Permafrost Thaw Research Articles

Patterns and rates of soil movement and shallow failures across several small watersheds on the Seward Peninsula, Alaska

Abstract. Thawing permafrost can alter topography, ecosystems, and sediment and carbon fluxes, but predicting landscape evolution of permafrost-influenced watersheds in response to warming and/or hydrological changes remains an unsolved challenge. Sediment flux and slope instability in sloping saturated soils have been commonly predicted from topographic metrics (e.g., slope, drainage area). In addition to topographic factors, cohesion imparted by soil and vegetation and melting ground ice may also control spatial trends in slope stability, but the distribution of ground ice is poorly constrained and hard to predict. To address whether slope stability and surface displacements follow topographic-based predictions, we document recent drivers of permafrost sediment flux present on a landscape in western Alaska that include creep, solifluction, gullying, and catastrophic hillslope failures ranging in size from a few meters to tens of meters, and we find evidence of rapid and substantial landscape change on an annual timescale. We quantify the timing and rate of surface movements using a multi-pronged, multi-scalar dataset including aerial surveys, interannual GPS surveys, synthetic aperture radar interferometry (InSAR), and climate data. Despite clear visual evidence of downslope soil transport of solifluction lobes, we find that the interannual downslope surface displacement of these features does not outpace downslope displacement of soil in locations where lobes are absent (downslope movement means: 7 cm yr−1 for lobes over 2 years vs. 10 cm yr−1 in landscape positions without lobes over 1 year). Annual displacements do not appear related to slope, drainage area, or modeled total solar radiation but are likely related to soil thickness, and volumetric sediment fluxes are high compared to temperate landscapes of comparable bedrock lithology. Time series of InSAR displacements show accelerated movement in late summer, associated with intense rainfall and/or deep thaw. While mapped slope failures do cluster at slope–area thresholds, a simple slope stability model driven with hydraulic conductivities representative of throughflow in mineral and organic soil drastically overpredicts the occurrence of slope failures. This mismatch implies permafrost hillslopes have unaccounted-for cohesion and/or throughflow pathways, perhaps modulated by vegetation, which stabilize slopes against high rainfall. Our results highlight the breadth and complexity of soil transport processes in Arctic landscapes and demonstrate the utility of using a range of synergistic data collection methods to observe multiple scales of landscape change, which can aid in predicting periglacial landscape evolution.

Mechanical Response of a Buried Pipeline to Permafrost Thawing Based on Sequential Coupling Method

Thawing permafrost has affected the structural integrity of buried warm pipelines in cold regions and poses an ongoing threat in the context of climate change. Therefore, characterizing variation in the engineering properties of pipeline foundation permafrost and its effect on the mechanical behavior of pipeline is important. In this paper, the ground temperature distributions around a buried warm pipeline and mechanical response of the pipeline to differential thaw settlement of foundation permafrost are investigated using thermal–mechanical sequential coupling simulation, based on the observational data collected from a selected monitoring site along the China-Russia crude oil pipelines in northeastern China. The results indicate that the thaw-induced settlement of pipeline foundation permafrost develops quickly with the formation and expansion of the thaw bulb in the first 10 years, and then increases slowly when the thaw bulb extends to the weathered granite. Differential thaw settlement will cause a significant change in the deformation and stress of the pipeline near the interface of strong and weak thaw settlement zones. When the length ratio of strong and weak thaw settlement zones is 1, the maximum stress of the pipeline with a thickness of 16 mm is approximately 45% of the allowable stress of X65 steel, and the pipeline remains safe for 30 years. However, the potential failure of the pipeline should be considered due to the continued ground thawing and warming and pipe material aging. Forthcoming research on this topic is needed to evaluate more carefully the structural integrity of buried pipelines in cold regions.

