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Lake Vostok, East Antarctica, represents an extensive water surface at the base of the ice sheet. Snow, ice and atmospheric pressure loads applied anywhere within the lake area produce a hydrostatic response, involving deformations of the ice surface, ice–water interface and particle horizons. A modelling scheme is developed to derive height changes of these surfaces for a given load pattern. It is applied to a series of load scenarios, and predictions based on load fields derived from a regional climate model are compared to observational datasets. Our results show that surface height changes due to snow-buildup anomalies are damped over the lake area, reducing the spatial standard deviation by one-third. The response to air pressure variations, in turn, adds surface height variability. Atmospheric pressure loads may produce height changes of up to 4 cm at daily resolution, but decay rapidly with integration time. The hydrostatic load response has no significant impact neither on ICESat laser campaign biases determined over the lake area nor on vertical particle movements derived from GNSS observations.

The evolving science of astrobiology for the search for life beyond Earth has put focus on the possibility of subsurface bodies of liquid water beneath the surface of icy moons within our Solar System. Specifically, the Jovian moon of Europa and the Saturnian moon of Enceladus show telling characteristics of endogenous heating through their complex tidal resonances with their parent planets – suggesting the presence of oceans of liquid water and therefore a potential for astrobiology. Today, the planning of upcoming satellite missions to guarantee the identification of completely unknown, possibly habitable environments has become an intriguing task for space agencies worldwide. A method of guidance, ground-truthing, debugging and testing of the viability of planned satellite missions is to utilize relatively accessible analogue environments on the Earth. Fortunately, the research of Antarctic subglacial water environments has been acknowledged as the most suitable analogues to the environments of Europa and Enceladus. Given its recorded motions of ice dynamics, erosion, complex thermal systems, and occasions of sampled microbes within – these sensitive, exotic environments beneath the Antarctic ice bed are currently assessed for their suitability as habitats for extremophilic microorganisms, which may provide important insights into the potential existence of habitats, perhaps even life, on icy moons. Ultimately, viable lessons can be drawn from the research of Antarctic subglacial environments given the uncovering of reliable sanitary methodologies that are going to be required during future approaches and sampling of natural systems of extraterrestrial subglacial environments in order to minimize irreparable anthropogenic disturbances on any potential astrobiology. Regardless of the question if the future investigation of Europa’s and Enceladus environments is going to reveal astrobiology within beneath their icy surfaces; this investigation is going to be of invaluable information for the improving familiarity of physical systems beyond Earth. / Sedan länge har forskare vetat att flytande/rinnande vatten är en av de viktigaste förutsättningar för allt liv på Jorden. Under de senaste decennierna har frågan om liv på Jupiters måne Europa, och Saturnus måne Enceladus aktualiserats, efter upptäckten av frusna islager på deras månytor. Evidens från respektive månars omloppsbanor, gör att forskare idag tror att månarnas djupa islager hettas upp inifrån av en aktiv, inre geologi, vilket i sin tur kan tyda på större mängder av flytande vatten. I och med omvandlingen av is till flytande vatten, får detta forskare att tro att cirkulationen av vatten, ihop med den aktiva geologin, möjligtvis har skapat och upprätthåller beboliga förutsättningar för mikroskopiskt liv. I dagsläget planeras ett flertal satelliter skjutas upp till månarna med uppdrag att bekräfta fallet om flytande vatten och beboliga miljöer på månarna, och därmed även undersöka frågan om månarna möjligtvis bär på liv. På den frusna kontinenten Antarktis har ett flertal motsvarande subglaciala miljöer undersökts för att fastställa deras kvalifikationer att upprätthålla liv under extrema levnadsförhållanden. Under dessa undersökningar har ett flertal mikroorganismer påträffats i prover från iskärnor från två subglaciala sjöar (Vostoksjön och Whillanssjön), varav några blivit omstritt påstådda som inhemska extremofiler. Ifall detta stämmer, betyder det att de två subglaciala sjöarna underhålls av ett unikt samspel av drivande faktorer som upprätthåller de nödvändigaste levnadsförutsättningarna för att mikroorganismer kan överleva, även frodas i uppenbart extrema miljöförhållanden. Med forskningen av Antarktis subglaciala sjöar som rapporterar ett unikt samspel mellan is, vatten och underliggande geologi; kan den fortsatta fastställningen av detta fysiska samspel, samt vidare identifikationer av inhemska mikroorganismer att bli en viktig lärdom för framtida satelliters datainsamling av subglaciala vattenmiljöer på Europa och Enceladus. För att underlätta och effektivisera sökandet efter potentiella livsmiljöer, samt möjligtvis hitta utomjordiskt liv på isiga månar bör framtidens satellituppdrag dra stor lärdom och väga in de erfarenheter och resultat som gjorts på subglaciala sjöar i Antarktis.

