Climate warming has the potential to alter the structure and function of Arctic ecosystems in ways that are not fully understood. Polygonal terrain is a widespread permafrost feature of Arctic landscapes that is likely to be impacted by warming ground temperatures. This is of particular relevance in the uplands in the Mackenzie Delta region, where high-centre ice wedge polygon fields comprise 10% of the terrestrial landscape, and mean annual ground temperatures have increased between 1 and 2°C over the last 40 years (Burn and Kokelj 2009). I used broad-scale airphoto analysis and fine-scale field studies to investigate the impacts and possible trajectories of ice wedge degradation in the upland tundra north of Inuvik, NWT. Field investigations were undertaken to characterize biotic and abiotic conditions and feedbacks in stable and degrading high-centre polygons. Field surveys were conducted along transects which crossed three polygon micropositions (centres, edges and troughs) and targeted a degradation sequence from stable troughs to ice wedge melt ponds. I measured surface microtopography, active layer depth, water depth, plant community composition, soil gravimetric moisture, late winter snow depth, and shallow annual ground temperatures. Field data showed that ice wedge degradation drove increases in soil moisture, standing water depth, ground surface collapse, ground temperature, and active layer thaw and snow pack compared to stable troughs. These changing abiotic conditions drove the shift from mesic upland tundra plant communities to unvegetated melt ponds. Interactions between abiotic and biotic factors in degrading troughs increase ground temperature and contribute to positive feedbacks for ice wedge degradation. Analysis of broad-scale factors affecting ice wedge degradation involved the mapping of high-centre polygon distribution across the study area and the distribution of ice wedge melt ponds using high-resolution aerial photographs from 2004. Recent changes in melt pond area were also mapped using imagery dating from 1972. Thermokarst activity in polygonal terrain adjacent to anthropogenic disturbances was also assessed. Polygon fields were more abundant and larger in the northern part of the study area, where ground temperature conditions were most favourable for ice wedge formation. Spatial variation in polygonal terrain density was also related to topography, drainage, and the distribution of lacustrine sediments. Melt pond mapping and assessment of thermokarst at anthropogenic disturbances showed that ice wedges at higher latitudes are more susceptible to degradation primarily because these areas are underlain by larger and more abundant ice wedges. Melt pond mapping confirmed that the polygonal fields north of 69.4°N have shown both large increases and decreases in area, and that polygons in the south have been relatively stable in recent decades. The increased thaw sensitivity of polygonal terrain at higher latitudes has implications for soil carbon dynamics, terrestrial ecosystems, and the planning and maintenance of infrastructure as air and ground temperatures continue to increase. / Graduate / 0329 / 0372 / 0388

This project investigates the near surface energy balance on the Murt`el-Corvatsch rock glacier in the Upper Engadine, Swiss Alps, using the 1D physical SNOWPACK model. A correct representation of the near surface energy balance is important to predict the long term evolution of permafrost below rock glaciers. This is of interest in the context of future water availability and management of water resources in a changing climate and also in the context of natural hazards. Some difficulties in modelling the thermal regime of rock glaciers are related to the large pore spaces between the blocks, which allow for different modes of heat transport. With this in mind, different modelling approaches were investigated: using the standard SNOWPACK (without advective heat flux, ventilation or canopy module), adding an advective heat flux, using the ventilation and canopy modules. The most promising results, i.e. the best match between measured and modelled temperatures, were obtained from the ventilation parameterisation. This parameterisation accounts for boundary-layer air penetrating into the blocky layer. Furthermore it was found that the most important input variables are the thickness of the the blocky layer, since this is where the additional modes of heat exchange take place, and the ice and void volume fraction together with the field capacity in the icy layer. The latter are particularly relevant for long term modelling as they determine the amount of ice melt and water transport in the icy layer. Measured and modelled temperatures at depths of 0.5 m, 2.5 m, 3.5 m and 7.5 m were compared. Generally good agreements between modelled and measured temperatures were obtained for the depths 0.5 m, 3.5 m and 7.5 m. The slight warming trend at the end of the modelled period (2012- 2016) that can be observed in the borehole data is also present in the modelled temperatures. The depth of 2.5 m shows the least agreement between modelled and measured temperatures with and overestimation during the snow free period and an underestimation during the snow covered period. However, agreement between modelled and measured temperatures improves for the snow covered period after a simulation period of about ten years.