Circum-Arctic peat soils resist priming by plant-derived compounds

Rapid Arctic warming increases permafrost thaw and CO2 production from soil organic matter decomposition, but also enhances CO2 uptake by plants. Conversely, plants can also stimulate soil organic matter decomposition near their roots, via rhizosphere priming. The recent PrimeSCale model suggests that this can accelerate Arctic soil carbon loss at a globally relevant rate, and points to large potential contributions from carbon-rich permafrost peatlands. At the same time, the high carbon content of peatlands might render them insusceptible to input of easily available organic compounds by plant roots, which is considered a key component of priming. We here investigated the sensitivity of permafrost peat soils to priming by plant compounds under aerobic conditions that resemble the dominant rooting zone, based on a 30-week laboratory incubation of peat soils from five circum-Arctic locations. No significant change in CO2 production from peat organic matter by organic carbon addition was observed, and an increase of 24% by organic nitrogen addition. Combining our data with a literature meta-analysis of priming studies showed similar, low priming sensitivity in organic layers of mineral soils, and significantly stronger priming in mineral horizons where organic carbon and nitrogen increased decomposition by 32% and 62%, respectively. Low sensitivity of permafrost peat to input of organic compounds was also supported under anaerobic conditions, by incubation of one soil type. In a new PrimeSCale sensitivity analysis, we show that excluding peatlands would reduce estimates of priming-induced carbon loss from the circum-Arctic by up to 40% (up to 18 Pg) until 2100, depending on peat priming sensitivity. While our study suggests a limited effect of plant-released organic compounds on peat decomposition, it does not preclude an effect of vegetation on decomposition under natural conditions, through other mechanisms. The large range of possible priming-induced peat carbon losses, and expected changes in vegetation and drainage, call for a sharpened focus on the combined effect of living plants on soil processes beyond carbon input, including changes in nutrient and water availability, aggregation, and microbial communities.

Quantifying the effect of a retrogressive thaw slump on soil freeze–thaw erosion in permafrost regions on the Qinghai–Tibet Plateau, China

AbstractThermokarst terrain is developing at an accelerating pace in the ice‐rich permafrost on the Qinghai–Tibet Plateau (QTP), China, and the most dramatic of these terrain‐altering thermokarsts is retrogressive thaw slump (RTS). The freeze–thaw erosion (FTE) impacts are sharply increasing on the Plateau due to RTS, especially as a result of the migration of fine sediments in cold climates, these impacts are still not quantified due to the limitation of hydro‐thermal‐mass transport laws in RTS development. Moreover, it is difficult to assess the impact of RTS on the ecology and environment, especially on soil erosion. This study developed a heat–water‐mass transport coupled model of a RTS in the Beiluhe River Region on the QTP, considering the actual topography, water‐ice phase change, latent heat, and surface heat exchange layer. Based on the observed data of ground temperature, unfrozen water content, and heat flux, the coupled model herein is practicable for presenting the geotemperature regime and groundwater flow in the RTS area, thereby quantifying the ice‐rich permafrost thaw and mass wasting. The results presented indicate that: (1) the seepage velocity of the superficial zone (0–1.5 m depth) is two orders of magnitude higher than that of the permafrost table; (2) the mean ice‐rich permafrost thaw volume was 13.4 m2 from 2016 to 2021; and (3) the cumulative mass transport volume was 22 m2 from July 2020 to September 2021. In addition, the relation between the FTE (shown as the migration of sediments) and the amount of ground ice ablation can be fitted by an exponential equation. This work proposes a reliable method for quantifying the effect of FTE and is helpful to assess the eco‐environmental impacts of RTS.

Optical remote sensing (Sentinel-3 OLCI) used to monitor dissolved organic carbon in the Lena River, Russia

In the past decades the Arctic has experienced stronger temperature increases than any other region globally. Shifts in hydrological regimes and accelerated permafrost thawing have been observed and are likely to increase mobilization of organic carbon and its transport through rivers into the Arctic Ocean. In order to better quantify changes to the carbon cycle, Arctic rivers such as the Lena River in Siberia need to be monitored closely. Since 2018, a sampling program provides frequent in situ observations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) of the Lena River. Here, we utilize this ground truth dataset and aim to test the potential of frequent satellite observations to spatially and temporally complement and expand these observations. We explored all available overpasses (~3250) of the Ocean and Land Colour Instrument (OLCI) on Sentinel-3 within the ice-free periods (May – October) for four years (2018 to 2021) to develop a new retrieval scheme to derive concentrations of DOC. OLCI observations with a spatial resolution of ~300 m were corrected for atmospheric effects using the Polymer algorithm. The results of this study show that using this new retrieval, remotely sensed DOC concentrations agree well with in situ DOC concentrations (MAPD=10.89%, RMSE=1.55 mg L−1, r²=0.92, n=489). The high revisit frequency and wide swath of OLCI allow it to capture the entire range of DOC concentrations and their seasonal variability. Estimated satellite-derived DOC export fluxes integrated over the ice-free periods of 2018 to 2021 show a high interannual variability and agree well with flux estimates from in situ data (RMSD=0.186 Tg C, MAPD=4.05%). In addition, 10-day OLCI composites covering the entire Lena River catchment revealed increasing DOC concentration and local sources of DOC along the Lena from south to north. We conclude that moderate resolution satellite imagers such as OLCI are very capable of observing DOC concentrations in large/wide rivers such as the Lena River despite the relatively coarse spatial resolution. The global coverage of remote sensing offers the expansion to more rivers in order to improve our understanding of the land-ocean carbon fluxes in a changing climate.

