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Organic carbon (OC) in permafrost interacts with the mineral fraction of soil and sediments, representing < 1% to ~80% of the total OC pool. Quantifying the nature and controls of mineral-OC interactions is therefore crucial for realistic assessments of permafrost-carbon-climate feedbacks, especially in ice-rich regions facing rapid thaw and the development of thermo-erosion landforms. Here, we analyzed sediment samples from the Batagay megaslump in East Siberia, and we present total element concentrations , mineralogy, and mineral-OC interactions in its different stratigraphic units. Our findings indicate that up to 34 ± 8% of the OC pool interacts with mineral surfaces or elements. Interglacial deposits exhibit enhanced OC-mineral interactions, where OC has undergone greater microbial transformation and has likely low degradability. We provide a first-order estimate of ~12,000 tons of OC mobilized annually downslope of the headwall (i.e., the approximate mass of 30 large aircrafts), with a maximum of 38% interacting with OC via complexation with metals or associations to poorly crystalline iron oxides. These data imply that over one-third of the OC exposed by the slump is not readily available for mineralization, potentially leading to prolonged OC residence time in soil and sediments under stable physicochemical conditions.

Mineral organic carbon (OC) interactions in soils and ice-rich sediments 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. Dissolution of mineral surfaces can release associated OC which contributes to carbon loss, whereas precipitation of new mineral surfaces can promote loci for OC interactions limiting carbon loss. Here we use radiogenic Sr isotopes to locate in situ dissolution-precipitation processes of mineral OC interactions along a gradient of permafrost thaw in soils and ice-rich sediments. In these materials, about 20% of OC is stabilized as organo-mineral associations (association between ferrihydrite and OC) and organo-metallic complexes (associations between Fe, Mn, Al, Ca polyvalent cations and organic acids). We target Sr adsorbed or occluded to metal oxides or participating in organo-metallic complexes. We hypothesize that a change in the Sr isotopic signature of such mineral OC interactions upon permafrost thaw indicates a destabilization of the binding between mineral surfaces and Sr, and hence OC. We show that it is in saturated layers that mineral OC interactions have remained undissociated and preserved since their formation. At the redox interface, the data highlight processes of dissolution and precipitation of the mineral OC interactions, supporting loci for the loss and gain in OC stabilization potential. Given the importance of stabilizing surfaces and cations involving Fe for the stabilization of OC in permafrost environments, we propose an approach to estimate at larger scale in the Arctic the proportion of this “reactive iron” in permafrost soils.

Between 30 and 80% of soil organic carbon (OC) in permafrost environments can be stabilized by interactions with mineral surfaces or metals such as iron. Iron-OC interactions may be modified by changing hydrological conditions upon permafrost thaw resulting in local subsidence. The challenge is to identify the early stage of thermokarst landforms, and to quantify the influence of thermokarst development on Fe and OC released in soil pore water upon thawing. We monitored the soil water content (SWC), soil temperature and soil electrical conductivity (EC) together with the chemical composition of the soil pore water along a gradient of thermokarst development and subsequent permafrost degradation at Abisko, Sweden (palsa-bog-fen). More specifically, we combined geophysical parameters (elevation, active layer depth, SWC and soil EC) and physico-chemical parameters (pH and soil pore water EC) at the profile and slope scales, with concentrations of Fe and dissolved organic carbon (DOC) in soil pore water at the profile scale. The results highlight that (i) at the profile scale, elevation, active layer depth and SWC are relevant geophysical criteria to discriminate between palsa, bog and fen; (ii) permafrost degradation leads to the mobilization of Fe and DOC in soil pore water; (iii) at the slope scale, landscape areas can be classified as palsa, intermediate or fen based on the three geophysical criteria and this can be used to derive the conditions for the mobility of Fe and DOC. These data support that physical degradation of permafrost and subsequent changes in SWC with thermokarst landform development from palsa to fen likely influences the geochemical conditions for the stability of Fe-OC interactions.

Upon permafrost thaw, the volume of soil accessible to plant roots increases which modifies the acquisition of plant-available resources. Tundra vegetation is actively responding to the changing environment with two major directions for vegetation shift across the Arctic: the expansion of deep-rooted sedges and the widespread increase in shallow-rooted shrubs. Changes in vegetation composition, density and distribution have large implications on the Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. A better understanding of these cumulated effects of changing vegetation on warming and permafrost requires assessing the changes in plant nutrient sources upon permafrost thaw, nutrient access being a limiting factor for the Arctic tundra vegetation development. In this study, we determined the influence of permafrost degradation on the base cation sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determined the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compared the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (Betula nana, Vaccinium vitisidaea, and Eriophorum vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) was consistent with a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflected a source of Sr from deeper soil horizons. There is a shift between poorly and highly thawed soil profiles towards lower foliar 87Sr/86Sr ratios in both deep- and shallow-rooted plant s

