability

Figure 2 caption page 6. As remarked earlier, I would replace the expression “acid”, here and through all the text.

Table 2 (page 6 and 7). Please, check the format of the analysed parameters (namely superscripts and symbol for “micro”).

Table 2 caption (page 6 and 7). I would add some details for the unit of measurement for dust in the Table or in the caption. Is it “#” referring to the total number of particles or to one particular size range?

Line 10 page 6. They are not “estimates”, actually; I would use the word “measurements”.

Line 11 page 7. “Lower estimate”: what do the Authors mean with it? The minimum value? A small percentile?

Line 15 page 7. Please, add the right symbol (±).

Line 20 page 7. “Counts mL^-1” is an unit of measurement for a signal, not for a concentration, which I find it more correct, to estimate a noise (signal is highly variable among different instruments, also in the case of dust measurements, I believe).

4. Seasonal cycles

Line 5 page 10 (also line 18 page 18). Please add brackets for publishing year for Gfeller et al. (2014).

Line 8 page 10. As above.

5. Temporal trends

Line 19 page 11. The reference does not appear in the Reference list.

Line 21 page 11. Please, correct of format of “micro”, also later in the section

Line 29 page 11. “assuming all spheres were perfectly round”: would rephrase f.i. “assuming all particles are perfectly round”.

Lines 4-5 page 12. Please, rewrite the sentence starting with “Thus”; it appears to be broken.

Line 6 page 12. I would complete the sentence this way: “…parting the data set this way…”

Table 3 page 13. check format (width of the first column, superscript in header of the second column, …)

6. Extreme events

Line 3 page 16 and line 5 page 17. Check format (superscript in km³).

Line 17 and line 31 page 16. Please, do not use the shortened expression “1986 Nov” and similar in the text

Line 5 page 17. After “…eruption signal” the sentence is not clear, please rewrite.

Line 23 page 18. Naming the sites located west of the ice divide would help the reader who is not extremely familiar with Greenland morphology.

Lines 28-29 page 18. Please check the format of p value.

Figure 6 page 19. Dotted lines for the fire records are not well visible.

Line 5 page 19. “>97.5% of full records”: I assume the Authors refer to the 97.5^th of each full record but it would be useful if they report it explicitly.

Line 18 page 20. No capital letter is needed for “levoglucosan”

Line 19 page 20. I believe “high concentration values” or “concentration peaks” are missing in the sentence. Same at line 10 for dehydroabietic acid and line 14 for fire tracers.

Line 13 page 20. NEEM is with capital letters.

Line 21 page 20. I am sure this correlation coefficient (is it R or R², by the way?) is highly significant but the Authors could report the associated significance and the number of data as well.

7. Conclusions

Line 7 page 21. Please correct the symbols of “micro”.

Lines 9-10 page 21. Please, correct the format of publication year for Nagatsuka et al. and Amino et al. Again, the sentence starting with “Thus” appears to be broken, please rephrase.

Data availability

Please check punctuation and core names.

References

Lines 24-26 page 24. This paper should be published now and not on TCD anymore; please, update.

Supplementary Material

Line 3 page 3. Please correct format for hydrogen peroxide (subscripts)

Review of manuscript cp-2021-99:

“NEEM to EastGRIP Traverse - spatial variability, seasonality, extreme events and trends in common ice core proxies over the past decades” by Helle Astrid Kjær et al.

This study presents new impurity data from six firn cores taken along a spatial transect in North Greenland and measured with a Continuous Flow Analysis system. The data are investigated in regard to their mean seasonal cycle, their temporal trends and their usability for indicating volcanic events and as a forest fire proxy. While the results could be of interest to the ice core and proxy community and hence to the Climate of the Past readers, I rate this paper to not sufficiently well communicate the novelty of the results and its overall presentation quality to be poor. In brief, I recommend the paper to be rejected.

Major comments

Overall structure and writing. In my opinion, the text seems carelessly assembled and is poorly written. Many parts and sections lack a clear structure, most notably the abstract and the various results sections. Especially regarding the latter, there is no clear distinction between the presentation of the study’s results and their discussion. While the journal offers the possibility of a combined results and discussion section, I find it unusual that the authors chose to start many sections with some kind of short literature review before they actually present their own new results. Additionally, results are very often stated or mentioned without any clear reference to a figure or table, which makes it difficult for the reader to retrace and verify a particular result. Regarding the writing, the text suffers from frequent grammatical mistakes and “orphan sentences” which lack a subject or syntactically just peter out.

Figure quality. The figure quality is very poor overall. The resolution is too low, making the graphics grainy already at standard zoom, the labeling is faint and too small, and the line plots are thin and are using color scales that are very hard to distinguish and are even indistinguishable for color-blind people. I would strongly recommend the authors to study how to produce higher-quality graphics from the computer program in use, either by using scalable vector graphics or by using a sufficiently high dpi value for raster graphics. In addition, color scales which are legible for color-blind people and sufficiently distinct both for on-screen viewing as well as printing can be looked up on resources such as https://colorbrewer2.org.

