the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2024
Enigmatic Fe-Mn-fueled Anaerobic Oxidation of Methane in sulfidic coastal sediments of the Eastern Arabian Sea
Abstract. Anaerobic oxidation of methane (AOM) coupled with Fe-Mn reduction (Fe-Mn-AOM) is considered a globally important biogeochemical process in marine sediments in addition to sulfate-driven AOM (SO42−-AOM) responsible for the consumption of methane, a strong greenhouse gas. Most existing studies have emphasized the significance of Fe-Mn-AOM activities in sediments below the depth of the sulfate methane transition zone (SMTZ) with insignificant dissolved sulfide and sulfate concentrations in the porewaters. Here, we report for the first time enigmatic geochemical evidence of focused Fe-Mn-AOM activity across the SMTZ in the presence of high dissolved sulfide concentrations in a sediment core collected within the seasonal coastal hypoxic zone of the Eastern Arabian Sea (West coast of India (WCI)). The Fe-Mn-AOM activity is evident from the concurrent decrease in CH4 concentrations, d13CCH4 and d13CDIC values coupled with the enrichment of porewater Fe2+ and Mn2+ concentrations at multiple depths below the seafloor. Since neither CH4 nor reactive Fe appears to be the limiting factor controlling the Fe-Mn-AOM activity, we hypothesize that the focused Fe-Mn-AOM at multiple depths is likely fueled by the localization of metal-reducing and methanotrophic microbial communities, leading to biogeochemical heterogeneity in a dynamic seasonally hypoxic coastal environment sensitive to climate change. This study highlights new insight into CH4-S-Fe-Mn biogeochemical cycling with far-reaching implications in climate studies linked to the estimation of sedimentary methane production and consumption.
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RC1: 'Comment on egusphere-2024-1829', Anonymous Referee #1, 21 Aug 2024
Review Sivan et al.
General comments
Sivan et al. investigate biogeochemical processes, in particular the anaerobic oxidation of methane coupled with Fe and/or Mn reduction (Fe-Mn-AOM), in seasonally hypoxic coastal sediments of the Eastern Arabian Sea. The main finding is that Fe-Mn-AOM is observed throughout the sediment core at several specific depths above and below the sulfate-methane transition zone (SMTZ). It is the first study to report Fe-Mn-AOM under sulfidic conditions. The authors argue that the activity of Fe-Mn-AOM at very specific depths is controlled by the distribution of metal-reducing and methanotrophic microbial communities in the sediment core.
While reading the manuscript, several concerns and questions arose. These could be addressed with major revisions. Overall, the article is of good quality, but the written language and some of the figures could be improved.
1) First of all, the present study lacks a clear/specific research question or hypothesis. In line 82-84, the authors only describe which aspects (driving factors fueling Fe-Mn-AOM) were investigated. However, the research gap and the specific research question of the study are not explained at this point.
2) The authors state that Fe-Mn-AOM is thermodynamically more favorable than SO42--AOM (line 52-53). Dissimilatory Fe reduction (DIR) however is thermodynamically more favorable compared to Fe-AOM. How do the authors rule out that the Fe peaks (and Mn peaks) above the SMTZ (where CH4 concentrations are close to zero) are not due to DIR (or Mn reduction)? The solid-phase data show that high levels of TOC and reactive Fe oxides are present. Furthermore, Mn-AOM is thermodynamically more favorable compared to Fe-AOM (Eq. 1 and 2); however the authors do not distinguish between Fe-AOM and Mn-AOM. Therefore, does it make sense that the Mn and Fe peaks are found at exactly the same depth (especially if the presence of Fe oxides is not the limiting factor)?
3) My main concern is that no information about the sediment composition/properties (e.g., lithology, grain size, bulk element content, porosity) is given. Only the sedimentation rate is reported (line 227). However, the given range (and hence the resulting sediment age) is also an order of magnitude different. Information on sediment composition is particularly important in an area with changing depositional conditions (as also described by the authors in line 310-320). How did the depositional conditions change in the past?
Looking at the Table S1 in the Supplemental Material, it is noticeable that the pore-water sample for the trace metal measurement above the Fe and Mn peaks is often missing. Is there a reason for this? Was less pore water extracted at the depths with the Fe and Mn peaks? Could this be related to differences in sediment composition? It is also striking that the Fe and Mn peaks are so regular.
In addition, Zones i to iii were defined only based on the presence or absence of sulfate and the Fe and Mn peaks. For example, are the zones homogeneous in terms of sediment composition?
