| 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.
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”