Large Arctic rivers are key locations for nitrogen processing, which controls the supply of this limiting nutrient to the Arctic Ocean. In a warming Arctic, longer ice-free periods increase riverine productivity and modulate nitrogen consumption and delivery to the ocean. In this study, the annual variability of nitrate concentrations at the Lena River outlet (Samoylov station) was investigated. Significantly higher nitrate concentrations in water were observed sub-ice (winter) than in the open water (summer), and the higher nitrate concentrations follow phases of colder air temperature at the Lena catchment scale (ERA5 reanalysis data). We hypothesize that colder phases result in thicker river ice leading to darker under-ice conditions preferred by nitrifying microbial communities, thereby inducing increasing sub-ice nitrification. We tested this hypothesis using silicon isotopes known to fractionate upon freezing. The high nitrate concentrations in the winter are associated with heavier silicon isotope compositions in river water. This can be explained by the supersaturation and precipitation of amorphous silica preferentially incorporating the lighter silicon isotopes, leaving the water isotopically heavier. Supersaturation of amorphous silica can result from thicker ice formation upon colder air temperature at catchment scale. The silicon isotope data support phases of thicker ice formation, and indirectly support darker sub-ice conditions at the river base creating pulses of increasing nitrification. Our hypothesis is also supported by a change in the value of an index for dissolved organic carbon aromaticity (SUVA) during the colder phases: this suggests that conditions favour the decomposition of dissolved organic matter during periods of thicker river ice. Air temperature, nitrate concentration, silicon isotopes and SUVA are supporting evidence for pulses of sub-ice microbial activity in the river during winter. It follows that decreasing ice cover duration

Mineral-organic carbon (OC) interactions account for 30 – 80 % of the total permafrost OC pool. Quantifying the nature and controls of mineral-OC interactions is necessary to better assess permafrost-carbon-climate feedbacks. This is particularly true for ice-rich environments that are impacted by rapid thaw and the development of thermokarst landforms. Retrogressive thaw slumps are amongst the most dynamic forms of slope thermokarst and they expand through the years due to the ablation of an ice-rich headwall. These phenomena are important to consider in the permafrost carbon budget since they expose deep OC sometimes tens of thousands of years old that would not have re-entered the modern carbon cycle if these disturbances had not occurred. Here, we analyzed sediment samples collected from the headwall of the Batagay megaslump, East Siberia, locally reaching 55 m high. The series of discontinuous deposits comprises also older sediment up to ~650 ka old. We present total element concentrations, mineralogy, and mineral-OC interactions in the different stratigraphic units. The mineralogy in the deposits is very similar across the sedimentary series. Our data show that up to 34 ± 8 % of the total OC pool is stabilized by mineral-OC interactions. For most of the analyzed samples, associations to poorly crystalline iron oxides do not have a significant role in OC stabilization. Hypothesizing a retreat rate of 26000 m²/yr and constant thickness of stratigraphic units within the headwall, we provide a first order estimate of ~2 × 10^7 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. These data support that more than one third of the organic carbon exposed by this massive thaw slump is not directly available for mineralization, but rather stabilized geochemically.

A 450 km crossing of the Greenland ice sheet on skis, over 400 km sea kayak descent along Greenland’s south and west coast and a 600 m vertical climb to open a new climbing route: this is the atypical triathlon led by the two Belgian explorers of Nanok Expedition, Gilles Denis and Nathan Goffart, between April and September 2022. Beyond their sportive objective, their goal was to serve climate research and engage with and communicate to a non-scientific audience about climate research. In partnership with five scientific institutions, they collected unique data and observations of the elevation changes of the ice sheet, as well as rare in situ samples of snow and fjords water to investigate the influence of ice sheet melting on dust input to the ecosystem. The data and samples collected will be analyzed by the universities and research centers. The expedition was documented with a movie “La Traversée” now shared to a general audience on national television and on different university campus. The two explorers are bridging different scientific disciplines (geophysics, geochemistry, glaciology) and bring the science at the table of a non-scientific audience by sharing their passion. Beyond the science, this is a source of inspiration to communicate about the importance of polar regions in the climate system., info:eu-repo/semantics/published

The stabilizing properties of mineral–organic carbon (OC) interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, and 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 OC is buried in deep sediments. Interactions between minerals and OC are important because OC is located very near the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physicochemical protection of OC. The dynamic changes in 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 on lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits on lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal to C molar ratio on thermokarst process, which may indicate an increase in metal–C bindings and could

Mineral elements bind to dissolved organic carbon (DOC) in permafrost soils, and this may contribute to the stabilisation or the degradation of organic carbon along the soil to river continuum. Permafrost thaw enlarges the pool of soil constituents available for soil to river transfer. The unknown is how changes in hydrology upon permafrost degradation affect the connection between soil-derived mineral element-bound DOC and headwater streams. Here, we study Al, Fe, Ca and DOC concentrations in water from a headwater stream at Eight Mile Lake, Alaska, USA (colloidal (0.22 µm–1 kDa) and truly dissolved (< 1 kDa) fractions) and in soil pore waters sampled across a gradient of permafrost degradation at the same location. We target the peak flow to base flow transition to show that there is a narrow window of mineral element-bound DOC colloid transport from soils to streams. We show that during spring thaw and maximum thaw there is an enhanced lateral transfer of mineral element-bound DOC colloids in extensively degraded sites compared to minimally degraded sites. This is explained by a more rapid response of hydrology at peak flow to base flow transition at degraded sites. Our results suggest that ongoing permafrost degradation and the associated response of soils to changing hydrology can be detected by targeting the composition and size of mineral element-DOC associations in soil waters and headwater streams during peak flow-baseflow transitions.