Methane hydrates are frozen deposits of methane and water found in high pressure or low temperature sediments. When these deposits destabilize, large quantities of methane can be emitted into the atmosphere. This is significant to climate change because methane has 25 times more greenhouse gas potential than Carbon Dioxide. Worldwide, it is estimated there are between 2500 and 10000 gigatons of methane stored in hydrate deposits. This represents more carbon than all fossil fuels on Earth. It is estimated that between 200 and 2000 gigatons of methane are stored in hydrates in Arctic waters acutely vulnerable to greenhouse warming. Over the last decade, researchers have identified instances of hydrate destabilization that have already begun. To gain insight into the potential climatic effects widespread hydrate dissociation would have, researchers have examined hydrate dissociation during the Paleocene Eocene Thermal Maximum 55 million years ago as a geologic precedent. In this period, large-scale hydrate dissociation contributed to 5-8 degree Celsius warming worldwide. If such a climatic shift were to transpire today, impacts on society would be enormous. There is currently a debate in the scientific community as to whether the risk of methane hydrate dissociation is relevant to the present generation. One side argues that not enough methane could be emitted into the atmosphere from today’s hydrate sources to have a meaningful impact on climate warming, where the other side contends that more than enough methane could be emitted from present day hydrate deposits to cause significant impacts to the global greenhouse effect. Given the information currently known about hydrates, it is reasonable to conclude there is a moderate risk of widespread destabilization that could impact global climate change in the coming decades. Significant acceleration of the conversion to alternative energies and implementation of geoengineering strategies should be considered.

The Arctic is warming rapidly as a result of global climate change. Permafrost - permanently frozen ground - plays a critical role in shaping arctic ecosystems and stores nearly one half of the global soil organic matter. Therefore, disturbance of permafrost will likely impact the carbon and related biogeochemical processes on local and global scales. In the Alaskan Arctic, fire and thermokarst (permafrost thaw) have become more common and have been hypothesized to accelerate the hydrological export of inorganic nutrients and sediment, as well as biodegradable dissolved organic carbon (BDOC), which may alter ecosystem processes of impacted streams. The biogeochemical characteristics of two tundra streams were quantified several years after the development of gully thermokarst features. The observed responses in sediment and nutrient loading four years after gully formation were more subtle than expected, likely due to the stabilization of the features and the dynamics controlling the hydrologic connectivity between the gully and the stream. The response of impacted streams may depend on the presence of water tracks, particularly their location in reference to the thermokarst and downslope aquatic ecosystem. We found evidence of altered ecosystem structure (benthic standing stocks, algal biomass, and macroinvertebrate composition) and function (stream metabolism and nutrient uptake), which may be attributable to the previous years' allochthonous gully inputs. The patterns between the reference and impacted reaches were different for both stream sites. Rates of ecosystem production and respiration and benthic chlorophyll-a in the impacted reaches of the alluvial and peat-lined streams were significantly lower and greater, respectively, compared to the reference reaches, even though minimal differences in sediment and nutrient loading were detected. Rates of ammonium and soluble reactive phosphorus uptake were consistently lower in the impacted reach at the alluvial site. The observed differences in metabolism, nutrient uptake and macroinvertebrate community composition suggest that even though the geochemical signal diminished, gully features may have long-lasting impacts on the biological aspects of downstream ecosystem function. In a separate study, a suite of streams impacted by thermokarst and fire were sampled seasonally and spatially. Regional differences in water chemistry and BDOC were more significant than the influences of fire or thermokarst, likely due to differences in glacial age and elevation of the landscape. The streams of the older (>700 ka), lower elevation landscape contained higher concentrations of dissolved nitrogen and phosphorus and DOC and lower BDOC compared to the streams of the younger (50-200 ka) landscapes that had lower dissolved nutrient and DOC quantity of higher biodegradability. The findings in this dissertation indicate that arctic stream ecosystems are more resilient than we expected to small-scale, rapidly stabilizing gully thermokarst features and disturbance caused by fire. Scaling the results of these types of studies should consider the size of thermokarst features in relation to the size of impacted rivers and streams. It remains to be determined how general permafrost thaw will affect the structure and function of arctic streams in the future.