4) Is the calculation in lines 247-256 based on the assumption that DIC release is due to Fe-AOM only? There are certainly other degradation processes of the organic material that lead to the release of DIC. Therefore, the value of 11000 µM is probably significantly overestimated. If so much Fe is supposed to have reacted with HS-, there should be a high enrichment of Fe sulfides in the corresponding layers. Was the Fe monosulfide, pyrite or total sulfur content also determined? The solid-phase contents might be essential to support the hypothesis.
5) If I understand correctly, the HS- concentrations in core SSK42/9 (Bhattacharya et al., 2021) are significantly lower than in this study. Are Fe and Mn concentrations also available for this core? Do the profiles also show the Fe and Mn peaks? Are the samples listed in Table S3 from layers with high Fe concentrations? Otherwise, the comparison with the microbial data from core SSK42/9 does not necessarily support the hypothesis that the Fe-Mn AOM activities in the specific zones are solely due to the presence of specific microbial communities.
5) The figures in the manuscript are very pixelated and some figures could be improved (please see specific comments).
Specific comments
Although not required by the journal, it would be useful to separate the Results and Discussion sections for an easier understanding.
Abstract
Line 22: δ13CCH4 and δ13CDIC were not introduced.
Introduction
Line 61: Aromokeye et al. was published in 2020.
Line 70: The sentence “The global distribution of Fe-Mn-AOM is plotted in Figure 1a” is a bit lost here. The global occurrence of Fe-AOM should be better integrated and discussed in the introduction.
Line 82-84: The specific research questions is missing. In addition, this is almost the same sentence as line 70-71.
Methods
Line 108: For headspace methane analysis, the sediment was extracted using 50 ml cut syringes at an interval of 10 cm and transferred into 20 ml headspace vials filled with 3 ml of KOH and 3 ml of NaN3 to trap CO2 and [stop instead of arrest] microbial activities respectively.
Line 115: Which constituents exactly?
Line 117: In what ratio were the samples acidified? How long after sampling were the trace metal aliquots acidified? Were the trace metal samples all diluted equally (i.e., 1:40)?
Line 124: GC was not introduced.
Line 161-162: Was the sediment thawed and homogenized prior to extraction? How much sediment and how much extraction solution was used? Was the water content taken into account when weighing the sediment?
Results and Discussion
Line 211-213: What does the negative correlation mean?
Line 232+245: “Tell-tale” is not the appropriate word here. I would write “clear” instead.
Line 234: In line 230, the authors state that AOM activity is not associated with significant pore water Fe2+ and Mn2+ peaks in Zone-iii, but in line 234 they state that Fe-Mn AOM occurs in Zone-iii. These two statements are contradictory.
Line 278-279: CH4 concentrations are generally low above the SMTZ (Otherwise there would be no SMTZ) and not only in the layers with Fe and Mn peaks.
Line 357-359: The last sentence is too general. Furthermore, SPOM is not the focus of this study and has only been mentioned once before. In the concluding sentence, the importance of the present study should be emphasized.
Figures
Figure 1b: What does the red line at the bottom right mean/indicate?
Figure 2: The figure is compressed. Labels a) to e) are not the same size and not at the same height.
Figure 3: Can these two figures be placed side by side?
Figure 4: The figure is also compressed. The TOC profile could also be added here.
Figure 5a: The Fe monosulfide and pyrite content is also shown schematically in this figure. Is it assumed to be constant with depth (see also General Comment 4)?
Figure 5c: The CH4 concentrations in the schematic representation reach near-zero values above the SMTZ (which is reasonable). How does Fe-Mn-AOM occur without any CH4 above the SMTZ?
Citation: https://doi.org/10.5194/egusphere-2024-1829-RC1 -
AC1: 'Reply on RC1', Aninda Mazumdar, 13 Sep 2024
Respected Reviewer 1
Sincere thanks for minutely going through our manuscript and giving valuable input. We have addressed each comment and have submitted point to point response. We sincerely hope that we will get an opportunity to revise the manuscript and submit it.
