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In northern high latitudes, rapid warming is set to amplify carbon-climate feedbacks by enhancing permafrost thaw and biogeochemical transformation of large amounts of soil organic carbon. However, between 30 % and 80 % of permafrost soil organic carbon is considered to be stabilized by geochemical interactions with the soil mineral pool and thus less susceptible to be emitted as greenhouse gases. Quantification of the nature of and controls on mineral-organic carbon interactions is needed to better constrain permafrost-carbon-climate feedbacks, particularly in ice-rich environments resulting in rapid thaw and development of thermokarst landforms. On sloping terrain, mass wasting features called retrogressive thaw slumps are amongst the most dynamic forms of thermokarst. These multi-decadal disturbances grow due to ablation of an ice-rich headwall, and their enlargement due to warming of the Arctic is mobilizing vast stores of previously frozen materials. Here, we investigate headwall profiles of seven retrogressive thaw slumps and sediments displaced from these mass wasting features from the Peel Plateau, western Canadian Arctic. The disturbances varied in their headwall height (2 to 25 m) and affected land surface area ( 30 ha). We present total and water extractable mineral element concentrations, mineralogy, and mineral-organic carbon interactions in the headwall layers (active layer, permafrost materials above an early Holocene thaw unconformity, and Pleistocene-aged permafrost tills) and in displaced material (suspended sediments in runoff and material accumulated on the debris tongue). Our data show that the main mechanism of organic carbon stabilization through mineral-organic carbon interactions within the headwall is the complexation with metals (mainly iron), which stabilizes 30 ± 15 % of the total organic carbon pool with higher concentrations in near-surface layers compared to deep permafrost. In the displaced material, this proportion drops to 18 ± 5 %. In addition, we estimate that up to 12 ± 5 % of the total organic carbon is stabilized by associations to poorly crystalline iron oxides, with no significant difference between near-surface layers, deep permafrost and displaced material. Our findings suggest that the organic carbon interacting with the sediment mineral pool in slump headwalls is preserved in the material mobilized by slumping and displaced as debris. Overall, up to 32 ± 6 % of the total organic carbon displaced by retrogressive thaw slumps is stabilized by organo-mineral interactions in this region. This indicates that organo-mineral interactions play a significant role in the preservation of organic carbon in the material displaced from retrogressive thaw slumps over years to decades after their development resulting in decadal to centennial scale sequestration of this retrogressive thaw slump-mobilized organic carbon interacting with the soil mineral pool., Geoderma, 433, ISSN:0016-7061, ISSN:1872-6259

The stabilizing properties of mineral-organic carbon interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, paddy soils). Recently, interest in their role in permafrost regions is increasing as permafrost was identified as a hotspot of change. In thawing ice-rich permafrost regions, such as the Yedoma domain, 327-466 Gt of frozen organic carbon (OC) is buried in deep sediments. Interactions between minerals and OC are important since the OC is located in close contact with the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physico-chemical protection of OC. The dynamic changes of redox and pH conditions associated with thermokarst lake formation and drainage, trigger metal-oxide dissolution and precipitation, likely influencing OC stabilization and microbial mineralization. However, the influence of thermokarst processes on mineral-OC interactions remains poorly constrained. In this study, we aim to characterize Fe, Mn, Al and Ca minerals and their potential protective role for OC. Total and selective extractions were used to assess the crystalline and amorphous oxides or complexed metal pools as well as the organic acids found within these pools. We analyzed four sediment cores from an ice-rich permafrost area in Central Yakutia, which were drilled i) in undisturbed Yedoma uplands, ii) beneath a recent lake formed within Yedoma deposits, iii) in a drained thermokarst lake basin, and iv) beneath a mature thermokarst lake from the early Holocene period. We find a decrease in the amount of reactive Fe, Mn, Al and Ca in the deposits upon lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits upon lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal:C molar ratio upon thermokarst process, which may indicate an increase of metal-C bindings and could provide a higher protective role against microbial mineralization of organic matter. Finally, we find that an increase in mineral-OC interactions corresponded to a decrease in CO2 and CH4 gas emissions upon thermokarst process. Mineral-OC interactions could mitigate greenhouse gas production from permafrost thaw as soon as lake drainage occurs.

Permafrost ecosystems are limited in nutrients for vegetation development and constrain the biological activity to the active layer. Upon Arctic warming, permafrost degradation exposes large amounts of soil organic carbon (SOC) to decomposition and minerals to weathering, but also releases organic and mineral soil material that may directly influence the soil exchange properties (cation exchange capacity and base saturation). The soil exchange properties are key for nutrient base cation supply (Ca2+, K+, Mg2+) for vegetation growth and development. In this study, we investigate the distribution of soil exchange properties within typical Arctic tundra permafrost soils at Eight Mile Lake (Interior Alaska, USA) because they will dictate the potential reservoir of newly thawed nutrients and thereby influence soil biological activity and vegetation nutrient sources. Our results highlight a difference in the SOC distribution within soil profiles according to the permafrost thaw. The poorly thawed permafrost soils (active layer thickness; ALT ≤ 60 cm) present more organic material in surface (i.e., organic layer thickness; OLT ≥ 40 cm) than the highly thawed permafrost soil (i.e., ALT > 60 cm and OLT < 40 cm). In turn, this difference in SOC distribution directly affects the soil exchange complex properties. However, the low bulk density of organic-rich soil layers leads to much lower CEC density in surface (~9 400 cmolc m-3) than in the mineral horizons of the active layer (~16 000 cmolc m-3) and in permafrost soil horizons (~12 000 cmolc m-3). As a result of the overall increase in CEC density with depth and the overall increase in base saturation with depth (from ~20 % in organic surface to 65 % in permafrost soil horizons), the average total stock in exchangeable base cations (Ca2+, K+, Mg2+ and Na+ in g m-3) is more than 2-times higher in the permafrost than in the active layer. More specifically, the stocks in base cations in the upper part of permafrost about to thaw in the following are ~ 860 g m-3 for Caexch, 45 g m-3 for Kexch, 200 g m-3 for Mgexch and 150 g m-3 for Naexch. This first order estimate is a needed step for future ecosystem prediction models to provide constraint on the size of the reservoir in exchangeable nutrients (Ca, K, Mg) about to thaw.