De nombreuses tentatives de reconstruction de l’étendue du paléo-pergélisol à partir de données de terrain montrent que de grandes parties de la France ont été affectées à la fin du Pléistocène. Cette étendue maximale a été attribuée au Dernier Maximum Glaciaire (DMG). Néanmoins, des contradictions existent entre les différentes reconstructions qui ont été réalisées pendant près d’un siècle ; elles résident en partie dans l’absence de consensus sur la signification paléoclimatique de certaines structures périglaciaires. De plus, le cadre chronologique utilisé pour ces reconstructions est principalement basé sur des datations relatives et/ou sur l’hypothèse que le maximum de froid durant le dernier glaciaire a été atteint pendant le DMG. Dans ce contexte, il était nécessaire de réévaluer les structures déjà décrites à la lumière de notre connaissance actuelle des processus périglaciaires et d’en chercher de nouvelles pour datation. L’approche développée pour résoudre ces problèmes a été divisée en trois parties. Tout d’abord, une base de données homogène fournissant un accès simple aux structures périglaciaires répertoriées sur le territoire français a été constituée. Celle-ci permet de remettre un site ou une structure dans un contexte régional pour éviter les interprétations simplistes et favorise une vision à l’échelle nationale. Cette base de données est accessible en ligne (https://afeqeng.hypotheses.org/48). Les données en coupe concernant les coins sableux, les pseudomorphoses de coin de glace et les coins composites ainsi que les données obtenues à partir de photos aériennes sur les polygones et les sols striés ont été compilées. Dans la deuxième partie de notre travail, nous nous sommes attachés à traiter les données recueillies. L’analyse à l’aide d’un SIG nous a apportée des informations sur l’influence de différents facteurs sur le développement des structures périglaciaires. Des comparaisons avec un ensemble de données du Nord de l’Europe a rendu possible la proposition d’une nouvelle carte des limites du pergélisol lors de son extension maximale en Europe de l’Ouest. La carte a ensuite été comparée avec des simulations du pergélisol issues de Modèles Globaux du Climat. Enfin, la troisième partie de cette thèse fournit le premier cadre chronologique pour la fissuration par contraction thermique du sol en France, en s’appuyant sur la datation par luminescence optiquement stimulée (OSL) du remplissage sableux des coins. / During the Mid to Late Pleistocene, the land area affected by periglacial conditions expanded and contracted repeatedly over large surfaces in mid-latitude Western Europe. In such environments, permafrost or deep seasonal freezing of the ground formed typical features, which have been the subject of abundant research by geomorphologists. In particular, researchers attempted to reconstruct the maximal extent of Pleistocene permafrost based on field evidence. Although most reconstructions suggest that permafrost spread over part of France during the coldest periods of the Pleistocene, there is no agreement regarding the land surface affected. This is mainly due to the scarcity of field data used for mapping and to the questionable palaeoclimatic significance of certain periglacial features. In addition, permafrost modelling during the Last Glacial Maximum using Global Climate Models does not seem consistent with field data. To solve these issues, a database of Pleistocene periglacial features has been compiled from a review of academic literature and unpublished reports, the analysis of aerial photographs and new field surveys. Polygons, soil stripes, ice-wedge pseudomorphs, sand wedges and composite wedge pseudomorphs were included in the database together with their geographic coordinates, geological context, description and references. The distribution of the identified features was analysed with a GIS software and clearly indicates that large areas in France were affected by periglaciation, apart from the southwesternmost part of France and the Languedoc. Ice-wedge pseudomorphs do not extend south of 47°N which indicates that widespread discontinuous permafrost did not affect the land south of the Paris basin. The exclusive presence of sand wedges with primary infill between 45 and 47°N, mainly in the periphery of coversands, suggests that thermal contraction cracking of the ground occurred together with sand drifting in a context of deep seasonal frost or sporadic discontinuous permafrost, unfavourable for the growth of significant ground-ice bodies. However, the description of composite-wedge pseudomorphs below 47°N indicates that at least locally ice veins formed probably during exceptionally cold winters. To provide a chronological framework for thermal contraction cracking single-grain OSL measurements were performed on 33 samples taken in the sandy infilling of sand-wedges and composite-wedge pseudomorphs. Results suggest that multiple events were recorded within wedges. The extraction of the datasets using the Finite Mixture Model, which was developed to analyse statistically data comprising multiple components, allowed calculating 86 ages. These age estimates show that wedge activity in France occurred at least 11 times over the last 100 ka. The most widespread events of thermal contraction cracking occurred between ca. 30 and 24 ka (Last Permafrost Maximum) and are concomitant with periods of high sand availability (MIS 2). Although most phases of sand-wedge growth correlate well with known Pleistocene cold periods, the identification of wedge activity during late MIS 5 and the very beginning of the Holocene strongly suggests that sand-wedges do not only indicate permafrost but also deep seasonal ground freezing in the context of low winter insolation. The previously published young ages yielded by North-European sand-wedges likely result from poor record of periglacial periods concomitant with low sand availability and/or age averaging inherent to standard luminescence methods. This work allowed us to propose a map of the maximum extent of Late Pleistocene permafrost in France, which partially reconciles field data with palaeoclimatic simulations. The remaining discrepancies may be linked with a potential time lag between the Last Permafrost Maximum (c. 31–24 ka) and the Last Glacial Maximum (21 ka) and to the already identified warm winter bias of the models.

