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Centrée au Pôle Sud, l’Antarctique constitue la plus grande étendue de glace continentale sur Terre,représentant le plus important réservoir d’eau douce. Cette masse de glace est constamment en mouvement.La glace se déforme sous l’action de la gravité, et glisse sur le substract rocheux sur lequel ellerepose. Depuis des années, l’Antarctique perd de la masse de glace en raison du réchauffement climatique.Les prédictions concernant l’évolution de sa masse de glace et sa contribution potentielle au niveau marinsont très incertaines. Une des principales sources d’incertitude réside dans comment exactement la glaceglisse-t-elle à la base? Le glissement basal inclut le glissement stricto sensu de la glace sur le lit rocheuxdans le cas d’un lit rigide, ainsi que la déformation du lit composé de sédiments dans le cas d’un lit déformable.Dans certaines zones, comme les ice streams, le glissement basal est le processus majoritaire dudéplacement de la glace. Les ice streams drainant la grande majorité de la calotte, il semble essentiel demieux comprendre le processus responsable de leur mouvement. Mathématiquement, le glissement basalest représenté par une loi de glissement reliant la vitesse du glacier à la contrainte exercée par celui-ci surle lit. Cette loi peut prendre plusieurs formes en fonction des phénomènes qu’elle englobe. L’eau sousglaciairejoue un rôle clef dans le glissement basal en lubrifiant le lit rocheux et en réduisant la résistance dusol à la déformation. Cette eau provient de la fonte de la glace à la base et s’organise en différents types desystèmes de drainage classés comme inefficaces ou efficaces. Les systèmes inefficaces se caractérisent parun faible flux en écoulement diffus, tandis que les systèmes efficaces présentent un fort flux en écoulementconcentré dans des conduits. Le premier objectif de cette thèse est de lier l’hydrologie sous-glaciaire auglissement basal à l’aide du modèle numérique Kori-ULB. Pour ce faire, nous évaluons quatre méthodesd, Centered at the South Pole, Antarctica constitutes the largest expanse of continental ice on Earth, representingthe most significant freshwater stock. This mass of ice is in constant motion, deforming underthe influence of gravity and sliding over the bedrock on which it is lying. Antarctica has been losing icemass for years due to global warming. Predictions regarding the evolution of its ice mass and its potentialcontribution to sea level rise are highly uncertain. One of the main sources of uncertainty lies in understandingexactly how the ice slides at the base. Basal sliding includes the sliding of the ice over the bedrockin the case of a hard bed, as well as the deformation of the bed composed of sediments in the case of adeformable and soft bed. In certain areas, such as ice streams, basal sliding is the predominant processin ice movement. As ice streams drain the majority of the ice sheet, it is essential to better understandthe process responsible for their motion. Mathematically, basal sliding is represented by a sliding law relatingthe glacier’s velocity to the stress it exerts on the bed. This law can take various forms dependingon the phenomena it encompasses. Subglacial water plays a key role in basal sliding by lubricating thebedrock and reducing the till’s resistance to deformation. This water comes from the melting of ice at thebase and organizes into different types of drainage systems classified as inefficient or efficient. Inefficientsystems are characterized by low-flow diffuse drainage, while efficient systems exhibit high-flow concentrateddrainage in conduits. The first objective of this thesis is to link subglacial hydrology to basal slidingusing the Kori-ULB numerical ice sheet model. To achieve this, we evaluate four methods developed inthe literature across the entire Antarctic ice sheet and compare them to simulations that do not considersubglacial hydrology. These methods differ in the representation of subglacial water: seawater pressure, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice‐sheet models projecting future sea‐level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre‐Oligocene, subaerial environment. Detrital hornblende 40 Ar/ 39 Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian‐Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high 143 Nd/ 144 Nd and low 87 Sr/ 86 Sr ratios. Detecting this “fingerprint” in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming., info:eu-repo/semantics/published

