Antoine Séjourné

In periglacial environments, water flowing on the surface of frozen ground can either thermally erode the soil or form successive ice layers with not only the water temperature as a controlling parameter. This study investigates the mechanism of water flows on permafrost creating thermal erosion or ice aggradation in periglacial environments. Our experimental setup mimicked the natural occurrence of meltwater flow over permafrost terrain during seasonal transitions in early spring. Our objective was to test different scenarios defined by different boundary conditions (water discharge, water stream temperature and frozen soil temperature). This study measured thawing and erosion depths with sensors and laser, revealing that water thermal gradients control heat propagation in the thawed substrates. Results show that water temperature is the key factor: stream temperature above +1.3°C leads to erosion, while lower one (< 1.3°C) favours ice aggradation. Stream discharge rate controls thermal exchange, with low flows promoting freezing and high flows enhancing erosion.

Sur Terre, les régions périglaciaires ayant un pergélisol riche en glace peuvent enregistrer les changements climatiques globaux. Ce pergélisol contenant 50-80 % de glace en volume s'est formé lors des grandes périodes glaciaires du Pléistocène. Par la suite, ce pergélisol riche en glace a subi une dégradation intense lors de réchauffements climatiques globaux au début de la période interglaciaire de l'Holocène.La planète Mars comporte un pergélisol à l'échelle planétaire dont la formation serait associée à des changements climatiques globaux provoqués par des variations chaotiques de son orbite durant les derniers millions d'années. La région d'Utopia Planitia située dans les moyennes latitudes nord de Mars présente différents modelés de surface (“ scalloped depressions ”, polygones, cavités à la jonction des polygones) interprétés comme s'étant formés à partir d'un pergélisol contenant potentiellement une grande quantité de glace. De la même manière que sur Terre, ce pergélisol a pu enregistrer les derniers changements climatiques globaux survenus sur Mars.Cette thèse propose d'étudier comparativement l'impact des changements climatiques sur le paysage des régions périglaciaires sur Terre et sur Mars. Dans ce but, nous avons conduit des études sur le terrain des processus et des modelés périglaciaires en Yakoutie Centrale (Sibérie) et dans le delta du Mackenzie (Canada) associées à une étude géomorphologique à haute résolution des modelés d'Utopia Planitia.Notre étude montre que l'ensemble des modelés d'Utopia Planitia est similaire en morphologie, taille et association spatiale à celui de la Yakoutie Centrale et du delta du Mackenzie (lacs thermokarstiques, polygones, mares à la jonction des polygones) indiquant que la région présenterait un pergélisol riche en glace. Le pergélisol serait composé de sédiments stratifiés et dont l'âge de formation minimale est estimé entre ~ 5 et 100 Ma. Le pergélisol contiendrait un volume de glace important (≥ 50 % en volume) sur une épaisseur de ~ 70 m.De part ses caractéristiques, ce pergélisol aurait une origine syngénétique : sa formation serait le résultat d'une accumulation importante de sédiments au sein du bassin d'Utopia Planitia sous des conditions froides permettant le gel in-situ des sédiments. Les sédiments peuvent avoir été déposés par des vallées de débâcles provenant d'Elysium Mons et/ou par une activité éolienne importante. Par ailleurs, la formation synchrone d'une calotte de glace régionale près d'Utopia Planitia lors de périodes de moyenne obliquité (~ 35°) de Mars pourrait avoir induit un dépôt éolien préférentiel dans Utopia Planitia.Par la suite, le pergélisol riche en glace aurait subi une dégradation régionale importante entre ~ 5 et 10 Ma. Ce thermokarst aurait été déclenché par une insolation accrue lors de périodes de haute obliquité (~ 45°) de Mars. L'augmentation des températures aurait provoqué une déstabilisation thermique du pergélisol entraînant une sublimation de la glace, modifiant profondément le paysage de la région.Ainsi, les variations importantes de l'obliquité de Mars ont généré des changements climatiques globaux qui ont permis la formation et la dégradation du pergélisol riche en glace d'Utopia Planitia entre ~ 5 et 10 Ma.

Arctic warming is leading to permafrost thawing which modifies, in cascade, hydrosystems at diverse levels. This study aimed to compare prokaryotic community structure and methane (CH ₄ ) dynamics across 16 sub‐Arctic waterbodies, and to assess how these features are shaped by permafrost thaw. The sampled waterbodies, located in an ice‐rich discontinuous permafrost region (Southwestern Yukon, Canada) differed in size, depth, stratification and degree of thaw influence. Prokaryotic communities were characterised through 16S rRNA gene sequencing and qPCR targeting mcrA (methanogenesis) and pmoA (methanotrophy) genes. Community structures differed significantly between shallow stratified, deep stratified and non‐stratified waterbodies. Methanogens, predominantly represented by the Methanobacterium genus, were mostly detected in shallow non‐stratified waterbodies. Methanotrophs, primarily represented by the Methylacidiphilaceae family, were more abundant in oxic layers whereas bacteria of Crenothrix and Methylobacter genera dominated in anoxic conditions. Our results showed that non‐stratified waterbodies directly affected by permafrost thaw harboured distinct prokaryotic communities, including specific methanogens and methanotrophs. The two sites with the highest CH ₄ emissions were affected by permafrost thaw, with fluxes reaching up to 1.7 × 10 ⁻¹ mg m ⁻² s ⁻¹ . Future investigations should address gaps in CH ₄ ‐related processes in thaw‐affected systems, as they are hotspots for methane emissions and harbour different microbial communities.

