the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Dec 2025
Reaction-transport modelling of methane cycling beneath the Greenland Ice Sheet
Abstract. Glacial and ice sheet advances have buried large amounts of organic matter (OM), which under anoxic subglacial conditions can be microbially converted into methane (CH4). Although CH4 emissions have been observed at glacier margins, the capacity of subglacial environments to sustain such fluxes remains uncertain. To address this, we developed a reaction–transport model (RTM) to simulate CH4 production, transformation, and transport in sediments beneath warm-based regions of the Greenland Ice Sheet (GrIS) margin. The model explores a wide range of environmental conditions, including sediment thickness, OM quantity and reactivity, O2 availability, and methanotrophic activity.
Model simulations show that subglacial sediments are largely anoxic. Oxygen (O2) penetration into subglacial sediments is generally restricted to the upper few tens of centimetres, with an average penetration depth of 22.8 cm. Microbial OM degradation and aerobic CH4 oxidation (AeOM) represent the main O2 sinks. Their relative contributions vary with CH4 availability. AeOM dominates in methane-rich sediments, whereas OM degradation prevails in methane-poor environments. Modeled depth-integrated methanogenesis rates range from 0.1 to 1600 mmol-CH4 m-2 yr-1 (mean 73 mmol-CH4 m-2 yr-1) and are primarily controlled by OM reactivity, with sediment depth and OM concentration exerting only a small secondary influence. This sensitivity of CH4 production rates to OM reactivity can produce sharp thresholds, where small decreases in reactivity strongly suppress CH4 fluxes. A highly variable fraction of the generated CH4 is consumed by AeOM within the shallow oxygenated zone, and is controlled by OM reactivity and the AeOM rate constant. Resulting net diffusive CH4 fluxes can range between 0–234.7 mmol m-2 yr-1. Results show that even shallow sediments (<1 m) can sustain a significant CH4 release into the subglacial environment when highly reactive OM is available, while oxidation efficiency tends to decline in thick, OM-rich deposits.
Comparison with field measurements of CH4 export data from southwest GrIS catchments suggests that observed fluxes could be already sustained by subglacial sediments that contain as little as 0.6 wt% of relatively unreactive OM assuming a catchment sediment cover of 10 % with sediment depths of 9 m.
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Status: open (until 13 Feb 2026)
- CC1: 'Comment on egusphere-2025-5506', Angelos Theodorou, 08 Jan 2026 reply
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RC1: 'Comment on egusphere-2025-5506', Anonymous Referee #1, 04 Feb 2026
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It was my pleasure to read “Reaction-transport modelling of methane cycling beneath the Greenland Ice Sheet”. This paper is an important step in understanding the methane fluxes observed from the Greenland Ice Sheet. As a modelling study, it necessarily makes some simplifying assumptions. Most critically, it assumes that oxygen is the only relevant terminal electron acceptor. The authors recognise this as unlikely, but quantifying the other electron acceptors would be difficult given the current state of science. The paper does give a sense that you can simulate almost any methane flux from the Greenland Ice Sheet by varying the relevant parameters within plausible boundaries. But it does, importantly, give us the tool to match this large range of methane fluxes to the conditions that might produce them. Thus, it offers something to refer back to as variable methane fluxes are observed. Reverse engineering geochemical fluxes under glaciers is tricky and I do think this paper contributes to solving this problem. In general, I think the effort here is solid and the paper should be published after moderate to major revisions.
The most significant issue is that Section 3 needs to be restructured. This section is supposedly methods, but it includes long digressions into background information that offer some sort of justification for the methods selected. This makes it almost impossible to properly read as section 3 as a methods section. All of the material that isn’t methods (which, by word count, is most of it) needs to be pulled into the introduction and ideally shortened. The authors could consider a separate “justification of model parameters” section, but they shouldn’t take this too far. We all know that this kind of modelling is only possible if simplifying assumptions are made.
I also think there needs to be substantial revision to section 4.5, where upscaling calculations are made. Because the authors find their model can produce almost any value depending on assumptions of sediment thickness and organic matter reactivity, plugging these into real glaciers gives fluxes that range over 6 to 7 orders of magnitude. This is not particularly useful as a result. Instead, the authors should present the actually measured values from these glaciers in terms of their model. I.e. explain the range of conditions that might have produced the range of measurements. It would be especially useful if they could analyse seasonal variability through these parameters.
I have some line-by-line comments below:
Line 10: My heart sank when I saw from the first line that the authors were planning to abbreviate organic matter throughout the paper. The use of nonstandard abbreviations of this sort can make the paper significantly harder to read, as the reader is always having to go back and look for the meaning of OM, RTM, AeOM, GrIS, TEAs, etc. I know this a personal pet peeve and other readers might not be as bothered but consider whether this actually necessary.
Line 14: Remove “a wide range of environmental conditions, including”. This is, in fact, the complete list of parameters that you consider in this paper.
Line 33: I might add the word to “potentially” to “burying vast stocks”. The literature cited is fairly speculative, and since glacier growth can be highly erosive, the extent of organic matter burial remains uncertain.
