the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Addition of brackish water to tundra soils does not inhibit methane production: implications for Arctic coastal methane production
Abstract. In Arctic regions where coastal sediments contain permafrost, global climate change drives processes such as erosion and subsidence. The contribution of these processes to carbon emissions are still uncertain. Relative sea level rise can lead to more waterlogged environments, promoting anoxic degradation of organic matter but it can also lead to a greater exposure of coastal sediments to seawater. This could alter methane (CH4) production dynamics, although the controls remain poorly understood. For instance, sulfates contained in seawater may have a tampering effect on methanogenesis through competitive inhibition but the increase in microbial abundance could enhance methanogenesis. In this study, we present CH4 production rates alongside geochemical analyses in a rapidly evolving coastal landscape near the community of Tuktoyaktuk, NWT, Canada, which is located in the continuous permafrost zone. To better constrain CH4 production dynamics along the land to ocean continuum, sediment profiles were collected from nearshore marine sediments, as well as from the active layer of the coastal (intertidal) zone and inland soils. Anoxic incubations were performed, amended with brackish water to simulate the effect of seawater on the breakdown of organic matter and the production of CH4. We found marine sediments expectedly led to negligible CH4 production rates, while the inland sites showed variable rates between null and 35 nmol cm-3 d-1. The coastal (intertidal) zone had the highest rates reaching 415 nmol cm-3 d-1. Interestingly, sulfate present in brackish water and sediments did not suppress methanogenesis in the incubations of the coastal and inland zones. Analyses of stable carbon isotopes from CH4 produced in the incubation experiment indicated greater acetotrophy and higher organic matter lability in the coastal zone, possibly contributing to higher CH4 production rates. This study highlights the potential for significant CH4 emissions even with high sulfate concentrations which are classically thought to inhibit methanogenesis. This suggests that Arctic coastal microbial CH4 production might be an understudied source to the atmosphere.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2570', Maren Jenrich, 25 Jul 2025
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RC2: 'Comment on egusphere-2025-2570', Anonymous Referee #2, 18 Aug 2025
Overall Evaluation
This paper focuses on the dynamics of methane production in permafrost regions of the Arctic coastal zone. Through multi-site sampling, anaerobic incubation experiments, and geochemical analyses, it explores the impact of brackish water input on methane production in permafrost soils. The research topic holds significant scientific value and practical significance. The study found that methane production rates in the coastal zone are high and that brackish water does not significantly inhibit methane production, challenging traditional perceptions and providing a new perspective for Arctic carbon cycle research.
However, there are still aspects in the experimental design, result analysis, and data presentation that need improvement.
Specific Evaluations
- No control experiment without brackish water addition was set up, making it impossible to clarify the specific impact of brackish water itself on methane production (e.g., whether there is a promoting effect). Additionally, no experimental groups with different sulfate concentration gradients were established, which weakens the persuasiveness of the key conclusions of this study.
- The incubation period lasted as long as 339 days, but the MS did not elaborate on how stable experimental conditions (such as temperature fluctuations and substrate consumption) were maintained during this process, which may affect the reliability of the results.
- No determination was made on sulfate reduction rates or related functional genes (such as the *dsrA* gene of desulfovibrio). The coexistence of sulfate reduction and methane production was only inferred from the "smell of sulfide," resulting in insufficiently direct and adequate evidence.
- In Figure 6, the correlation analysis between the stable carbon isotope composition of methane and methane production pathways only cites the research of Hornibrook et al. (1997, 2000), without conducting in-depth cross-validation with other data in this paper (such as TOC content and sulfate concentration).
- Regarding the explanation for the high methane production rate in the coastal zone, it is mentioned that "it may be related to the labile organic matter brought by goose feces" in lines 445-446, but there is a lack of direct experimental evidence (such as analytical data on organic matter in goose feces), making this explanation somewhat tenuous.
- The study found significant differences in methane production rates among different landforms (such as high-centered and low-centered polygons), but key environmental factors of various landforms (such as organic matter degradation rate, microbial community composition, and pore water chemical gradients) were not systematically measured. That is, it is unclear whether factors such as hydrological differences caused by landforms (e.g., water saturation), organic matter activity (as indicated by δ¹³C-TOC), or microbial community structure dominate the spatial heterogeneity of methane production.
- In the discussion, the explanation of the mechanism by which sulfate does not inhibit methane production (such as non-competitive methanogenesis and syntrophic methanogenesis) was not analyzed in combination with the specific data of this study, making it relatively general.
- For the determination of sulfate and chloride ion concentrations (Method 2.2), it is clearly stated that "only one measurement was performed for each sample." Although stability tests showed a variation rate of < 3%, there is a lack of biological replicates (such as different sampling points of the same landform type), making it impossible to rule out the interference of spatial heterogeneity.
- In the stable carbon isotope analysis (δ¹³C-CH₄), only one incubation vial was used for each depth. Although 2-3 instrument replicate measurements were conducted, no biological replicates (such as parallel incubation vials for the same treatment) were set up, making it difficult to distinguish between real differences among samples and experimental errors.
- All figures and tables (such as Figure 4, Figure 5, and Table 1) only display means and standard deviations, without statistical tests (such as t-test and ANOVA) to verify the significance of differences between groups (such as different landform types and different sites).
Citation: https://doi.org/10.5194/egusphere-2025-2570-RC2
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- 1
General comments:
The authors present a novel, valuable and well-executed study on methane production dynamics in Arctic coastal soils under the influence of brackish water, simulating coastal processes such as tidal flooding and storm surges.
