Distribution of mineral constituents in Yedoma permafrost: implications for Yedoma formation

Ice-rich permafrost deposits such as Yedoma are highly sensitive to thaw and given that they contain up to one third of the organic carbon content of the Northern circumpolar permafrost region, their degradation is considered to be a potential climate tipping point on Earth. Accurately predicting the impact of climate warming on the fate of organic carbon in Yedoma requires better constraints on the mineral element reserve in these deposits. This study provides evidence for the homogeneity of chemical composition and mineralogy of Yedoma deposits with depth. This suggests that upon deep thaw through thermokarst or thermo-erosion a high reserve in mineral nutrients is likely to be exposed also from deeper deposits

The permafrost mineral reserve: identify potential mineral nutrient hotspots upon thawing

The thawing of permafrost exposes organic matter to decomposition but also mineral constituents to water. To evaluate the potential to create mineral nutrients hotspots upon thawing, an inventory of the mineral element content and its local variability in permafrost terrain is needed. Based on measurements from major Arctic regions (Alaska, Greenland, Svalbard and Siberia), it is suggested that the mineral reserve in permafrost is firstly controlled by the local lithology. More specifically, the data highlight the potential for mineral nutrient hotspots to be generated upon thawing in soils derived from deltaic deposits, but not in thermokarst deposits. Finally, we suggest that portable X-ray fluorescence (pXRF) may present a quick and low-cost alternative to total digestion and ICP-AES measurements to build a mineral element inventory in permafrost terrain at a large spatial scale

What are the impacts of shifting Arctic tundra vegetation?

Elisabeth Mauclet from the Earth and Life Institute at UCLouvain, Belgium, brings to light the ways in which Arctic tundra vegetation mirrors the complex landscape response to climate chang

Influence of permafrost degradation on the mineral nutrient cycling by Arctic tundra vegetation

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, 202

Mineral element and organic carbon transport from permafrost soils to a headwater stream under contrasting flow regimes and permafrost degradation

Organic carbon transport from permafrost soils to headwater streams is an important vector in the quantification of permafrost soil organic carbon (SOC) stocks, with 5.4 % of SOC lost per year by lateral transport at Eight Mile Lake, Alaska1. A portion of SOC is transported as dissolved organic carbon (DOC), comprising micron to nano-sized organic carbon and organic carbon bound with mineral elements. These DOC pools are transported unaltered into headwater streams or transformed, (e.g. by metabolism and photo-oxidation) perturbing the initial SOC composition. Here we ask: how do temporal changes in hydrology and permafrost degradation effect mineral element-bound DOC transport from permafrost soils to streams at Eight Mile Lake, Alaska? Fe, Al and DOC concentrations were determined in the colloidal (0.22 μm – 1 nm) and truly dissolved (< 1 nm) fractions of a headwater stream before, during and after snowmelt in May 2018 and in a headwater stream and active layer groundwaters during and after a summer rain event in August 2019. Fe, Al and DOC concentrations are highest during peak flow events (snowmelt and rain event) and mainly transported in colloidal form. When comparing Fe, Al and DOC size separation in active layer ground waters and stream, ~ 90 % of Al and DOC are transported as colloids in soil waters and stream during the rain event, but only ~ 60 % of Al and DOC transported in colloidal form in the headwater stream under baseflow conditions, evidence for the transformation of colloidal SOC during transport. The relatively brief snowmelt and rain events are the hydrological drivers connecting mineral element-bound DOC in soils and headwater streams. Fe, Al and DOC concentrations were determined in soil pore waters (< 0.2 μm) sampled during spring thaw (depth of 20 cm) and maximum thaw (depth of 120 cm) at sites of extensive and minimal permafrost degradation. There is a factor of 5 - 10 decrease in DOC, Fe and Al in soil pore waters from degraded soils compared to minimally degraded soils suggesting that long-term permafrost degradation has depleted the pool of Fe and Al available to form complexes with SOC. When the hydrological driver (snow melt and rain event) occurs synchronously with permafrost soil degradation, the transformed pool of colloidal SOC may be detected in headwater streams

Iron dynamics upon thermokarst formation in the Yedoma domain

Ice-rich permafrost is subject to abrupt thaw, during past and present global warming. Ice-rich domains encompass Yedoma Ice Complex deposits that have never thawed since deposition and Alas deposits that have undergone previous thermokarst processes during the Late glacial and Holocene warming periods. Upon thaw of these deep ice-rich deposits, organic carbon (OC) and minerals are unlocked and OC is exposed to mineralization. A portion of this OC is associated with iron (Fe), that provides physico-chemical protection of OC or drives the mineralization of OC by redox processes. We hypothesize that post-depositional thaw processes induce changes in redox conditions in Alas deposits and so affect the role that Fe plays in mediating present day OC mineralization. To test this hypothesis, we quantify the vertical distribution of Fe concentrations and Fe mineralogy in unthawed Yedoma and previously thawed Alas deposits from the Yedoma domain (Alaska, the Kolyma region, the Indigirka region, the New Siberian Archipelago, the Laptev Sea coastal region, and Central Yakutia). Portable XRF-measured concentrations trueness were calibrated from alkaline fusion and inductively coupled plasma optical emission spectrometry (ICP-OES) measurement method on a subset of 144 samples (R² = 0.95). Iron extractions of unthawed and previously thawed deposits show that, ~25% of the total iron is a reactive species, composed of crystalline or amorphous oxides, or complexed with OC, with no significant difference between Yedoma and Alas deposits. We observe a consistenttotal Fe concentration in Yedoma deposits, but a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events. These results suggest that redox driven processes during the Lateglacial and Holocene thermokarst formation impact the present-day distribution of reactive Fe and its association with OC in ice-rich permafrost

