Sophie Opfergelt

Taliks, unfrozen ground within permafrost, sustain year-round biogeochemical processes, mobilizing carbon, nutrients, and contaminants through lateral and open taliks, while closed and isolated taliks confine reactions locally. Yet, while talik physical and thermal dynamics have been increasingly documented, their biogeochemical processes remain understudied, particularly in winter. In this manuscript, we argue that taliks represent a critical and overlooked driver of Arctic and sub-Arctic biogeochemical processes. We put forward three testable hypotheses to advance the study of talik biogeochemical functioning and we propose a related road map: year-round sampling, standardized protocols, pan-Arctic talik mapping, and community-building.

Taliks, unfrozen ground within permafrost, sustain year-round biogeochemical processes, mobilizing carbon, nutrients, and contaminants through lateral and open taliks, while closed and isolated taliks confine reactions locally. Yet, while talik physical and thermal dynamics have been increasingly documented, their biogeochemical processes remain understudied, particularly in winter. In this manuscript, we argue that taliks represent a critical and overlooked driver of Arctic and sub-Arctic biogeochemical processes. We put forward three testable hypotheses to advance the study of talik biogeochemical functioning and we propose a related road map: year-round sampling, standardized protocols, pan-Arctic talik mapping, and community-building.

Taliks, unfrozen ground within permafrost, sustain year-round biogeochemical processes, mobilizing carbon, nutrients, and contaminants through lateral and open taliks, while closed and isolated taliks confine reactions locally. Yet, while talik physical and thermal dynamics have been increasingly documented, their biogeochemical processes remain understudied, particularly in winter. In this manuscript, we argue that taliks represent a critical and overlooked driver of Arctic and sub-Arctic biogeochemical processes. We put forward three testable hypotheses to advance the study of talik biogeochemical functioning and we propose a related road map: year-round sampling, standardized protocols, pan-Arctic talik mapping, and community-building.

Upland peatlands are a major terrestrial carbon reservoir that may play an important role in the global carbon cycle. However, knowledge of upland peatlands before the Holocene remains speculative because of the poor long-term preservation potential of peat in upland environments. Here, we explore the use of paired ²⁶ Al and ¹⁰ Be to simultaneously determine denudation rates and peat thicknesses averaged over multiple glacial-interglacial cycles. We report cosmogenic ²⁶ Al and ¹⁰ Be concentrations in quartz from saprolite underlying the modern peat cover along a hillslope transect and from stream sediment in the Hautes Fagnes, an upland peatland in the Belgian Ardennes. The measured ²⁶ Al / ¹⁰ Be ratios are lower than expected for steady-state denudation under the modern peat cover, which we interpret as evidence of thicker peat in the past. To quantify long-term average peat thicknesses and denudation rates and identify secular changes in overburden, we inverse-model the measured ²⁶ Al and ¹⁰ Be concentrations. Modelled denudation rates of the saprolite, reflecting landscape lowering rates, are exceptionally low (0.3–4.9 tons km ⁻² yr ⁻¹ , equivalent to approximately 0.1–1.9 m Myr ⁻¹ ). The median probability long-term overburden thicknesses exceed modern overburden thicknesses by 190–350 g cm ⁻² along the hillslope transect, approximately equivalent to 1.8–3.4 m of saturated peat. Peat degradation from historical land use, including peat extraction, drainage, and afforestation, may explain much of the discrepancy. Inverse modeling of the sample with the slowest denudation rate, and thus the longest near-surface residence time of quartz and signal integration timescale, suggests that a secular increase in overburden thickness, potentially reflecting the onset of peat cover, coincided with mid-Pleistocene uplift of the Ardennes. These results demonstrate the utility of cosmogenic nuclides in inferring the long-term history of peat cover where geomorphic process rates are slow and differential radioactive decay is non-negligible.

Taliks, unfrozen ground within permafrost, sustain year-round biogeochemical processes, mobilizing carbon, nutrients, and contaminants through lateral and open taliks, while closed and isolated taliks confine reactions locally. Yet, while talik physical and thermal dynamics have been increasingly documented, their biogeochemical processes remain understudied, particularly in winter. In this manuscript, we argue that taliks represent a critical and overlooked driver of Arctic and sub-Arctic biogeochemical processes. We put forward three testable hypotheses to advance the study of talik biogeochemical functioning and we propose a related road map: year-round sampling, standardized protocols, pan-Arctic talik mapping, and community-building.

