the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Aug 2025
Seafloor chemosynthetic habitats and AOM-influenced sediment microbiome at a cold-water coral site off the Vesterålen coast, northern Norway
Abstract. Cold seeps associated with cold-water corals have been reported worldwide. Yet, there are still knowledge gaps regarding ecological relationships due to contrasting observations. Here, we report the results from a multidisciplinary study on cold seeps off the Vesterålen coast (northern Norway) hosting coral mounds. We discuss the geochemical results from sediment (carbon-nitrogen systematics, foraminifera) and pore fluids (sulfate, dissolved inorganic carbon, methane) in relation to seafloor habitats (orthomosaics and habitat maps). Microbial mats are the dominant seep-related community, forming white patches of a few ten cm in diameter, mostly distributed along the edges of methane-derived authigenic carbonates and cracks on top of them. Foraminifera tests in the sediment display negative δ13C values down to − 18.5 ‰, suggesting ongoing authigenic carbonate precipitation. We also report the discovery of a macroscopic white biofilm, observed while slicing a pushcore onboard. Organic matter analyses indicated that the sediment interval hosting this biofilm is associated with a sharp drop in δ13C values, as negative as −43.4 ‰. Results from 16S rRNA gene analyses on the uppermost 10 cm in the same core showed a significant shift in microbial community. Protebacteria-dominated communities near the seafloor transition to a Halobacterota-dominated composition mainly consisting of ANME-1b anaerobic methanotrophs in correspondence of the biofilm interval. Corals in this area are spatially associated with seafloor chemosynthetic habitats and bubbling, but not vice versa, suggesting that seafloor emissions do not influence coral distribution. Instead, the presence of a methane-charged sediment substrate leading carbonate crust formation and food supply by high-energy currents appears to be a prerequisite for cold-water corals development in this area.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-3906', Anonymous Referee #1, 05 Oct 2025
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AC2: 'Reply on RC1', Claudio Argentino, 31 Oct 2025
Authors: We thank the reviewer for the valuable feedback on the manuscript. As also noted by Reviewer 2, we understand that the main concerns are related to the inclusion of CWCs in the narrative and the fragmented nature of the discussion of datasets.
For clarity, we are reiterating the same introductory comment here to place our reply in the correct context. This is a multidisciplinary study in biogeosciences investigating the shallow-subsurface biogeochemical environment, which directly influences the microhabitat types and their spatial distribution observed at the seafloor. CWCs are a significant habitat component (widespread, not only the two mounds shown in Fig. 1, and threated by ocean acidification with major potential implications, see comments below) in this seepage area, which is why this site was initially chosen for a major multidisciplinary project (EMAN7). What is the role of seeps, if any, on the development of CWC habitats in the Lofoten-Vesterålen area? what is the biogeochemical relationship between methane seepage and seafloor ecosystems? That is why, although this is not a paper about biology of CWCs, they are indeed the reason why we conducted such studies. We will clarify this in the introduction and place less emphasis on CWCs to avoid conveying a misleading message to reader. Several datasets are challenging to combine but each of them is important to depict a different aspect of the methane-dominated sediment biogeochemical system. We will improve the main text based on the valuable comments provided by the reviewers.
Please find below the replies to specific comments from Reviewer 2.
Best regards
REVIEWER1
R1: I appreciate the opportunity to review the manuscript by Argentino et al., which documents the seafloor features and sediment/porewater geochemistry of a cold seep and cold-water coral site off Vesterålen, northern Norway. The authors present ROV observations together with geochemical and microbial analyses of the sediments and porewaters. The study compiles a diverse dataset, including seafloor imagery and mosaics, porewater concentrations and isotopic compositions, foraminiferal and bulk sediment geochemistry, as well as DNA data.
However, the conclusions drawn from these various datasets appear rather fragmented.
For example, the authors discuss existing hypotheses regarding (1) the association between cold-water corals and hard substrates in cold-seep environments (e.g., methane-derived carbonates), (2) the occurrence and control of macroscopic biofilms, and (3) the origin of organic matter with anomalously low stable carbon isotopic signatures. Despite these discussions, a coherent overarching conclusion is lacking. It also remains unclear how these seemingly unconnected findings advance our understanding of cold seep or coral–seep interactions. Does this case study reveal an overlooked process or mechanism? If so, how? Or does it challenge an existing paradigm? The broader scientific significance and impact of the study should be articulated more clearly.
