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: open (until 14 Oct 2025)
- RC1: 'Comment on egusphere-2025-3906', Anonymous Referee #1, 05 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-3906', Anonymous Referee #2, 10 Oct 2025
reply
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
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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.