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AC1: 'Reply on RC1', Aninda Mazumdar, 13 Sep 2024
-
RC2: 'Comment on egusphere-2024-1829', Anonymous Referee #2, 23 Aug 2024
This manuscript presents geochemical data for sediments from a long core from the Eastern Arabian Sea. The authors combine porewater profiles of methane, sulfide, Fe2+ and Mn2+ and DIC with isotopic data for methane and DIC and results of sediment Fe speciation (2 steps). The results are used to argue that anaerobic oxidation of methane is coupled to Fe and Mn oxide reduction in a series of discrete layers in the sediment. While the topic is important and of interest to the BG readership, I have many concerns about the data and interpretation, as detailed below.
Methods:
- Iron and manganese in porewaters and sediments are highly sensitive to oxidation artefacts. The depth trends of both dissolved Fe and Mn show a range of spikes that are typical for profiles with such artefacts. The procedure for porewater collection and processing described in the methods refers to “a stream of argon gas” used to avoid oxidation of dissolved sulfide. The measures taken to avoid oxidation of dissolved Fe and Mn are not described. Notably, such a stream of gas does not generally prevent Fe2+ from oxidation. This is why sediment and porewater samples used for Fe2+ and Mn2+ analyses should be shielded fully from the atmosphere (e.g. in a glove box filled with argon or nitrogen) until the porewater samples are placed into in the vial in which the acid will be added to keep them in solution. Furthermore, no measures to avoid oxidation of the solid phase samples are mentioned in the text. FeS can easily oxidize to Fe oxides upon brief exposure to oxygen. The authors should clarify what they did and address the implications for their results.
- Methane is known to degas from sediments upon sample retrieval (see, for example: Jorgensen, 2021. Geochemical Perspectives 10 (2)) The potential impact of this process on the methane profile and other results should be discussed.
Presentation and interpretation
- The presentation and discussion of the results could be done in a much more structured and balanced way and many of the conclusions are speculative. The combination of the results and discussion makes it hard to obtain an overview of the data. Many interpretations in the text do not appear to be fully supported by the data. For example, the separation into “zones” with different “diagenetic regimes” based on porewater data alone is rather arbitrary. A firm case for Fe- and Mn-AOM in a series of specific zones (which is highly unusual!) requires strong support from solid phase and microbial data for the same sediment intervals and appropriate stoichiometric calculations and a scenario with a timeline of deposition and diagenesis that can also explain the generation of the spikes in Fe and Mn. Such data and such a scenario are not provided in the manuscript. In fact, based on the data presented, I don't see a clear case for Fe-AOM or Mn-AOM.
- Various terms are used that are not well-defined, such as “vital”, “tell-tale”, “biogeochemical phenomenon” etc
Detailed comments:
- Line 13: the authors write that metal-driven AOM is a “globally important biogeochemical process….”. To my knowledge, such a global role has so far not been shown.
- Line 18: from the data presented, the conclusion that there is Fe-Mn-AOM is speculative. Further evidence should be provided, including additional solid phase data and porewater profiles that allow a case to made that the spikes are real and not oxidation artefacts.
- Line 19. Change to in the sediment, not in the core
- Line 21, 22. Here, an increase in delta 13C-CH4 would be expected.
- Line 25: Microbes follow substrates, so this is rather speculative without data to support this.
- Line 29: If there are far-reaching implications, please specify them.
- Line 32: “a vital process” => what does “vital” refer to?
- Line 33-35: Please be more specific about the impact on global cycles or remove it.
- Line 38: Better to remove “syntrophic” since ANMEs may also do this alone as mentioned later in the text.
- Line 54: Please be more specific about the type of kinetics.
- Line 62: This is more complex than described: sulfate reduction coupled to organic matter degradation can generate sulfide that can react with the Fe and Mn oxides
- Line 70: Sources of the data used for figure 1B should be given
- Line 71. Remove tell-tale
- Line 73. Not clear what the number 1,80,000 refers to.
- Line 78. Add comma before “respectively”
- Line 88. I would suggest to use the same marker for the study site in both figures
- Line 93-94: I would propose to only add sites that are relevant to this study
- Line 109. Were the vials stored upside down?
- Line 111-112 using a stream of gas is not sufficient to avoid oxidation artefacts
- Line 113: add the material of the filter
- Line 114 helium headspace
- Line 116. Please add a reference for the method for sulfide trapping with Cd
- Line 117 Please provide the amount of acid per volume added
- Line 142 Please list the metals that are presented in the manuscript
- This is sulfate and not the sum of HS-
- Line 152. The detection limit is relevant here and should be given.
- Line 161. Explain what Fe minerals the asc and dithionite fractions are a measure of
- Line 176. Please add how the sample was desalinated.