Figure 1: I know this is a conceptual figure, but the image of a meltwater channel equal in thickness to the subglacial sediment package is wrong – especially when you later consider sediment packages up to 15 m thick! Also adjust the text to not overlap with the drawing and change the font to match the font used by the journal.
Lines 70-72: This is not needed here. Equations, etc., should appear in the main text.
Line 95: I thought the top was water, per the conceptual model.
Line 96: There certainly would not be cryoturbation in the subglacial environment! Cryoturbation is caused by seasonal freezing and thawing. The bed of the Greenland Ice Sheet is variably frozen or warm, but it does not vary seasonally. However, there very well could be sediment shear from the driving stress of the glacier overhead. I think it’s okay ignore sediment shear, but the reference to cryoturbation has me worried.
Lines 98-99: You generally should avoid calling out sections in advance.
Line 101: Remove the word “environments”.
Line 110: Salinity wouldn’t exactly be zero, but maybe it’s close enough to zero that it might as well be?
Section 3.3: This is the first of several sections composed more of background information than strictly methods. I would try to take the discussion about electron acceptors, methanogenesis pathways, and previous work into the introduction and have this section just be a description of how the model was implemented in the study (i.e. have the subsection start at line 149). I also think this goes into far too much detail justifying why simplifying assumptions were made. Readers will perfectly well understand why you can’t include every possible electron acceptor or methanogenesis pathway into the model.
Line 117: You can just have “(Table C1)”.
Line 119: I had to go back and look up what TEAs meant, highlighting my earlier point about acronyms.
Line 124: Sulphur is the one electron acceptor that you don’t explore in any further detail here. Of all the electron acceptors, sulphur is by far the most abundant in Greenland Ice Sheet waters.
Line 126: “Statham et al.”; I assume The Cryosphere doesn’t expect three author lists.
Line 137: There’s a missing link here or the logic of the paragraph needs to be reordered. The terms acetoclastic and hydrogenotrophic need context.
Line 138: Does The Cryosphere allow for citation of papers in review?
Line 165: I wonder to what extent the normal aging process might not apply or might not apply in the same way to the subglacial environment.
Line 166: I don’t think it’s necessary to call out upcoming sections in advance.
Lines 168-186: This is also introductory material. The methods section should focus on the methods.
Line 170: Again, you don’t need a preamble before citing a table.
Line 171: Parentheses should only be around the year when the authors names form part of the sentnece.
Line 180: You do not define AOM. (Again, too many acronyms.)
Lines 180-183: To me, these values for sulphur are not particularly low. What are you comparing this to here? As far as I know, there’s never been any evidence for sulphur reduction under the Greenland Ice Sheet. But I don’t think that’s for lack of sulphur; sulphate is one of the major anions found in subglacial waters in Greenland and in glacial environments in general. There’s a missing conceptual piece here. It might be the mechanochemistry angle. Subglacial environments are generally highly oxidative.
Line 185: Again, can you site manuscripts in preparation in The Cryosphere?
Line 194: Not necessary.
Section 3.6.1: It might be worth pointing out the glacial forefield in the study area also consists of a mix of rocky and sediment mantled regions.
Line 241: This is very unlikely. It’s far more plausibly that there are patches where organic matter is highly concentrated due to minimal erosion for whatever happenstance of the subglacial environment alongside vast areas with almost nothing. On the other hand, uniformity is far easier to model (especially in a nonarbitrary way).
Line 247: RCM is undefined.
Line 254-272; Figure 2: I needed to re-read this several times to understand the main point. State the main point – i.e. how all of this affects your model – much more clearly and consider omitting some of the detail. The figure could maybe go to the supplement.
Line 287: Is there a good reason not to have the temperature increase by 0.5°C over the 15 m sediment package? Unlike the other simplifying assumptions, I don’t see how this would make the model any harder to run.
Line 323: The pressure in the subglacial environment is usually considerably below ice overburden pressure, at least during the summer when water is discharging from the proglacial environment. You might want to look into borehole studies to see what is actually more realistic. This fluctuating pressure would also have implications for the stability of clathrates, should they form. I.e. if a clathrate formed in winter, when water pressure was high, it might well melt in summer as pressures drop.
Section 3.6.5: This should at least briefing discuss all the work Jon Telling’s group has been putting out on the mechanochemistry of glaciers, which may be critically important to why they’re so highly oxidative (e.g. Stone et al., 2023, Gill-Olivas et al., 2024, etc.).
Line 385: This goes back to the hard bed / soft bed question. In the areas with thick sediment packages, methanogenesis is potentially extensive. But it remains unclear how prevalent those are – i.e. it could be mostly a hard bed with only patches of thick sediment.
Line 394: I keep coming back to this, but to me GSA is Geological Society of America or perhaps General Services Administration.
Lines 396-400: I find this section very confusing. You first say that saturation is rarely reached and then say that it is often exceeded. Which is it?