The increased temperatures in the Arctic accelerate the loss of land based ice stored in glaciers. The Greenland Ice Sheet is the largest ice mass in the Northern Hemisphere and holds ~10% of all the freshwater on Earth, equivalent to ~7 metres of global sea level rise. A few decades ago, the mass balance of the Greenland Ice Sheet was poorly known and assumed to have little impact on global sea level rise. The development of regional climate models and remote sensing of the ice sheet during the past decade have revealed a significant mass loss. To monitor how the Greenland Ice Sheet will affect sea levels in the future requires understanding the physical processes that govern its mass balance and movement. In the southeastern and central western regions, mass loss is dominated by the dynamic behaviour of ice streams calving into the ocean. Changes in surface mass balance dominate mass loss from the Greenland Ice Sheet in the central northern, southwestern and northeastern regions. Little is known about what the hydrological system looks like beneath the ice sheet; how well the hydrological system is developed decides the water’s impact on ice movement. In this thesis, I have focused on radar sounding measurements to map the subglacial topography in detail for a land-terminating section of the western Greenland Ice Sheet. This knowledge is a critical prerequisite for any subglacial hydrological modelling. Using the high-resolution ice thickness and bed topography data, I have made the following specific studies: First, I have analysed the geological setting and glaciological history of the region by comparing proglacial and subglacial spectral roughness. Second, I have analysed the subglacial water drainage routing and revealed a potential for subglacial water piracy between adjacent subglacial water catchments with changes in the subglacial water pressure regime. Finally, I have looked in more detail into englacial features that are commonly observed in radar sounding data from western Greenland. In all, the thesis highlights the need not only for accurate high-resolution subglacial digital elevation models, but also for regionally optimised interpolation when conducting detailed hydrological studies of the Greenland Ice Sheet. / De ökade temperaturerna i Arktis påskyndar förlusten av landbaserad is lagrad i glaciärer och permafrost. Grönlands inlandsis är den största ismassan på norra halvklotet och lagrar ca 10% av allt sötvatten på jorden, vilket motsvarar ca 7 meter global havsnivåhöjning. För ett par decennier sedan var inlandsisens massbalans dåligt känd och antogs ha liten inverkan på dagens havsnivåhöjning. Utvecklingen av regionala klimatmodeller och satellitbaserad fjärranalys av inlandsisen har under de senaste decenniet påvisat en betydande massförlust. För att förutse vilken inverkan inlandsisen har på framtida havsnivåhöjningar krävs en förståelse för de fysikaliska processerna som styr dess massbalans och isrörelse. I de sydöstra och centrala västra delarna av inlandsisen domineras massförlusten av dynamiska processer i isströmmar som kalvar ut i havet. Massförlusten i de centrala norra, sydvästra och nordöstra delarna domineras av isytans massbalans. Ytterst lite är känt om hur det hydrologiska systemet ser ut under inlandsisen; hur väl det hydrologiska systemet är utvecklat avgör vattnets påverkan på isrörelsen. I denna doktorsavhandling har jag använt markbaserade radarmätningar för att kartlägga den subglaciala topografin för en del av den västra landbaserade inlandsisen. Denna kunskap är en viktig förutsättning för att kunna modellera den subglaciala hydrologin. Med hjälp av rumsligt högupplöst data över istjockleken och bottentopografin har jag gjort följande specifika studier: Först har jag analyserat de geologiska och glaciologiska förhållandena i regionen genom att jämföra proglacial och subglacial spektralanalys av terrängens ytojämnheter. Sedan har jag analyserat den subglaciala vattenavrinningen och påvisat en potential för att avrinningsområdena kan ändras beroende på vattentryckförhållandena på botten. Slutligen har jag tittat mer i detalj på englaciala radarstrukturer som ofta observerats i radardata från västra Grönland. Sammanfattningsvis belyser avhandlingen behovet av inte bara noggranna rumsligt högupplösta subglaciala digitala höjdmodeller, utan även regionalt optimerad interpolering när detaljerade hydrologiska studier ska utföras på Grönlands inlandsis.

