the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2023
The marine methane cycle in the Canadian Arctic Archipelago during summer
Abstract. In the Arctic Ocean region, methane (CH4) concentrations are higher than the global average, with particularly high concentrations of dissolved CH4 observed along many subarctic and Arctic continental shelf margins. Despite this, the Arctic Ocean emits only minimal methane fluxes to the atmosphere across the air-sea interface, suggesting that water column oxidation of methane may be an important process.
In this study, we paired thermohaline, chemical, and biological data collected during the Northwest Passage Project transit through the Canadian Arctic Archipelago (CAA) waters in the summer of 2019 with in-situ and in-vitro methane data. Our findings suggested that the most elevated in-situ concentration of dissolved methane was present in the near-surface waters of the Pacific, particularly in meltwater regions. The highest methane concentrations were observed within shallow waters, averaging at 5.8 ± 2.5 nM within the upper 30 m depth. Furthermore, the methane distribution showed a distinct pattern from east to west, with higher concentrations and oxidation rate potential in the western region. In our study, we observed generally low methane oxidation rate constants, averaging at 0.006 ± 0.002 d−1. However, surface waters from Wellington Channel and Croker Bay exhibited relatively higher methane oxidation rates, averaging at 0.01 ± 0.0004 d−1. These regions were distinguished by a significant proportion of meltwater, including both meteoric water and sea ice meltwater, mixed with water of Pacific origin. We identified microbial taxa of Pacific-origin likely associated with methane oxidation, including Oleispira (γ-proteobacteria) and Aurantivirga (Flavobacteria), in the Pacific and meteoric waters. In contrast, deeper layers (> 200 m depth) showed lower methane concentrations (av. 3.1 ± 1.1 nM) and lower methane oxidation rate constants (av. 0.005 ± 0.001 d−1). Within the sea ice, dissolved methane concentrations were found to be higher than the concentrations at equilibrium with atmospheric capacity, with an average of [CH4] = 9.2 ± 5 nM. The sea ice temperature data (Table S2) indicated the presence of ice permeability, which likely facilitated the release of dissolved methane that was either trapped or produced since the previous freezing period. Notably, methane concentrations were 25 % higher in waters collected in the western CAA in comparison to the ice-free waters (eq. S1).
The overall picture suggested supersaturation of in-situ methane in shallow waters, coupled with faster oxidation rates in meltwater and Pacific dominant layers, suggesting rapid seasonal cycling of methane and prevention of the methane migration into the atmosphere.
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RC1: 'Comment on bg-2023-157', Anonymous Referee #1, 25 Oct 2023
I recognize the amount of work performed that led to this manuscript and the efforts of the authors to answer the comments. However, they answered the reviews quickly after the first reviews, mentioning future improvements, but many were not provided. It would have been wiser to answer reviewers' comments once the improvements were done so the authors could refer their answers to the text. These improvements promised but not done include:
- Ranges of temperature, salinity and nutrients were not given in the introduction.
- The distinction between the experimental and discrete samples is still not clearly explained
- The method lacks the duplicate, meaning that it is still unclear where the 56 number (28 x 2) is coming from. Also, the authors mention a leaking sample yet the 56 samples remain.
- The explanation of the 9 points instead of 18 in figure 10 is still missing.
- The authors answered my comment regarding the missing transect on the figure by saying that they only show the stations where methane was investigated. My question remains: where is the transect that the authors mention?
I wish a detailed revision was provided with a response to each question or comment. Here are my comments per section:
Abstract:
Lines 20-23: make it one sentence.
Only in the abstract is shown the maximum averaged methane concentration in the first 30m.
The abstract should be void of any reference to figure, table, or equation.
Introduction:
Line 52-53: wrong references: Steinle et al and Ferré et al do show elevated methane concentrations but don’t mention air-sea interface. Also, instead of showing limited gas exchanges to the atmosphere, Shakhova et al demonstrate the exact opposite.
Line 54-57: Please explain how methane concentration in the water column can inform on past processes. Yamamoto et al., 1976 don’t mention anything regarding the stability or persistence of methane, and their reference is missing.
Line 83: The authors mention the uniqueness of the study in the comparison
Figure 1: typo in the y-axis (latitude degrees W). Both panels in this figure are hard to see, but I assume the quality will be higher. The range of bathymetry in the right panel could also highlight more contrasts at the seafloor. The text mentions a maximum of 2000m depth but the color scale goes down to 10 000m depth. At least, the isobaths should be labeled.
Line 112: the IPCC report should be referred to.
Lines 121 and 125: I asked in my previous review to give a range. “Low” or “warm” are relative terms.
Line 142: like I asked in my previous review, where is this transect? If it includes all the “single points”, then mention it or point it out on the figure. However, this sentence is strange because transects consist in single points. Consider removing the sentence.
Line 153: already pointed out in my first review: explain experimental and discrete.
Line 233: replace “1 to 2.5 mL” by “1 or 2.5 mL”
Line 236: consider merging Tables S3 and S4. Table S4 is also sited before Table S3.
Line 253: the text presents the Spearman’s correlation and refers to figure 2, whereas figure 2 presents the linear correlation coefficient. Figure 2 is also surprising, as one would expect the linear trend to be on the other side of the 1:1 regression line. I couldn’t reproduce an of the R2 or Spearman’s correlation.
Results:
Line 450: this reads like the average methane concentration is 3.4 nM below 100m at WNBI, which is not shown in figure S4.
Line 534: like in 1st review, there is a discrepancy between the text (isotopic value of -51.4) and figure 8 (>-47.8). This minimum value is only shown in table S2. Consider showing the extreme values (or all) in figure 8 to avoid this discrepancy.
Part 4.1: consider referring to Silyakova et al. 2022 who showed that winter storms can break out the ice and allow methane to be released into the atmosphere. (Silyakova, A., D. Nomura, M. Kotovitch, A. Fransson, B. Delille, M. Chierici and M. A. Granskog (2022). "Methane release from open leads and new ice following an Arctic winter storm event." Polar Science.)
Line 605: in-situ dissolved methane
Line 607: study area
Line 609: the chl-a data are not shown
Line 627: wrong choice of publications. Damm, Graves and Silyakova don’t mention a link between high turbidity and methane excess. In fact, I don’t know any publication showing this except in hydrothermal vents. A turbidity of 0.005 ntu is low.
Figure 11: please explain where the sea ice concentration data is coming from, and indicate whether this is representative of the time when the survey occurred. It is not clear if this is satellite-derived from Spreen et al.
Line 698: again, the calculation from table 2 shows 0.005
Line 715: replace thesis with theory
Conclusion:
Line 731: see comment above regarding turbidity
Line 741: see comment above regarding winter storms
Citation: https://doi.org/10.5194/bg-2023-157-RC1 -
AC1: 'Reply on RC1', Alessandra D'Angelo, 24 Nov 2023
AC: We would like to express our sincere appreciation for the thorough review of our manuscript. We have carefully considered each of your recommendations and we are committed to incorporating them into the manuscript.
RC1:
I recognize the amount of work performed that led to this manuscript and the efforts of the authors to answer the comments. However, they answered the reviews quickly after the first reviews, mentioning future improvements, but many were not provided. It would have been wiser to answer reviewers' comments once the improvements were done so the authors could refer their answers to the text.
AC: As per the journal's policy promoting open discussion, it is highly recommend providing timely feedback to facilitate an open and constructive dialogue. Consequently, we have crafted our responses within the recommended timeframe.
We wish to underscore that the revised manuscript aligns closely with the revisions requested during the initial round of reviews. However, it incorporates several newly generated sections, making it distinct from the initial version. Therefore, it should not be anticipated to be identical to the original submission.
These improvements promised but not done include:
Ranges of temperature, salinity and nutrients were not given in the introduction.
AC: After careful consideration, we have determined that, in the broader context of the study area description, it is not essential to present indicative values of temperature, salinity, nutrients, and oxygen saturation. Instead, we have appropriately cited the existing literature where all these values are comprehensively reported (see lines 119-126).
The distinction between the experimental and discrete samples is still not clearly explained
AC: We have incorporated several sentences to provide clarity regarding the distinction between discrete (in-situ) and experimental (in-vitro) samples. These explanations can be found in various sections of the manuscript, such as lines 54 - 62 and 159 – 162. Furthermore, in Chapter 2, "Materials and Methods," we have separated the descriptions for in-situ and in-vitro samples, detailing their processing and analysis independently to prevent any potential confusion. We have also restructured the paragraphs throughout the text to ensure a clear differentiation between the in-situ and in-vitro samples.
In the upcoming revised version, we will make extra efforts to enhance this distinction for the reader's benefit.
The method lacks the duplicate, meaning that it is still unclear where the 56 number (28 x 2) is coming from. Also, the authors mention a leaking sample yet the 56 samples remain.
AC: In line 53 and Table S1, we have provided a description of the total number of samples collected for the methane in-vitro experiments, stating, "A total of 132 seawater samples (28x2 experimental and 76 discrete)." The term "28x2" is used to denote the duplicates, as further explained in the caption of Table S1. It's important to note that one of the samples experienced a leakage issue during the final measurement, and as a result, we did not include this particular sample in the final calculation of the kox. However, we retained this sample in Table S1 to maintain a record of all the samples processed.
We recognize that this may potentially lead to confusion, and in the forthcoming revised version, we are committed to removing this sample from the table and text to ensure clarity and accuracy.
The explanation of the 9 points instead of 18 in figure 10 is still missing.
AC: In the caption of Figure 10, we have clarified that the values presented in the plot were obtained by averaging the kox values by transect and depth, with the exclusion of non-methane metabolism data. Based on the information presented and referencing Table 2, we can observe that we have 2 data points for CB, 4 data points for WNBI (including the two data points at 70m depth), 1 data point for WC, and 2 data points for JS.
We acknowledge that the caption may require further refinement for clarity, and in the forthcoming revised version, we will include the provided information to improve its clarity and readability.
The authors answered my comment regarding the missing transect on the figure by saying that they only show the stations where methane was investigated. My question remains: where is the transect that the authors mention?
AC: Among the sampling stations, we conducted investigations at two specific transects during our campaign, as visually depicted in Fig. 1, namely West Navy Board Inlet and Wellington Channel. In the interest of clarity and ease of reference, we employed the term "transect" in Fig. 10 to denote the individual sampling sites, as our calculations involved averaging kox values by site and depth.