- Line 188. Please focus on the sediment, not the core
- Figure 2. It is unusual that H2S and Fe2+ co-occur in porewaters since they would be expected to co-precipitate as FeS. The spikes in Fe and Mn are typical for oxidation artefacts. This warrants further research (e.g. repeated sampling at the same site while taking measures to fully exclude oxidation during sampling).
- Line 209: It is critical to be able to exclude methane degassing artefacts. Such spikes in methane are unusual and require large variations in methane production and removal over short depth intervals if the interpretation here is correct. Some quantification of the corresponding processes is needed to support the interpretation. The stoichiometry of Fe-AOM is such that you need quite some more change in dissolved Fe2+ than is seen here.
- Line 212 Please clarify what is meant by “Fe-Mn-AOM specific points”
- Line 229 Please clarify what the basis is for the conclusion that there is AOM in these layers.
- Line 232 See above. To the reader, it is not clear that there is evidence for Fe-Mn-AOM.
- Line 245-246 and later. See earlier comments. Fe2+ and H2S do not generally co-occur. You should assess your methods to exclude potential artefacts.
- Line 256 more solid phase data are needed e.g. S or FeS and FeS2 data
- Line 256. Mn-sulfide formation is rare. Here and elsewhere, the manuscript would benefit from Mn speciation for the sediment.
- Line 258. Change to “colloids”.
- Line 263 Oxidation artefacts often lead to erratic profiles so you cannot exclude artefacts here.
- Line 266. Convert to umol/g.
- Line 275. Rephrase to “throughout the sediment depth assessed”
- Line 286. See the comment above: a plausible scenario with a timeline of deposition and diagenesis is needed to explain what the sediment in these 500 cm represents, the depositional regime and how it has been altered upon burial.
- What about Mn oxides? Note that the TOC data belongs in the main manuscript
- Line 299. The relevance of the delta 15 N data and TOC/TON data to this paper is not clear.
- Line 304. Data on microorganisms in the water column cannot be directly coupled to those in the sediment
- The conclusion on focusing of microbial communities in distinct layers is speculative.
- The metagenomic data for a core from a site in the vicinity cannot be used. The data do not support the conclusion that there are distinct layers with microbes that can carry out Fe-Mn-AOM
- Line 344-346: The data set is not comprehensive enough and does not provide evidence for Fe-Mn-AOM, see comments above.
- Line 347-348. Unless this is FeS that has passed the filter or an artefact, see above.
- Line 349. The meaning of “Significant biogeochemical phenomenon” is not clear.
- Line 358. The link with seasonal hypoxia is not clear
Supplement:
- Presentation of the TOC data in wt% would allow for more easy comparison to other published work.
Citation: https://doi.org/10.5194/egusphere-2024-1829-RC2 -
AC2: 'Reply on RC2', Aninda Mazumdar, 13 Sep 2024
Respected Reviewer 2
Thank you for minutely going through our manuscript and providing valuable feedback. We have carefully addressed each of your comments and provided a detailed, point-by-point response. We sincerely hope that we will get an opportunity to revise the manuscript and submit it.
Status: closed
-
RC1: 'Comment on egusphere-2024-1829', Anonymous Referee #1, 21 Aug 2024
Review Sivan et al.
General comments
Sivan et al. investigate biogeochemical processes, in particular the anaerobic oxidation of methane coupled with Fe and/or Mn reduction (Fe-Mn-AOM), in seasonally hypoxic coastal sediments of the Eastern Arabian Sea. The main finding is that Fe-Mn-AOM is observed throughout the sediment core at several specific depths above and below the sulfate-methane transition zone (SMTZ). It is the first study to report Fe-Mn-AOM under sulfidic conditions. The authors argue that the activity of Fe-Mn-AOM at very specific depths is controlled by the distribution of metal-reducing and methanotrophic microbial communities in the sediment core.
While reading the manuscript, several concerns and questions arose. These could be addressed with major revisions. Overall, the article is of good quality, but the written language and some of the figures could be improved.
1) First of all, the present study lacks a clear/specific research question or hypothesis. In line 82-84, the authors only describe which aspects (driving factors fueling Fe-Mn-AOM) were investigated. However, the research gap and the specific research question of the study are not explained at this point.