Authors reply: The scope of the manuscript will be refined based on the key questions presented in the initial comment and also considering the recently-published papers in the same site (Sauer et al., 2015; Ferre’ et al., 2024; Sert et al., 2025). The spatial heterogeneity of methane biogeochemistry influences the distribution of seafloor chemosynthetic communities but does not appear to control coral distribution. Instead, coral distribution appears to be primarily restricted by topographic constraints. The geological relationship has been recently highlighted by other authors, and we will ensure it is clearly presented in the manuscript.
R1: In addition, some datasets reported do not appear to contribute meaningfully to the main objectives (for instance, the rationale for including foraminiferal stable isotope data is not explained).
Authors reply: The foraminifera datasets are meant to investigate the fluctuations of the SMTZ through time, as foraminifera can record methane-derived carbonate overgrowth induced by alkalinity produced via AOM. Foraminifera can be used as a proxy for paleo-seepage reconstructions. Our results show a rather stable modern SMTZ keeping the pace of sedimentation. We will ensure the connections between the datasets are clearly presented in the revised manuscript.
R1: When examining the specific conclusions more closely, not all are fully supported by the data presented. Additional considerations and possibly further analyses may be needed to substantiate some interpretations. Certain analytical methods should be verified, and the principles and assumptions underlying the calculations or models should be described explicitly. In particular, where correlations between microbial DNA data and environmental parameters are discussed, the authors often infer causation from correlation.
Authors reply: Organic matter parameters, pore water gradients and microbiological data are pointing to the same interpretation, which is a methanotrophic-dominated sediment interval at around 10 cm bsf. To avoid wrong conclusions about the biofilm itself we carefully avoided using direct interpretations extrapolated from the sediment analyses (we did not conduct the measurements on the biofilm but on the sediment from that interval, same sediment slice collected onboard). Regarding the correlation of DNA data with the environmental parameters, we will make it clear that the sequencing data cannot designate methanotrophy as a function but only infer that taxa associated with methanotrophy are present in different relative abundances throughout the sediment. This part and Fig. 8 will be moved to the Supplementary Information. The modeling of the AOM end-member is a well-established approach used for extrapolating isotopic end-members of pore water DIC (e.g. Martin et al., 2000 https://doi.org/10.1016/S0016-7037(99)00424-X, Thomas et al., 2002 https://doi.org/10.1016/S0967-0645(02)00135-2) as well as sedimentary TOC (e.g. Coffin et al., 2017, Argentino et al., 2023). Here, the assumption is that the methanotrophic biomass is being added to a background bulk TOC pool which remains the same over the investigated layer corresponding to a few cm interval dominated by methane-related geochemical signals. Additional information and relevant literature will be included in the manuscript.
R1: These interpretations should be revisited and revised as necessary. Detailed comments are provided in the annotated PDF file attached.
Authors reply: We thank the reviewer for the comments in the attached pdf. We agree on most of the comments and will correct accordingly. Just a couple of clarifications here: Precision value for IC was provided by external laboratory and accuracy was assured by running certified materials as unknowns which are in line with certified values. Most importantly, if you look at the measured sulfate concentrations in the 0-2 cm interval reported in the excel made available to the reviewers as well as in the plots, they consistently show seawater values (~29 mM), confirming the reliability of the data. MDAC were measured previously by Cremiere et al., 2016 10.1038/ncomms11509 demonstrating that these are indeed seep carbonates. We also collected samples which will be included in a dedicated geochemical paper on proxy development, since their methane-derived origin was already established.
R1: There are a few minor issues with the figures, but overall the figure quality is good.
Citation: https://doi.org/10.5194/egusphere-2025-3906-AC2
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AC2: 'Reply on RC1', Claudio Argentino, 31 Oct 2025
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RC2: 'Comment on egusphere-2025-3906', Anonymous Referee #2, 10 Oct 2025
The manuscript by Argentino et al. presents a compilation of several data sets (seafloor imaging, sediment solid-phase and pore water analysis from push cores, as well as genetic information from one of them, including a white biofilm dominatly composed of anaerobic methanotrophs) from sediment habitats at/near cold seeps in the Atlantic Ocean off northern Norway, which are located near a cold-water coral (CWC) site. The idea behind this is that there is an interrelationship between the CWCs and the cold seeps, but I see no evidence of this in the current version. I also do not understand the addition of foraminiferal data (including d18O, d15N, and C/N), as these are not discussed at all. There is no data shown for the CWC site except for the image in Figure 1.