I wish a detailed revision was provided with a response to each question or comment. Here are my comments per section:Abstract:
Lines 20-23: make it one sentence.
AC: We will edit it.
Only in the abstract is shown the maximum averaged methane concentration in the first 30m.
AC: We will incorporate this information into the Results section.
The abstract should be void of any reference to figure, table, or equation.
AC: In response to a previous comment highlighting the absence of the simple calculation (lines 25-26) in the manuscript, we included a reference to the equation in the abstract to address that specific concern. We appreciate your guidance and would be happy to make further adjustments based on your recommendations.Introduction:
Line 52-53: wrong references: Steinle et al and Ferré et al do show elevated methane concentrations but don’t mention air-sea interface. Also, instead of showing limited gas exchanges to the atmosphere, Shakhova et al demonstrate the exact opposite.
AC: We have incorporated all the references related to the processes mentioned in lines 50-52 at the end of the sentence. To eliminate any potential confusion, we will categorize each reference according to the specific processes described in that sentence.Line 54-57: Please explain how methane concentration in the water column can inform on past processes. Yamamoto et al., 1976 don’t mention anything regarding the stability or persistence of methane, and their reference is missing.
AC: Yamamoto et al. (1976) provided a comprehensive account of methane solubility in seawater across varying salinities and temperatures. The stability of methane within the water column is intrinsically linked to its solubility in seawater. This inherent quality allows methane to remain in a dissolved and stable state for an extended duration before undergoing subsequent processes, such as microbial oxidation or physical transport. Consequently, gas concentrations provide insights into historical processes. In summary, the citation of Yamamoto et al. (1976) serves as a valuable indirect endorsement of the concept of methane's stability in the water column.
Line 83: The authors mention the uniqueness of the study in the comparison
AC: Yes, the distinctiveness of this study is rooted in its examination of paired datasets encompassing dissolved methane, potential oxidation rates, marine microbial communities, and the physical-chemical characteristics of water masses in the CAA.
Figure 1: typo in the y-axis (latitude degrees W). Both panels in this figure are hard to see, but I assume the quality will be higher. The range of bathymetry in the right panel could also highlight more contrasts at the seafloor. The text mentions a maximum of 2000m depth but the color scale goes down to 10 000m depth. At least, the isobaths should be labeled.
AC: Regarding the y-axis, we have noted the label "Latitude (˚N)." We are committed to enhancing the quality of the figure, and we sincerely appreciate your suggestions.
Line 112: the IPCC report should be referred to.
AC: Thank you for your suggestion. We will incorporate the reference.
Lines 121 and 125: I asked in my previous review to give a range. “Low” or “warm” are relative terms.
AC: As aforementioned, we have determined that in the broader context of the study area description it is not essential to present indicative values of temperature, salinity, nutrients, and oxygen saturation. Instead, we have appropriately cited the existing literature where all these values are comprehensively reported (see lines 119-126).
Line 142: like I asked in my previous review, where is this transect? If it includes all the “single points”, then mention it or point it out on the figure. However, this sentence is strange because transects consist in single points. Consider removing the sentence.
AC: In this sentence, we are referring to the two investigated transects, namely WC and WNBI.
Line 153: already pointed out in my first review: explain experimental and discrete.
AC: If the use of those terms causes confusion, we will replace them with "in-situ" and "in-vitro," consistent with the terminology used throughout the rest of the text.
Line 233: replace “1 to 2.5 mL” by “1 or 2.5 mL”
AC: We will make the necessary edit.
Line 236: consider merging Tables S3 and S4. Table S4 is also sited before Table S3.
AC: We appreciate your suggestion, and we will proceed with making this edit.
Line 253: the text presents the Spearman’s correlation and refers to figure 2, whereas figure 2 presents the linear correlation coefficient. Figure 2 is also surprising, as one would expect the linear trend to be on the other side of the 1:1 regression line. I couldn’t reproduce an of the R2 or Spearman’s correlation.
AC: Indeed, in the text, we presented the Spearman’s correlation coefficient value, while in the plot, we exhibited the linear model. Both of these coefficients were computed by considering all values of kox.mass.balance and kox.isotope.ratio. We were not surprised by the result and good fit of the data, as the study by Uhlig and Loose (2017) had already assessed the low uncertainty of this method.
Results:
Line 450: this reads like the average methane concentration is 3.4 nM below 100m at WNBI, which is not shown in figure S4.
AC: Yes, we indicated the average [CH4] below 100m in WNBI as 3.4 nM, with Figure S4 illustrating the individual profiles of [CH4] for each site, from which this result is derived. In the text, we make reference to Figure S4 to emphasize the methane enrichment in deep water.Line 534: like in 1st review, there is a discrepancy between the text (isotopic value of -51.4) and figure 8 (>-47.8). This minimum value is only shown in table S2. Consider showing the extreme values (or all) in figure 8 to avoid this discrepancy.
AC: In Figure 8, we exclusively presented the two samples that exhibited methane depletion (core 1 and core 2). However, the sentence in line 534 pertains to all sea ice methane data, as documented in table S2.
We appreciate your feedback, and in the revised version of the manuscript, we will make the reference to table S2 clearer in relation to all isotope ratio data.
Part 4.1: consider referring to Silyakova et al. 2022 who showed that winter storms can break out the ice and allow methane to be released into the atmosphere. (Silyakova, A., D. Nomura, M. Kotovitch, A. Fransson, B. Delille, M. Chierici and M. A. Granskog (2022). "Methane release from open leads and new ice following an Arctic winter storm event." Polar Science.)
AC: Thank you for the suggestion. We will incorporate the recent paper into our references and aligning our discussion to ensure that it is well-informed by this new data.
Line 605: in-situ dissolved methane
AC: we will edit it
Line 607: study area
AC: we will edit it.
Line 609: the chl-a data are not shown
AC: We did not display the chl-a fluorescence data; nevertheless, the chl-a fluorescence data was incorporated in the Spearman's rank calculation presented in Figures 8 and 9. In line 609, our intent was solely to emphasize that the highest dissolved methane concentrations coincided with elevated chlorophyll-a fluorescence data, as referenced throughout the text via https://doi.org/10.18739/A2BN9X45M.
Line 627: wrong choice of publications. Damm, Graves and Silyakova don’t mention a link between high turbidity and methane excess. In fact, I don’t know any publication showing this except in hydrothermal vents. A turbidity of 0.005 ntu is low.
AC: We included these specific references in our manuscript despite their lack of direct mention of a connection between high turbidity and methane excess. Our rationale for doing so was to provide fundamental background information on the processes contributing to methane excess in the deep-water column and seabed. We will revise the text to enhance clarity in this regard.
Regarding the relatively high turbidity value of 0.005 NTU that we reported, we understand that it may seem low compared to some other environments, such as hydrothermal vents. However, it's important to note that in the context of our study and the specific depths we investigated, the turbidity values below 100m averaged 0.009 NTU. In this context, 0.005 NTU can be considered relatively high.
Figure 11: please explain where the sea ice concentration data is coming from, and indicate whether this is representative of the time when the survey occurred. It is not clear if this is satellite-derived from Spreen et al.
AC: Yes, the sea ice concentration data was obtained from the University of Bremen data archive, and we specifically downloaded the data corresponding to our sampling dates.
We will ensure to include this information in the caption as suggested, and we appreciate your guidance.
Line 698: again, the calculation from table 2 shows 0.005
AC: Thank you for pointing this out. Yes, it's because Table 2 includes kox=0 (non-methane metabolism), which affects the calculated average. In line 699 we specified in parenthesis “excluding non-methane metabolism”.
Line 715: replace thesis with theory
AC: We will edit it.
Conclusion:Line 731: see comment above regarding turbidity
AC: See above.
Line 741: see comment above regarding winter storms
AC: We will update the text accordingly, as explained above.
Citation: https://doi.org/10.5194/bg-2023-157-AC1
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RC2: 'Comment on bg-2023-157', Anonymous Referee #2, 27 Oct 2023
Review: The marine methane cycle in the Canadian Arctic Archipelago during summer.
I can see the effort behind this study from sample acquisition, analyses, evaluations and last but not least the writing of such a manuscript, which is well written in terms of writing style and structure. However, I see big gaps in the data set and issues in the way data are presented and interpreted.
GENERAL COMMENTS
The term CAA
This study presents results from a set of samples collected at 12 seawater and 5 sea ice stations along the eastern part of the Parry Channel and adjacent Sounds. This low number of stations does not represent the entire CAA and therefore the term should be adjusted to e.g. “Central Canadian Arctic Archipelago” and “Eastern part of the Parry Channel” or similar.
Propositions like this “In the waters of the Canadian Arctic Archipelago, dissolved methane showed the strongest variations vertically in the water column, [..]” (L577) are not accurate, because one cannot extrapolate an assumption made on the basis of a very limited data set to a field as broad and very diverse as that of CAA.
Abbreviations
only use them when they are needed like in tables or Figures or if they are used really a lot e.g. CAA., but AO for Arctic Ocean and SWM for source water masses, is only used 5 times in the manuscript, so please write it out. Using PW, AW, ANP, SIM and similar in tables and equations is good, but Throughout the manuscript, I would appreciate to read the full wording, otherwise it gets too complicated to remember all the different meanings for the many Abbreviations.
The use of PW, AW, ANP, SIM, and the like in tables and equations is fine, but throughout the manuscript I would appreciate reading the full text, otherwise it becomes too complicated to remember all the different meanings for the many abbreviations. I am not a water mass specialist; acronyms like SIM, MW, or SWM are not natural to me.
L120: remove PML since it is not used elsewhere in the manuscript. Abbreviations like SPRS, URI, GSO, UIC are irrelevant.
Design of the study
How come the sampling scheme for seawater and ice cores does not follow a uniform concept for the depth profiles? In some cores the first sample was taken at 10 cm, in others at 20 cm. The same discontinuity applies to the greater depths of the cores. Similarly, the surface samples of seawater were taken at 1, 2, 5, or 7 m below the sea surface, followed by a wild combination of depths below. Is there a logical explanation for this? How can we ensure that data from different sites are comparable?