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. / On Earth, periglacial regions where an ice-rich permafrost is present provide a record of global climate changes. For example, the ice-rich permafrost (50-80 % of ice by volume) that occurs in Central Yakutia (Siberia) and in the Mackenzie River Delta (Canada) was formed during the glacial periods of the Pleistocene. This permafrost was subsequently degraded during global warming at the early Holocene interglacial period.Global and possibly ice-rich permafrost occurs on Mars as well. It is thought to be the product of obliquity-driven and relatively recent global climate change (i.e. dozens of Ma). Utopia Planitia, situated in the northern mid-latitudes, is dotted with possible periglacial landforms (scalloped depressions, polygons and polygon-junction pits) that could indicate the presence of an ice-rich permafrost. Similarly to Earth, this permafrost could be marker of recent global climate changes.This thesis focuses on the impact of global climate changes on the periglacial regions of Earth and Mars. With this aim in view, we conducted (i) field studies of the periglacial processes and landforms in the Central Yakutia (Siberia) and in the Mackenzie River Delta (Canada) and, (ii) a geomorphological study (based on high-resolution images) of the putative-periglacial landforms of Utopia Planitia.Our study shows that the assemblage of landforms in Utopia Planitia share traits of form, scale and spatial association with the landforms of the Central Yakutia and of the Mackenzie Delta (thermokarst lakes, polygons and polygon-junction ponds) indicating that Utopia Planitia has an ice-rich permafrost. The permafrost is composed of stratified sediments ~ 70 m thick with a high ice-content (possibly ≥ 50 % by volume).The permafrost appears to have a syngenetic origin: it was formed by an accumulation of sediments in the basin of Utopia Planitia under cold climate conditions that leaded to the in-situ freezing of the sediments. The sediments could have been deposited by outflow valleys from Elysium Mons and/or by an eolian activity. With regard to the latter, the synchronous formation of a possible regional ice-sheet near Utopia Planitia during medium-obliquity (~ 35°) periods of Mars could have induced a preferential eolian deposition in Utopia Planitia.Subsequently, the ice-rich permafrost was regionally degraded between ~ 5 and 10 Ma. The thermokarst was triggered by an increase of insolation during high-obliquity (~ 45°) periods of Mars. The increase of temperature caused the thermal destabilization of the permafrost inducing the sublimation of ground-ice, deeply modifying the landscape.Thus, important obliquity variations of Mars caused global climate changes that could have induced the formation and the degradation of the ice-rich permafrost of Utopia Planitia between ~ 5 and 10 Ma.

Temperatures strongly affect physical, chemical, and biological processes in rivers and streams. The processes that influence river temperatures are known across most geographic regions, but the relative importance varies significantly. Little is known about what controls water temperature Arctic rivers, limiting our ability to understand the impacts of climate change. This dissertation addresses this knowledge gap by incorporating field measurements with river temperature modeling to estimate the relative importance of key factors that affect Arctic river temperatures. Results indicate that shortwave radiation (e.g., sunlight) and net longwave radiation are significant throughout an Arctic watershed in all flow conditions. In areas where the river is smaller, however, exchange of water with the riverbed and inputs of water from the landscape become significant under low-flow and high-flow conditions, respectively. Additional field observations and modeling were used to quantify the water and heat exchanges between the river and the riverbed. These heat exchanges were found to cool the river and reduce the daily range of temperatures. To better estimate the flow of water from the landscape to the river, a new method for estimating river flow was developed using high-resolution aerial imagery. This method allows us to estimate river flow without depending on field measurements, and presents a potential solution to barriers in performing river temperature modeling in other parts of the Arctic.