Overall, this paper attempts to summarize all the data in a single manuscript, which in most cases is not a good decision, as it causes the context to be lost and the reader to lose track of the big picture. The current version results in a highly fragmented manuscript that does not even provide clues as to the interrelationship between cold seeps and CWCs, except that they are in close vicinity. I would rather suggest removing unnecessary data and pursuing the “one article, one story” approach. Therefore, I recommend that the authors fundamentally revise the current submission and reduce the data sets to those that are relevant to the most important interpretations supported by the results.
Specific comments:
Title: By adding CWCs, the authors attempt to construct an argument that is not subsequently presented. Based on what I have read, such a combination is not necessary. Although CWCs are evident in this region (and there is a push core, but which was not used), the discussion of the data does not benefit from this fact. Removing this construction will increase the likelihood of arriving at some kind of context.
Line 23: Complicated sentence. Rewrite.
Paragraph starting at line 40: Is it really necessary to expand the focus of the manuscript to include ocean acidification and coral reefs/CWCs (see further comments below)?
Line 44: I don't understand this sentence.
Paragraph from line 50 (to line 58): Again, not much data on this topic is presented later on. Is this part necessary?
Figure 1: The CWCs are outside the range of the push cores used later for interpretation. I do not understand the general correlation announced in the title.
Lines 87 and 119: How can I obtain information about methane dynamics in a core if this is not provided? Simply by measuring sulfate and assuming that this is caused exclusively by AOM? No data on concentration profiles or d13C of methane in the core are provided.
Line 123: Where can I find information on this in the current version? Please provide an echo sounder image.
Line 190: An accuracy of 1.2 mM for the DIC concentration seems rather high. This leads to a certain degree of uncertainty in the definition of the diagenetic zones in the cores.
Lines 193–197: Where can I find the data on methane concentration in the core? These are not displayed.
Line 199: The data from this is not used later in the discussion, but only displayed in the results section.
Line 208: d13C of the three internal reference materials?
Line 212: d15N of the three internal reference materials?
Line 273: I find it difficult to discern this clear definition of curvatures and interpretations from the diagrams.
Line 286: What can the reader deduce from these concentrations? Are these integrated values over the entire core length? Maxima in a specific horizon? Which one exactly? Depth profiles should be specified.
Figure 3: The names of the push cores are difficult to read in the images. The compilation is good, but do we need them all? Reducing the number of figures would improve readability. Adding methane profiles seems reasonable. This would define the SMTZ horizon much better. If there is a dominant advective component, this is also good to know. Why are there question marks? What do they mean?
Figure 4: These data with information on the deeper sections are not discussed at all in the manuscript. I think they can be deleted or moved to the supplementary information. As in Figure 3, the details are difficult to read if the size is retained.
Figure 5: The caption with information on sampling can be deleted. The figure may be good information that can be easily included in the SI.
Figure 6: Differences in color coding and the associated microbes are difficult to discern. Please change.
Lines 349 to 370: This is a general and lengthy introductory section that is unnecessary here. Why should this be specific to CWC-associated cold seep habitats?
Line 371: What other parameters did the other studies use to identify the SMTZ?
Lines 384 to 387: I do not see any connection to the data obtained in the current study.
Line 392: How high is the “methane charge” in mM? Must be defined or deleted.
Line 396: This statement contradicts any previously established connection to CWC. Delete this line and adjust the entire manuscript accordingly.
Lines 402 to 413: Another introductory section that is not needed here.
Line 436: This is indeed a very interesting finding, and I would like to see more data on it.
Figure 8: Is this figure really helpful? Is there a correlation with depth/core position?
Line 461: To my knowledge, ANMEs predominantly use DIC and not OM.
Line 468: What “heavier” carbon sources do the authors mean? Please clarify this. Does this make sense and is it supported by previous literature?
Line 474: How can greater incorporation of carbon derived from methane, i.e., DIC, be tested? Isn't this simply a pure biofilm without a sediment matrix from which ANMEs cannot be easily extracted?
Line 482: The only point at which the authors return to the foraminifera data.
Line 483: The general statement is not justified given the location studied by the authors.
Line 490: Proximity does not imply interdependence.
Line 492: Is there any new data on carbonate crusts other than those from OBIA in this article?
Lines 502 to 505: This general conclusion is not supported by the current data and should be removed.