Why did you decide to work with duplicate? In order to obtain statistically evaluable data, I would always recommend measuring at least triplicates, especially for samples where low methane concentrations or low methane oxidation rates are expected.
Microbial Community
The manuscript does not contain any information on how the sequencing dataset was processed.
In Chapter 2, I miss the description of sequencing data analysis such as initial processing, quality filtering, assembly, clustering, classification, statistical evaluation. It is unclear what databases and thresholds were used to identify OTUs. I also wonder how it is that the occurrence of chloroplast genomes is highlighted (L512); usually sequences classified as chloroplasts are removed from the dataset early before any kind of statistics are performed.
In Section 3.3, I cannot find any quantitative or qualitative data on the microbial community described in this manuscript, such as the percentage of each group in each sample or any type of distribution of microbes along the water column or sample area. No figures or tables visualizing bacterial community structures, such as relative abundances, hierarchical clusters, or correlation with methane concentrations and oxidation rates, which would be beneficial to the MS.
L512: “In summary, our dataset highlights the occurrence of Chloroplast genomes, Oleispira, Planctomarina, and Aurantivirga in samples showing potential methane oxidation, consistent with the findings of Uhlig et al. (2018) and Gründger et al. (2021).”
There are two problems with this sentence.
First, it suggests that the two publications given as references for this statement also show a high occurrence of chloroplast genomes (whatever that means, given the lack of data), which is not the case and is simply wrong. In both Uhlig et al. and Gründger et al. the sequences identified as chloroplasts were removed from the sequence dataset before the reads were assigned to OTUs.
Second, both publications including SIs do not confirm the presence of Planctomarina. I can only find affiliations with Planctomycetes (Uhlig) or Planctomycetes and Planctomycetaceae (Gründger).
As mentioned earlier, the section on microbial community composition is extremely sloppy and lacks any kind of baseline data. This is especially surprising since Brice Loose was also the last author to contribute to the Uhlig publication in 2018 and therefore should know what this publication says and what it does not say and what a good study linking oxidation rates and microbial composition might look like.
In many places in the manuscript (except in the Discussion), it is not clear what is meant by the term "microbial methane metabolism" (e.g. L558, 562, 569, 590) - it could be methane production (methanogenesis) and/or methane consumption (methane oxidation). Be more precise in the wording.
References
PLEASE, make sure your reference list is up to BG standards. Seriously, this is a resubmission and the references are still not up to the standards e.g. L794 the Boetius paper misses the journal. There is no consistent format apparent.
DETAILED COMMENTS
L34, 37: References to SI do not belong in an abstract.
L54: I do not understand this sentence. What “information about past processes” are meant here?
L80: The combination of methane data, oxidation rate measurements and water column properties is not unique.
L86: I cannot find the Parry Channel on the maps in Fig.1.
Fig.1: Left panel – the figure is too small and the study area is not recognizable. The figure should be the same size as the one to the right. The depth legend and the place names are extremely difficult to read. I suggest placing the legend outside the globe, increasing the font size of the place names, and submitting a higher resolution graphic.
Right panel - In general, the quality of this map is very poor. I have trouble reading place names in white text with a grayish shadow on a light blue background - I can't identify the westernmost Sound (I assume the Viscount Melville Sound is meant, but the "Viscount" got lost), because the text is above the red dots of the sea ice sampling stations. Make sure the written and dots do not overlap on the map! Please make the sampling station numbers legible and make sure that each dot can be clearly assigned to the correct station name. What does the legend mean? I assume it is depth/heights in meters. Use a more descriptive legend; choose a range that makes sense for the map you are showing - I can't find depths greater than -5000 m or heights greater than 5000 m.
L93: Change “August 2019, with seawater sampling stations […]”.
L118: Since this is a new section of text, you could give a hint about the region. Like in the previous chapter, which was about the Arctic Ocean. What is this chapter about? Also about the Arctic Ocean? The Parry Channel?
L136: Barrow East is missing in the list of sampling locations. Is there a reason why it is not listed?
L140: The term “in the vicinity of Parry Channel” is a bit too unspecific. Especially since you only took samples from the eastern part of the Parry Channel anyway.
L142: I do not see any transects in Fig.1, there is only a string of single points.
L145: Remove “owned and”. The information of ownership is irrelevant for the study. Omit “(SPRS)” since this abbreviation is not used further on.
L197: “room temperature” is a general term for what people prefer for indoor settings and is around 20–22 °C. Since your room was colder, I suggest to give the 10 °C only.
L240-242: I do not understand the meaning of this sentence.
L261: “each sample by adding 0.1M NaOH and injecting it into the sample after the final measurement.” – should a killed control not being killed at the beginning of the measurement?
Fig. S1: Is there a specific order in which the results for each station are displayed in c and d? Why don't you arrange the stations in the order of longitude, as in Fig. 4, then it is easier to compare the data.
Tab.2: It is unclear whether the depth data listed under station WNBI are from one cast with 6 sampling horizons or from multiple casts with fewer horizons at different depths. I can only assume that WNBI refers to three individual stations according to the map in Fig. 1. However, stations 1, 2, and 3 are not clearly assignable to the depths listed in the table. It may be that this is not important to the overall story, but I think that good presentation of the data is important to avoid irritation and confusion.
Also, make sure to use the terms location, site, and station consistently.
Fig.10: To perform an interpolation of methane oxidation rates (consisting of three measurement points according to Fig.10) over a distance of 500 km, then make statements about the extent of methane oxidation activity in the entire water column down to 200-300m water depth is borderline. Unfortunately, the interpolation algorithm is not described anywhere. I do not claim that the data are wrong, but the figure gives the impression that the whole water column in the western part of the study area is characterized by high oxidation rates, which definitely cannot be proven by data.
L609: Where are Chlorophyll-a data coming from?
L670: “it is likely that dissolved methane in the CAA waters during the summer is primarily driven by microbial metabolism, […]” what does it mean? The presence, the concentration, the distribution,…? What is driven?
L732: “[…] likely following one of the paths explained by Repeta et al. (2016) and Sosa et al. (2020).” – Could you please specify which paths you are referring to, as this is the conclusion and the statements should be clearly stated?
L736: “The community structure likely responsible for the methane oxidation was characterized by three main groups that have been recently associated with the methane metabolism” – the microbial groups might be somehow related to methane oxidation, but I doubt they are "likely responsible". Since no abundance data are given, I can only speculate, but usually methane oxidizers do not show up as dominant OTUs in 16S sequencing data. To be able to describe a MOB community, sequencing should be done with specific primers targeting genes involved in MOx processes.
Citation: https://doi.org/10.5194/bg-2023-157-RC2 -
AC2: 'Reply on RC2', Alessandra D'Angelo, 24 Nov 2023
AC: Thank you sincerely for your thoughtful review of our manuscript. We are committed to incorporating your suggestions to improve the clarity and overall quality of the paper.
RC2:
I can see the effort behind this study from sample acquisition, analyses, evaluations and last but not least the writing of such a manuscript, which is well written in terms of writing style and structure. However, I see big gaps in the data set and issues in the way data are presented and interpreted.
GENERAL COMMENTS
The term CAA
This study presents results from a set of samples collected at 12 seawater and 5 sea ice stations along the eastern part of the Parry Channel and adjacent Sounds. This low number of stations does not represent the entire CAA and therefore the term should be adjusted to e.g. “Central Canadian Arctic Archipelago” and “Eastern part of the Parry Channel” or similar.
Propositions like this “In the waters of the Canadian Arctic Archipelago, dissolved methane showed the strongest variations vertically in the water column, [..]” (L577) are not accurate, because one cannot extrapolate an assumption made on the basis of a very limited data set to a field as broad and very diverse as that of CAA.
AC: The data set includes 12 incubations for methane oxidation, but many more measurements of in-situ methane. Under sampling is a challenge in nearly every field-based study, when confronted with the complexity of the environment. This study took place in the CAA, and that is an adequate description of the field site, so we prefer to stay with CAA.
Abbreviationsonly use them when they are needed like in tables or Figures or if they are used really a lot e.g. CAA., but AO for Arctic Ocean and SWM for source water masses, is only used 5 times in the manuscript, so please write it out. Using PW, AW, ANP, SIM and similar in tables and equations is good, but Throughout the manuscript, I would appreciate to read the full wording, otherwise it gets too complicated to remember all the different meanings for the many Abbreviations.
The use of PW, AW, ANP, SIM, and the like in tables and equations is fine, but throughout the manuscript I would appreciate reading the full text, otherwise it becomes too complicated to remember all the different meanings for the many abbreviations. I am not a water mass specialist; acronyms like SIM, MW, or SWM are not natural to me.
L120: remove PML since it is not used elsewhere in the manuscript. Abbreviations like SPRS, URI, GSO, UIC are irrelevant.
AC: Thanks for your suggestion, we will make the necessary edits to the text accordingly.
Design of the study
How come the sampling scheme for seawater and ice cores does not follow a uniform concept for the depth profiles? In some cores the first sample was taken at 10 cm, in others at 20 cm. The same discontinuity applies to the greater depths of the cores. Similarly, the surface samples of seawater were taken at 1, 2, 5, or 7 m below the sea surface, followed by a wild combination of depths below. Is there a logical explanation for this? How can we ensure that data from different sites are comparable?
AC: We acknowledge the concern regarding the lack of uniformity in our sampling scheme for seawater and ice cores. The sea ice sampling strategy was influenced by two key factors: the length of the sea ice core and the logistical constraints. Access to sea ice was difficult and did not permit us to sample a wide geographic range nor examine different ice types. As a result, we opted to analyze a 20cm segment, for example, in cases where the ice core exhibited particular characteristics when melted at the top.
Seawater samples were determined by examining the CTD trace to identify specific water masses or other oceanographic/biogeochemical features. This is a very common approach and often produces sample distributions that are not uniform between profiles. Our choice of sampling depths was informed by critical oceanographic parameters, such as maximum temperature, maximum chlorophyll-a fluorescence, and minimum dissolved oxygen levels.Why did you decide to work with duplicate? In order to obtain statistically evaluable data, I would always recommend measuring at least triplicates, especially for samples where low methane concentrations or low methane oxidation rates are expected.