The study shows that methane production is significantly higher in coastal permafrost soils compared to inland sites. By combining field sampling along a land-to-ocean gradient near Tuktoyaktuk, NWT, Canada with long-term anoxic incubations and stable CH4 isotope analyses, the study offers new insights into methane dynamics in underexplored coastal permafrost environments.
Contrary to traditional assumptions, the presence of sulfate-rich brackish water does not suppress CH₄ production in coastal and inland sites. Instead, sea level rise and coastal processes such as tides and storm surges may enhance emissions by increasing the lability of organic matter. The findings also suggest that methanogenesis can coexist with sulfate reduction, potentially through syntrophic or methylotrophic pathways.
Strengths of the study:
- Innovative experimental design that simulates realistic coastal environmental change.
- Comprehensive data collection across inland, coastal, and marine sites, combined with robust geochemical and isotopic analyses.
- The study provides important insight into underexplored Arctic coastal methane sources.
- The extrapolation of findings to landscape-scale CH₄ production enhances the relevance of the results.
- The authors discuss the potential overestimation due to closed-system incubations.
Specific comments and recommendations:
1) Did you include control incubations without brackish water? If not, I would suggest this for the next incubation study as the comparison would strengthen the interpretation of the brackish water effect and sulfate inhibition hypothesis.
2) I am missing a discussion on methane oxidation since in the ocean a large percentage of the produced CH4 is oxidised before reaching the atmosphere.
3) Consider discussing the seasonality or temporal dynamics of coastal CH₄ production, even if only conceptually.
4) In my opinion, you are using the term "active layer" also for unfrozen zones beneath water bodies, which are referred to as "taliks" (for example, at the Harbour site). This should be expressed more precisely throughout the manuscript.
Line 15: I agree in general that the processes of erosion and subsidence on carbon emissions are understudied but I would be careful with the wording here. There are some publications e.g. Tanski et al., 2019 measuring C release caused by erosion.
Line 42: For a more concrete statement, could you give examples for rapid environmental changes?
Line 55: I like this figure which represents your study sites. In my mind it would fit better in section 2.1. Consider moving.
Line 71: A reference from an Arctic study would fit better here.
Line 114 – Figure 2: For a better orientation please add a dot for Tuktoyaktuk or add to the description that the Harbour is located in Tuktoyaktuk. Further I strongly recommend to add detail maps of the individual study sites (eg. high resolution satellite images where the coring locations are marked) to get a better understanding of the landscape and the exact sampling positions.
Line 118: I like the clear explanation on why you chose the sampling sites.
Line 125: Please add a range of the core lengths.
Line 132: In figure 1 pondlets are labled as thaw ponds. For consistency I would recommend to use the same terms. You could also label each core and refer to that in the text.
Line 141: If I’m not mistaken you are using the terms profile and core equally but they represent different approaches. I guess from a trough you collected a core? Please explain and/or revise the manuscript accordingly. It would be helpful to have profile/ core pictures and some additional information such as a brief description and profile depth /core length for all sites. Especially for the profiles numbered with 10 a detail map would be ideal to get an understanding of the location and the environment.
Line 183-186: Do I understand correctly that the gas concentration measurements were not continuous throughout the entire 339-day period? Was the final measurement on day 339? What do you mean by back calculating? Were the CH₄ production rates calculated solely from the linear accumulation observed during the first 16 weeks?
Line 217: I am no expert on stable methane isotopy
Line 238-244 (Figure 3): I like your figures but to me the graphs in plot c and d are hard to distinguish, especially 10A and 10C. Think of adjusting the colours. All information needed is written in the figure caption but I think the legend could be improved by adding some more
information such as location name. You are talking about the active layer at the harbour site. Do you really mean active layer or rather talik? Below waterbodies the unfrozen layer usually is called talik.
Line 266 (Figure 4): Add to the caption that the error bars are the grey lines. To me the error bars miss the end point marking but if you note in the caption that the grey lines are the error bars, it’s clear.
Line 303 (Figure 6): I suggest to relocate some of the information given in the caption to the figure itself, such as “acetoclastic” and “hydrogenotrophic methanogenesis” you also could add "permafrost" to panel b. Please label the vertical line. This makes it easier for the reader to understand your figure.
Line 342: In general, it is true that long-term sulfate input is inhibiting methanogenesis, but there are field studies which show that low sulfate concentrations or recent inundation is either not impacting or even promoting methanogenesis. Please distinguish this statement and incorporate more recent findings. For example, Yang et al., 2023 (https://onlinelibrary.wiley.com/doi/10.1111/gcb.16649); Jenrich et al., 2024 (https://onlinelibrary.wiley.com/doi/abs/10.1002/ppp.2251) and Jenrich et al., 2025 (https://bg.copernicus.org/articles/22/2069/2025/).
Line 355: You could add Yang et al., 2023 as an example for thermokarst lagoons which are a transition zone from terrestrial to marine environments.
Line 414-417: Very cool finding! I agree with that. In a recent study I also found that CH4 and CO2 production is highest during the first stages of land-sea transition and that CH4 production decreases with increasing marine influence.
Line 456: Great to see that in situ flux measurements and incubation results are in line.
Line 468-474: I like the comparison and reasoning, and I agree that Arctic soils represent an important source of CH₄. However, to strengthen the argument on a global scale, it would be helpful to include a size comparison between Arctic coastal wetlands and tropical coastal wetlands.
494: I like the upscaling approach to show the impact of sea water inundation on a bigger landscape scale.