Citation: https://doi.org/10.5194/egusphere-2025-3906-RC2 -
AC1: 'Reply on RC2', Claudio Argentino, 31 Oct 2025
Authors: We thank the reviewer for their valuable feedback on the manuscript. We understand that the main concerns pertain to the presence of CWCs in the story and the fragmented datasets.
We want to highlight the fact that this is a multidisciplinary study in biogeosciences investigating the shallow-subsurface biogeochemical environment, which directly influences the microhabitat types and their spatial distribution observed on the seafloor. CWCs are a significant habitat component (widespread, not only the two mounds shown in Fig. 1, and threated by ocean acidification with major potential implications, see comments below) in this seepage area, which is why this site was initially chosen for a major multidisciplinary project (EMAN7). What is the role of seeps, if any, on the development of CWC habitats in the Lofoten-Vesterålen area?, what is the biogeochemical relationship between methane seepage and seafloor ecosystems? That is why, although this is not a paper about biology of CWCs, they are indeed the reason why we conducted such studies. We will clarify this in the introduction and place less emphasis on CWCs to avoid conveying a misleading message to reader.
As we write in our point-by -point replies below, several datasets are challenging to combine but each of them is important to depict a different aspect of the methane-dominated sediment biogeochemical system. We will improve the main text based on the valuable comments provided by the reviewers.
Please find below the replies to specific comments.
Best regards
REVIEWER 2
R2: The manuscript by Argentino et al. presents a compilation of several data sets (seafloor imaging, sediment solid-phase and pore water analysis from push cores, as well as genetic information from one of them, including a white biofilm dominatly composed of anaerobic methanotrophs) from sediment habitats at/near cold seeps in the Atlantic Ocean off northern Norway, which are located near a cold-water coral (CWC) site.
The idea behind this is that there is an interrelationship between the CWCs and the cold seeps, but I see no evidence of this in the current version.
I also do not understand the addition of foraminiferal data (including d18O, d15N, and C/N), as these are not discussed at all.
Authors reply: The geochemical analyses on foraminifera are important for tracking the precipitation of methane-derived carbonate at modern and ancient sulfate-methane transition zones. They must be combined with pore water data for us to be able to determine whether a SMTZ anomaly in foraminifera is associated to a modern or paleo-SMTZ and interpret SMTZ fluctuations in terms of methane seep dynamics. Our foraminifera results show anomalies which we ascribe to AOM at the modern SMTZ as indicated by pore water profiles. We acknowledge that the foraminiferal data (δ13C, δ18O) are not clearly introduced or fully integrated in the current version of the manuscript. We will address this by further developing this concept, emphasizing the importance of foraminiferal proxies for biogeochemical interpretations.
Sedimentary organic matter analyses (carbon and nitrogen geochemistry) are also meant to track the methanotrophic activity at modern and ancient SMTZs at cold seeps (in combination with pore water data again). The effectiveness of this proxy to track the accumulation of AOM-related biomass in sediment cores has been established in recent papers (e.g. Argentino et al., 2023 https://doi.org/10.1016/j.chemgeo.2023.121638; Gosch et al., 2025 https://doi.org/10.1029/2025GL116435; Szymczycha et al., 2025 https://doi.org/10.1016/j.marchem.2025.104570) and it’s combination with foraminiferal geochemistry as well (e.g. Argentino et al., 2024 https://doi.org/10.1016/j.marpetgeo.2024.106761).
The robustness of biogeochemical interpretations of the shallow subsurface conditions at this site based on a multiproxy approach is supported by having multiple datasets showing different, yet interrelated, aspects of sediment biogeochemistry. We believe it is important to retain the foraminiferal profiles in the manuscript. The products of these processes are essential for interpreting the observed microhabitat types and their distribution at the seafloor.
R2: There is no data shown for the CWC site except for the image in Figure 1.
Authors reply: Corals form two main mounds shown in fig. 1 but minor coral build-ups are also present over a larger area hosting gas seeps (Ferre’ et al., 2024 https://doi.org/10.1029/2024JC020949). Sediment samplings in correspondence of the mounds and adjacent sediment were hindered by the presence of a stiff layer of coral debris and carbonate pavements partially exposed. Moving away from the mounds, we obtained cores for biogeochemical investigations which are used here to determine the lateral variations of subsurface biogeochemical processes and methane dynamics, which explain microhabitat distribution seen at the seafloor. The corals settled inside the seepage area that we characterized using several cores and different biogeochemical proxies. We will revise the introduction and abstract to clarify that we do not present biological studies on corals but on the biogeochemical setting over which they thrive.