AC: As detailed in the methods section, we adhered to the protocol outlined in Uhlig and Loose (2017), in which duplicated samples were analyzed to evaluate the statistical robustness of the results. Duplicates give a measure of the incubation sample uncertainty but must be balanced against the limited time and resources for analysis. We felt triplicate samples were not warranted and, more importantly, would impose limitations on our ability increase the number oxidation measurements, which was critiqued in the comment above.
Microbial Community
The manuscript does not contain any information on how the sequencing dataset was processed.
In Chapter 2, I miss the description of sequencing data analysis such as initial processing, quality filtering, assembly, clustering, classification, statistical evaluation. It is unclear what databases and thresholds were used to identify OTUs. I also wonder how it is that the occurrence of chloroplast genomes is highlighted (L512); usually sequences classified as chloroplasts are removed from the dataset early before any kind of statistics are performed.
AC: We will revise paragraph 2.4 to provide a more detailed description of the sequence analysis. For clarification, we employed an opti-clust clustering analysis at 97% sequence similarity. We processed and analyzed the sequences as 97%-similar OTUs (Schloss et al., 2009; Kozich et al., 2013, revision 6/24/19) and the clustering process (Weiss et al., 2017; Kerrigan et al., 2019).
While we acknowledge that the presence of chloroplast genomes could introduce certain biases, its inclusion provides a comprehensive analysis. It allows us to integrate our findings with chlorophyll-a fluorescence data from CTD, providing a more holistic understanding of the relationship between photosynthetic activity and the methane cycle.
In Section 3.3, I cannot find any quantitative or qualitative data on the microbial community described in this manuscript, such as the percentage of each group in each sample or any type of distribution of microbes along the water column or sample area. No figures or tables visualizing bacterial community structures, such as relative abundances, hierarchical clusters, or correlation with methane concentrations and oxidation rates, which would be beneficial to the MS.
AC: We will incorporate a figure displaying Genus Abundance in the supplemental material to enhance the manuscript's clarity.
L512: “In summary, our dataset highlights the occurrence of Chloroplast genomes, Oleispira, Planctomarina, and Aurantivirga in samples showing potential methane oxidation, consistent with the findings of Uhlig et al. (2018) and Gründger et al. (2021).”
There are two problems with this sentence.
First, it suggests that the two publications given as references for this statement also show a high occurrence of chloroplast genomes (whatever that means, given the lack of data), which is not the case and is simply wrong. In both Uhlig et al. and Gründger et al. the sequences identified as chloroplasts were removed from the sequence dataset before the reads were assigned to OTUs.
Second, both publications including SIs do not confirm the presence of Planctomarina. I can only find affiliations with Planctomycetes (Uhlig) or Planctomycetes and Planctomycetaceae (Gründger).AC: We recognize that the current sentence may be potentially misleading. Our intention was to convey that certain species identified in our samples, such as Oleispira and Aurantivirga, were previously reported in Uhlig et al. (2018) and Gründger et al. (2021), respectively. We apologize for any confusion, and we will rectify this in the revised version of the manuscript.
As mentioned earlier, the section on microbial community composition is extremely sloppy and lacks any kind of baseline data. This is especially surprising since Brice Loose was also the last author to contribute to the Uhlig publication in 2018 and therefore should know what this publication says and what it does not say and what a good study linking oxidation rates and microbial composition might look like.
AC: We acknowledge that the section on the microbial community requires additional information to effectively link oxidation rates and microbial composition, and we will incorporate these details in the revised version of the manuscript.In many places in the manuscript (except in the Discussion), it is not clear what is meant by the term "microbial methane metabolism" (e.g. L558, 562, 569, 590) - it could be methane production (methanogenesis) and/or methane consumption (methane oxidation). Be more precise in the wording.
AC: Thank you for bringing this to our attention. In the revised version of the manuscript, we will define the term at the outset to ensure clarity and understanding.References
PLEASE, make sure your reference list is up to BG standards. Seriously, this is a resubmission and the references are still not up to the standards e.g. L794 the Boetius paper misses the journal. There is no consistent format apparent.
AC: We apologize for this oversight. Although we double-checked the reference style before submission, it appears that we may have missed some incorrect references. We will promptly rectify this error. Thank you for bringing it to our attention.
DETAILED COMMENTS
L34, 37: References to SI do not belong in an abstract.
AC: In response to a previous comment highlighting the absence of the simple calculation (lines 25-26) in the manuscript, we included a reference to the equation in the abstract to address that specific concern. We appreciate your guidance and would be happy to make further adjustments based on your recommendations.
L54: I do not understand this sentence. What “information about past processes” are meant here?
AC: The stability of methane within the water column is inherently tied to its solubility in seawater. This property enables methane to persist in a dissolved and stable state over extended periods, providing insights into past processes within the water column, including methane production and consumption, which can be inferred from its present concentrations. Further elaboration on this concept will be provided in the text.
L80: The combination of methane data, oxidation rate measurements and water column properties is not unique.
AC: To the best of our knowledge, no study to date has presented concurrent data encompassing dissolved methane, methane oxidation rates, chemical and physical water column parameters, and water mass composition. However, we would welcome the opportunity to become aware of any existing publications that have explored these comprehensive datasets in combination.
L86: I cannot find the Parry Channel on the maps in Fig.1.
AC: Thank you for bringing this to our attention. We will include the label in the revised version.
Fig.1: Left panel – the figure is too small and the study area is not recognizable. The figure should be the same size as the one to the right. The depth legend and the place names are extremely difficult to read. I suggest placing the legend outside the globe, increasing the font size of the place names, and submitting a higher resolution graphic. Right panel - In general, the quality of this map is very poor. I have trouble reading place names in white text with a grayish shadow on a light blue background - I can't identify the westernmost Sound (I assume the Viscount Melville Sound is meant, but the "Viscount" got lost), because the text is above the red dots of the sea ice sampling stations. Make sure the written and dots do not overlap on the map! Please make the sampling station numbers legible and make sure that each dot can be clearly assigned to the correct station name. What does the legend mean? I assume it is depth/heights in meters. Use a more descriptive legend; choose a range that makes sense for the map you are showing - I can't find depths greater than -5000 m or heights greater than 5000 m.
AC: We appreciate your suggestion, and we will make the necessary edits accordingly.L93: Change “August 2019, with seawater sampling stations […]”.
AC: We will edit it.
L118: Since this is a new section of text, you could give a hint about the region. Like in the previous chapter, which was about the Arctic Ocean. What is this chapter about? Also about the Arctic Ocean? The Parry Channel?
AC: Yes, it indeed refers to the Parry Channel. We will restructure the paragraph to enhance clarity.
L136: Barrow East is missing in the list of sampling locations. Is there a reason why it is not listed?
AC: Thank you for pointing that out. It was an oversight on our part. We will include it in the revised version.
L140: The term “in the vicinity of Parry Channel” is a bit too unspecific. Especially since you only took samples from the eastern part of the Parry Channel anyway.
AC: We will provide more specific details.
L142: I do not see any transects in Fig.1, there is only a string of single points.
AC: In this sentence, we are referring to the two investigated transects, namely Wellington Channel and West of Navy Board Inlet.
L145: Remove “owned and”. The information of ownership is irrelevant for the study. Omit “(SPRS)” since this abbreviation is not used further on.
AC: We will edit it.
L197: “room temperature” is a general term for what people prefer for indoor settings and is around 20–22 °C. Since your room was colder, I suggest to give the 10 °C only.
AC: Thanks for the suggestion. We will edit it.
L240-242: I do not understand the meaning of this sentence.
AC: We are referring to the method used for assessing the error introduced by the incubation time constraints. This method is fully described in Uhlig et al. (2017), which is why we cited the reference instead of providing a detailed description.
L261: “each sample by adding 0.1M NaOH and injecting it into the sample after the final measurement.” – should a killed control not being killed at the beginning of the measurement?
AC: During this cruise, the killed control samples were created after the final time point to conserve samples throughout the expedition. Subsequently, after sample analysis, we introduced NaOH and measured methane concentrations within a defined time frame. Notably, the control samples exhibited a consistent, flat trend from the initial measurement, indicating their good quality.
Fig. S1: Is there a specific order in which the results for each station are displayed in c and d? Why don't you arrange the stations in the order of longitude, as in Fig. 4, then it is easier to compare the data.
AC: The current order follows the sampling dates, but we can revise the order in the updated version of the manuscript.
Tab.2: It is unclear whether the depth data listed under station WNBI are from one cast with 6 sampling horizons or from multiple casts with fewer horizons at different depths. I can only assume that WNBI refers to three individual stations according to the map in Fig. 1. However, stations 1, 2, and 3 are not clearly assignable to the depths listed in the table. It may be that this is not important to the overall story, but I think that good presentation of the data is important to avoid irritation and confusion.
Also, make sure to use the terms location, site, and station consistently.
AC: Yes, the site WNBI comprises three separate sampling collections, which may result in some depths being repeated. We will make an effort to provide a clearer description of this in the text to ensure better clarity.
Fig.10: To perform an interpolation of methane oxidation rates (consisting of three measurement points according to Fig.10) over a distance of 500 km, then make statements about the extent of methane oxidation activity in the entire water column down to 200-300m water depth is borderline. Unfortunately, the interpolation algorithm is not described anywhere. I do not claim that the data are wrong, but the figure gives the impression that the whole water column in the western part of the study area is characterized by high oxidation rates, which definitely cannot be proven by data.
AC: The interpolation algorithm employed to create fig. 10 is based on Delaunay triangulation, and we utilized 9 data points for this purpose. Delaunay triangulation-based interpolation can produce reasonably accurate results with 9 strategically distributed data points. Nonetheless, upon examining our comprehensive dataset, it becomes evident that higher kox values were consistently recorded to the west of WNBI. Consequently, the contour plot aligns well with our observed outcomes.
L609: Where are Chlorophyll-a data coming from?
AC: The chl-a fluorescence data was referred in the text via the following DOI: https://doi.org/10.18739/A2BN9X45M and was an integral part of the Spearman's rank calculations presented in Figures 8 and 9. We will enhance the clarity of this reference in the text.
L670: “it is likely that dissolved methane in the CAA waters during the summer is primarily driven by microbial metabolism, […]” what does it mean? The presence, the concentration, the distribution,…? What is driven?
AC: We were referring to the concentration. We will provide a clear clarification of this in the text.