2) The authors state that Fe-Mn-AOM is thermodynamically more favorable than SO42--AOM (line 52-53). Dissimilatory Fe reduction (DIR) however is thermodynamically more favorable compared to Fe-AOM. How do the authors rule out that the Fe peaks (and Mn peaks) above the SMTZ (where CH4 concentrations are close to zero) are not due to DIR (or Mn reduction)? The solid-phase data show that high levels of TOC and reactive Fe oxides are present. Furthermore, Mn-AOM is thermodynamically more favorable compared to Fe-AOM (Eq. 1 and 2); however the authors do not distinguish between Fe-AOM and Mn-AOM. Therefore, does it make sense that the Mn and Fe peaks are found at exactly the same depth (especially if the presence of Fe oxides is not the limiting factor)?
3) My main concern is that no information about the sediment composition/properties (e.g., lithology, grain size, bulk element content, porosity) is given. Only the sedimentation rate is reported (line 227). However, the given range (and hence the resulting sediment age) is also an order of magnitude different. Information on sediment composition is particularly important in an area with changing depositional conditions (as also described by the authors in line 310-320). How did the depositional conditions change in the past?
Looking at the Table S1 in the Supplemental Material, it is noticeable that the pore-water sample for the trace metal measurement above the Fe and Mn peaks is often missing. Is there a reason for this? Was less pore water extracted at the depths with the Fe and Mn peaks? Could this be related to differences in sediment composition? It is also striking that the Fe and Mn peaks are so regular.
In addition, Zones i to iii were defined only based on the presence or absence of sulfate and the Fe and Mn peaks. For example, are the zones homogeneous in terms of sediment composition?
4) Is the calculation in lines 247-256 based on the assumption that DIC release is due to Fe-AOM only? There are certainly other degradation processes of the organic material that lead to the release of DIC. Therefore, the value of 11000 µM is probably significantly overestimated. If so much Fe is supposed to have reacted with HS-, there should be a high enrichment of Fe sulfides in the corresponding layers. Was the Fe monosulfide, pyrite or total sulfur content also determined? The solid-phase contents might be essential to support the hypothesis.
5) If I understand correctly, the HS- concentrations in core SSK42/9 (Bhattacharya et al., 2021) are significantly lower than in this study. Are Fe and Mn concentrations also available for this core? Do the profiles also show the Fe and Mn peaks? Are the samples listed in Table S3 from layers with high Fe concentrations? Otherwise, the comparison with the microbial data from core SSK42/9 does not necessarily support the hypothesis that the Fe-Mn AOM activities in the specific zones are solely due to the presence of specific microbial communities.
5) The figures in the manuscript are very pixelated and some figures could be improved (please see specific comments).
Specific comments
Although not required by the journal, it would be useful to separate the Results and Discussion sections for an easier understanding.
Abstract
Line 22: δ13CCH4 and δ13CDIC were not introduced.
Introduction
Line 61: Aromokeye et al. was published in 2020.
Line 70: The sentence “The global distribution of Fe-Mn-AOM is plotted in Figure 1a” is a bit lost here. The global occurrence of Fe-AOM should be better integrated and discussed in the introduction.
Line 82-84: The specific research questions is missing. In addition, this is almost the same sentence as line 70-71.
Methods
Line 108: For headspace methane analysis, the sediment was extracted using 50 ml cut syringes at an interval of 10 cm and transferred into 20 ml headspace vials filled with 3 ml of KOH and 3 ml of NaN3 to trap CO2 and [stop instead of arrest] microbial activities respectively.
Line 115: Which constituents exactly?
Line 117: In what ratio were the samples acidified? How long after sampling were the trace metal aliquots acidified? Were the trace metal samples all diluted equally (i.e., 1:40)?
Line 124: GC was not introduced.
Line 161-162: Was the sediment thawed and homogenized prior to extraction? How much sediment and how much extraction solution was used? Was the water content taken into account when weighing the sediment?
Results and Discussion
Line 211-213: What does the negative correlation mean?
Line 232+245: “Tell-tale” is not the appropriate word here. I would write “clear” instead.
Line 234: In line 230, the authors state that AOM activity is not associated with significant pore water Fe2+ and Mn2+ peaks in Zone-iii, but in line 234 they state that Fe-Mn AOM occurs in Zone-iii. These two statements are contradictory.
Line 278-279: CH4 concentrations are generally low above the SMTZ (Otherwise there would be no SMTZ) and not only in the layers with Fe and Mn peaks.
Line 357-359: The last sentence is too general. Furthermore, SPOM is not the focus of this study and has only been mentioned once before. In the concluding sentence, the importance of the present study should be emphasized.
