.La densification du névé en glace est un processus essentiel à comprendre pour interpréter les enregistrements climatiques. Une bonne connaissance des mécanismes permet une datation précise de l'air capturé dans la glace lors de la fermeture des pores. Celle-ci est plus vieille que l'air capturé à cause du transport des gaz dans la colonne de névé plus rapide que la densification de celui-ci. Cette différence d'âge entre la glace et le gaz est généralement appelé le Δage. La densification de la neige consiste en un processus complexe de réarrangement de grains, de frittage et de déformation viscoplastique. Bien que le comportement viscoplastique du cristal de glace soit fortement anisotrope, les modèles de densification actuels ne tiennent pas compte de cette anisotropie. De plus, le caractère granulaire du névé affecte aussi sa densification. La relation entre la fermeture des pores et les mécanismes microstructuraux sous-jacents est encore méconnue. Le but de cette thèse est d'incorporer l'aspect granulaire ainsi que l'anisotropie du cristal de glace dans une approche de modélisation innovante de la densification. Des expériences sur l'indentation viscoplastique de cylindres monocristallins de glace ont été réalisées pour proposer une loi de contact basée sur la théorie de l'indentation, et prenant en compte la déformation préférentielle du cristal de glace sur les plans basaux. Cette loi de contact a été implémentée dans un code utilisant la méthode des éléments discrets pour prédire la densification du névé.La micro-tomographie aux rayons X a été utilisée pour caractériser ex situ le névé polaire en trois dimensions à différentes étapes de la densification (ρ= 0.55-0.88 g/cm3), i.e. pour différentes profondeurs (~23 à 130m). Une étude fine de la fermeture des pores et de différentes caractéristiques morphologiques et physiques a été réalisée pour les sites polaires Dome C et Lock In. Des essais mécaniques ont aussi été réalisés in situ sur du névé extrait de Dome C dans le but de modéliser la densification du névé. Les observations microstructurales des expériences ex situ et in situ révèlent d'importantes différences dues aux vitesses relativement importantes utilisées lors des essais mécaniques. Ces vitesses rapides permettent de découpler la contribution des cinétiques de diffusion de la contribution viscoplastique de la déformation. Les effets de ces contributions sur la morphologie des pores et leurs fermetures sont discutés. Pour caractériser la fermeture des pores, cette thèse propose un indice de connectivité définit par le ratio entre le volume du plus gros pore sur la porosité totale. En effet, cet indice est plus approprié lors de l'utilisation de la tomographie aux rayons X que le ratio de pores fermés pour prédire la densité au close-off. / Densification from firn to ice is an essential phenomenon to understand for the interpretation of the climate record. A good knowledge of this mechanism enables the precise dating of the air embedded in the ice. The step at which the air becomes entrapped is the pore closure (or close-off). Because of gas flow in the firn column, the ice is older than the entrapped air. The difference between ice and gas is generally defined as Δage.Snow densification consists of grain rearrangements, sintering and viscoplastic deformation. Although the viscoplastic behaviour of the ice crystal is strongly anisotropic, densification models do not take into account this anisotropy. Firn also bears some granular characteristics that may affect its densification. The interactions between pore closure and microstructural mechanisms in the firn are still misunderstood.The aim of this PhD thesis is to incorporate both the granular aspect of firn and its anisotropy into an innovating approach of firn densification modelling. The mutual indentation of viscoplastic monocrystalline ice cylinders was experimentally carried out to propose a contact law that is based on indentation theory and that takes into account the preferential viscoplastic deformation on the basal plane. We have integrated this contact law into a DEM (Discrete Element Method) code for the prediction of firn densification.3D X-ray micro-tomography was performed on polar firn at different stages of the densification (ρ= 0.55-0.88 g/cm3) and depths (~23 to 130m). A thorough investigation of pore closure and of different morphological and physical parameters was achieved for the Dome C and the newly drilled Lock In polar sites. In addition to these ex situ analyses, in situ X-ray micro-mechanical experiments were carried out on firn extracted from Dome C in order to model its densification. Ex situ and in situ microstructural observations indicate significant differences that can be explained by the relatively large strain-rates imposed to the firn during in situ tests. These large strain rates allow for a decoupling of the effects of diffusion kinetics and of viscoplastic deformation. Their relative weights on the morphology of pores and on their closure are discussed. To measure pore closure, we propose a connectivity index, which is the ratio of the largest pore volume over the total pore volume. We show that this index is better suited for X-ray tomography analysis than the classic closed porosity ratio to predict the close-off density