Interactions between minerals and organic carbon (OC) in soils are key to stabilize OC and mitigate greenhouse gas emissions upon permafrost thaw. However, changes in soil water pathways upon permafrost thaw are likely to affect the stability of mineral OC interactions by inducing their dissolution and precipitation. This study aims to assess and quantify how mineral OC interactions are affected by dissolution and precipitation in thawed relative to unthawed layers. We hypothesize that a change in the radiogenic strontium (Sr) isotopic ratio (87Sr/86Sr) involved in mineral OC interactions upon changing water saturation conditions implies a destabilization of the mineral OC interaction. We quantified mineral OC interactions using selective extractions in soils facing gradual thaw (Eight Mile Lake, AK, USA) and in sediments with a thawing history of abrupt thaw (Duvanny Yar, Russia), and we measured the 87Sr/86Sr ratio of the selective extracts targeting the Sr associated to mineral OC interactions. Firstly, for water saturated layers with a higher proportion of mineral OC interactions, we found a difference in the 87Sr/86Sr ratio relative to the surrounding layers, and this supports the preservation of a Sr “stable” pool in these mineral OC interactions. We estimated that a portion of these mineral OC interactions have remained undissociated since their formation (between 4% and 64% by Sr isotope mass balance). Secondly, we found no difference in 87Sr/86Sr ratio between layers accumulating Fe oxides at redox interfaces regularly affected by water table changes (or upon thermokarst processes) relative to surrounding layers. This supports the dominance of a Sr “labile” pool inherited from processes of dissolution and precipitation of the mineral OC interactions. Thirdly, our estimations based on a Sr isotope mass balance support that, as a consequence of permafrost thaw, a larger proportion of Sr from primary mineral weathering (>80%) controls the Sr in mineral OC interactions in the saturated zone of deeply thawed soils relative to poorly thawed soils (∼50%). In conclusion, we found that the radiogenic Sr isotope method, applied for the first time in this context, is promising to trace dissolution-precipitation processes of mineral OC interaction in thawing permafrost.

Arctic is warming close to four times faster than the global average and, as a direct outcome, permafrost temperatures have increased by up to 0.39 ± 0.15 °C in the years 2007-2016. This increased warming is expected to generate a permafrost carbon feedback on the climate by promoting permafrost thaw and by emitting additional volumes of greenhouse gases into the atmosphere. Still, it is estimated that between 30% and 80% of soil organic carbon (OC) in permafrost is stabilized by geochemical interactions with mineral elements such as iron and thus less likely to be emitted as greenhouse gases. Quantifying the nature and controls of mineral-OC interactions is necessary to better frame permafrost-carbon-climate feedbacks, particularly in ice-rich environments that result in rapid thawing and the development of thermokarst landforms. Thaw slumps are amongst the most dynamic forms of slope thermokarst and expand through the years due to the ablation of an ice-rich headwall each summer. These phenomena are important to consider in the permafrost carbon budget since they expose a deep OC pool that may reach tens of thousands of years old and that would not have re-entered the modern carbon cycle if these disturbances had not occurred. Here, we collected samples from the Batagaika crater, Siberia, on a headwall reaching locally 55 m high - every half a meter for the upper 10 m of the headwall and then every meter. We present total element concentrations, mineralogy, and mineral-organic carbon interactions in the different stratigraphic units, i.e., from the top to the bottom, i) the organic surface layer, ii) the Holocene cover, iii) the upper ice complex, also called Yedoma, which is dominated by large ice wedges, iv) the woody debris layer which consists of macroscopic terrestrial plant remains, v) the lower sand unit of pore-ice-cemented aeolian-sourced fine sand, and vi) the lower ice complex which reveals ice-rich deposits of ice-wedges and provides access to ancient permafrost up to ~650 ka old. Our data show that the main mechanism of organic carbon stabilization through mineral-OC interactions is the complexation with metals, which stabilizes 35 ± 18% of the total organic carbon (TOC) pool. Associations to poorly crystalline iron oxides do not have a significant role in OC stabilization as we estimate a maximum of 5 ± 2% of TOC to be stabilized by this mechanism, with the exception of the Holocene cover which stabilizes up to 29 ± 14% of the TOC via associations with poorly crystalline iron oxide. From a budget perspective, we estimate that a mass of 1.65 × 107 kg of OC is exported annually downslope of the headwall with ~ 38% being geochemically stabilized by complexation with metals or associations to poorly crystalline iron oxides. Climatic and geochemical conditions at the time of deposition appear to be the key parameters influencing OC geochemical stability as the mineralogy in the deposits is very similar despite a sedimentary depositional series spanning ~650 ka old.