Climate warming has direct and indirect effects on microbes associated with carbon cycling in northern lakes.

Northern lakes disproportionately influence the global carbon cycle, and may do so more in the future depending on how their microbial communities respond to climate warming. Microbial communities can change because of the direct effects of climate warming on their metabolism and the indirect effects of climate warming on groundwater connectivity from thawing of surrounding permafrost, especially at lower landscape positions. Here we used shotgun metagenomics to compare the taxonomic and functional gene composition of sediment microbes in 19 peatland lakes across a 1600-km permafrost transect in boreal western Canada. We found microbes responded differently to the loss of regional permafrost cover than to increases in local groundwater connectivity. These results suggest that both the direct and indirect effects of climate warming, which were respectively associated with loss of permafrost and subsequent changes in groundwater connectivity interact to change microbial composition and function. Archaeal methanogens and genes involved in all major methanogenesis pathways were more abundant in warmer regions with less permafrost, but higher groundwater connectivity partly offset these effects. Bacterial community composition and methanotrophy genes did not vary with regional permafrost cover, and the latter changed similarly to methanogenesis with groundwater connectivity. Finally, we found an increase in sugar utilization genes in regions with less permafrost, which may further fuel methanogenesis. These results provide the microbial mechanism for observed increases in methane emissions associated with loss of permafrost cover in this region and suggest that future emissions will primarily be controlled by archaeal methanogens over methanotrophic bacteria as northern lakes warm. Our study more generally suggests that future predictions of aquatic carbon cycling will be improved by considering how climate warming exerts both direct effects associated with regional-scale permafrost thaw and indirect effects associated with local hydrology.

Patterns and drivers of anaerobic nitrogen transformations in sediments of thermokarst lakes.

Significant attention has been given to the way in which the soil nitrogen (N) cycle responds to permafrost thaw in recent years, yet little is known about anaerobic N transformations in thermokarst lakes, which account for more than one-third of thermokarst landforms across permafrost regions. Based on the N isotope dilution and tracing technique, combined with qPCR and high-throughput sequencing, we presented large-scale measurements of anaerobic N transformations of sediments across 30 thermokarst lakes over the Tibetan alpine permafrost region. Our results showed that gross N mineralization, ammonium immobilization, and dissimilatory nitrate reduction rates in thermokarst lakes were higher in the eastern part of our study area than in the west. Denitrification dominated in the dissimilatory nitrate reduction processes, being two and one orders of magnitude higher than anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA), respectively. The abundances of the dissimilatory nitrate reduction genes (nirK, nirS, hzsB, and nrfA) exhibited patterns consistent with sediment N transformation rates, while α diversity did not. The inter-lake variability in gross N mineralization and ammonium immobilization was dominantly driven by microbial biomass, while the variability in anammox and DNRA was driven by substrate supply and organic carbon content, respectively. Denitrification was jointly affected by nirS abundance and organic carbon content. Overall, the patterns and drivers of anaerobic N transformation rates detected in this study provide a new perspective on potential N release, retention, and removal upon the formation and development of thermokarst lakes.