Figures
Figure 1b: What does the red line at the bottom right mean/indicate?
Figure 2: The figure is compressed. Labels a) to e) are not the same size and not at the same height.
Figure 3: Can these two figures be placed side by side?
Figure 4: The figure is also compressed. The TOC profile could also be added here.
Figure 5a: The Fe monosulfide and pyrite content is also shown schematically in this figure. Is it assumed to be constant with depth (see also General Comment 4)?
Figure 5c: The CH4 concentrations in the schematic representation reach near-zero values above the SMTZ (which is reasonable). How does Fe-Mn-AOM occur without any CH4 above the SMTZ?
Citation: https://doi.org/10.5194/egusphere-2024-1829-RC1 -
AC1: 'Reply on RC1', Aninda Mazumdar, 13 Sep 2024
Respected Reviewer 1
Sincere thanks for minutely going through our manuscript and giving valuable input. We have addressed each comment and have submitted point to point response. We sincerely hope that we will get an opportunity to revise the manuscript and submit it.
-
AC1: 'Reply on RC1', Aninda Mazumdar, 13 Sep 2024
-
RC2: 'Comment on egusphere-2024-1829', Anonymous Referee #2, 23 Aug 2024
This manuscript presents geochemical data for sediments from a long core from the Eastern Arabian Sea. The authors combine porewater profiles of methane, sulfide, Fe2+ and Mn2+ and DIC with isotopic data for methane and DIC and results of sediment Fe speciation (2 steps). The results are used to argue that anaerobic oxidation of methane is coupled to Fe and Mn oxide reduction in a series of discrete layers in the sediment. While the topic is important and of interest to the BG readership, I have many concerns about the data and interpretation, as detailed below.
Methods:
- Iron and manganese in porewaters and sediments are highly sensitive to oxidation artefacts. The depth trends of both dissolved Fe and Mn show a range of spikes that are typical for profiles with such artefacts. The procedure for porewater collection and processing described in the methods refers to “a stream of argon gas” used to avoid oxidation of dissolved sulfide. The measures taken to avoid oxidation of dissolved Fe and Mn are not described. Notably, such a stream of gas does not generally prevent Fe2+ from oxidation. This is why sediment and porewater samples used for Fe2+ and Mn2+ analyses should be shielded fully from the atmosphere (e.g. in a glove box filled with argon or nitrogen) until the porewater samples are placed into in the vial in which the acid will be added to keep them in solution. Furthermore, no measures to avoid oxidation of the solid phase samples are mentioned in the text. FeS can easily oxidize to Fe oxides upon brief exposure to oxygen. The authors should clarify what they did and address the implications for their results.
- Methane is known to degas from sediments upon sample retrieval (see, for example: Jorgensen, 2021. Geochemical Perspectives 10 (2)) The potential impact of this process on the methane profile and other results should be discussed.
Presentation and interpretation
- The presentation and discussion of the results could be done in a much more structured and balanced way and many of the conclusions are speculative. The combination of the results and discussion makes it hard to obtain an overview of the data. Many interpretations in the text do not appear to be fully supported by the data. For example, the separation into “zones” with different “diagenetic regimes” based on porewater data alone is rather arbitrary. A firm case for Fe- and Mn-AOM in a series of specific zones (which is highly unusual!) requires strong support from solid phase and microbial data for the same sediment intervals and appropriate stoichiometric calculations and a scenario with a timeline of deposition and diagenesis that can also explain the generation of the spikes in Fe and Mn. Such data and such a scenario are not provided in the manuscript. In fact, based on the data presented, I don't see a clear case for Fe-AOM or Mn-AOM.
- Various terms are used that are not well-defined, such as “vital”, “tell-tale”, “biogeochemical phenomenon” etc
Detailed comments:
- Line 13: the authors write that metal-driven AOM is a “globally important biogeochemical process….”. To my knowledge, such a global role has so far not been shown.
- Line 18: from the data presented, the conclusion that there is Fe-Mn-AOM is speculative. Further evidence should be provided, including additional solid phase data and porewater profiles that allow a case to made that the spikes are real and not oxidation artefacts.
- Line 19. Change to in the sediment, not in the core
- Line 21, 22. Here, an increase in delta 13C-CH4 would be expected.
- Line 25: Microbes follow substrates, so this is rather speculative without data to support this.
- Line 29: If there are far-reaching implications, please specify them.
