the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Oct 2025
Methane releases across the Laptev Sea signaled by time-integrated biomarkers of aerobic methane oxidation
Abstract. Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the δ13C-composition of specific C30 hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation and thereby provide a proxy for methane release. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low δ13C-C30 hopenes signals, ranging between −57.5 to −37.1 ‰ (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane release with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted δ13C-C30 hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane emissions. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier δ13C-C30 hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the δ13C-C30 hopenes were still much lower than δ13C-OC, indicating aerobic methane oxidation and a clear methane release signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of methane releases in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs with the molecular-isotopic pattern being preserved in the sedimentary archive.
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Status: open (until 15 Dec 2025)
- RC1: 'Comment on egusphere-2025-4756', Anonymous Referee #1, 04 Nov 2025 reply
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- 1
This study utilizes lipid biomarker signatures from surface sediment samples to obtain a time-integrated signal of methane release events in the Laptev Sea. The results suggest that methane oxidation occurs across the Laptev Sea shelf, including the mid-shelf region that was previously known as a region of low methane emissions. This study is relevant for the community, and for better understanding methane cycling in the Arctic. Overall, it is very well-written and very thorough. I have several minor comments and suggestions regarding the manuscript.
Comments:
Lines 23-24, 76, and 81: The authors interpret the d13C values of the hopanoids as a proxy for “methane release”. In principle, yes, there does appear to be a correlation between methane concentrations and d13C values, however, this has not been thoroughly tested in diverse environmental settings and with different methanotroph communities. It would be more accurate to say that this proxy reflects aerobic methane oxidation, and therefore enhanced methane cycling. I would recommend rephrasing this in both the abstract and the manuscript by replacing “methane release” with “enhanced methane cycling” or “methane oxidation”.
Lines 86-87: Based on previous studies, does methane-oxidation predominantly occur in the sediment or the water column? Or does this vary based on the site?
Lines 268-275: Based on these measurements the methane concentrations are highly variable between sites as you mentioned. Is most of the methane being emitted from the Laptev Sea from a diffusive or ebullitive flux? Previous studies have shown that aerobic methanotrophs are usually less efficient with oxidizing methane when the methane flux is mostly ebullitive. If the flux is mostly ebullitive rather than diffusive, do you expect this to influence the proxy signatures and your interpretations? Coming back to my previous comment, is the water fully oxygenated at all these sites? If so, do you expect most of the methane-oxidation to occur in the water column and sediment, and how would this influence the proxy signatures you obtain?
Lines 335-336: Not all members of the family Methyloligellaceae are considered methanotrophs. The majority of the classified strains appear to be methylotrophs, including Methyloligella and certain strains of Methyloceanibacter. You can still mention these as potential candidates for Type II methanotrophs, but you should add a sentence somewhere indicating that these are not all necessarily methanotrophs. Further, do you know whether Methyloligella and Methyloceanibacter have the capacity to produce hopanoids? It would be good to confirm this as this can influence some of your interpretations. You can check this by searching for the sqhc gene (accession no. WP_038942977.1) on the NCBI database, and then checking to see if either of these species contain the gene (see Richter et al. 2023 Biogeosciences for more details).
Lines 344-354: The title of this subsection and the content seem unrelated. This section also sounds like it belongs more in the conclusions rather than at the start of the discussion.
Line 381-383: Are there hydrothermal vents in the ILS region?
Lines 382-383: The reference that you site here (Takeuchi et al. 2014) says the strain of Methyloceanibacter is a methylotroph and not a methanotroph. Going back to my previous comment, maybe check to see if they produce hopanoids.
Lines 383-387: Since it is unclear whether some of the MOB you have classified as Type II MOB are methanotrophs, I would say it is difficult to fully exclude terrestrial inputs in the ILS region. Do you have any other independent biomarkers that can give an indication of how much of your signal here is derived from terrestrial sources? In your next paragraph, you seem to indicate that terrestrial inputs are relatively high, so you might still have a terrestrial signal from your methanotrophs.
Line 392: Check your reference for the “10% bacteria”. Belin et al (2018) was not the first paper to report this, this was already shown in previous studies.
Line 401: Could the high methane concentrations in the ILS region also be derived from the Lena River rather than in situ production in the ILS, or a combination of both? Based on your figure 3, it seems like the methane in this region should be very depleted. Could this tell us a bit more about the source of the methane in this region?
Line 417: change to “here we show”
Line 429: Change “Our display” to “Our biomarkers”
Comments on figures:
Figure 1: The numbers for the stations are hard to read in the figure and against the subsea permafrost shading. Consider making the numbers black to make them easier to read. It would also be helpful if you could indicate the outer, mid-, and inner Laptev Sea regions in this figure.
Figure 3: In the caption you say the shaded “gray zone” but in the figure it looks “green” to me. Consider changing this. All of these figures are showing the same things but the varying scales are a bit confusing. Consider making this into one large figure that contains all of the same information to make it easier to read.
Figures 4 & 5: It would be helpful if you could indicate the ILS, MLS, and OLS regions on these figures. It would make the figures easier to interpret and to know which station numbers and data points belong to which region.