From the moment a snowflake touches down on the surface of a glacier, it begins a process of transformation. Fresh snow, made up of single-grained snowflakes is compacted into glacial ice by the weight of subsequent snowfall and by sintering, grain boundary sliding and diffusion. At first, snow grains accommodate the stress through mechanical failure and by changing their shapes and positions. Fragile, dendritic structures on the edges of snowflakes break off, and grains round into lower free energy configurations. Rounded grains slip into air pockets. As time passes, increasing overburden of a load to bear, and it is, for a single snowflake. But it is precisely this stress that creates a glacier. Stress, in this case, is a catalyst for transformation. But don't worry. I am not going to make an overly forced metaphor for what happens during a doctorate program.} Pressure causes the grains to merge, large grains absorbing small ones. As ice grains squeeze and grow into all the available pore space, grains trap air bubbles and cut them off from the atmosphere, preserving records of climate conditions. Eventually, these processes densify the snow so thoroughly that it metamorphoses into glacial ice, and from a crumbly collection of snowflakes emerges a cohesive crystalline matrix. This process, firn densification, is the subject of my first chapter. From measurements of englacial strain rates by repeat phase-sensitive radar deployments, we show it is possible to extract densification rates that match modeled predictions. The formation of ice is just the beginning of the story of a glacier. As and after ice forms, gravity pulls on the body of the glacier; ice flows under its own weight, becoming a viscous river that meanders from high elevations toward the sea level. Along the way, various other forces act on the ice (e.g., friction at the ice-bed causes ice to shear, narrowing valley walls create compressive stresses, etc.). This history can be written into the ice in the orientation and configuration of its molecular structure. Ice is made of a regular crystal matrix of water molecules. Covalently bonded oxygen and hydrogen molecules assemble into sheets of hexagons, held to each other by hydrogen bonds. The relative orientation of these hexagonal sheets is called the "ice fabric”, and its importance lies in the fact that ice’s asymmetric molecular structure gives rise to asymmetric properties. For example, ice is softer—more deformable—when stress is applied parallel to the hexagonal planes, like playing cards sliding over one another. Over hundreds or thousands of years, this asymmetric response to stress causes the hexagonal planes to rotate so that they lie perpendicular to the direction of compressive stress. This, in turn, changes which relative direction a glacier is the “softest”. In short, the history of the glacier is written into its fabric. Ice remembers the stress it has undergone, and that memory changes its resistance to (or accommodation of) stress in the present and future. In chapter two, I use an autonomous phase-sensitive radar to measure the ice fabric along a central transect of Thwaites Glacier. Thwaites drains ice from West Antarctica and is one of the fastest changing glaciers on the continent. Locked up in Thwaites is at least half a meter of sea level rise, as well as much of the buttressing that holds back WAIS. Measurements of the fabric of Thwaites tell us about the history of stress undergone by the glacier, as well as any change in relative direction of the "softest" ice. As a glaciologist, I have dedicated my life to studying how glaciers form, flow, and disappear. As an artist and writer, I am interested in material memory, with a particular orientation toward ice itself and in the way the language and mathematics used to describe ice mimic processes that happen in body, mind, and society. My fourth chapter is centered on the creative research and art produced during my dissertation, particularly focused on a visual autoethnography of my body I created during my first field season in Antarctica in 2022-2023. In it, I try to grapple with whether/how, even as positivist science demands I remove as much of myself as possible from my scientific research, my body/myself show up in small ways in my data. I consider how