Using Numerical Reservoir Simulation to Assess CO2 Capture and Underground Storage, Case Study on a Romanian Power Plant and Its Surrounding Hydrocarbon Reservoirs

During current times, it is acknowledged that there is the often presence of extreme meteorological phenomena including floods and landslides, due to heavy rains, large wildfires, due to heat or droughts, permafrost melting, etc. At this stage, the world admits that anthropic activities have an important impact on these phenomena and considers that greenhouse gases are at the core of this climate change. The most common greenhouse gasses have general formulae COX and/or NOX, and they are released during different energy generating/conversion processes such as electric energy generation from fossil fuels or mechanical energy obtained from by-products of fossil fuels. Once acknowledged, the world’s countries have developed long-term strategies to eliminate gradually the release of these gases directly into Earth’s atmosphere. E.g., the EU aims to be climate-neutral by 2050; i.e., its economy will have net-zero greenhouse gas emissions. For this to happen, different effective methodologies have been drafted and implemented with underground gas storage in hydrocarbon depleted geological formations and/or saline aquifers being ones of significance when it comes to electric energy generation from fossil fuels in controlled spaces. The paper presents the simulation of capturing and injecting of these greenhouse gases through injection wells in neighboring depleted natural gas reservoirs using commercial numerical simulators for the Iernut natural gas (CH4) burning power plant which is one of Romania’s most important gas plants. Within this simulation study, the total CO2 quantity that can be stored via the proposed carbon capture and sequestration study and the proportion of each of the three CO2 storage mechanisms involved in the process (physical trapping, hydrodynamic trapping, and geochemical trapping) were determined and presented. Even though previous local studies investigated the potential of CO2 storage and sequestration into the Romanian underground reservoirs, none of it considered using the depleted hydrocarbon reservoirs surrounding the Iernut power plant for this process.

Pan-Arctic soil element bioavailability estimations

Abstract. Arctic soils store large amounts of organic carbon and other elements, such as amorphous silicon, silicon, calcium, iron, aluminum, and phosphorous. Global warming is projected to be most pronounced in the Arctic, leading to thawing permafrost which, in turn, changes the soil element availability. To project how biogeochemical cycling in Arctic ecosystems will be affected by climate change, there is a need for data on element availability. Here, we analyzed the amorphous silicon (ASi) content as a solid fraction of the soils as well as Mehlich III extractions for the bioavailability of silicon (Si), calcium (Ca), iron (Fe), phosphorus (P), and aluminum (Al) from 574 soil samples from the circumpolar Arctic region. We show large differences in the ASi fraction and in Si, Ca, Fe, Al, and P availability among different lithologies and Arctic regions. We summarize these data in pan-Arctic maps of the ASi fraction and available Si, Ca, Fe, P, and Al concentrations, focusing on the top 100 cm of Arctic soil. Furthermore, we provide element availability values for the organic and mineral layers of the seasonally thawing active layer as well as for the uppermost permafrost layer. Our spatially explicit data on differences in the availability of elements between the different lithological classes and regions now and in the future will improve Arctic Earth system models for estimating current and future carbon and nutrient feedbacks under climate change (https://doi.org/10.17617/3.8KGQUN, Schaller and Goeckede, 2022).

Current and Future Global Lake Methane Emissions: A Process‐Based Modeling Analysis

AbstractFreshwater ecosystem contributions to the global methane budget remains the most uncertain among natural sources. With warming and accompanying carbon release from thawed permafrost and thermokarst lake expansion, the increase of methane emissions could be large. However, the impact and relative importance of various factors related to warming remain uncertain. Based on diverse lake characteristics incorporated in modeling and observational data, we calibrate and verify a lake biogeochemistry model. The model is then applied to estimate global lake methane emissions and examine the impacts of temperature increase for the first and the last decades of the 21st century under different climate scenarios. We find that current emissions are 24.0 ± 8.4 Tg CH4 yr−1 from lakes larger than 0.1 km2, accounting for 11% of the global total natural source as estimated based on atmospheric inversion. Future projections under the RCP8.5 scenario suggest a 58%–86% growth in emissions from lakes. Our model sensitivity analysis indicates that additional carbon substrates from thawing permafrost may enhance methane production under warming in the Arctic. Warming enhanced methane oxidation in lake water can be an effective sink to reduce the net release from global lakes.