R2: Overall, this paper attempts to summarize all the data in a single manuscript, which in most cases is not a good decision, as it causes the context to be lost and the reader to lose track of the big picture.
Authors reply: The strength of our interdisciplinary study is the fact of having different datasets depicting different biogeochemical aspects of the investigated environment. We agree that these datasets should be better integrated, and the interpretations combined into a cohesive narrative. We will revise the manuscript to improve its coherence and explicitly highlight the connections between the datasets.
R2: The current version results in a highly fragmented manuscript that does not even provide clues as to the interrelationship between cold seeps and CWCs, except that they are in close vicinity. I would rather suggest removing unnecessary data and pursuing the “one article, one story” approach. Therefore, I recommend that the authors fundamentally revise the current submission and reduce the data sets to those that are relevant to the most important interpretations supported by the results.
Authors reply: please see previous comments.
R2: Title: By adding CWCs, the authors attempt to construct an argument that is not subsequently presented. Based on what I have read, such a combination is not necessary. Although CWCs are evident in this region (and there is a push core, but which was not used), the discussion of the data does not benefit from this fact. Removing this construction will increase the likelihood of arriving at some kind of context.
Authors reply: we can rephrase the title to place less emphasis on CWCs.
R2: Line 23: Complicated sentence. Rewrite.
Authors reply: we will rephrase, thank you.
R2: Paragraph starting at line 40: Is it really necessary to expand the focus of the manuscript to include ocean acidification and coral reefs/CWCs (see further comments below)?
Authors reply: We understand the reviewer’s perspective. This paragraph was meant to put the study into the context of why we are interested in this place, which is the EMAN7 project. This paragraph was intended to provide context for why we are interested in this location, specifically in relation to the EMAN7 project. We will revise the introduction to place less emphasis on the corals and focus more on seep biogeochemistry.
R2: Line 44: I don't understand this sentence.
Authors reply: That was just connected to the previous one about consequences of coral loss. We will modify the introduction.
R2: Paragraph from line 50 (to line 58): Again, not much data on this topic is presented later on. Is this part necessary?
Authors reply: The link between CWCs and seep carbonates is fundamental to understanding their occurrence in this seepage area. Our manuscript is a follow-up of others (Ferre’ et al., 2024, Serth et al., 2025) integrating the deeper subsurface to the seafloor.
R2: Figure 1 : The CWCs are outside the range of the push cores used later for interpretation. I do not understand the general correlation announced in the title.
Authors reply: please see previous comments.
R2: Lines 87 and 119: How can I obtain information about methane dynamics in a core if this is not provided? Simply by measuring sulfate and assuming that this is caused exclusively by AOM? No data on concentration profiles or d13C of methane in the core are provided.
Authors reply: We agree that the word “dynamics” is not clear. We confirmed high methane concentrations by measuring headspace samples from the bottom of the cores. Methane concentrations will be also reported in a dedicated table to be more accessible. Shallow sulfate methane transition zones in sediment with low TOC and high methane concentrations are controlled by sulfate-driven AOM here. Instead of “methane dynamics” we suggest modifying to “methane biogeochemistry”. The gas isotopes were presented in a previous study Sauer et al., 2015 showing a thermogenic origin.
R2: Line 123: Where can I find information on this in the current version? Please provide an echo sounder image.
Authors reply: Multibeam echosounder maps are shown in Fig. 1 and uploaded to a database. Not sure what the reviewer is referring to, but it can be certainly fixed.
R2: Line 190: An accuracy of 1.2 mM for the DIC concentration seems rather high. This leads to a certain degree of uncertainty in the definition of the diagenetic zones in the cores.
Authors reply: This is not an accuracy. This is the highest value of repeatability (precision) obtained on 5 duplicates, following the terminology recommended by the International Association of Geoanalysts (IAG). This is a conservative value to report, as is commonly accepted in geoscientific metrology. DIC values in our dataset range from 2 to 23 mM (PusC04), the magnitude of our repeatability does not compromise biogeochemical zonation interpretations.
R2: Lines 193–197: Where can I find the data on methane concentration in the core? These are not displayed.
Authors reply: The data only appear in the main text but we will report the values in a table.
R2: Line 199: The data from this is not used later in the discussion, but only displayed in the results section.