ice's response to stress—to soften or harden, to flow or crack—is in many ways, a mirror for how we as humans respond to stress. Other work in Chapter 4 was created in direct response to the beauty of glaciated landscapes and the grief I struggle to manage in response to their rapid change. Biome I is a short zine that uses faux-color satellite imagery overlain with text and meshes of glaciers from Grand Teton National Park (GRTE). In 2021, I spent six months as a Scientists-in-Parks fellow through AmeriCorps, joining the park's physical science team in Wyoming to expand their glacier monitoring program. From this work emerged Chapter 3 a history of glacial change in the park over the last 70 years from in situ and remotely sensed observations. This work, while quite different from my previous scientific output, allowed me to learn and explore other glaciological techniques as well as template methodologies and provide information that is immediately useful for education and action in GRTE and other rapidly deglaciating landscapes. Much of the way I have come to understand glacial geophysics is by considering the ways they connect more broadly to our lived experiences. In the Tetons, this involved understanding how deglaciation affects the park's ecological systems and the evolving safety for visitors given the changing ice conditions. In pursuit of both expanding my own understanding and hoping to share with others the joy and beauty of the study of ice, I have developed numerous education efforts to make the study of glaciers, climate, and the earth physical, tangible, less abstract, emotional, joyful, and intuitive. Chapter 5 concludes the thesis by taking a step back to look at education and teaching, the thread that has carried through my doctorate, from prior to starting graduate school and, I hope, that will continue long after. I discuss the influences of teacher-philosophers like Shannon Mattern, Lynda Barry, and bell hooks, who have all, in their own way, striven to reshape the (idea of the) classroom into forms that better serve the learner. This work has taken place on the seat of a bicycle riding across the country, on an icefield in Juneau, Alaska, and in my own backyard, in classrooms across New York City. To conclude, I hope this thesis is not only a scientific effort, but one that draws the curtain back on the broader work we do as glaciologists. We are also artists and educators, caretakers, archivists, and public figures. Our work can be physically, mentally, and emotionally demanding, and it is as often full of grief as it is of awe.

The Greenland ice sheet is the largest Northern Hemisphere store of fresh water, and it is responding rapidly to the warming climate. In situ observations document the changing ice sheet properties in the lower accumulation area, Southwest Greenland. Firn densities from 1840 meters above sea level retrieved in May 2012 revealed the existence of a 5.5-meter-thick, near-surface ice layer in response to the recent increased melt and refreezing in firn. As a consequence, vertical meltwater percolation in the extreme summer 2012 was inefficient, resulting in surface runoff. Meltwater percolated and refroze at six meters depth only after the end of the melt season. This prolonged autumn refreezing under the newly accumulated snowpack resulted in unprecedented firn warming with temperature at ten meters depth increased by more than four degrees Celsius. Simulations confirm that meltwater reached nine meters depth at most. The refrozen meltwater was estimated at 0.23 meters water equivalent, amounting to 25 % of the total 2012 ablation. A surface energy balance model was used to evaluate the seasonal and interannual variability of all surface energy fluxes at that elevation in the years 2009 to 2013. Due to the meltwater presence at the surface in 2012, the summer-averaged albedo was significantly reduced (0.71 in 2012; typically 0.78). A sensitivity analysis revealed that 71 % of the subsequent additional solar radiation in 2012 was used for melt, corresponding to 36 % of the total 2012 surface lowering. This interplay between melt and firn properties highlights that the lower accumulation area of the Greenland ice sheet will be responding rapidly in a warming climate. / Stability and Variations of Arctic Land Ice (SVALI) / Programme for Monitoring of the Greenland Ice Sheet (PROMICE) / Greenland Analogue Project (GAP)