TYPOLOGY OF TERRITORIAL VEGETATION UNITS ON THE EXAMPLE OF THERMOKARST MASSIFS ON KOTELNY ISLAND (NEW SIBERIAN ISLANDS)

In the Arctic, permafrost and underground ice melting is one of the modern problems that requires constant monitoring. In the areas with spreading thick ice-wedge polygonal networks, the territories occupied by groups of mounds separated by trenches formed in the place of melted ice veins are a widespread result of thermokarst. The vegetation of such territories (referred to as massifs) is represented by territorial units of the first hierarchy levels with an intra- (mosaic of patches within communities) and supraphytocoenotic (micro- and mesokombinations: complexes, their aggregates and series) heterogeneity. The shape, size and vegetation of the mounds and trenches change under the influence of thermal denudation, and the vegetation of a whole massif also changes, so it can serve as an indicator of thermokarst intensity. The relief formed by thermokarst is typical of the landscapes of New Siberian Islands. According to the descriptions of 38 massifs made on Kotelny Island in 1974–1975, they were for the first time typified as territorial units of vegetation. In total, 7 types of massifs belonging to 2 classes are identified and characterized: baidzharakh massifs proper (with mineral mounds) and massifs with peat mounds (including 1 type of massifs). The first class comprises 2 groups of types: weakly differentiated massifs (microcombinations) and differentiated massifs (mesocombinations). In the first group, there are 2 types of massifs, in the second one – 4 types. Six distinguished types of baidzharakh massifs correspond to successive stages of massif transformation (a scheme is given). The heterogeneity of massif vegetation becomes more complicated with time, and becomes simpler again, when the massif is destroyed. Embryonal massifs are represented by microcombinations, a group of developed (differentiated) massifs – by mesocombinations, collapsing massifs – by microcombinations. The developed typology of the baidzharakh massifs as territorial vegetation units is convenient for practical work and using in monitoring and comparative assessment of thermokarst intensity in the Arctic areas with different flora and vegetation.

Climate change and emerging pathogens: Toward nature-based solutions

Our planet has experienced several drastic changes in climate during its history. Nonetheless, human activity in the last two hundred years of industrialization has brought about a significant rise in greenhouse gases, which have led to inexorable global warming and climate change, a direct threat to our ecosystem. Global warming is precipitating the thawing of peri-glacial permafrost, which then releases additional greenhouse gasses, which can be toxic to our health, and alter fresh water supplies and crops. Melting permafrost also releases ancient and novel pathogens potentially harmful to human health. This paper outlines certain nature-based solutions that could bring about carbon-neutral energy generation, as well as immune protective interventions.

The thermal response of permafrost to coastal floodplain flooding

Flooding of low-lying Arctic regions has the potential to warm and thaw permafrost by changing the surface reflectance of solar insolation, increasing subsurface soil moisture, and increasing soil thermal conductivity. However, the impact of flooding on permafrost in the continuous permafrost environment remains poorly understood. To address this knowledge gap, we used a combination of available flooding data on the Ikpikpuk delta and a numerical model to simulate the hydro-thermal processes under coastal floodplain flooding. We first constructed the three most common flood events based on water level data on the Ikpikpuk: snowmelt floods in the late spring and early summer, middle and late summer floods, and floods throughout the whole spring and summer. Then the impact of these flooding events on the permafrost was simulated for one-dimensional permafrost columns using the Advanced Terrestrial Simulator (ATSv1.0), a fully coupled permafrost-hydrology and thermal dynamic model. Our results show that coastal floods have an important impact on coastal permafrost dynamics with a cooling effect on the surficial soil and a warming effect on the deeper soil. Cumulative flooding events over several years can cause continuous warming of the deep subsurface but cool down the surficial layer. Flood timing is a primary control of the vertical extent of the permafrost thaw and the active layer deepening.

Persistence and Potential Atmospheric Ramifications of Ice-Nucleating Particles Released from Thawing Permafrost.

Permafrost underlies approximately a quarter of the Northern Hemisphere and is changing amidst a warming climate. Thawed permafrost can enter water bodies through top-down thaw, thermokarst erosion, and slumping. Recent work revealed that permafrost contains ice-nucleating particles (INPs) with concentrations comparable to midlatitude topsoil. These INPs may impact the surface energy budget of the Arctic by affecting mixed-phase clouds, if emitted into the atmosphere. In two 3-4-week experiments, we placed 30,000- and 1000-year-old ice-rich silt permafrost in a tank with artificial freshwater and monitored aerosol INP emissions and water INP concentrations as the water's salinity and temperature were varied to mimic aging and transport of thawed material into seawater. We also tracked aerosol and water INP composition through thermal treatments and peroxide digestions and bacterial community composition with DNA sequencing. We found that the older permafrost produced the highest and most stable airborne INP concentrations, with levels comparable to desert dust when normalized to particle surface area. Both samples showed that the transfer of INPs to air persisted during simulated transport to the ocean, demonstrating a potential to influence the Arctic INP budget. This suggests an urgent need for quantifying permafrost INP sources and airborne emission mechanisms in climate models.