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Sea ice is an active component of the Earth's climate system, interacting with both the atmosphere and the ocean. Arctic sea ice is commonly covered by melt ponds during late spring and summer, strongly affecting sea ice physical and optical properties. How melt pond formation affects sea ice gas dynamics and exchanges between sea ice and the atmosphere, with potential feedbacks on climate, is not well known. Here we measured concentrations of N2, O2, and Ar, total alkalinity, and dissolved inorganic carbon within sea ice of Young Sound, NE Greenland, to examine how melt pond formation and meltwater drainage through the ice affect its physical properties and gas composition, including impacts on CO2 exchange with the atmosphere. Sea ice gas composition was controlled mainly by physical processes, with most of the gas initially in gaseous form in the upper ice layer. A minor contribution from biological processes was associated with positive estimates of net community production (up to 2.6 mmol Lice-1 d-1), indicating that the ice was net autotrophic. As the sea ice warmed, the upper ice gas concentrations decreased, suggesting a release of gas bubbles to the atmosphere. However, as melt ponds formed, the ice surface became strongly depleted in gases. Due to melt pond development, meltwater permeated through the ice, resulting in the formation of an underwater ice layer also depleted in gases. Sea ice, including brine, slush, and melt ponds, was undersaturated in CO2 compared to the atmosphere, supporting an uptake of up to -4.26 mmol m-2 d-1 of atmospheric CO2. As melt pond formation progressed, however, this uptake weakened in the strongly altered remaining ice surface (the “white ice”), averaging -0.04 mmol m-2 d-1. This study reveals the importance of melt pond formation and dynamics for sea ice gas composition., SCOPUS: ar.j, info:eu-repo/semantics/published

The future contributions of the Antarctic Ice Sheet to sea level rise will depend on the evolution of its surface mass balance (SMB), which could amplify/dampen mass losses increasingly observed at the ice sheet's edge. In situ constraints of SMB over annual-to-decadal timescales consist mostly of firn/ice cores that have a surface footprint cm. SMB constraints also come from climate models, which have a higher temporal resolution but a larger surface footprint of several km. We use ice-penetrating radar data to obtain an intermediate spatial and temporal resolution SMB record over three ice rises along the Princess Ragnhild Coast. The co-located ice cores allow us to obtain absolute radar-derived SMB rates at a multi-annual-to-decadal temporal resolution. By comparing the ice core SMB measurements and the radar-derived SMB records, we determine that pointwise measurements of SMB are representative of a small surface area, m radius extending from the ice core drill site for the ice rises studied here, and that the pointwise measurements are systematically 7–15 cm w.e. a lower than the mean SMB value calculated for the whole ice rises. However, ice core records are representative of an entire ice rise's temporal variability at the temporal resolution examined.

The future contributions of the Antarctic Ice Sheet to sea level rise will depend on the evolution of its surface mass balance (SMB), which could amplify/dampen mass losses increasingly observed at the ice sheet's edge. In situ constraints of SMB over annual-to-decadal timescales consist mostly of firn/ice cores that have a surface footprint $sim$ cm $^{2}$ .SMB constraints also come from climate models, which have a higher temporal resolution but a larger surface footprint of several km $^{2}$ .We use ice-penetrating radar data to obtain an intermediate spatial and temporal resolution SMB record over three ice rises along the Princess Ragnhild Coast. The co-located ice cores allow us to obtain absolute radar-derived SMB rates at a multi-annual-to-decadal temporal resolution. By comparing the ice core SMB measurements and the radar-derived SMB records, we determine that pointwise measurements of SMB are representative of a small surface area, $sim 200-500$ m radius extending from the ice core drill site for the ice rises studied here, and that the pointwise measurements are systematically 7–15 cm w.e. a $^{-1}$ lower than the mean SMB value calculated for the whole ice rises. However, ice core records are representative of an entire ice rise's temporal variability at the temporal resolution examined., SCOPUS: ar.j, info:eu-repo/semantics/published

Convective dynamics developing below growing sea ice are studied experimentally by freezing salt water from above in a quasi-two-dimensional Hele-Shaw cell. Observations of the convective processes are made with Schlieren and direct imaging systems, allowing visualization both under and within the growing ice. Buoyancy-driven flows are seen to develop under the ice layer via two different mechanisms: On one hand, brine diffuses out from the ice layer creating a denser boundary layer of enhanced salinity, which triggers boundary layer convection resulting in small-scale interfacial fingers. On the other hand, internal flow within brine drainage channels inside the ice is observed flushing out longer-scale convective streamers at given locations at the ice-water interface. Streamers descend in the bulk aqueous layer faster and for longer distances than fingers. Simulations confirm that, despite nonlinear interactions between fingers and streamers, the different speeds observed can be correlated to different density differences between the interfacial or internal rejection and the underlying bulk salt water. Estimates of relative mass fluxes through the interface by the two mechanisms suggest that, when streamers are active, the mass of salt rejected through the streamer pathway can be larger than the one expelled through the finger pathway. However, as fingers are maintained throughout the ice growth while the rejection from brine channels features an intermittent "on-off" behavior, there are certain periods of time when the mass flux of the two mechanisms is similar, but also some time intervals during which the flux due to interfacial short fingers becomes dominant., SCOPUS: ar.j, info:eu-repo/semantics/published