<p>This thesis outlines the development and decay of palsas in northern Sweden in relation to climatic and environmental factors. Palsas were investigated on morphology, vegetation, peat porosity, ground temperature and local climate in four bogs situated on a north-south gradient in northern Sweden. A new classification for palsa stages (embryo, young, mature, degrading and remnant stage), based on morphological and vegetational characteristics, is proposed and described in the thesis. The start of palsa growth seems to be favoured by a low vegetation height and a high cover of <i>Sphagnum</i> mosses. Very high porosities were measured in this peat type, resulting in a high insulation capacity preserving the frozen ground in summer. A decrease of porosities in the surface peat layer from the embryo palsa stage to the mature and degrading palsa stage was found. This contributes to the degrading of palsas, due to higher thermal diffusivities in palsas with lower porosities. The aggradation of the ice core can also cease due to the observed change in vegetation from low vegetation on embryo and young palsas to vegetation with tall <i>Betula nana</i> shrubs on degrading palsas. This was found to cause thicker snow cover that prevents frost penetration into the palsa core. The present climate conditions (with mean annual temperature of –1.5°C and annual precipitation of 737 mm) at the boundary of palsa distribution in Sweden were found to be unsuitable for palsa development. Palsa growth started here during a cold period in the last part of the Little Ice Age, with estimated mean annual and mean winter temperatures of –2.3°C and –10.0°C, respectively. </p><p>The major conclusions are that besides air temperature, depth of snow cover, summer precipitation and hydrology, also vegetation cover and peat characteristics are important for palsa growth and decay.</p>

This thesis outlines the development and decay of palsas in northern Sweden in relation to climatic and environmental factors. Palsas were investigated on morphology, vegetation, peat porosity, ground temperature and local climate in four bogs situated on a north-south gradient in northern Sweden. A new classification for palsa stages (embryo, young, mature, degrading and remnant stage), based on morphological and vegetational characteristics, is proposed and described in the thesis. The start of palsa growth seems to be favoured by a low vegetation height and a high cover of Sphagnum mosses. Very high porosities were measured in this peat type, resulting in a high insulation capacity preserving the frozen ground in summer. A decrease of porosities in the surface peat layer from the embryo palsa stage to the mature and degrading palsa stage was found. This contributes to the degrading of palsas, due to higher thermal diffusivities in palsas with lower porosities. The aggradation of the ice core can also cease due to the observed change in vegetation from low vegetation on embryo and young palsas to vegetation with tall Betula nana shrubs on degrading palsas. This was found to cause thicker snow cover that prevents frost penetration into the palsa core. The present climate conditions (with mean annual temperature of –1.5°C and annual precipitation of 737 mm) at the boundary of palsa distribution in Sweden were found to be unsuitable for palsa development. Palsa growth started here during a cold period in the last part of the Little Ice Age, with estimated mean annual and mean winter temperatures of –2.3°C and –10.0°C, respectively. The major conclusions are that besides air temperature, depth of snow cover, summer precipitation and hydrology, also vegetation cover and peat characteristics are important for palsa growth and decay.

This thesis presents an assessment of the variation in solifluction occurrence, morphometry and movement rates in the Abisko region, northern Sweden. Variations in movement rates are analyzed both on a regional and local scale. The main methodological contributions of this thesis have been to provide new techniques of analyzing spatial and temporal variations of solifluction in order to detect long term temporal trends and to regionalize the variations in movement rates. The spatial analysis is achieved by using a combination of field measurements, GIS and remote sensing techniques and statistical analysis. The results are presented in six papers, focusing on the morphometry of solifluction landforms (paper I), the occurrence of permafrost (paper II), the spatial and temporal variations of lobe front movement rates using aerial photographs (paper III), the temporal, regional and local spatial variations in movement rates (paper IV – VI) and statistical modelling of the occurrence of solifluction landforms and calculation of geomorphic work (paper V and VI). The results show that, on a regional scale, vegetation patterns are a major control on the occurrence of turf-banked solifluction landforms, with high NDVI-values (vegetation) associated with the presence of forms. Elevation is also a major control on a regional scale with a decrease in lobe dimensions and movement rates with increased elevation. High soil moisture values are associated with larger landforms and increased movement rates. Movement rates are generally higher in the western part of the region and appear to increase with higher MAAT. Equally, geomorphic work is greatest in the western part of the region. The important controls on a local scale vary from site to site, but include vegetation, slope angle and soil moisture. The photo analysis indicates that annual movement rates of lobe fronts in Kärkevagge and Låktatjåkka valley over the period 1959-2000 ranges from not-detectable to 63mm/yr. The permafrost model shows probabilities &gt;0.8 for permafrost at elevations above 1300 m a.s.l. in the western part of the region, decreasing to altitudes over 850 m a.s.l. in the eastern part of the region. Calculated geomorphic work suggests that solifluction is a significant denudational agent in the sub-Arctic mountains of northern Sweden, but less so than previously estimated.