Greater regulation of permafrost organic matter composition by enzymes and redox than temperature

Accelerating permafrost thaw across high-latitude ecosystems increases organic matter (OM) availability to soil microorganisms. The chemical composition of permafrost-derived OM, which influences microbial metabolic efficiency, may determine whether the Arctic remains a critical carbon sink in the future or shifts to net a source of greenhouse gases. During a 90-day, simulated thaw experiment, we measured shifts in the chemistry of soil and dissolved organic matter pools sourced from surface (0–10 cm) and subsurface (16–25 cm) permafrost layers. We manipulated temperature (1 versus 15 °C) and redox (drained versus saturated) to mimic field-relevant conditions and tested whether variability in OM chemistry could be explained by the activities of six hydrolytic enzymes that catalyze carbon, nitrogen, and phosphorus mineralization. We found that permafrost depth and enzyme activities were significantly correlated with shifts in soil OM (SOM) functional group chemistries, as measured by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Greater variability in dissolved OM (DOM) chemistries were driven by redox and pH. Our results suggest depth-resolved analyses of permafrost vulnerability to enzymatic hydrolysis may help explain distinct patterns in SOM versus DOM cycling that can be used to constrain modeled projections of Arctic carbon storage during global change.

Recording Permafrost Thaw and Thaw Lake Degradation in Northern Siberia: School Science in Action

Arctic landscapes are changing dramatically in response to climate changes that are regionally four times faster than the global average. However, these vast lands are sparsely populated and ground-based measurements of environmental change impacts on land and lakes are few compared with the impacted areas. In the Tazovsky District of the Yamalo-Nenets Autonomous Okrug, school science has been put into practice for over 20 years to determine thermokarst lake changes and their causes. We describe the contributions of school science in recording these lake changes and also their local impacts as a contribution to the Siberian Environmental Change Network. Describing the process and results are particularly relevant to the generation that will experience the greatest environmental change impacts. In contrast to the use of traditional indigenous knowledge, we report on a conventional science methodology used by local people that further empowers a new generation to determine for themselves the changes in their environment and associated implications.

Tundra Soil Viruses Mediate Responses of Microbial Communities to Climate Warming.

The rise of global temperature causes the degradation of the substantial reserves of carbon (C) stored in tundra soils, in which microbial processes play critical roles. Viruses are known to influence the soil C cycle by encoding auxiliary metabolic genes and infecting key microorganisms, but their regulation of microbial communities under climate warming remains unexplored. In this study, we evaluated the responses of viral communities for about 5 years of experimental warming at two depths (15 to 25 cm and 45 to 55 cm) in the Alaskan permafrost region. Our results showed that the viral community and functional gene composition and abundances (including viral functional genes related to replication, structure, infection, and lysis) were significantly influenced by environmental conditions such as total nitrogen (N), total C, and soil thawing duration. Although long-term warming did not impact the viral community composition at the two depths, some glycoside hydrolases encoded by viruses were more abundant at both depths of the warmed plots. With the continuous reduction of total C, viruses may alleviate methane release by altering infection strategies on methanogens. Importantly, viruses can adopt lysogenic and lytic lifestyles to manipulate microbial communities at different soil depths, respectively, which could be one of the major factors causing the differences in microbial responses to warming. This study provides a new ecological perspective on how viruses regulate the responses of microbes to warming at community and functional scales. IMPORTANCE Permafrost thawing causes microbial release of greenhouse gases, exacerbating climate warming. Some previous studies examined the responses of the microbial communities and functions to warming in permafrost region, but the roles of viruses in mediating the responses of microbial communities to warming are poorly understood. This study revealed that warming induced changes in some viral functional classes and in the virus/microbe ratios for specific lineages, which might influence the entire microbial community. Furthermore, differences in viral communities and functions, along with soil depths, are important factors influencing microbial responses to warming. Collectively, our study revealed the regulation of microbial communities by viruses and demonstrated the importance of viruses in the microbial ecology research.