Local deposition noise. Local deposition noise, and also the noise from intermittent precipitation, is an important issue but not treated appropriately in this study. It is either mentioned somewhat unmotivated, as is the case for example on P10 Line 29, or only briefly referred to at instances scattered throughout the text. In the recent years quite some literature was published on these topics, both for Greenland and Antarctica, which could be used to put the current data into context. While the available data might be a bit limited for this purpose, one could perform at least some statistical investigations, e.g., looking at the correlations between profiles at seasonal and annual resolution to see if there is any common signal among the cores along the traverse, depending on the impurity species.

Trends (Section 5). In general, it is very difficult to follow from where you derive your results and conclusions about the various trends. Overall, the paper would benefit from showing additionally a plot with the annual mean time series for the individual impurities, maybe even showing only stacked annual mean time series from averaging across the firn cores in favor of a clear presentation. Then, clearly stating the results from linear regressions on the data, including slope uncertainty and p values, might help creating a concise picture on the overall trends. In case of insoluble dust fluxes, you do use annual mean time series, but for unknown reasons they are relegated to the supplement, and the trend results mentioned in the text are hard to verify by looking at Fig. S3. Maybe a logarithmic y axis scaling and adding the trend lines to the plot might help here.

Section 6. Overall, these sections are overly detailed, making it hard for the reader to grasp the main conclusions you want to convey here. One idea could be to put all of the results concerning the determined extreme and volcanic events, and the possible sources thereof, into a table, maybe also giving some indication for how certain you can be on relating a specific event in the records to a known eruption or other source. Then, the text could be significantly shortened to concisely present the main findings and conclusions from this table, which could make it much clearer for the reader how the new data can possibly advance our knowledge on the mentioned topics.

Minor comments

General. Frequently, the term “excess” is used to refer to specific data series, however, what this terminology means is nowhere explained. This is problematic since it is firstly not a common terminology for data series, and secondly it might be confused with the quantity of “deuterium excess” commonly measured on firn and ice cores. From what I understand, your usage of “excess” refers to either the deviations from the mean of the seasonal cycle data (Fig. 3), which more commonly would be referred to as “anomalies”, or to the residuals after subtracting a five-year running mean from the data series (e.g., Fig. 4). I would suggest to adopt a more appropriate terminology or to clearly define your usage of the term “excess” in the Methods.

Title. I find the title too long and too general, merely listing key words rather than naming the key essence of the paper. In addition, the title should not have a full stop (I am referring to the pdf version here).

Abstract. In my opinion, the abstract could be significantly shortened to convey only a brief introduction as well as the key messages and results of the study. There are several unnecessary filler sentences, e.g., “The temporal variability of the records is further assessed”, “By creating a composite based on excess ammonium compared to the five year running average…” etc.

P2L17. Maybe here a word about possible complications with CFA measurements is appropriate, such as the intrinsic diffusion-like smoothing of the CFA system.

 P2L19-23. I find the here-stated motivation for the paper a bit vague, e.g., “constraining proxies analysed by means of CFA” could be understood in a technical sense from a measurement quality point of view, which is I guess not what you have in mind. Could you elaborate more precisely on the main aims of the study?

P4L6. I guess by “acid” you refer to the H⁺ measurements here, which is, however, unclear at this point, since you use one or the other term throughout the text, and it is also a bit misleading, since in normal language acid could mean any kind of acid (I guess you refer to the Brønsted–Lowry acid definition here?). Please choose one terminology, introduce it here and then use it consistently throughout the text. The same goes for the other species, which you alternatingly refer to either by their chemical composition (e.g., NH₄⁺) or by the common name (i.e., ammonium). The text would be much easier to follow if you sticked to one option throughout.

P5L1-11. This method description is hard to follow and seems incomplete. What I understand you do in essence is seasonal layer counting to derive an age-depth relationship for your cores, for which you use the peroxide mainly, and additionally calcium, if the former has not good enough quality. What remains a bit unclear is how you derive the age scale in Fig. 2; I guess you use the age-depth relationship from the peroxide peaks to interpolate your depth series into a time series using the constant accumulation assumption stated in the second paragraph. However, this is not entirely clear since you mention the equal accumulation assumption and formal month definition only in relation to “investigating the seasonality” (Fig. 3). In addition, from the caption of Table 2 it seems that you block-average your depth series data into monthly means following the formal month definition; is that correct? If so, it should be mentioned here.

P5L6-9. But could you maybe give an educated guess for how far off you might be with the constant accumulation assumption from the actual seasonal accumulation variations?