Documenter la variabilité climatique récente est nécessaire à la compréhension des mécanismes en jeu, associés au rôle du bilan de masse de l’Antarctique pour l’élévation du niveau des mers globale. Les enregistrements issus des carottes peu profondes d’Antarctique sont des données précieuses, complémentaires aux observations instrumentales et satellitaires, pour couvrir en continu l’ensemble du continent. Mesurés le long de ces carottes de glace, les isotopes stables de l’eau sont traditionnellement utilisés pour quantifier les changements passés de la température locale.Cette thèse doctorale a été initiée dans le cadre du programme de l’Agence Nationale de la Recherche ASUMA (“Improving the Accurancy of SUrface Mass balance of Antarctica”), ayant pour objectif de reconstruire et identifier les processus contrôlant la variabilité spatio-temporelle du bilan de masse de surface (BMS) de la Terre Adélie. J’ai utilisé des données d’isotopes stables de l’eau enregistrées dans des carottes de névé, des simulations atmosphériques produites par le modèle atmosphérique de circulation générale de haute résolution ECHAM5-wiso équipé des isotopes stables de l’eau, des réanalyses atmosphériques, des rétro-trajectoires, ainsi que des observations instrumentales satellitaires et de surface.Dans une première partie, j’ai évalué les capacités du modèle ECHAM5-wiso à simuler les températures de l’Antarctique, le BMS, le δ18O et le d-excess (ci-après, d-excess), comme prérequis à l’exploitation du modèle pour interpréter les compositions isotopiques. J’ai développé des diagnostics pour les relations δ18O-température et d-excess- δ18O sur l’ensemble du continent de l’Antarctique, en montrant que les différences issues des pentes des relations δ18O-température spatiales, inter-annuelles et saisonnières. Au sein du groupe de travail international de PAGES (Past Global Changes) Antarctica2k, j’ai utilisé des calibrations établies issues du modèle ECHAM5-wiso pour reconstruire la température de 7 régions d’Antarctique à partir d’une synthèse d’enregistrements de δ18O issus de carottes de glace couvrant les 2 000 dernières années.Dans une seconde partie, de nouveaux enregistrements issus de deux carottes de névé extraites en Terre Adélie, la S1C1 et la TA192A, ont été exploités, couvrant respectivement les périodes 1947-2007 et 1998-2014. Les BMS reconstruits décrivent une grande variabilité spatiale (74,11 ± 14,1 cm w.e. y-1 et 21,8 ± 6,9 cm w.e. y-1 pour la TA192A et la S1C1 respectivement), cohérente avec les données de balise disponibles. En utilisant une base de données mise à jour des isotopes stables de l’eau de l’Antarctique, j’ai montré que les valeurs moyennes isotopiques de Terre Adélie appartiennent à l’intervalle des valeurs côtières de l’Antarctique. Des analyses statistiques montrent une absence de relation entre nos enregistrements avec la température de surface locale à l’échelle inter-annuelle, mais des relations significatives avec des rétro-trajectoires atmosphériques et des simulations isotopiques suggérant que les isotopes de l’eau de la Terre Adélie fournissent des indications de la variabilité de la dynamique atmosphérique et du transport d’humidité, aux échelles saisonnière et inter-annuelle.Les analyses de cette thèse ont été limitées par la quantité d’enregistrements isotopiques disponibles pour la Terre Adélie, ainsi que par le manque de compréhension des effets de dépôt et de post-dépôt. Il est donc nécessaire d’exploiter les nouvelles carottes de névé extraites au cours du programme ASUMA, et d’effectuer en continu des mesures de la composition isotopique des précipitations, de la vapeur d’eau et de la neige de surface de Terre Adélie, en combinaison avec des outils de simulations atmosphériques, tels que des rétro-trajectoires associées à un diagnostic des sources d’humidité, et des modèles atmosphériques de circulation générale et régionaux équipés des isotopes stables de l’eau. / Documenting recent Antarctic climate variability is needed in order to understand the mechanisms at play, associated with the role of Antarctic mass balance for global sea level rise. Proxy records from Antarctic shallow firn cores are precious data, which complement instrumental and remote sensing observations to continuously cover the whole continent. Within these ice cores, water stable isotopes are commonly used to quantify past changes in local temperature.This PhD thesis was initiated within the French Agence Nationale de la Recherche “Improving the Accurancy of SUrface Mass balance of Antarctica” (ASUMA) project, which aims to reconstruct and to identify the processes controlling the spatio-temporal variability of the surface mass balance (SMB) in Adélie Land. I used water stable isotopes records from recently drilled shallow firn cores, as well as atmospheric simulations performed with the high resolution atmospheric general circulation model ECHAM5-wiso model, equipped with water stable isotopes, atmospheric reanalyses and back-trajectories, instrumental and remote sensing climate observations.In a first part, I assessed the skills of the ECHAM5-wiso with respect to Antarctic temperature, SMB, δ18O and deuterium excess (hereafter