Influence of permafrost degradation and shift in vegetation on litter and soil properties: case study in Central Alaska

Global warming affects the Arctic and sub-Arctic regions by exposing previously frozen permafrost to thaw, unlocking mineral nutrients, stimulating microbial activity, and boosting plant growth. Initially composed of graminoids, forbs, deciduous and evergreen shrubs, mosses and lichens, Arctic tundra is subject to a shrub, forb and moss expansion, at the expense of graminoid species. For now, little is known about the intrinsic foliar properties of these plant species and how they may influence properties of the underlying litter. Therefore, we investigated vegetation foliar properties and litter properties (organic carbon, C/N ratio, cation exchange capacity (CEC), pH, and concentrations of major and trace elements), from a typical Arctic tundra across a natural gradient of permafrost degradation. Results show that vegetation foliar properties are intrinsic to plant species and do not depend on the permafrost degradation stage. Furthermore, the natural gradient of permafrost degradation showed contrasts in litter mineral element concentrations, related to the occurring shift in vegetation. For example, we found a decrease in silicon concentrations in the litter between the least and the most degraded permafrost site, which is concurrent with the decrease in graminoids (Si-rich plant species) and the increase in shrubs biomass (Si-poor plant species), upon permafrost degradation. Therefore, changes in vegetation composition may influence litter properties, such as C/N ratio, CEC and mineral element concentrations (Figure 1), which could in turn influence C dynamics with the change of nutrient availability for plant cover

WeThaw: Mineral Weathering in Thawing Permafrost

Enhanced thawing of the permafrost in response to warming of the Earth’s high latitude regions exposes previously frozen soil organic carbon (SOC) to microbial decomposition, liberating carbon to the atmosphere and creating a dangerous positive feedback on climate warming. Thawing the permafrost may also unlock a cascade of mineral weathering reactions. These will be accompanied by mineral nutrient release and generation of reactive surfaces which will influence plant growth, microbial SOC degradation and SOC stabilisation. Arguably, weathering is an important but hitherto neglected component for correctly assessing and predicting the permafrost carbon feedback. The goal of the project WeThaw is to provide a comprehensive assessment of the mineral weathering response in permafrost regions subject to thawing, including ice-rich permafrost subjected to abrupt disturbances called thermokarst, and permafrost with lower ground-ice content subjected to a gradual but persistent deeper thaw of the active layer. The aim is to contribute to augment our capacity to develop models that can accurately predict the permafrost carbon feedback

Mineral organic carbon interactions in dry versus wet tundra soils

Mineral organic carbon interactions (aggregation, organo-mineral associations and organo-metallic complexes) enhance the protection of organic carbon (OC) from microbial degradation in soils. The northern circumpolar permafrost region stores between 1,440 and 1,600 Pg OC of which a significant portion is already thawed or about to thaw in coming years. In the light of this tipping point for climate change, any mechanism that can promote OC stabilization and hence mitigate OC mineralization and greenhouse gas emissions is of crucial interest. Here, we study interactions between metals (Fe, Al, Mn and Ca) and OC in the moist acidic tundra ecosystem of Eight Mile Lake, near Healy, AK, USA. We collected thirteen cores (124 soil samples) in late summer 2019 with shallow and deep active layers (45 to 109 cm deep) and varying water table depths. We find that between 6% and 59% of total OC in Eight Mile Lake tundra soils is mineral-associated (mean 20%), in organomineral associations (association between poorly crystalline oxides and OC) and in organo-metallic complexes (associations between Fe, Mn, Al, Ca polyvalent cations and organic acids). We find that total Fe and Mn concentrations can be used as good proxies to assess the reactive pool of these metals able to form associations with OC, i.e., poorly crystalline oxides or metals complexed with OC. We observe that in the active layer, mineral OC interactions are mostly as organo-metallic complexes with Fe cations, with an accumulation at the water table level acting as a soil redox interface. In waterlogged soils with a water table level above surface, no such accumulation of OC-Fe complexes is found due to the absence of a redox interface below soil surface. In the permafrost layer, we find that a combination of complexed metals and poorly crystalline Fe oxides act as reactive phases towards OC. Knowing that upon permafrost thaw tundra soils will become wetter or drier, the assessment of mineral-bound OC in drier or wetter tundra soils is a needed step to better constrain the changes in the proportion of non-protected OC more likely to contribute to C emissions from tundra soils