L732: “[…] likely following one of the paths explained by Repeta et al. (2016) and Sosa et al. (2020).” – Could you please specify which paths you are referring to, as this is the conclusion and the statements should be clearly stated?
AC: We were referring to methane production within phosphate-limited Pacific waters. We will incorporate additional clarification in the text to avoid any ambiguity. Thank you for the input.
L736: “The community structure likely responsible for the methane oxidation was characterized by three main groups that have been recently associated with the methane metabolism” – the microbial groups might be somehow related to methane oxidation, but I doubt they are "likely responsible". Since no abundance data are given, I can only speculate, but usually methane oxidizers do not show up as dominant OTUs in 16S sequencing data. To be able to describe a MOB community, sequencing should be done with specific primers targeting genes involved in MOx processes.
AC: We appreciate your guidance and will exercise greater care in selecting appropriate terminology for the revised manuscript. The term 'likely responsible' will be removed, and we will introduce a new figure in the supplemental material depicting Genus Abundance to better illustrate our findings, enhancing clarity for the reader.Citation: https://doi.org/10.5194/bg-2023-157-AC2
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AC2: 'Reply on RC2', Alessandra D'Angelo, 24 Nov 2023
Status: closed
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RC1: 'Comment on bg-2023-157', Anonymous Referee #1, 25 Oct 2023
I recognize the amount of work performed that led to this manuscript and the efforts of the authors to answer the comments. However, they answered the reviews quickly after the first reviews, mentioning future improvements, but many were not provided. It would have been wiser to answer reviewers' comments once the improvements were done so the authors could refer their answers to the text. These improvements promised but not done include:
- Ranges of temperature, salinity and nutrients were not given in the introduction.
- The distinction between the experimental and discrete samples is still not clearly explained
- The method lacks the duplicate, meaning that it is still unclear where the 56 number (28 x 2) is coming from. Also, the authors mention a leaking sample yet the 56 samples remain.
- The explanation of the 9 points instead of 18 in figure 10 is still missing.
- The authors answered my comment regarding the missing transect on the figure by saying that they only show the stations where methane was investigated. My question remains: where is the transect that the authors mention?
I wish a detailed revision was provided with a response to each question or comment. Here are my comments per section:
Abstract:
Lines 20-23: make it one sentence.
Only in the abstract is shown the maximum averaged methane concentration in the first 30m.
The abstract should be void of any reference to figure, table, or equation.
Introduction:
Line 52-53: wrong references: Steinle et al and Ferré et al do show elevated methane concentrations but don’t mention air-sea interface. Also, instead of showing limited gas exchanges to the atmosphere, Shakhova et al demonstrate the exact opposite.
Line 54-57: Please explain how methane concentration in the water column can inform on past processes. Yamamoto et al., 1976 don’t mention anything regarding the stability or persistence of methane, and their reference is missing.
Line 83: The authors mention the uniqueness of the study in the comparison
Figure 1: typo in the y-axis (latitude degrees W). Both panels in this figure are hard to see, but I assume the quality will be higher. The range of bathymetry in the right panel could also highlight more contrasts at the seafloor. The text mentions a maximum of 2000m depth but the color scale goes down to 10 000m depth. At least, the isobaths should be labeled.
Line 112: the IPCC report should be referred to.
Lines 121 and 125: I asked in my previous review to give a range. “Low” or “warm” are relative terms.
Line 142: like I asked in my previous review, where is this transect? If it includes all the “single points”, then mention it or point it out on the figure. However, this sentence is strange because transects consist in single points. Consider removing the sentence.
Line 153: already pointed out in my first review: explain experimental and discrete.
Line 233: replace “1 to 2.5 mL” by “1 or 2.5 mL”
Line 236: consider merging Tables S3 and S4. Table S4 is also sited before Table S3.
Line 253: the text presents the Spearman’s correlation and refers to figure 2, whereas figure 2 presents the linear correlation coefficient. Figure 2 is also surprising, as one would expect the linear trend to be on the other side of the 1:1 regression line. I couldn’t reproduce an of the R2 or Spearman’s correlation.
Results:
Line 450: this reads like the average methane concentration is 3.4 nM below 100m at WNBI, which is not shown in figure S4.
Line 534: like in 1st review, there is a discrepancy between the text (isotopic value of -51.4) and figure 8 (>-47.8). This minimum value is only shown in table S2. Consider showing the extreme values (or all) in figure 8 to avoid this discrepancy.
Part 4.1: consider referring to Silyakova et al. 2022 who showed that winter storms can break out the ice and allow methane to be released into the atmosphere. (Silyakova, A., D. Nomura, M. Kotovitch, A. Fransson, B. Delille, M. Chierici and M. A. Granskog (2022). "Methane release from open leads and new ice following an Arctic winter storm event." Polar Science.)
Line 605: in-situ dissolved methane
Line 607: study area
Line 609: the chl-a data are not shown
Line 627: wrong choice of publications. Damm, Graves and Silyakova don’t mention a link between high turbidity and methane excess. In fact, I don’t know any publication showing this except in hydrothermal vents. A turbidity of 0.005 ntu is low.
Figure 11: please explain where the sea ice concentration data is coming from, and indicate whether this is representative of the time when the survey occurred. It is not clear if this is satellite-derived from Spreen et al.
Line 698: again, the calculation from table 2 shows 0.005
Line 715: replace thesis with theory
Conclusion:
Line 731: see comment above regarding turbidity
Line 741: see comment above regarding winter storms
Citation: https://doi.org/10.5194/bg-2023-157-RC1 -
AC1: 'Reply on RC1', Alessandra D'Angelo, 24 Nov 2023
AC: We would like to express our sincere appreciation for the thorough review of our manuscript. We have carefully considered each of your recommendations and we are committed to incorporating them into the manuscript.
RC1:
I recognize the amount of work performed that led to this manuscript and the efforts of the authors to answer the comments. However, they answered the reviews quickly after the first reviews, mentioning future improvements, but many were not provided. It would have been wiser to answer reviewers' comments once the improvements were done so the authors could refer their answers to the text.
AC: As per the journal's policy promoting open discussion, it is highly recommend providing timely feedback to facilitate an open and constructive dialogue. Consequently, we have crafted our responses within the recommended timeframe.
We wish to underscore that the revised manuscript aligns closely with the revisions requested during the initial round of reviews. However, it incorporates several newly generated sections, making it distinct from the initial version. Therefore, it should not be anticipated to be identical to the original submission.
These improvements promised but not done include:
Ranges of temperature, salinity and nutrients were not given in the introduction.
AC: After careful consideration, we have determined that, in the broader context of the study area description, it is not essential to present indicative values of temperature, salinity, nutrients, and oxygen saturation. Instead, we have appropriately cited the existing literature where all these values are comprehensively reported (see lines 119-126).
The distinction between the experimental and discrete samples is still not clearly explained
AC: We have incorporated several sentences to provide clarity regarding the distinction between discrete (in-situ) and experimental (in-vitro) samples. These explanations can be found in various sections of the manuscript, such as lines 54 - 62 and 159 – 162. Furthermore, in Chapter 2, "Materials and Methods," we have separated the descriptions for in-situ and in-vitro samples, detailing their processing and analysis independently to prevent any potential confusion. We have also restructured the paragraphs throughout the text to ensure a clear differentiation between the in-situ and in-vitro samples.
In the upcoming revised version, we will make extra efforts to enhance this distinction for the reader's benefit.
The method lacks the duplicate, meaning that it is still unclear where the 56 number (28 x 2) is coming from. Also, the authors mention a leaking sample yet the 56 samples remain.
AC: In line 53 and Table S1, we have provided a description of the total number of samples collected for the methane in-vitro experiments, stating, "A total of 132 seawater samples (28x2 experimental and 76 discrete)." The term "28x2" is used to denote the duplicates, as further explained in the caption of Table S1. It's important to note that one of the samples experienced a leakage issue during the final measurement, and as a result, we did not include this particular sample in the final calculation of the kox. However, we retained this sample in Table S1 to maintain a record of all the samples processed.
We recognize that this may potentially lead to confusion, and in the forthcoming revised version, we are committed to removing this sample from the table and text to ensure clarity and accuracy.
The explanation of the 9 points instead of 18 in figure 10 is still missing.
AC: In the caption of Figure 10, we have clarified that the values presented in the plot were obtained by averaging the kox values by transect and depth, with the exclusion of non-methane metabolism data. Based on the information presented and referencing Table 2, we can observe that we have 2 data points for CB, 4 data points for WNBI (including the two data points at 70m depth), 1 data point for WC, and 2 data points for JS.
We acknowledge that the caption may require further refinement for clarity, and in the forthcoming revised version, we will include the provided information to improve its clarity and readability.
The authors answered my comment regarding the missing transect on the figure by saying that they only show the stations where methane was investigated. My question remains: where is the transect that the authors mention?
AC: Among the sampling stations, we conducted investigations at two specific transects during our campaign, as visually depicted in Fig. 1, namely West Navy Board Inlet and Wellington Channel. In the interest of clarity and ease of reference, we employed the term "transect" in Fig. 10 to denote the individual sampling sites, as our calculations involved averaging kox values by site and depth.
I wish a detailed revision was provided with a response to each question or comment. Here are my comments per section:Abstract:
Lines 20-23: make it one sentence.
AC: We will edit it.
Only in the abstract is shown the maximum averaged methane concentration in the first 30m.
AC: We will incorporate this information into the Results section.
The abstract should be void of any reference to figure, table, or equation.
AC: In response to a previous comment highlighting the absence of the simple calculation (lines 25-26) in the manuscript, we included a reference to the equation in the abstract to address that specific concern. We appreciate your guidance and would be happy to make further adjustments based on your recommendations.Introduction:
Line 52-53: wrong references: Steinle et al and Ferré et al do show elevated methane concentrations but don’t mention air-sea interface. Also, instead of showing limited gas exchanges to the atmosphere, Shakhova et al demonstrate the exact opposite.
AC: We have incorporated all the references related to the processes mentioned in lines 50-52 at the end of the sentence. To eliminate any potential confusion, we will categorize each reference according to the specific processes described in that sentence.Line 54-57: Please explain how methane concentration in the water column can inform on past processes. Yamamoto et al., 1976 don’t mention anything regarding the stability or persistence of methane, and their reference is missing.
