the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jul 2025
Mineral-bound organic carbon exposed by hillslope thermokarst terrain: case study in Cape Bounty, Canadian High Arctic
Abstract. Arctic landscapes could add 55–230 Pg of carbon (in CO2 equivalent) to the atmosphere, through CO2 and CH4 emissions, by the end of this century. These estimates could be quantified more accurately by constraining the contribution of rapid thawing processes such as thermokarst landscapes to permafrost carbon loss, and by investigating the exposed organic carbon (OC) interacting with mineral surfaces or metallic cations, i.e., the nature of these interactions and what controls their relative abundance. Here, we investigate two contrasted types of hillslope thermokarst landscapes: an Active Layer Detachment (ALD) which is a one-time event, and a Retrogressive Thaw Slump (RTS) which repeats annually during summer months in the Cape Bounty Arctic Watershed Observatory (Melville Island, Canada). We analyzed mineralogy, total and soluble element concentrations, total OC and mineral-OC interactions within the headwalls of both disturbances, and within corresponding undisturbed profiles. Our results show that OC stabilized by chemical bonds account for 13 ± 5 % of total OC in the form of organo-metallic complexes and up to 6 ± 2 % associated with poorly crystalline iron oxides. If we add the mechanisms of physical protection of particulate organic matter in aggregates and larger molecules stabilized by chemical bonds, we reach 64 ± 10 % of the total OC being stabilized. Importantly, we observe a decrease in the proportion of mineral-bound OC in the deeper layers exposed by the retrogressive thaw slump: the proportion of organo-metallic complexes drops from ~18 % in surface samples to ~1 % in the deepest samples. These results therefore suggest that the OC exposed by thermokarst disturbances at Cape Bounty is protected by interactions with minerals to a certain extent, but that deep thaw features could expose OC more readily accessible to degradation.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3428', Anonymous Referee #1, 15 Aug 2025
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RC2: 'Comment on egusphere-2025-3428', Adrian A Wackett, 19 Oct 2025
GENERAL COMMENTS –
The authors present a case study from the Cape Bounty Arctic Observatory on Mellville Island in the Canadian high Arctic, where they have measured the quantity and quality of organic carbon (OC) in permafrost exposed by two contrasting types of abrupt thaw disturbances: active layer detachments (ALD) and retrogressive thaw slumps (RTS). The authors find that roughly 20% of the total OC pool is chemically-bound and another ~45% is physically occluded, which they discuss in the context of other circumpolar sites where abrupt thaw disturbances are occurring. Their study is well-motivated by the wealth of literature linking permafrost thaw to the carbon-climate feedback, which typically (and simplistically) presumes gradual exposure of thermally-stabilized permafrost C as the active layer steadily thickens with high-latitude warming. This is an important and interesting research question that is well-contextualized by their introduction, and in general I find their writing easy to follow and their main inferences to be well-documented. I also greatly appreciate their efforts to make their data easily interpretable and verifiable by including a comprehensive set of figures and tables in the supplement.
As currently written the discussion section starts to drag at times, and I see some opportunities to streamline the text there by incorporating some relatively straightforward analyses (some of which they already allude to). Shortening the discussion of their measurements in the context of other Arctic sites would also free up some space to discuss the unique characteristics of permafrost soils and how this might impact the stability and behavior of chemical and/or physical OC-mineral interactions under different thaw scenarios, which seems critical given their emphasis on linking mineral (and specifically sorptive) protection to OC stabilization.