Permafrost is a defining feature of the Arctic and sub-Arctic environments. The extent of the permafrost region accounts for about a quarter of the Northern Hemisphere’s terrestrial surface. While most research on permafrost has focused on the summer season—when the active layer is thawed and carbon emissions peak—the late shoulder season, marking the transition season between summer and winter and ending by surface freeze-back at the large scale, has received less attention. Yet, about 14% of the annual mean methane emissions from the permafrost occur during the refreezing period of the active layer. Understanding the seasonality, interannual variability and long-term trends of the surface freeze-back is therefore crucial to better constrain the high-latitude atmospheric carbon budget and improve Earth System Model projections. In this study, we analyze the evolution of surface freeze-back onset from 1950 to 2020 using the ERA5-Land reanalysis (0.1° spatial resolution) over a large region of Siberia encompassing the four main permafrost types. We find that surface freeze-back onset has been delayed by five days on average over that 70-year period. Through spatial regression modeling, we show that while several climatic and geographic factors influence freeze-back timing, the principal control on freeze-back seasonality at the large scale (~kilometers) is the date when the 2 m air temperature first falls below 0 °C, followed by the snow cover depth. These findings complement previous research that focused on the small scale (~meters), which emphasized the importance of the vegetation type and the snow cover characteristics at these spatial scales. Our results provide new insights into changes during the late shoulder season in one of the world’s fastest-warming regions and identify key variables to monitor for improving sub-seasonal forecasts that could become relevant to infrastructure upgrade and logistics planning in permafrost-affected areas.

Organic matter decomposition in permafrost soils depends on factors such as soil pH, temperature, and redox conditions. Over lowland permafrost soils, these conditions are shaped by microtopography, which evolves with physical degradation, i.e., lowland thermokarst development. A dynamic quantification of lowland thermokarst development – still poorly constrained – is therefore a critical prerequisite for predictive models of permafrost carbon balance in these areas. Here we provide such a quantification, updated for the Stordalen mire in Abisko, Sweden (68°21 ^′ 20 ^′′ N, 19°02 ^′ 38 ^′′ E), which displays a gradient from well-drained stable palsas to inundated fens, which have undergone ground subsidence. We produced RGB orthomosaics and digital surface models from very high resolution (10 cm) unoccupied aircraft system (UAS) photogrammetry as well as a spatially continuous map of soil electrical conductivity (EC) based on electromagnetic induction (EMI) measurements. We classified the land cover following the degradation gradient using a support vector machine algorithm and derived palsa loss rates. Our findings confirm that topography is a key variable for monitoring palsa loss, nearly doubling the overall accuracy of the classification, while slope enabled the identification of early-stage degradation. We show a clear acceleration of degradation for the period 2019–2021, with a decrease in palsa area of 3.3–3.6 % a ⁻¹ (% reduction per year relative to the initial palsa areal extent) compared to previous estimates of ∼ 0.3 % a ⁻¹ (1970–2000) and ∼ 0.1 % a ⁻¹ (2000–2014). EMI data show that this degradation leads to an increase in soil moisture, which in turn likely decreases organic carbon geochemical stability and potentially increases methane emissions. With a palsa loss of 3.3–3.6 % a ⁻¹ , we estimate accordingly that surface degradation at Stordalen might lead to a pool of 12 × 10 ³ kg of organic carbon exposed annually within the topsoil (23 cm depth), of which ∼ 25 % is mineral-interacting organic carbon. These results demonstrate that UAS monitoring can robustly quantify rapid thermokarst development that coarser methods miss, and provide a transferable framework for tracking permafrost degradation rates at other lowland sites.

Increased land-water connectivity of northern landscapes driven by permafrost thaw is shifting the bioavailability of dissolved organic matter (DOM) in surface waters, with implications for northern food webs and regional and global carbon balances. However, sorption of DOM to previously frozen sediments has received little attention as a mechanism of regulating the bioavailability of organic matter in thaw-affected freshwater ecosystems. Using batch sorption experiments, we assessed sorption potential, water-extractable dissolved organic carbon (DOC) concentration, and the impact of sorption on DOM composition of six different permafrost sediment types common throughout northwestern Canada, reflecting variation in geologic and permafrost histories. A principal component analysis revealed that sediment biogeochemical characteristics reflected geologic origin, and past thaw increased within-type variation. Sorption was positively correlated with organo-reactive forms of Al and Fe and negatively correlated with sediment pH. Proportion of bulk sediment organic carbon released as water extractable DOC ranged from 1.0 % to 62.0 %, with yedoma sediments from the Klondike region releasing substantially more than sediments from other regions. Preferential sorption of larger, humic-like compounds and displacement of mineral-bound small, aliphatic molecules enriched the DOM pool in labile compounds. Bio-incubations verified that exposure to sediments increased rates of biodegradation, corresponding with shifts in DOM composition and increased nutrient concentrations. Our experiments demonstrate that organo-mineral interactions have the potential to decrease DOC concentrations while increasing DOM bioavailability following exposure to permafrost-origin sediments, but that the strength of this response varies with sediment characteristics that are reflective of landscape history.