AC: Yamamoto et al. (1976) provided a comprehensive account of methane solubility in seawater across varying salinities and temperatures. The stability of methane within the water column is intrinsically linked to its solubility in seawater. This inherent quality allows methane to remain in a dissolved and stable state for an extended duration before undergoing subsequent processes, such as microbial oxidation or physical transport. Consequently, gas concentrations provide insights into historical processes. In summary, the citation of Yamamoto et al. (1976) serves as a valuable indirect endorsement of the concept of methane's stability in the water column.
Line 83: The authors mention the uniqueness of the study in the comparison
AC: Yes, the distinctiveness of this study is rooted in its examination of paired datasets encompassing dissolved methane, potential oxidation rates, marine microbial communities, and the physical-chemical characteristics of water masses in the CAA.
Figure 1: typo in the y-axis (latitude degrees W). Both panels in this figure are hard to see, but I assume the quality will be higher. The range of bathymetry in the right panel could also highlight more contrasts at the seafloor. The text mentions a maximum of 2000m depth but the color scale goes down to 10 000m depth. At least, the isobaths should be labeled.
AC: Regarding the y-axis, we have noted the label "Latitude (˚N)." We are committed to enhancing the quality of the figure, and we sincerely appreciate your suggestions.
Line 112: the IPCC report should be referred to.
AC: Thank you for your suggestion. We will incorporate the reference.
Lines 121 and 125: I asked in my previous review to give a range. “Low” or “warm” are relative terms.
AC: As aforementioned, we have determined that in the broader context of the study area description it is not essential to present indicative values of temperature, salinity, nutrients, and oxygen saturation. Instead, we have appropriately cited the existing literature where all these values are comprehensively reported (see lines 119-126).
Line 142: like I asked in my previous review, where is this transect? If it includes all the “single points”, then mention it or point it out on the figure. However, this sentence is strange because transects consist in single points. Consider removing the sentence.
AC: In this sentence, we are referring to the two investigated transects, namely WC and WNBI.
Line 153: already pointed out in my first review: explain experimental and discrete.
AC: If the use of those terms causes confusion, we will replace them with "in-situ" and "in-vitro," consistent with the terminology used throughout the rest of the text.
Line 233: replace “1 to 2.5 mL” by “1 or 2.5 mL”
AC: We will make the necessary edit.
Line 236: consider merging Tables S3 and S4. Table S4 is also sited before Table S3.
AC: We appreciate your suggestion, and we will proceed with making this edit.
Line 253: the text presents the Spearman’s correlation and refers to figure 2, whereas figure 2 presents the linear correlation coefficient. Figure 2 is also surprising, as one would expect the linear trend to be on the other side of the 1:1 regression line. I couldn’t reproduce an of the R2 or Spearman’s correlation.
AC: Indeed, in the text, we presented the Spearman’s correlation coefficient value, while in the plot, we exhibited the linear model. Both of these coefficients were computed by considering all values of kox.mass.balance and kox.isotope.ratio. We were not surprised by the result and good fit of the data, as the study by Uhlig and Loose (2017) had already assessed the low uncertainty of this method.
Results:
Line 450: this reads like the average methane concentration is 3.4 nM below 100m at WNBI, which is not shown in figure S4.
AC: Yes, we indicated the average [CH4] below 100m in WNBI as 3.4 nM, with Figure S4 illustrating the individual profiles of [CH4] for each site, from which this result is derived. In the text, we make reference to Figure S4 to emphasize the methane enrichment in deep water.Line 534: like in 1st review, there is a discrepancy between the text (isotopic value of -51.4) and figure 8 (>-47.8). This minimum value is only shown in table S2. Consider showing the extreme values (or all) in figure 8 to avoid this discrepancy.
AC: In Figure 8, we exclusively presented the two samples that exhibited methane depletion (core 1 and core 2). However, the sentence in line 534 pertains to all sea ice methane data, as documented in table S2.
We appreciate your feedback, and in the revised version of the manuscript, we will make the reference to table S2 clearer in relation to all isotope ratio data.
Part 4.1: consider referring to Silyakova et al. 2022 who showed that winter storms can break out the ice and allow methane to be released into the atmosphere. (Silyakova, A., D. Nomura, M. Kotovitch, A. Fransson, B. Delille, M. Chierici and M. A. Granskog (2022). "Methane release from open leads and new ice following an Arctic winter storm event." Polar Science.)
AC: Thank you for the suggestion. We will incorporate the recent paper into our references and aligning our discussion to ensure that it is well-informed by this new data.
Line 605: in-situ dissolved methane
AC: we will edit it
Line 607: study area
AC: we will edit it.
Line 609: the chl-a data are not shown
AC: We did not display the chl-a fluorescence data; nevertheless, the chl-a fluorescence data was incorporated in the Spearman's rank calculation presented in Figures 8 and 9. In line 609, our intent was solely to emphasize that the highest dissolved methane concentrations coincided with elevated chlorophyll-a fluorescence data, as referenced throughout the text via https://doi.org/10.18739/A2BN9X45M.
Line 627: wrong choice of publications. Damm, Graves and Silyakova don’t mention a link between high turbidity and methane excess. In fact, I don’t know any publication showing this except in hydrothermal vents. A turbidity of 0.005 ntu is low.
AC: We included these specific references in our manuscript despite their lack of direct mention of a connection between high turbidity and methane excess. Our rationale for doing so was to provide fundamental background information on the processes contributing to methane excess in the deep-water column and seabed. We will revise the text to enhance clarity in this regard.
Regarding the relatively high turbidity value of 0.005 NTU that we reported, we understand that it may seem low compared to some other environments, such as hydrothermal vents. However, it's important to note that in the context of our study and the specific depths we investigated, the turbidity values below 100m averaged 0.009 NTU. In this context, 0.005 NTU can be considered relatively high.
Figure 11: please explain where the sea ice concentration data is coming from, and indicate whether this is representative of the time when the survey occurred. It is not clear if this is satellite-derived from Spreen et al.
AC: Yes, the sea ice concentration data was obtained from the University of Bremen data archive, and we specifically downloaded the data corresponding to our sampling dates.
We will ensure to include this information in the caption as suggested, and we appreciate your guidance.
Line 698: again, the calculation from table 2 shows 0.005
AC: Thank you for pointing this out. Yes, it's because Table 2 includes kox=0 (non-methane metabolism), which affects the calculated average. In line 699 we specified in parenthesis “excluding non-methane metabolism”.
Line 715: replace thesis with theory
AC: We will edit it.
Conclusion:Line 731: see comment above regarding turbidity
AC: See above.
Line 741: see comment above regarding winter storms
AC: We will update the text accordingly, as explained above.
Citation: https://doi.org/10.5194/bg-2023-157-AC1
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RC2: 'Comment on bg-2023-157', Anonymous Referee #2, 27 Oct 2023
Review: The marine methane cycle in the Canadian Arctic Archipelago during summer.
I can see the effort behind this study from sample acquisition, analyses, evaluations and last but not least the writing of such a manuscript, which is well written in terms of writing style and structure. However, I see big gaps in the data set and issues in the way data are presented and interpreted.
GENERAL COMMENTS
The term CAA
This study presents results from a set of samples collected at 12 seawater and 5 sea ice stations along the eastern part of the Parry Channel and adjacent Sounds. This low number of stations does not represent the entire CAA and therefore the term should be adjusted to e.g. “Central Canadian Arctic Archipelago” and “Eastern part of the Parry Channel” or similar.
Propositions like this “In the waters of the Canadian Arctic Archipelago, dissolved methane showed the strongest variations vertically in the water column, [..]” (L577) are not accurate, because one cannot extrapolate an assumption made on the basis of a very limited data set to a field as broad and very diverse as that of CAA.
Abbreviations
only use them when they are needed like in tables or Figures or if they are used really a lot e.g. CAA., but AO for Arctic Ocean and SWM for source water masses, is only used 5 times in the manuscript, so please write it out. Using PW, AW, ANP, SIM and similar in tables and equations is good, but Throughout the manuscript, I would appreciate to read the full wording, otherwise it gets too complicated to remember all the different meanings for the many Abbreviations.
The use of PW, AW, ANP, SIM, and the like in tables and equations is fine, but throughout the manuscript I would appreciate reading the full text, otherwise it becomes too complicated to remember all the different meanings for the many abbreviations. I am not a water mass specialist; acronyms like SIM, MW, or SWM are not natural to me.
L120: remove PML since it is not used elsewhere in the manuscript. Abbreviations like SPRS, URI, GSO, UIC are irrelevant.
Design of the study
How come the sampling scheme for seawater and ice cores does not follow a uniform concept for the depth profiles? In some cores the first sample was taken at 10 cm, in others at 20 cm. The same discontinuity applies to the greater depths of the cores. Similarly, the surface samples of seawater were taken at 1, 2, 5, or 7 m below the sea surface, followed by a wild combination of depths below. Is there a logical explanation for this? How can we ensure that data from different sites are comparable?
Why did you decide to work with duplicate? In order to obtain statistically evaluable data, I would always recommend measuring at least triplicates, especially for samples where low methane concentrations or low methane oxidation rates are expected.
Microbial Community
The manuscript does not contain any information on how the sequencing dataset was processed.
In Chapter 2, I miss the description of sequencing data analysis such as initial processing, quality filtering, assembly, clustering, classification, statistical evaluation. It is unclear what databases and thresholds were used to identify OTUs. I also wonder how it is that the occurrence of chloroplast genomes is highlighted (L512); usually sequences classified as chloroplasts are removed from the dataset early before any kind of statistics are performed.
In Section 3.3, I cannot find any quantitative or qualitative data on the microbial community described in this manuscript, such as the percentage of each group in each sample or any type of distribution of microbes along the water column or sample area. No figures or tables visualizing bacterial community structures, such as relative abundances, hierarchical clusters, or correlation with methane concentrations and oxidation rates, which would be beneficial to the MS.
L512: “In summary, our dataset highlights the occurrence of Chloroplast genomes, Oleispira, Planctomarina, and Aurantivirga in samples showing potential methane oxidation, consistent with the findings of Uhlig et al. (2018) and Gründger et al. (2021).”
There are two problems with this sentence.