There are some minor statistical errors to address and I highlight a few opportunities to incorporate additional analyses/measurements that would further improve the study, although I fully recognize that additional measurements may not be viable
SPECIFIC COMMENTS –
Organic carbon concentrations vs. stocks/inventories: Have you measured bulk density and/or coarse fraction contents for the four profiles? It seems likely that these abrupt thaw events would alter the volume of soil contained within a given depth scale (i.e., from 0 to 70 cm), even if sampling was restricted to headwall features. I recognize that much of your focus is on the form of OC contained within the profiles, but it seems important to discuss these inferences on a volumetric basis as well given that this is what ultimately controls the quantity of potentially mineralizable OC. If these measurements have been made it would be great to see them included alongside some discussion of how changes in OC stabilization might be augmented (or neutralized) by concomitant volumetric changes, especially in the lowermost RTS-D sample where the most profound differences are observed
Regression modeling: Given that you are regressing multiple observations from individual pedons, I recommend a linear mixed modeling framework with profile and site included as nested random factors. Given the small sample size there may not be enough degrees of freedom available to build a nested model, but at minimum I suggest re-running the regression models with profile included as a random intercept and/or slope term. This seems critical considering that there are several instances where there are visible variations in the relationship (i.e., slope) at the profile scale (see Fig. D1c, D7b-f, etc.) A hierarchical modeling framework would help tease this out and enrich the ensuing discussion. The robust regression approach they opted for (rather than OLS) can still be exported into a mixed modeling framework (see robustlmm package in R for one example)
Streamlining discussion section: As mentioned earlier, the authors do a commendable job of placing their results from Cape Bounty in the context of other circumpolar sites that they (and others) have studied, but the writing in the discussion begins to drag on at times. Could you shorten the writing and instead insert some relatively simple analyses examining the dominant environmental and edaphic controls on mineral-associated OC across the permafrost thaw sites that you reference? For example, perhaps take the cumulative portion of stabilized (both physical and chemical) OC and plot these site-level averages against mean annual temperature, precipitation, or NPP. It’s probably also worth exploring some edaphic controls of interest like extractable Fe+Al content, silt+clay content (see Georgiou et al., 2022 https://doi.org/10.1029/2009JG000947), pH, cation exchange capacity, etc. As the authors already allude to when discussing the relatively low proportion of stabilized OC at Cape Bounty, I suspect precipitation likely emerges as a major (if not dominant) control (see e.g., Klaminder et al., 2009 https://doi.org/10.1029/2009JG000947). It seems more useful to explore these other probable controls rather than focusing on the latitude of the various sites, which the authors reference repeatedly.
Discussing unique characteristics of permafrost OC: It would be nice to see some additional discussion of the unique characteristics of permafrost soils that then explicitly links how these distinct attributes might impact the different modes of OC stabilization laid out in current Fig. 1. There is a strong temperate bias in the literature relating different modes of OC stabilization to C turnover times. Currently I noted only one very brief mention (line 470) of the possibility that some of this mineral-associated OC could be solubilized if redox/pH conditions change as soils become increasingly saturated during (and after) extensive thawing. I recognize that the specific OM-mineral complexes in these Cape Bounty soils have not been investigated directly through incubations and/or other manipulations, radiocarbon measurements, etc., which would have been optimal to more directly quantify their stability. While I would of course welcome such additional measurements, I fully recognize this would be a major undertaking and is likely untenable. But at the very least it would be helpful to include some discussion of these possible interactions, and to bring this discussion forward in the text so that it appears sooner than the final sentences of the discussion section.
Importance of pH dependence in mineral-OM stabilization: Somewhat related to the point above, I think it’s worth diving just a little deeper into the pH dependence of the different OM-mineral stabilization modes. For example, isn’t it somewhat surprising that pyrophosphate-extractable Al is the strongest predictor of OC in organometallic complexes, whereas Ca carries no predictive power? I would typically expect low Al (and Fe) solubility and abundant Ca2+ ions available to form cation bridges with negatively charged DOM under circumneutral pH conditions. Again, some additional discussion of how this might differ between the Cape Bounty sites versus the soil types where much of the literature on organo-mineral interactions has been established (which carries a significant temperate and acidic forest soil bias) would be fruitful. This pH dependence could be interesting to explore in association with the site-level compilation/analysis discussed above.
TECHNICAL CORRECTIONS –
Line 35: comparable to emissions over what timescales? Annual? Please specify.
Line 44: I suggest re-ordering the figures and relocating the current figure 3 (photos of the ALD and RTS) into figure 1, which can then be cited here. I have some additional recommendations for improving this figure below (see comments on line 135).
Line 56: Can you include an estimate of what proportion of permafrost landscapes are susceptible to these alternative (and rapid) thaw mechanisms? Is this value known from the literature? If not, perhaps it could be worth trying to make an initial first order estimate (see discussion of this in general comments).
Line 76: This is not always true (re: that MAOC is stabilized over decadal to millennial timescales). For a great discussion see Jillig et al., 2025 (https://doi.org/10.1038/s43247-025-02681-8). This fast-cycling MAOM pool might be expected to be particularly large in permafrost landscapes where saturated soil conditions are prevalent following thaw (see discussion in general comments above).
Line 100: Beautiful figure!! Very nicely done to capture so much information in a digestible and aesthetically pleasing way.
Line 130: I recommend moving current Fig. 9 and making this panels e, f, and g of this figure. It’s an excellent conceptual figure but it seems best suited to depict the two disturbance types and show how you sampled. I find that it doesn’t add very much when included so late in the manuscript, but it could be very helpful if moved forward and included as a set of additional panels here in (current) Fig. 2 to augment the site/sampling descriptions.