Authors reply: We acknowledge that the foraminiferal data (δ13C, δ18O) are not clearly introduced or fully integrated in the current version of the manuscript. We will address this by further developing this concept, emphasizing the importance of foraminiferal proxies for biogeochemical interpretations.
R2: Line 208: d13C of the three internal reference materials?
Authors reply: the d13C values of the in-house standards are 1.96 ‰, −10.21‰ and – 48.95 ‰. These will be specified in the text.
R2: Line 212: d15N of the three internal reference materials?
Authors reply: the d15N values of the in-house standards are 20.73 ‰, 6.67 ‰ and −4.91‰. These values will be specified in the text.
R2: Line 273: I find it difficult to discern this clear definition of curvatures and interpretations from the diagrams.
Authors reply: We agree that some profiles with fewer data cannot be accurately described in terms of shapes. We will only leave the more evident ones in the figure and move the others to the Supplementary Information.
R2: Line 286: What can the reader deduce from these concentrations? Are these integrated values over the entire core length? Maxima in a specific horizon? Which one exactly? Depth profiles should be specified.
Authors reply: We agree that these values need more context and explanation in the text.
R2: Figure 3: The names of the push cores are difficult to read in the images. The compilation is good, but do we need them all? Reducing the number of figures would improve readability. Adding methane profiles seems reasonable. This would define the SMTZ horizon much better. If there is a dominant advective component, this is also good to know. Why are there question marks? What do they mean?
Authors reply: We agree that the names are hard to read and we will improve the readability of this figure. We included a large dataset to better represent lateral heterogeneities in this area in terms of microhabitats and subsurface processes. Advection is confirmed by ROV observations of gas bubbling and piezometer measurements reported in Ferre’ et al., 2024. In pink we marked the intervals influenced by AOM as indicated by both sulfate and DIC datasets, as we say in the caption “ identified in correspondence of a drop in sulfate concentrations and δ13CDIC, and increases in DIC concentrations”. The question marks just refer to the uncertainty on intervals where either sulfate or DIC data are missing. We will clarify the meaning of this symbol.
R2: Figure 4: These data with information on the deeper sections are not discussed at all in the manuscript. I think they can be deleted or moved to the supplementary information. As in Figure 3, the details are difficult to read if the size is retained.
Authors reply: We will improve the readability by increasing font size of these figures. The foram data will be presented more in detail and better integrated in the discussion.
R2: Figure 5: The caption with information on sampling can be deleted. The figure may be good information that can be easily included in the SI.
Authors reply: We agree and will move Fig. 5 to the Supplementary Information. The biofilm is mainly included as visual observation and the discussion will also be modified accordingly.
R2: Figure 6: Differences in color coding and the associated microbes are difficult to discern. Please change.
Authors reply: We will modify color palette of this figure.
R2: Lines 349 to 370: This is a general and lengthy introductory section that is unnecessary here. Why should this be specific to CWC-associated cold seep habitats?
Authors reply: We agree that the title here is misleading, we do not mean of course that the described conditions are representative of all CWC-associated seep habitats but that they pertain specifically to this case study. We will make this clarification.
R2: Line 371: What other parameters did the other studies use to identify the SMTZ?
Authors reply: The approach used to determine the SMTZ can vary depending on whether you conduct modeling or just track the SMTZ stratigraphic location. For example, Lee et al., 2019 published on Biogeosciences used only sulfate profiles to determine SMTZs; Sen et al., 2018 on Biogeosciences used several parameters to conduct modeling. Fischer et al., 2012 used sulfide and sulfate. Sulfate is the key parameter when enough data points to plot.
R2: Lines 384 to 387: I do not see any connection to the data obtained in the current study.
Authors reply: It is known that the dynamic nature of cold seeps can be traced at different timescales (e.g. Ferre’ et al., 2024 and references therein), this sentence can be omitted as it does not add much to the discussion.
R2: Line 392: How high is the “methane charge” in mM? Must be defined or deleted.
Authors reply: Compared to background conditions found in areas with no seep indications nor chemosynthetic habitats. We can define values.
R2: Line 396: This statement contradicts any previously established connection to CWC. Delete this line and adjust the entire manuscript accordingly.
Authors reply: We will rephrase as follows: The CWCs in this area are consistently associated with chemosynthetic substrates and bubbling. However, not all those areas host CWCs, indicating that additional factors, such as the availability of hard substrates and suitable topography, are required for their development.