First, it suggests that the two publications given as references for this statement also show a high occurrence of chloroplast genomes (whatever that means, given the lack of data), which is not the case and is simply wrong. In both Uhlig et al. and Gründger et al. the sequences identified as chloroplasts were removed from the sequence dataset before the reads were assigned to OTUs.
Second, both publications including SIs do not confirm the presence of Planctomarina. I can only find affiliations with Planctomycetes (Uhlig) or Planctomycetes and Planctomycetaceae (Gründger).
As mentioned earlier, the section on microbial community composition is extremely sloppy and lacks any kind of baseline data. This is especially surprising since Brice Loose was also the last author to contribute to the Uhlig publication in 2018 and therefore should know what this publication says and what it does not say and what a good study linking oxidation rates and microbial composition might look like.
In many places in the manuscript (except in the Discussion), it is not clear what is meant by the term "microbial methane metabolism" (e.g. L558, 562, 569, 590) - it could be methane production (methanogenesis) and/or methane consumption (methane oxidation). Be more precise in the wording.
References
PLEASE, make sure your reference list is up to BG standards. Seriously, this is a resubmission and the references are still not up to the standards e.g. L794 the Boetius paper misses the journal. There is no consistent format apparent.
DETAILED COMMENTS
L34, 37: References to SI do not belong in an abstract.
L54: I do not understand this sentence. What “information about past processes” are meant here?
L80: The combination of methane data, oxidation rate measurements and water column properties is not unique.
L86: I cannot find the Parry Channel on the maps in Fig.1.
Fig.1: Left panel – the figure is too small and the study area is not recognizable. The figure should be the same size as the one to the right. The depth legend and the place names are extremely difficult to read. I suggest placing the legend outside the globe, increasing the font size of the place names, and submitting a higher resolution graphic.
Right panel - In general, the quality of this map is very poor. I have trouble reading place names in white text with a grayish shadow on a light blue background - I can't identify the westernmost Sound (I assume the Viscount Melville Sound is meant, but the "Viscount" got lost), because the text is above the red dots of the sea ice sampling stations. Make sure the written and dots do not overlap on the map! Please make the sampling station numbers legible and make sure that each dot can be clearly assigned to the correct station name. What does the legend mean? I assume it is depth/heights in meters. Use a more descriptive legend; choose a range that makes sense for the map you are showing - I can't find depths greater than -5000 m or heights greater than 5000 m.
L93: Change “August 2019, with seawater sampling stations […]”.
L118: Since this is a new section of text, you could give a hint about the region. Like in the previous chapter, which was about the Arctic Ocean. What is this chapter about? Also about the Arctic Ocean? The Parry Channel?
L136: Barrow East is missing in the list of sampling locations. Is there a reason why it is not listed?
L140: The term “in the vicinity of Parry Channel” is a bit too unspecific. Especially since you only took samples from the eastern part of the Parry Channel anyway.
L142: I do not see any transects in Fig.1, there is only a string of single points.
L145: Remove “owned and”. The information of ownership is irrelevant for the study. Omit “(SPRS)” since this abbreviation is not used further on.
L197: “room temperature” is a general term for what people prefer for indoor settings and is around 20–22 °C. Since your room was colder, I suggest to give the 10 °C only.
L240-242: I do not understand the meaning of this sentence.
L261: “each sample by adding 0.1M NaOH and injecting it into the sample after the final measurement.” – should a killed control not being killed at the beginning of the measurement?
Fig. S1: Is there a specific order in which the results for each station are displayed in c and d? Why don't you arrange the stations in the order of longitude, as in Fig. 4, then it is easier to compare the data.
Tab.2: It is unclear whether the depth data listed under station WNBI are from one cast with 6 sampling horizons or from multiple casts with fewer horizons at different depths. I can only assume that WNBI refers to three individual stations according to the map in Fig. 1. However, stations 1, 2, and 3 are not clearly assignable to the depths listed in the table. It may be that this is not important to the overall story, but I think that good presentation of the data is important to avoid irritation and confusion.
Also, make sure to use the terms location, site, and station consistently.
Fig.10: To perform an interpolation of methane oxidation rates (consisting of three measurement points according to Fig.10) over a distance of 500 km, then make statements about the extent of methane oxidation activity in the entire water column down to 200-300m water depth is borderline. Unfortunately, the interpolation algorithm is not described anywhere. I do not claim that the data are wrong, but the figure gives the impression that the whole water column in the western part of the study area is characterized by high oxidation rates, which definitely cannot be proven by data.
L609: Where are Chlorophyll-a data coming from?
L670: “it is likely that dissolved methane in the CAA waters during the summer is primarily driven by microbial metabolism, […]” what does it mean? The presence, the concentration, the distribution,…? What is driven?
L732: “[…] likely following one of the paths explained by Repeta et al. (2016) and Sosa et al. (2020).” – Could you please specify which paths you are referring to, as this is the conclusion and the statements should be clearly stated?
L736: “The community structure likely responsible for the methane oxidation was characterized by three main groups that have been recently associated with the methane metabolism” – the microbial groups might be somehow related to methane oxidation, but I doubt they are "likely responsible". Since no abundance data are given, I can only speculate, but usually methane oxidizers do not show up as dominant OTUs in 16S sequencing data. To be able to describe a MOB community, sequencing should be done with specific primers targeting genes involved in MOx processes.
Citation: https://doi.org/10.5194/bg-2023-157-RC2 -
AC2: 'Reply on RC2', Alessandra D'Angelo, 24 Nov 2023
AC: Thank you sincerely for your thoughtful review of our manuscript. We are committed to incorporating your suggestions to improve the clarity and overall quality of the paper.
RC2:
I can see the effort behind this study from sample acquisition, analyses, evaluations and last but not least the writing of such a manuscript, which is well written in terms of writing style and structure. However, I see big gaps in the data set and issues in the way data are presented and interpreted.
GENERAL COMMENTS
The term CAA
This study presents results from a set of samples collected at 12 seawater and 5 sea ice stations along the eastern part of the Parry Channel and adjacent Sounds. This low number of stations does not represent the entire CAA and therefore the term should be adjusted to e.g. “Central Canadian Arctic Archipelago” and “Eastern part of the Parry Channel” or similar.
Propositions like this “In the waters of the Canadian Arctic Archipelago, dissolved methane showed the strongest variations vertically in the water column, [..]” (L577) are not accurate, because one cannot extrapolate an assumption made on the basis of a very limited data set to a field as broad and very diverse as that of CAA.
AC: The data set includes 12 incubations for methane oxidation, but many more measurements of in-situ methane. Under sampling is a challenge in nearly every field-based study, when confronted with the complexity of the environment. This study took place in the CAA, and that is an adequate description of the field site, so we prefer to stay with CAA.
Abbreviationsonly use them when they are needed like in tables or Figures or if they are used really a lot e.g. CAA., but AO for Arctic Ocean and SWM for source water masses, is only used 5 times in the manuscript, so please write it out. Using PW, AW, ANP, SIM and similar in tables and equations is good, but Throughout the manuscript, I would appreciate to read the full wording, otherwise it gets too complicated to remember all the different meanings for the many Abbreviations.
The use of PW, AW, ANP, SIM, and the like in tables and equations is fine, but throughout the manuscript I would appreciate reading the full text, otherwise it becomes too complicated to remember all the different meanings for the many abbreviations. I am not a water mass specialist; acronyms like SIM, MW, or SWM are not natural to me.
L120: remove PML since it is not used elsewhere in the manuscript. Abbreviations like SPRS, URI, GSO, UIC are irrelevant.
AC: Thanks for your suggestion, we will make the necessary edits to the text accordingly.
Design of the study
How come the sampling scheme for seawater and ice cores does not follow a uniform concept for the depth profiles? In some cores the first sample was taken at 10 cm, in others at 20 cm. The same discontinuity applies to the greater depths of the cores. Similarly, the surface samples of seawater were taken at 1, 2, 5, or 7 m below the sea surface, followed by a wild combination of depths below. Is there a logical explanation for this? How can we ensure that data from different sites are comparable?
AC: We acknowledge the concern regarding the lack of uniformity in our sampling scheme for seawater and ice cores. The sea ice sampling strategy was influenced by two key factors: the length of the sea ice core and the logistical constraints. Access to sea ice was difficult and did not permit us to sample a wide geographic range nor examine different ice types. As a result, we opted to analyze a 20cm segment, for example, in cases where the ice core exhibited particular characteristics when melted at the top.
Seawater samples were determined by examining the CTD trace to identify specific water masses or other oceanographic/biogeochemical features. This is a very common approach and often produces sample distributions that are not uniform between profiles. Our choice of sampling depths was informed by critical oceanographic parameters, such as maximum temperature, maximum chlorophyll-a fluorescence, and minimum dissolved oxygen levels.Why did you decide to work with duplicate? In order to obtain statistically evaluable data, I would always recommend measuring at least triplicates, especially for samples where low methane concentrations or low methane oxidation rates are expected.
AC: As detailed in the methods section, we adhered to the protocol outlined in Uhlig and Loose (2017), in which duplicated samples were analyzed to evaluate the statistical robustness of the results. Duplicates give a measure of the incubation sample uncertainty but must be balanced against the limited time and resources for analysis. We felt triplicate samples were not warranted and, more importantly, would impose limitations on our ability increase the number oxidation measurements, which was critiqued in the comment above.
Microbial Community
The manuscript does not contain any information on how the sequencing dataset was processed.
In Chapter 2, I miss the description of sequencing data analysis such as initial processing, quality filtering, assembly, clustering, classification, statistical evaluation. It is unclear what databases and thresholds were used to identify OTUs. I also wonder how it is that the occurrence of chloroplast genomes is highlighted (L512); usually sequences classified as chloroplasts are removed from the dataset early before any kind of statistics are performed.
AC: We will revise paragraph 2.4 to provide a more detailed description of the sequence analysis. For clarification, we employed an opti-clust clustering analysis at 97% sequence similarity. We processed and analyzed the sequences as 97%-similar OTUs (Schloss et al., 2009; Kozich et al., 2013, revision 6/24/19) and the clustering process (Weiss et al., 2017; Kerrigan et al., 2019).