- Line 32: “a vital process” => what does “vital” refer to?
- Line 33-35: Please be more specific about the impact on global cycles or remove it.
- Line 38: Better to remove “syntrophic” since ANMEs may also do this alone as mentioned later in the text.
- Line 54: Please be more specific about the type of kinetics.
- Line 62: This is more complex than described: sulfate reduction coupled to organic matter degradation can generate sulfide that can react with the Fe and Mn oxides
- Line 70: Sources of the data used for figure 1B should be given
- Line 71. Remove tell-tale
- Line 73. Not clear what the number 1,80,000 refers to.
- Line 78. Add comma before “respectively”
- Line 88. I would suggest to use the same marker for the study site in both figures
- Line 93-94: I would propose to only add sites that are relevant to this study
- Line 109. Were the vials stored upside down?
- Line 111-112 using a stream of gas is not sufficient to avoid oxidation artefacts
- Line 113: add the material of the filter
- Line 114 helium headspace
- Line 116. Please add a reference for the method for sulfide trapping with Cd
- Line 117 Please provide the amount of acid per volume added
- Line 142 Please list the metals that are presented in the manuscript
- This is sulfate and not the sum of HS-
- Line 152. The detection limit is relevant here and should be given.
- Line 161. Explain what Fe minerals the asc and dithionite fractions are a measure of
- Line 176. Please add how the sample was desalinated.
- Line 188. Please focus on the sediment, not the core
- Figure 2. It is unusual that H2S and Fe2+ co-occur in porewaters since they would be expected to co-precipitate as FeS. The spikes in Fe and Mn are typical for oxidation artefacts. This warrants further research (e.g. repeated sampling at the same site while taking measures to fully exclude oxidation during sampling).
- Line 209: It is critical to be able to exclude methane degassing artefacts. Such spikes in methane are unusual and require large variations in methane production and removal over short depth intervals if the interpretation here is correct. Some quantification of the corresponding processes is needed to support the interpretation. The stoichiometry of Fe-AOM is such that you need quite some more change in dissolved Fe2+ than is seen here.
- Line 212 Please clarify what is meant by “Fe-Mn-AOM specific points”
- Line 229 Please clarify what the basis is for the conclusion that there is AOM in these layers.
- Line 232 See above. To the reader, it is not clear that there is evidence for Fe-Mn-AOM.
- Line 245-246 and later. See earlier comments. Fe2+ and H2S do not generally co-occur. You should assess your methods to exclude potential artefacts.
- Line 256 more solid phase data are needed e.g. S or FeS and FeS2 data
- Line 256. Mn-sulfide formation is rare. Here and elsewhere, the manuscript would benefit from Mn speciation for the sediment.
- Line 258. Change to “colloids”.
- Line 263 Oxidation artefacts often lead to erratic profiles so you cannot exclude artefacts here.
- Line 266. Convert to umol/g.
- Line 275. Rephrase to “throughout the sediment depth assessed”
- Line 286. See the comment above: a plausible scenario with a timeline of deposition and diagenesis is needed to explain what the sediment in these 500 cm represents, the depositional regime and how it has been altered upon burial.
- What about Mn oxides? Note that the TOC data belongs in the main manuscript
- Line 299. The relevance of the delta 15 N data and TOC/TON data to this paper is not clear.
- Line 304. Data on microorganisms in the water column cannot be directly coupled to those in the sediment
- The conclusion on focusing of microbial communities in distinct layers is speculative.
- The metagenomic data for a core from a site in the vicinity cannot be used. The data do not support the conclusion that there are distinct layers with microbes that can carry out Fe-Mn-AOM
- Line 344-346: The data set is not comprehensive enough and does not provide evidence for Fe-Mn-AOM, see comments above.
- Line 347-348. Unless this is FeS that has passed the filter or an artefact, see above.
- Line 349. The meaning of “Significant biogeochemical phenomenon” is not clear.
- Line 358. The link with seasonal hypoxia is not clear
Supplement:
- Presentation of the TOC data in wt% would allow for more easy comparison to other published work.
Citation: https://doi.org/10.5194/egusphere-2024-1829-RC2 -
AC2: 'Reply on RC2', Aninda Mazumdar, 13 Sep 2024
Respected Reviewer 2
Thank you for minutely going through our manuscript and providing valuable feedback. We have carefully addressed each of your comments and provided a detailed, point-by-point response. We sincerely hope that we will get an opportunity to revise the manuscript and submit it.
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