Line 399: I would not point the reader back to the methods here. If the reader goes back there, they will also find the suspiciously even number of 50 mM proposed without any citation or justification.
Line 400: That is a huge range.
Line 439: Russell has a much smaller catchment than Isunnguata Sermia or Leverett. Perhaps relevant to Figure 6.
Line 448: Missing “The”.
Line 448: The dominant role of organic matter reactivity is also clear just by scrutinising your figures. Are there not any datasets available to put greater constraints on this? I.e. studies of particulate organic matter in the outflow?
Line 454: This again brings us back to the question of characterising sediment thickness under the ice.
Line 490: You need to say how this estimate is derived. Cowton got to 600 km2 by just assuming the active area started below 1200 m. Did you make a similar assumption here? If so, I find this assumption dubious. Anywhere where meltwater contacts the bed is “active”, cutting the hydrologic catchment off at a certain elevation doesn’t make any sense unless there is no surface melt above that elevation.
Line 494: You must be using the entire catchments not the hydrological active ones to get these values. Otherwise Isunnguata Sermia would only be twice Leverett, not an order of magnitude larger.
Line 495: I know the Hatton paper is still under review, but how do they get a flux ranging from 0.002- 28.9 Mg per year. That’s a 4 order of magnitude range. This must be a measurement range not error bars on a single estimate of flux. If there really is a 4-order-of-magnitude measurement range at Isunnguata Sermia, that’s actually something very meaty that your model could explore. When do you get the low values, when do you get the high values? Over what temporal and spatial scale does this vary? If your model is right, it would imply that the glacier is accessing different parts of the bed that either have thicker sediment or more reactive sediment over these timescales. Or perhaps it’s simply a function of water residence time per figure 6.
Line 498: This needs to be completely rethought. You can’t make any “suggestion” based on a model output that varies over 6 orders of magnitude depending on model input.
Line 501: I’m having trouble following your math here. How do you get from 0.006-0.13 micromolarity multiplied by 3.1-7.9 cubic km of water flux to 0.002-28.9 Mg per year for Isunnguata Sermia? Whether multiplying the top and bottom end members or doing formal error propagation, you cannot get such a large range in the final value. A mistake seems to have been made somewhere.
Line 520: I would remove “or evades”. It seems very unlikely that gas could evade from the subglacial environments of Greenland.
Line 540: Or it may be substantially smaller; you produced values over an almost 7 order of magnitude range. I don’t think this model can be used in this way. I think it can be used to interpret measurements, but not to postulate about fluxes that haven’t been measured.
Lines 544-547: I don’t follow this argument. This distance between moulins in the interior and the subglacial outflow can be 10s of km, I don’t see how potential evasion at the outlet could have anything to do with diffusive flux to the meltwater channel in the interior.
Line 549: There isn’t an unpressurised zone under any flow condition at Isunnguata Sermia, which is highly overdeepened and terminates in a fountain that sprays the subglacial water several meters into the air. There might be under certain conditions at Leverrett, but I find Chandler’s evidence for it (which is based on the behaviour of a tracer gas) less than convincing.
Line 552-557: This is highly, highly unlikely. The routing of subglacial water is entirely driven by the surface slope of the glacier and the shape of the bed. The notion that the large outlets might flow backward against these gradients is incredibly doubtful and isn’t based on anything observed in subglacial hydrology (e.g. in borehole studies). On the contrary, borehole studies and fundamental physics (e.g. Rothlisberger, 1972) suggest that pressure is lower in the large outlets during high flow, so if there were to be any rerouting of methane, it would be to the large outlets, not away from them. But, then again, there is no evidence that the small outlets ever flow up ice overburden pressure gradients either.
I might point out the authors have presented a very good reason why the big outlets have less methane – longer water residence times. This seems fully explanatory as to why the Russell Glacier study and the Christiansen study (which was of a small, minor outlet) both present far higher methane concentrations than Leverett and Russell. It’s unclear why the authors seem to want to run away from this conclusion by coming up with highly dubious explanations of where the missing methane might have gone.
Line 580: This is very repetitive with what you wrote on line 448.
Lines 592-598: I would bring up the water residence time finding here too. In my view this, as much as anything, explains why methane fluxes are sometimes miniscule (especially in large systems like Isunnguata Sermia).
Table C2: The values for the relative proportions of carbon, nitrogen, and phosphorus in subglacial organic matter are from a 1934 study of plankton? Surely there must be some more current and relevant values to go by? Say, studies of arctic soil organic matter and/or plant material?
Citation: https://doi.org/10.5194/egusphere-2025-5506-RC1
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Interesting read. I do not recall coming across such type of research. Considering methane's high warming potential, it has been a hot topic, specifically for permafrost degradation in peatlands and in other periglacial systems.
Could your model be applied to cold-based ice-sheets/glaciers, or do you think that it does not make any sense? We have seen from peatlands that there are some fluxes even during the winter season when grounds are frozen (in subarctic Finland and northern Sweden at least), so maybe there could be some fluxes even below cold-based glaciers?
Angelos