d-excess), as a prerequisite for the exploitation of the model to interpret isotope compositions. I developed Antarctic-wide diagnostics of the δ18O-temperature and d-excess- δ18O relationships, showing differences in the spatial, seasonal and interannual δ18O-temperature slopes. Within the international working group of PAGES (Past Global Changes) Antarctica 2k, I used the calibrations inferred from ECHAM5-wiso to reconstruct temperatures over 7 Antarctic regions from a synthesis of ice core δ18O records spanning the past 2,000 years.In a second part, new water stable isotope records from two firn core drilled in Adélie Land, the S1C1 and the TA192A, were investigated, covering the periods 1947-2007 and 1998-2014 respectively. The reconstructed SMB display a high spatial variability (74.1 ± 14.1 cm w.e. y-1 and 21.8 ± 6.9 cm w.e. y-1 for the TA192A and S1C1 respectively), consistent with Adélie Land stake data. Using an updated database of Antarctic water stable isotope datasets, I showed that the mean isotopic values (δ18O and d-excess) in Adélie Land are in line with the range of Antarctic coastal values. Statistical analyses show no relationship between our records and local surface air temperature, at the inter-annual scale, but significant relationships with atmospheric back-trajectories and isotopic simulations, suggesting that water stable isotopes in Adélie Land provide fingerprints of the variability of atmospheric dynamics and moisture transport, at the seasonal and inter-annual scales.The analyses performed during this PhD thesis have been limited by the few available Adélie Land water stable isotope records, and by the lack of understanding of deposition and post-deposition processes. Further work is thus needed to exploit the new firn cores drilled within the ASUMA project, and to monitor continuously Adélie Land water stable isotopes in precipitation, surface water vapour and surface snow, in combination with tools of atmospheric simulations such as back-trajectory simulations provided with moisture sources diagnostics, as well as water stable isotopes-enabled atmospheric general and regional circulation models.

Glaciers cover ca 10% of the Earth's land and are found in the high altitudes and latitudes. They are important components of environmental systems due to the multiple feedbacks linking them with the atmosphere, hydrosphere and periglacial landscapes. The cold sloping surfaces of glaciers change the patterns of atmospheric circulation at different scales and at the same time glaciers are largely controlled by climate. They are commonly used as climatic archives for reconstruction of the past environmental changes based on evidences from the areas affected by glaciation at the moment and in the past. Glaciers are the largest fresh-water reservoirs on our planet and runoff thereof significantly affects the global sea level and life in glaciated catchments. However, melt- and rain-induced runoff from glaciers greatly depends on the subsurface conditions which thus need to be taken into account, particularly in a changing climate. This thesis focuses on the processes of subsurface mass and energy exchange in the accumulation zones of glaciers, which are largely driven by the climate at the surface. Results are largely based on empirical data from Lomonosovfonna ice cap, Svalbard, collected during field campaigns in 2012-2017. Observations of subsurface density and stratigraphy using shallow cores, video records from boreholes and radar surveys returned detailed descriptions of the snow and firn layering. The subsurface temperature data collected using multiple thermistor strings provided insights into several subsurface processes. The temperature values measured during three summer seasons were used to constrain the suggested parameterization of deep preferential water flow through snow and firn. The part of data recorded during the cold seasons was employed for an inverse modelling exercise resulting in optimized values of effective thermal conductivity of the subsurface profile. These results are then used to compute the subsurface water content by comparing the simulated and measured rates of freezing front propagation after the melt season in 2014. The field observations and quantitative estimates provide further empirical evidences of preferential water flow in snow/firn packs at glaciers. Results presented in the thesis call for implementation of description of the process in layered models simulating the subsurface fluxes of energy and mass at glaciers. This will result in a better understanding of glacier response to the past and future climatic changes and more accurate estimates of glacier runoff. / Stability and Variations of Arctic Land Ice (SVALI)