While we acknowledge that the presence of chloroplast genomes could introduce certain biases, its inclusion provides a comprehensive analysis. It allows us to integrate our findings with chlorophyll-a fluorescence data from CTD, providing a more holistic understanding of the relationship between photosynthetic activity and the methane cycle.
In Section 3.3, I cannot find any quantitative or qualitative data on the microbial community described in this manuscript, such as the percentage of each group in each sample or any type of distribution of microbes along the water column or sample area. No figures or tables visualizing bacterial community structures, such as relative abundances, hierarchical clusters, or correlation with methane concentrations and oxidation rates, which would be beneficial to the MS.
AC: We will incorporate a figure displaying Genus Abundance in the supplemental material to enhance the manuscript's clarity.
L512: “In summary, our dataset highlights the occurrence of Chloroplast genomes, Oleispira, Planctomarina, and Aurantivirga in samples showing potential methane oxidation, consistent with the findings of Uhlig et al. (2018) and Gründger et al. (2021).”
There are two problems with this sentence.
First, it suggests that the two publications given as references for this statement also show a high occurrence of chloroplast genomes (whatever that means, given the lack of data), which is not the case and is simply wrong. In both Uhlig et al. and Gründger et al. the sequences identified as chloroplasts were removed from the sequence dataset before the reads were assigned to OTUs.
Second, both publications including SIs do not confirm the presence of Planctomarina. I can only find affiliations with Planctomycetes (Uhlig) or Planctomycetes and Planctomycetaceae (Gründger).AC: We recognize that the current sentence may be potentially misleading. Our intention was to convey that certain species identified in our samples, such as Oleispira and Aurantivirga, were previously reported in Uhlig et al. (2018) and Gründger et al. (2021), respectively. We apologize for any confusion, and we will rectify this in the revised version of the manuscript.
As mentioned earlier, the section on microbial community composition is extremely sloppy and lacks any kind of baseline data. This is especially surprising since Brice Loose was also the last author to contribute to the Uhlig publication in 2018 and therefore should know what this publication says and what it does not say and what a good study linking oxidation rates and microbial composition might look like.
AC: We acknowledge that the section on the microbial community requires additional information to effectively link oxidation rates and microbial composition, and we will incorporate these details in the revised version of the manuscript.In many places in the manuscript (except in the Discussion), it is not clear what is meant by the term "microbial methane metabolism" (e.g. L558, 562, 569, 590) - it could be methane production (methanogenesis) and/or methane consumption (methane oxidation). Be more precise in the wording.
AC: Thank you for bringing this to our attention. In the revised version of the manuscript, we will define the term at the outset to ensure clarity and understanding.References
PLEASE, make sure your reference list is up to BG standards. Seriously, this is a resubmission and the references are still not up to the standards e.g. L794 the Boetius paper misses the journal. There is no consistent format apparent.
AC: We apologize for this oversight. Although we double-checked the reference style before submission, it appears that we may have missed some incorrect references. We will promptly rectify this error. Thank you for bringing it to our attention.
DETAILED COMMENTS
L34, 37: References to SI do not belong in an abstract.
AC: In response to a previous comment highlighting the absence of the simple calculation (lines 25-26) in the manuscript, we included a reference to the equation in the abstract to address that specific concern. We appreciate your guidance and would be happy to make further adjustments based on your recommendations.
L54: I do not understand this sentence. What “information about past processes” are meant here?
AC: The stability of methane within the water column is inherently tied to its solubility in seawater. This property enables methane to persist in a dissolved and stable state over extended periods, providing insights into past processes within the water column, including methane production and consumption, which can be inferred from its present concentrations. Further elaboration on this concept will be provided in the text.
L80: The combination of methane data, oxidation rate measurements and water column properties is not unique.
AC: To the best of our knowledge, no study to date has presented concurrent data encompassing dissolved methane, methane oxidation rates, chemical and physical water column parameters, and water mass composition. However, we would welcome the opportunity to become aware of any existing publications that have explored these comprehensive datasets in combination.
L86: I cannot find the Parry Channel on the maps in Fig.1.
AC: Thank you for bringing this to our attention. We will include the label in the revised version.
Fig.1: Left panel – the figure is too small and the study area is not recognizable. The figure should be the same size as the one to the right. The depth legend and the place names are extremely difficult to read. I suggest placing the legend outside the globe, increasing the font size of the place names, and submitting a higher resolution graphic. Right panel - In general, the quality of this map is very poor. I have trouble reading place names in white text with a grayish shadow on a light blue background - I can't identify the westernmost Sound (I assume the Viscount Melville Sound is meant, but the "Viscount" got lost), because the text is above the red dots of the sea ice sampling stations. Make sure the written and dots do not overlap on the map! Please make the sampling station numbers legible and make sure that each dot can be clearly assigned to the correct station name. What does the legend mean? I assume it is depth/heights in meters. Use a more descriptive legend; choose a range that makes sense for the map you are showing - I can't find depths greater than -5000 m or heights greater than 5000 m.
AC: We appreciate your suggestion, and we will make the necessary edits accordingly.L93: Change “August 2019, with seawater sampling stations […]”.
AC: We will edit it.
L118: Since this is a new section of text, you could give a hint about the region. Like in the previous chapter, which was about the Arctic Ocean. What is this chapter about? Also about the Arctic Ocean? The Parry Channel?
AC: Yes, it indeed refers to the Parry Channel. We will restructure the paragraph to enhance clarity.
L136: Barrow East is missing in the list of sampling locations. Is there a reason why it is not listed?
AC: Thank you for pointing that out. It was an oversight on our part. We will include it in the revised version.
L140: The term “in the vicinity of Parry Channel” is a bit too unspecific. Especially since you only took samples from the eastern part of the Parry Channel anyway.
AC: We will provide more specific details.
L142: I do not see any transects in Fig.1, there is only a string of single points.
AC: In this sentence, we are referring to the two investigated transects, namely Wellington Channel and West of Navy Board Inlet.
L145: Remove “owned and”. The information of ownership is irrelevant for the study. Omit “(SPRS)” since this abbreviation is not used further on.
AC: We will edit it.
L197: “room temperature” is a general term for what people prefer for indoor settings and is around 20–22 °C. Since your room was colder, I suggest to give the 10 °C only.
AC: Thanks for the suggestion. We will edit it.
L240-242: I do not understand the meaning of this sentence.
AC: We are referring to the method used for assessing the error introduced by the incubation time constraints. This method is fully described in Uhlig et al. (2017), which is why we cited the reference instead of providing a detailed description.
L261: “each sample by adding 0.1M NaOH and injecting it into the sample after the final measurement.” – should a killed control not being killed at the beginning of the measurement?
AC: During this cruise, the killed control samples were created after the final time point to conserve samples throughout the expedition. Subsequently, after sample analysis, we introduced NaOH and measured methane concentrations within a defined time frame. Notably, the control samples exhibited a consistent, flat trend from the initial measurement, indicating their good quality.
Fig. S1: Is there a specific order in which the results for each station are displayed in c and d? Why don't you arrange the stations in the order of longitude, as in Fig. 4, then it is easier to compare the data.
AC: The current order follows the sampling dates, but we can revise the order in the updated version of the manuscript.
Tab.2: It is unclear whether the depth data listed under station WNBI are from one cast with 6 sampling horizons or from multiple casts with fewer horizons at different depths. I can only assume that WNBI refers to three individual stations according to the map in Fig. 1. However, stations 1, 2, and 3 are not clearly assignable to the depths listed in the table. It may be that this is not important to the overall story, but I think that good presentation of the data is important to avoid irritation and confusion.
Also, make sure to use the terms location, site, and station consistently.
AC: Yes, the site WNBI comprises three separate sampling collections, which may result in some depths being repeated. We will make an effort to provide a clearer description of this in the text to ensure better clarity.
Fig.10: To perform an interpolation of methane oxidation rates (consisting of three measurement points according to Fig.10) over a distance of 500 km, then make statements about the extent of methane oxidation activity in the entire water column down to 200-300m water depth is borderline. Unfortunately, the interpolation algorithm is not described anywhere. I do not claim that the data are wrong, but the figure gives the impression that the whole water column in the western part of the study area is characterized by high oxidation rates, which definitely cannot be proven by data.
AC: The interpolation algorithm employed to create fig. 10 is based on Delaunay triangulation, and we utilized 9 data points for this purpose. Delaunay triangulation-based interpolation can produce reasonably accurate results with 9 strategically distributed data points. Nonetheless, upon examining our comprehensive dataset, it becomes evident that higher kox values were consistently recorded to the west of WNBI. Consequently, the contour plot aligns well with our observed outcomes.
L609: Where are Chlorophyll-a data coming from?
AC: The chl-a fluorescence data was referred in the text via the following DOI: https://doi.org/10.18739/A2BN9X45M and was an integral part of the Spearman's rank calculations presented in Figures 8 and 9. We will enhance the clarity of this reference in the text.
L670: “it is likely that dissolved methane in the CAA waters during the summer is primarily driven by microbial metabolism, […]” what does it mean? The presence, the concentration, the distribution,…? What is driven?
AC: We were referring to the concentration. We will provide a clear clarification of this in the text.
L732: “[…] likely following one of the paths explained by Repeta et al. (2016) and Sosa et al. (2020).” – Could you please specify which paths you are referring to, as this is the conclusion and the statements should be clearly stated?
AC: We were referring to methane production within phosphate-limited Pacific waters. We will incorporate additional clarification in the text to avoid any ambiguity. Thank you for the input.
L736: “The community structure likely responsible for the methane oxidation was characterized by three main groups that have been recently associated with the methane metabolism” – the microbial groups might be somehow related to methane oxidation, but I doubt they are "likely responsible". Since no abundance data are given, I can only speculate, but usually methane oxidizers do not show up as dominant OTUs in 16S sequencing data. To be able to describe a MOB community, sequencing should be done with specific primers targeting genes involved in MOx processes.
AC: We appreciate your guidance and will exercise greater care in selecting appropriate terminology for the revised manuscript. The term 'likely responsible' will be removed, and we will introduce a new figure in the supplemental material depicting Genus Abundance to better illustrate our findings, enhancing clarity for the reader.Citation: https://doi.org/10.5194/bg-2023-157-AC2
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AC2: 'Reply on RC2', Alessandra D'Angelo, 24 Nov 2023
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