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The future contributions of the Antarctic Ice Sheet to sea level rise will depend on the evolution of its surface mass balance (SMB), which could amplify/dampen mass losses increasingly observed at the ice sheet's edge. In situ constraints of SMB over annual-to-decadal timescales consist mostly of firn/ice cores that have a surface footprint cm. SMB constraints also come from climate models, which have a higher temporal resolution but a larger surface footprint of several km. We use ice-penetrating radar data to obtain an intermediate spatial and temporal resolution SMB record over three ice rises along the Princess Ragnhild Coast. The co-located ice cores allow us to obtain absolute radar-derived SMB rates at a multi-annual-to-decadal temporal resolution. By comparing the ice core SMB measurements and the radar-derived SMB records, we determine that pointwise measurements of SMB are representative of a small surface area, m radius extending from the ice core drill site for the ice rises studied here, and that the pointwise measurements are systematically 7–15 cm w.e. a lower than the mean SMB value calculated for the whole ice rises. However, ice core records are representative of an entire ice rise's temporal variability at the temporal resolution examined., Mathematical Geodesy and Positioning

The Antarctic ice sheet (AIS) is the largest and yet the most uncertain potential contributor to future sea-level rise. While recent satellite observations have highlighted that the AIS is currently losing mass at an accelerating rate, projections of the future evolution of the ice sheet in a warming climate remain highly uncertain. As future sea-level rise is probably one of the biggest threats imposed on us by climate change, predicting it with the lowest possible uncertainty is of capital societal importance. Uncertainties in the future evolution of the AIS can be explained, notably, by the fact that the ice sheet is capable of abrupt and self-sustained changes associated with several positive feedback mechanisms, especially in its marine areas, i.e. where the ice lies on bedrock below sea level. This is the case for most of the West Antarctic ice sheet (WAIS) as well as for some basins of the East Antarctic ice sheet (EAIS). The interactions between the ice sheet and its surrounding environment (namely the ocean, the atmosphere, and the solid Earth) have been shown to strongly influence its stability, more particularly by triggering or dampening the instabilities threatening the ice sheet. Despite the uncertainties, recent studies suggest that the WAIS will lose mass in the future and eventually (partially) collapse. The uncertainties pertain to when, and to whether the weak Earth structure beneath that area of the ice sheet may be a stabilising factor, as a rapid bedrock uplift in response to ice mass loss has been shown to delay or even limit mass loss. The fate of the EAIS is less clear. A pending question is: will the EAIS lose or gain mass in the future? More specifically, will the grounding line retreat in its marine basins, and if so, can the associated mass loss be compensated by sufficient mass gain due to increased snow accumulation in the interior of the ice sheet?In this thesis, we contribute to clarifying and providing new insights to these question, La calotte glaciaire Antarctique est le plus gros contributeur potentiel à l'élévation future du niveau marin global, mais aussi le plus incertain. De plus en plus d’observations satellitaires mettent en évidence une actuelle perte de masse de l'Antarctique, et ce à un rythme accéléré. Malgré cela, les projections de l'évolution future de la calotte Antarcique en réponse aux changements climatiques actuels et à venir restent très incertaines. Pourtant, être en mesure de prédire avec la plus faible incertitude possible l'élévation future du niveau de la mer est d'une importance sociétale capitale. Les incertitudes sur l'évolution future de l'Antarctique s'expliquent notamment par le fait que la calotte glaciaire est capable de changements brusques associés à plusieurs mécanismes de rétroactions positives, et ce particulièrement dans les régions où la glace repose sur un socle rocheux situé sous le niveau de la mer. C'est le cas pour la majorité de l'Antarctique de l’Ouest ainsi que dans certains bassins de l’Antarctique de l’Est. Il a été démontré que les interactions entre la calotte glaciaire et son environnement (à savoir l'océan, l'atmosphère et le lit rocheux sous-jacent) influencent fortement sa stabilité, notamment en déclenchant ou en atténuant les instabilités qui la menacent. Malgré les incertitudes, des études récentes suggèrent que, à l'avenir, l’Antarctique de l’Ouest perdra de la masse et finira par (au moins partiellement) s'effondrer. Les incertitudes concernent dès lors le moment de cet effondrement, mais également une potentielle stabilisation de la calotte par le lit rocheux (qui repose sur une région du manteau terrestre particulièrement peu visqueuse) situé sous cette région occidentale. En effet, il a été démontré qu'un rebond rapide du lit rocheux pouvait ralentir voire limiter la perte de masse de la calotte. Le sort de l'Antarctique de l’Est est moins clair. Une des questions en suspens est la suivante :l'Antarctique de l’Est perdra-t-elle ou, Doctorat en Sciences, info:eu-repo/semantics/nonPublished