R2: Lines 402 to 413: Another introductory section that is not needed here.
Authors reply: We believe that providing more context of isotopic thresholds helps to interpret the results.
R2: Line 436: This is indeed a very interesting finding, and I would like to see more data on it.
R2: Figure 8 : Is this figure really helpful? Is there a correlation with depth/core position?
Authors reply: We can move it to the Supplementary Information to reduce the number of figures as suggested earlier.
R2: Line 461: To my knowledge, ANMEs predominantly use DIC and not OM.
Authors reply: Thank you for noticing this, we will correct.
R2: Line 468: What “heavier” carbon sources do the authors mean? Please clarify this. Does this make sense and is it supported by previous literature?
Authors reply: We will support this with relevant literature and cite the sources.
R2: Line 474: How can greater incorporation of carbon derived from methane, i.e., DIC, be tested? Isn't this simply a pure biofilm without a sediment matrix from which ANMEs cannot be easily extracted?
Authors reply: We have to clarify in the text that this modelled d13C value refers to the “added” organic matter in this interval showing isotopic anomalies. It is based on measurements on bulk sediment geochemistry, not pure biofilm. So we need to rephrase this.
R2: Line 482: The only point at which the authors return to the foraminifera data.
Authors reply: Please see previous replies about foraminifera.
R2: Line 483: The general statement is not justified given the location studied by the authors.
Authors reply: The sentence “Cold seeps offshore Vesterålen islands revealed a unique Arctic environment where CWCs coexist with seafloor chemosynthetic communities and local sedimentary hotspots of methane oxidation.” describes what we found in this study, as written in previous replies.
R2: Line 490: Proximity does not imply interdependence.
Authors reply: The connection of CWCs and subsurface gas migration pathways was demonstrated in Ferre’ et al., 2024. We agree that this concept should be better presented at the beginning of the manuscript.
R2: Line 492: Is there any new data on carbonate crusts other than those from OBIA in this article?
Authors reply: The carbonate crusts geochemistry will be covered by a separate manuscript focused on geochemistry. The reason for this choice is that carbonate pavements d13C and d18O from this site were already reported by Cremiere et al., 2016 and already proved the methane-derived origin of these crusts.
R2: Lines 502 to 505: This general conclusion is not supported by the current data and should be removed.
Authors reply: We will rephrase this to include the findings from previous studies and provide an overall picture of the biogeochemical influence of seepage on seafloor ecosystems.
Citation: https://doi.org/10.5194/egusphere-2025-3906-AC1
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AC1: 'Reply on RC2', Claudio Argentino, 31 Oct 2025
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I appreciate the opportunity to review the manuscript by Argentino et al., which documents the seafloor features and sediment/porewater geochemistry of a cold seep and cold-water coral site off Vesterålen, northern Norway. The authors present ROV observations together with geochemical and microbial analyses of the sediments and porewaters. The study compiles a diverse dataset, including seafloor imagery and mosaics, porewater concentrations and isotopic compositions, foraminiferal and bulk sediment geochemistry, as well as DNA data.
However, the conclusions drawn from these various datasets appear rather fragmented. For example, the authors discuss existing hypotheses regarding (1) the association between cold-water corals and hard substrates in cold-seep environments (e.g., methane-derived carbonates), (2) the occurrence and control of macroscopic biofilms, and (3) the origin of organic matter with anomalously low stable carbon isotopic signatures. Despite these discussions, a coherent overarching conclusion is lacking. It also remains unclear how these seemingly unconnected findings advance our understanding of cold seep or coral–seep interactions. Does this case study reveal an overlooked process or mechanism? If so, how? Or does it challenge an existing paradigm? The broader scientific significance and impact of the study should be articulated more clearly. In addition, some datasets reported do not appear to contribute meaningfully to the main objectives (for instance, the rationale for including foraminiferal stable isotope data is not explained).
When examining the specific conclusions more closely, not all are fully supported by the data presented. Additional considerations and possibly further analyses may be needed to substantiate some interpretations. Certain analytical methods should be verified, and the principles and assumptions underlying the calculations or models should be described explicitly. In particular, where correlations between microbial DNA data and environmental parameters are discussed, the authors often infer causation from correlation. These interpretations should be revisited and revised as necessary. Detailed comments are provided in the annotated PDF file attached.
There are a few minor issues with the figures, but overall the figure quality is good.