In evaluating the unique geomorphology of Hebrus Valles on Mars, an outflow channel southeast of Utopia Planitia, we investigated the role of fluvial dynamics in forming subsurface conduits. Unlike typical Martian channels that terminate under younger sediment rocks, Hebrus Valles retains its downstream features, thus offering a window into surface and subsurface interactions between liquid water and regolith. Employing physical lab simulations, we utilized buried polygonal tunneling configurations in a sandy matrix to replicate the incision features observed at the channel's terminus. This study provides the first lab‐based evidence demonstrating the role of conduits in conveying large floods in the Hebrus Valles. Our results lend support to the interpretation that Hebrus Valles conveyed high‐volume floods by demonstrating that conduits are necessary to form the features observed on the surface of Hebrus Valles. The experiments also highlight the role of sinkholes as floodwater capture points, which suggests the formation of a regional interconnected network of subsurface channels and caverns. These findings affirm that fluvial dynamics in Hebrus Valles could have expanded subsurface discontinuities into functioning conduit systems, potentially directing floodwaters through an interconnected subterranean network.

Arctic regions are highly impacted by the global temperature rising and its consequences and influences on the thermo-hydro processes and their feedbacks. Theses processes are especially not very well understood in the context of river–permafrost interactions and permafrost degradation. This paper focuses on the thermal characterization of a river–valley system in a continuous permafrost area (Syrdakh, Yakutia, Eastern Siberia) that is subject to intense thawing, with major consequences on water resources and quality. We investigated this Yakutian area through two transects crossing the river using classical tools such as in–situ temperature measurements, direct active layer thickness estimations, unscrewed aerial vehicle (UAV) imagery, heat transfer numerical experiments, Ground-Penetrating Radar (GPR), and Electrical Resistivity Tomography (ERT). Of these two transects, one was closely investigated with a long-term temperature time series from 2012 to 2018, while both of them were surveyed by geophysical and UAV data acquisition in 2017 and 2018. Thermodynamical numerical simulations were run based on the long-term temperature series and are in agreement with river thermal influence on permafrost and active layer extensions retrieved from GPR and ERT profiles. An electrical resistivity-temperature relationship highlights the predominant role of water in such a complicated system and paves the way to coupled thermo-hydro-geophysical modeling for understanding permafrost–river system evolution.

The current rate and magnitude of temperature rise in the Arctic are disproportionately high compared to global averages. Along with other natural and anthropogenic disturbances, this warming has caused widespread permafrost degradation and soil subsidence, resulting in the formation of thermokarst (thaw) lakes in areas of ice-rich permafrost. These lakes are hotspots of greenhouse gas emissions (CO₂ and CH₄), but with substantial spatial and temporal heterogeneity across Arctic and sub-Arctic regions. In Central Yakutia (Eastern Siberia, Russia), nearly half of the landscape has been affected by thermokarst processes since the early Holocene, resulting in the formation of more than 10,000 partly drained lake depressions (alas lakes). It is not yet clear how recent changes in temperature and precipitation will affect existing lakes and the formation of new thermokarst lakes. A multi-decadal remote sensing analysis of lake formation and development was conducted for two large study areas (~1200 km² each) in Central Yakutia. Mask Region-Based Convolutional Neural Networks (R-CNN) instance segmentation was used to semi-automate lake detection in Satellite pour l’Observation de la Terre (SPOT) and declassified US military (CORONA) images (1967–2019). Using these techniques, we quantified changes in lake surface area for three different lake types (unconnected alas lake, connected alas lake, and recent thermokarst lake) since the 1960s. Our results indicate that unconnected alas lakes are the dominant lake type, both in the number of lakes and total surface area coverage. Unconnected alas lakes appear to be more susceptible to changes in precipitation compared to the other two lake types. The majority of recent thermokarst lakes form within 1 km of observable human disturbance and their surface area is directly related to air temperature increases. These results suggest that climate change and human disturbances are having a strong impact on the landscape and hydrology of Central Yakutia. This will likely affect regional and global carbon cycles, with implications for positive feedback scenarios in a continued climate warming situation.

Rapid permafrost degradation is observed in northern regions as a result of climate change and expanding economic development. Associated increases in active layer depth lead to thermokarst development, resulting in irregular surface topography. In Central Yakutia, significant areas of the land surface have been deteriorated by thermokarst; however, no mitigation or land rehabilitation efforts are undertaken. This paper presents the results of numerical modeling of the thermal response of permafrost to changes in the active layer hydrothermal regime using field data from the village of Amga, Republic of Sakha (Yakutia), and mathematical analysis. The results suggest that restoring a thick ice-enriched layer will require increasing the pre-winter soil moisture contents in order to increase the effective heat capacity of the active layer. Snow removal or compaction during the winter is recommended to maximize permafrost cooling. The thickness of the restored transition layer varies from 0.3 to 1.3 m depending on soil moisture contents in the active layer. The modeling results demonstrate that damaged lands can be restored through a set of measures to lower the subsurface temperatures. A combination of the insulating layer (forest vegetation) and the high heat capacity layer (transition layer) in the atmosphere–ground system would be more effective in providing stable geocryological conditions.

In the ice‐rich permafrost of the Arctic regions, thermokarst erosion on slopes induces the formation of large‐scale retrogressive thaw slumps (RTS). They have significant geomorphological, hydrological, and biogeochemical impacts on the landscape. Further research is thus needed to better understand the respective effect of ice content and permafrost heterogeneities on the dynamics of these erosional features. Here, we present results of a full‐scale physical modeling of RTS development in a cold room. The experimental setup was designed to simulate and compare two ground‐ice settings (ice wedges, icy layers) with the thawing of ice‐poor permafrost (i.e., reference model). Our results show that the melting of the icy layers induces a loss of decohesion of the overlapping frozen soil. The heterogeneous frozen soil with ice wedges needs a longer time until degradation, but undergoes a stronger and faster decohesion of its structure during the thawing phase.