P5L13. “profiles”: If I understand your methods correctly, Fig. 2 actually shows monthly mean time series for the individual impurity species and cores. This should be explicitly mentioned/repeated here to ease understanding and to avoid confusion with the original depth series.

P7L5. Do you mean the interannual variability here? Where do I see that the variability is large, and what do you mean with the “concentration variability between sites is masked”?

P7L19-20. How do you derive that conclusion, based on the values in Table 2? Did you perform any statistical test to check whether the null hypothesis of identical mean and/or distribution cannot be rejected?

P8L2-4. Speaking of spatial variability here is misleading, since variability is more commonly understood to mean random variations. I could imagine what you instead observe here is a spatial gradient in concentration due to a gradient in accumulation.

P8L13. Rather use “average seasonality”, “average seasonal cycle” or “climatology” for describing these results.

P10L3-4. “The variability is high and unevenly distributed” – again, where can I see this? Can you quantify it, i.e., it is high relative to what? What means unevenly distributed?

P10L14. It is unclear to which species you refer to here. What means “high deviations in adjacent months”? Isn’t that in general the case for a seasonal cycle?

Technical comments (by far not exhaustive)

Throughout text. The core names are inconsistently labelled either T2015-A1 or 2015T-A1, and so on. Please use one consistent nomenclature.

P1L24. Change “70’s” to “1970s” (more similar instances throughout the text).

P1L25-26. The sentence “After detrending using…” is difficult to understand and should be rephrased.

P2L3. “intricate”: I would avoid such an evaluative adjective in a scientific text.

P2L11. Should be changed to “at the deposition site”.

P2L5 and L12. Please note the hyphenation needed in phrases such as “large-scale atmospheric circulation patterns” or “high-resolution climatic signals”. This is a frequent mistake needing correction throughout the text.

P2L18. “sample decontamination”: To my understanding this means cleaning from toxic components or from radioactive radiation. Is this what you actually mean here?

P2L25. “Neem” should be “NEEM”; “May to June” of which year do you mean? You also should explain the various site acronyms at some point in the manuscript (preferably at their first respective instance).

P2L26,27. Please format the site coordinates correctly. Besides, I would welcome giving the coordinates in decimal degrees, since that is easier to handle in a numerical context.

P2L31. A comma is missing between “Greenland” and “and then shipped”.

P3L2. Please rephrase to “prior to the CFA measurements”.

Fig. 1 caption. Please mention the information relevant to the study first, i.e., first the firn cores, then afterwards the information on the surface elevation data set.

Table 1 caption. The column of core depths is not mentioned in the caption. Additionally, you write that the core labels go till “2015T-A5”, but there is another core (“2015T-A6”) listed in the table.

P4L4. “in 2017”: this could be mistaken to mean that you only measured the impurity content for the year 2017; I guess instead you mean the CFA measurements took place in 2017; please rephrase.

P4L5. Add a comma before “by adding”.

P4L8. Please change to “were converted into units of concentration”.

P4L9-10.

• Do you mean “A baseline was established”?
• You should explain what “milliq water” means; not every reader might be familiar with the laboratory terminologies.
• What is “8eight 55 cm pieces stacked” supposed to mean?
• “Although” is not the correct wording here; I guess you mean something around “In general the baseline was established by… However, for the top 1.65 metres, where the core was fragile […], the baseline was established…”. Please clarify.

P4L13/15. The firn cannot “suck anything”. Melt water can flow or percolate into the firn driven by capillary forces; please use the correct physical terminology.

P4L15. I don’t understand how excess water can be limited to an amount of 0.5-1 cm; what does this unit mean here? Please bear in mind that not every reader might have worked with a CFA system him- or herself.

P4L18. “response time”: Again, a reader not familiar with the CFA technique will have problems understanding this; what do you mean by response time and how does this affect the effective depth resolution?

P4L20. Please change to “at a sufficient resolution”.

P4L21. Do you mean “which are used to constrain…”?

P4L22-22. Please change to “as it is produced by a photochemically-derived”.

P4L27. Please change to “this exchange can cause smoothing”.

P5L3. Please change “invoked” to “used”.

P5L9-11. Why not? If you mention this explicitly here then you should give a reason for not doing it.

Figure 3. It is rather counterintuitive to display the formal months in the reversed temporal order summer – spring – winter – autumn.

P20 L27. “dissolves” is the wrong wording, please use “resolves” instead.

P20L29. As mentioned earlier, you mix up spatial variability (random variations) with spatial gradients or spatial variations. Please be careful to use the appropriate wording throughout the text.

P21L4. Please change to “We thus highlight” and to “of using the same methods”.

P21L6. Please change to “in the acid and conductivity profiles”.

P21L7. Please change to “an increase over time, especially for the large …”.

P21L9-11. Please change the reference to the standard format; the final sentence is grammatically wrong.