The seasonal retreat of sea ice in the austral spring and summer around Antarctica has a significant effect on phytoplankton activity, mainly due to light availability, meltwater input of dissolved iron, and surface water stratification. Phytoplankton produce dimethylsulfoniopropionate, the precursor to the climate-cooling gas dimethyl sulphide, which is ventilated to the atmosphere and oxidised to methane sulphonic acid (MSA). MSA is preserved in firn and ice cores from both the Arctic and Antarctica. Attempts to reconstruct sea ice conditions in different regions of Antarctica with the help of MSA records from ice cores have had varying success, highlighting the often-regional relationship between ice core MSA and sea ice. This study uses MSA records from three firn cores and one ice core drilled on Fimbul Ice Shelf in Dronning Maud Land, East Antarctica, to investigate the relationship to satellite-derived sea ice extent (SIE) in five sectors of the Southern Ocean. Chlorophyll-a concentrations, serving as a measure of phytoplankton biomass, are correlated to the MSA records to further test the MSA – SIE relationship. The firn cores are named after the ice rise where they were drilled: Kupol Ciolkovskogo (KC), Kupol Moskovskij (KM), and Blåskimen Island (BI). The ice core is named S100. The results show that there is a significant, yet weak positive correlation between summer MSA in the KM core and winter SIE in the Weddell Sea Sector. There is also a significant, weak positive correlation between summer MSA in the BI core and summer chlorophyll-a concentrations in the Weddell Sea Sector. There are no significant correlations between MSA in the low-accumulation KC or S100 cores and SIE or chlorophyll-a concentrations. Furthermore, the two high-accumulation core sites in this study, BI and KM, do not display the same relationship between MSA and SIE or MSA and chlorophyll-a, which is likely due to very local wind patterns. Surface winds on Fimbul Ice Shelf are easterly or north-easterly which results in a more coastal influence at the KM site compared to the BI site, likely introducing the differences observed when comparing the two MSA records. More research aimed at evaluating the meteorological conditions that prevail at the core sites is needed to further assess the use of the MSA records from the high-accumulation ice rise cores BI and KM as proxies for SIE in the Weddell Sea region, but in their current state these MSA records are not suitable to use for sea ice reconstruction. / Havsisen kring Antarktis smälter årligen under vår- och sommarmånaderna, vilket har en betydande inverkan på fytoplankton eftersom isen reglerar tillgången till solljus, det viktiga näringsämnet järn samt vattenkolumnens stabilitet. Fytoplankton producerar ämnet dimetylsulfid som oxideras till metansulfonsyra (MSA) i atmosfären. MSA kan sedan transporteras till Antarktis där det avsätts och bevaras i snön. Genom att borra upp iskärnor kan man erhålla ett daterat MSA-arkiv, som i flera fall har använts för att försöka rekonstruera havsisens utbredning. Dessa försök har haft varierande framgång, vilket beror på att förhållandet mellan MSA och havsis ofta är regionalt betingat. I den här studien har MSA-arkiven från tre firnkärnor och en iskärna tagna från Fimbulisen i Dronning Maud Land, Östra Antarktis, använts för att undersöka förhållandet till havsisutbredning i Antarktiska Oceanen. Dessutom har klorofyll-a, ett sätt att mäta fytoplanktonens biomassa i havet, också korrelerats till MSA-arkiven för att ytterligare testa förhållandet mellan MSA och havsis. Firnkärnorna är döpta efter platsen de borrades på: Kupol Ciolkovskogo (KC), Kupol Moskovskij (KM) och Blåskimen Island (BI). Iskärnan kallas S100. Resultaten av korrelationsberäkningarna påvisar en signifikant men svagt positiv korrelation mellan sommar-MSA i KM-kärnan och havsisutbredning under vintern i Weddellhavet. Dessutom finns det en signifikant, svag korrelation mellan sommar-MSA i BI-kärnan och klorofyll-a under sommaren i Weddellhavet. Inga signifikanta korrelationer mellan MSA i KC- eller S100-kärnorna och havsis eller klorofyll-a kan påvisas. Det faktum att MSA-arkiven från BI- och KM-kärnorna inte uppvisar samma förhållande till havsisutbredning eller klorofyll-a kan förklaras av de lokala vind- och transportmönstren som är aktiva på olika delar av Fimbulisen. Marknära vindar är ostliga eller nordostliga i det här området vilket resulterar i ett högre inflytande av kustliga vindar vid KM jämfört med vid BI. Detta är förmodligen tillräckligt för att påverka MSA-arkiven att uppvisa olika korrelationsmönster till havsis och klorofyll-a. För att fortsatt utreda lämpligheten av MSA-arkiven från KM och BI för att rekonstruera havsisutbredning i Weddellhavet behövs mer forskning kring de specifika meteorologiska förhållanden som är aktiva på Fimbulisen.