Drying of tundra landscapes will limit subsidence-induced acceleration of permafrost thaw

We used a model for permafrost hydrology informed by detailed measurements of soil ice content to better understand the potential risk of abrupt permafrost thaw triggered by melting ground ice, a key open question associated with permafrost response to a warming Arctic. Our spatially resolved simulations of a well-characterized site in polygonal tundra near Utqiaġvik, Alaska, agree well with multiple types of observations in the current climate. Projections indicate 63cm of bulk subsidence from 2006 to 2100 in the strong-warming RepresentativeConcentration Pathway 8.5 climate. Permafrost thaw as measured by the increase in active layer thickness (ALT)-the thickness of the soil layer that thaws each summer-is accelerated by subsidence, but the effect is relatively small. The ALT increases from the current-day value of approximately 50cm to approximately 180cm by 2100 when subsidence is included compared to about 160cm when it is neglected. In these simulations, previously identified positive feedbacks between subsidence and thaw are self-limiting on decadal time frames because landscape runoff and increasing evapotranspiration result in drier tundra with weaker surface/atmosphere coupling. These results for a tundra site that is representative of large swathes of the Alaska North Slope suggest that subsidence is unlikely to lead to abrupt thaw over large areas. However, subsidence does have significant effects on the hydrology of polygonal tundra. Specifically, subsidence increases landscape runoff, which helps maintain streamflow in the face of increased evapotranspiration but also causes drier tundra conditions that could have deleterious effects on sensitive Arctic wetland ecosystems.

Mapping retrogressive thaw slumps using deep neural networks

Retrogressive thaw slumps (RTS) are thermokarst features in ice-rich hillslope permafrost terrain, and their occurrence in the warming Arctic is increasingly frequent and has caused dynamic changes to the landscape. RTS can significantly impact permafrost stability and generate substantial carbon emissions. Understanding the spatial and temporal distribution of RTS is a critical step to understanding and modelling greenhouse gas emissions from permafrost thaw. Mapping RTS using conventional Earth observation approaches is challenging due to the highly dynamic nature and often small scale of RTS in the Arctic. In this study, we trained deep neural network models to map RTS across several landscapes in Siberia and Canada. Convolutional neural networks were trained with 965 RTS features, where 509 were from the Yamal and Gydan peninsulas in Siberia, and 456 from six other pan-Arctic regions including Canada and Northeastern Siberia. We further tested the impact of negative data on the model performance. We used 4-m Maxar commercial imagery as the base map, 10-m NDVI derived from Sentinel-2 and 2-m elevation data from the ArcticDEM as model inputs and applied image augmentation techniques to enhance training. The best-performing model reached a validation Intersection over Union (IoU) score of 0.74 and a test IoU score of 0.71. Compared to past efforts to map RTS features, this represents one of the best-performing models and generalises well for mapping RTS in different permafrost regions, representing a critical step towards pan-Arctic deployment. The predicted RTS matched very well with the ground truth labels visually. We also tested how model performance varied across different regional contexts. The result shows an overall positive impact on the model performance when data from different regions were incorporated into the training. We propose this method as an effective, accurate and computationally undemanding approach for RTS mapping.

A Tale of Two Seasons: Distinct Seasonal Viral Communities in a Thermokarst Lake

Thermokarst lakes are important features of subarctic landscapes and are a substantial source of greenhouse gases, although the extent of gas produced varies seasonally. Microbial communities are responsible for the production of methane and CO2 but the "top down" forces that influence microbial dynamics (i.e., grazers and viruses) and how they vary temporally within these lakes are still poorly understood. The aim of this study was to examine viral diversity over time to elucidate the seasonal structure of the viral communities in thermokarst lakes. We produced virus-enriched metagenomes from a subarctic peatland thermokarst lake in the summer and winter over three years. The vast majority of vOTUs assigned to viral families belonged to Caudovirales (Caudoviricetes), notably the morphological groups myovirus, siphovirus and podovirus. We identified two distinct communities: a dynamic, seasonal community in the oxygenated surface layer during the summer and a stable community found in the anoxic water layer at the bottom of the lake in summer and throughout much of the water column in winter. Comparison with other permafrost and northern lake metagenomes highlighted the distinct composition of viral communities in this permafrost thaw lake ecosystem.