Climate change affects the Arctic and sub-Arctic regions by exposing previously frozen permafrost to thaw, unlocking soil nutrients, changing hydrological processes, and boosting plant growth. As a result, sub-Arctic tundra is subject to a shrub expansion, called “shrubification”, at the expense of sedge species. Depending on the intrinsic foliar properties of these plant species, changes in foliar mineral element fluxes with shrubification in the context of permafrost degradation may influence topsoil mineral element composition. Despite the potential implications of changes in topsoil mineral element concentrations for the fate of organic carbon, this remains poorly quantified. Here, we investigate vegetation foliar and topsoil mineral element composition (Si, K, Ca, P, Mn, Zn, Cu, Mo, V) across a natural gradient of permafrost degradation at a typical sub-Arctic tundra at Eight Mile Lake (Alaska, USA). Results show that foliar mineral element concentrations are higher (up to 9 times; Si, K, Mo for all species, and for some species Zn) or lower (up to 2 times; Ca, P, Mn, Cu, V for all species, and for some species Zn) in sedge than in shrub species. As a result, a vegetation shift over ~40 years has resulted in lower topsoil concentrations in Si, K, Zn, and Mo (respectively of 52, 24, 20, and 51%) in highly degraded permafrost sites compared to poorly degraded permafrost sites due to lower foliar fluxes of these elements. For other elements (Ca, P, Mn, Cu, and V), the vegetation shift has not induced a marked change in topsoil concentrations at this current stage of permafrost degradation. A modeled amplified shrubification associated with a further permafrost degradation is expected to increase foliar Ca, P, Mn, Cu, and V fluxes, which will likely change these element concentrations in topsoil. These data can serve as a first estimate to assess the influence of other shifts in vegetation in Arctic and sub-Arctic tundra such as sedge expansion under wetter soil co

Arctic warming and permafrost degradation are modifying northern ecosystems through changes in microtopography, soil water dynamics, nutrient availability, and vegetation succession. Upon permafrost degradation, the release of deep stores of nutrients, such as nitrogen and phosphorus, from newly thawed permafrost stimulates Arctic vegetation production. More specifically, wetter lowlands show an increase in sedges (as part of graminoids), whereas drier uplands favor shrub expansion. These shifts in the composition of vegetation may influence local mineral element cycling through litter production. In this study, we evaluate the influence of permafrost degradation on mineral element foliar stocks and potential annual fluxes upon litterfall. We measured the foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typical tundra plant species from two contrasting Alaskan tundra sites, i.e., an experimental sedge-dominated site (Carbon in Permafrost Experimental Heating Research, CiPEHR) and natural shrub-dominated site (Gradient). The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration and shrub leaves having relatively high Ca and Mn concentrations. Therefore, changes in the species biomass composition of the Arctic tundra in response to permafrost thaw are expected to be the main factors that dictate changes in elemental composition of foliar stocks and maximum potential foliar fluxes upon litterfall. We observed an increase in the mineral element foliar stocks and potential annual litterfall fluxes, with Si increasing with sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing with shrub expansion in drier sites (Gradient). Consequently, we expect that sedge and shrub expansion upon permafrost thaw will lead to changes in litter elemental composition and therefore affect nutrient cycling across the sub-Arctic tundra with potential implications for fur

Arctic warming and permafrost loss modify northern ecosystems through soil subsidence, changes in soil hydrology, nutrient availability and vegetation succession. In particular, warming and soil moisture conditions influence Arctic tundra vegetation production and distribution: wetter soil conditions favor sedge expansion and drier soil conditions drive woody shrub expansion. While these shifts in vegetation may alter permafrost integrity by modifying the surface energy transfers, they may also be responsible for further changes in the tundra vegetation production and distribution by influencing plant nutrient cycling. Specifically, this PhD thesis investigates the influence of permafrost degradation on mineral nutrient distribution in soils and the influence of tundra vegetation shift on mineral nutrient cycling. Along a permafrost thaw gradient in Interior Alaska, our results demonstrate that permafrost is an important frozen reservoir of nutrients for plants, with stocks of total and exchangeable base cations in the permafrost soil layers more than twice as high as in the seasonally thawed active layer. Moreover, we observe that foliar elemental stocks and annual fluxes from leaf to soil litter change with vegetation shift; sedge expansion promotes Si, P and Fe foliar cycling and shrubification promotes Ca and Mn foliar cycling. Lastly, we highlight that shrub and sedge species both take up nutrients from deeper soil horizons upon permafrost thaw, with the deeply rooted sedge benefitting first from the release of nutrients at depth. These findings provide insights to improve the integration of changes in mineral nutrient cycling into models predicting Arctic ecosystem evolution and feedbacks on climate., (AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 2022