Line 135: See comment above about this figure. I recommend moving this up to become a new fig 1 that can be referenced in the introductory discussion about the ALD and RTS disturbances. It would also be ideal to include a scale and potentially demarcate where on the ALD and RTS features these samples came from.
Line 156: I believe that these are better suited for a linear mixed model than simple linear regression, given that the samples are not truly independent from one another (i.e., multiple samples from the same profile, which are genetically linked and therefore cannot be considered independent). I recommend a linear mixed modeling framework with profile and/or site included as random factor(s). See discussion in general comments
Line 175: Which samples were analyzed for IC? How many of the 33 total samples were screened?
Line 176: Did you measure organic (and/or inorganic) N on any of the samples? It would be nice to know if there are any variations in OM quality down the profiles (which could be at least superficially surmised through C:N ratios and ON content). Was there too long of a gap between sampling and measurement to obtain reliable N estimates?
Line 225: See general comments for discussion of regression models
Line 303: I would change to say ‘amorphous’ or specify ‘poorly crystalline Fe oxides’. Reading crystalline Fe oxides makes me think of citrate-dithionite extractable oxides (which were not measured here?)
Citation: https://doi.org/10.5194/egusphere-2025-3428-RC2
Data sets
Mineral-bound organic carbon exposed by hillslope thermokarst terrain: case study in Cape Bounty, Canadian High Arctic Maxime Thomas et al. https://doi.org/10.14428/DVN/5O6FJ3
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General Comments
The manuscript details a case study of organic carbon fractions with varying degrees of mineral-complexation in layers exposed by contrasting thaw slump or detachment types (active layer detachment vs. retrogressive thaw slump) in the Canadian High Arctic for four distinct profiles. The authors find that chemically stabilized organic carbon makes up about 20% of total OC, mostly as organo–metal complexes and C bound to poorly crystalline iron oxides. This is low compared to other Arctic sites, which they attribute to local temperature and humidity. Deep sediments exposed by retrogressive thaw slumps contain even less chemically stabilized C, making them more susceptible to decomposition. In contrast, physically protected C in aggregates and large, stable molecules represents a larger fraction (45%) of total OC. The study analyzes a wide breadth of mineral phases, elemental compositions, and OC fractions and makes good use of the complementary methods selective chemical extractions and density fractionation.
The manuscript is well-written and the study design is well-conceptualized, and this is a topic of great interest and importance in the context of the warming of the arctic. The conceptual diagrams and figures are useful and clearly convey the information (with a couple small exceptions noted in specific comments section below).
The discussion is quite long, although the authors do a good job of putting their results within the context of other studies. Perhaps each discussion subsection could be shortened to improve readability.
I appreciate and commend the inclusion of all data and figures in the supplementary information.
The authors acknowledge the limitations of the small sample size.
Specific Comments
As you state, selective extractions omit large biopolymers and physically occluded OC, so the conclusion that 20% of TOC is chemically stabilized may be an underestimate. The wording could be tweaked to acknowledge this, since it’s a central conclusion.
Regarding the RTS deep sediments, could it be possible that cryoturbation and/or post-thaw mobilization of OC or FE/Al from the profile could be contributing to the high soluble ions, pH, and low Cp in RTS-D deep layers?
L89-90: This isn’t really a hypothesis. It should be more specific as to the magnitude and direction of the change expected, and the hypothesized mechanism for that change.
L224-5: Please describe how robust R2 was calculated, since it can differ between type of statistical test and by software package. This is important since correlations between OC fractions and metal concentrations are central to some conclusions.
L 470-2: This is speculative and perhaps should not be included since the study didn’t measure it.
L491-3: The wording makes this sentence a little unclear. Recommend to reword to something like: “Only about one-fifth of the total organic carbon (20 ± 4%) is chemically stabilized through strong associations with minerals, yet this fraction likely persists the longest in soils. Physical protection, which traps carbon within aggregates or in large, chemically stable molecules, accounts for a larger portion (45 ± 8%) and spans a wider variety of carbon forms.”
On Fig. 9, it would be useful to include typical depths of the various features for comparison among the slump detachment types.
Fig. 10: Are the error bars in fact standard deviation or are they standard error?
Technical Corrections
L 126: Change to SI convention (decimal point, not comma)
L 377: Please correct plural vs. singular: “In particular, organometallic complexes, an efficient mechanisms for OC protection”