Modern Pine Island and Thwaites Glaciers, which both drain into Pine Island Bay, are some of the fastest moving portions of the cryosphere and may be the most unstable ice streams in Antarctica. I examined over 133 cores to conduct a detailed sedimentological facies analysis. These data, augmented by new radiocarbon and 210Pb dates, and bathymetric data, are used to reconstruct the post-LGM deglacial history of PIB and gain a better understanding of the causes of ice sheet retreat. My results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (plumite), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, in response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB that spans ∼12.3–10.6 k cal yr BP. Widespread impingement of warm water onto the continental shelf caused an abrupt change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The uppermost sediments in Pine Island Bay were hydrodynamically sorted by meltwater plumes. Inner Pine Island Bay contains several large basins that are linked by channels. The most recent release of sediment coincides with rapid retreat of the grounding line, and has an order of magnitude greater flux relative to the entire unit, indicating episodic sedimentation. This is the first identification of a meltwater-derived deposit in Antarctica and demonstrates that punctuated meltwater-intensive glacial retreat occurred at least three times throughout the Holocene in this region. Quartz sand grains were used to conduct an analysis of mode of transport for sediments in the Antarctic Peninsula region from the Eocene to present to record the onset of glaciation. Glacial transport imparts a unique suite of microtextures on quartz grains from high shear-stresses. Eocene samples are free of glacial influence. Late Eocene samples show the inception of glacially derived high-stress microtextures, marking the onset of alpine glaciation. Oligocene grains are similar to the late Eocene samples. Middle Miocene microtextures are characteristic of transport from far-field large ice sheets, originating from ice rafting from the West Antarctic Ice Sheet. The Pliocene and Pleistocene samples indicate the existence of the northern Antarctic Peninsula Ice Sheet at this time, consistent with other proxies.

A travers deux champs d'étude, nous nous intéressons au couplage mécanique entre le magma et l'encaissant, utilisant des méthodes numériques. Tout d'abord, nous étudions l'influence de perturbations de contraintes sur les réservoirs magmatiques, avec comme exemple deux volcans sous-glaciaires (Islande). Au volcan Grímsvötn, notre modèle montre que les vidanges, du lac sous-glaciaire (jökulhlaup) présent dans la caldera, peuvent déclencher une éruption comme ce fut le cas en 2004, en favorisant la rupture du réservoir magmatique. L'effet est cependant faible, ce qui implique que le système magmatique doit déjà être proche des conditions de rupture avant que le jökulhlaup se produise. Au volcan Katla, notre modèle indique que les conditions de rupture sont favorisées en été durant la fonte du glacier Mýrdalsjökull. Les changements de contrainte de Coulomb montrent aussi une plus forte probabilité de séismes durant la même période, résultats en accord avec la sismicité enregistrée sous le Mýrdalsjökull. Il existe une modulation à la fois de l'activité volcanique et sismique au Katla, en relation avec la variation saisonnière de la charge glaciaire. Dans un second temps, nous travaillons sur les écoulements de magma dans les conduits andésitiques. Améliorer nos connaissances sur la dynamique du magma durant son ascension est nécessaire, car les processus dans le conduit volcanique semblent gouverner l'évolution de l'activité éruptive de ces volcans. Des précédents modèles ont montré que la viscosité du magma augmente dans la partie supérieure du conduit lors de l'écoulement, ce qui cause la formation d'un plug visqueux. Mais la relation entre la mise en place du plug et les signaux précurseurs, telles que la déformation ou la sismicité, n'est pas totalement établie. A partir de nos modèles de plug, nous trouvons que les déplacements de surface sont contrôlés par la géométrie du conduit et du plug ainsi que le contraste de viscosité entre le plug et la colonne de magma. Nous montrons que l'évolution de la taille du plug est une hypothèse possible pour expliquer les rapides transitions inflation/subsidence observées à la surface des volcans andésitiques. / Through two different applications, we focus on the mechanical coupling between magma and host rocks, using numerical method. First, we study the influence of stress perturbations on shallow magma chambers, with the exemple of two subglacial volcanoes (Iceland). A variation in the stress field acts to modify the magma pressure within the reservoir as well as the failure conditions to initiate an intrusion at the reservoir wall. At Grímsvötn volcano, subglacial lake discharges (so-called "jökulhlaup") often occur in relation to eruptions. Our models show that jökulhlaup promote the failure of the magma reservoir and thus trigger eruptions, as observed for 2004 eruption. The triggering effect is small, so magmatic system must be already pressurized and close to failure before the discharge of the lake occurs. At Katla volcano, our models indicate that reservoir failure is highest in summer period when the ice load at Mýrdalsjökull icecap is reduced. Coulomb stress calculation predicts also an increase of earthquake occurrence at the caldera rim at the same time. A seasonal modulation of volcanic and seismic activity at Katla related to the loading/unloading of the icecap can exist. Secondly, we focus on magma conduit flow for andesitic volcanoes. Improving our knowledge of magma dynamic during ascent is a challenge, because processes occurring in the conduit seem to govern the temporal evolution of eruptive activity. Previous models showed that magma viscosity increases during flow in the upper part of the conduit, which causes the emplacement of a viscous plug. But the relationship between the plug emplacement and precursors signals, such as ground deformation or seismicity, is not yet fully described. From our plug flow model, we find that surface deformation is mainly controlled by the geometry of the conduit, the ratio between the plug length and the total conduit length and the viscosity contrast between the plug and the magma column. We show that the evolution of the plug size is a possible explanation for rapid transition between ground inflation and ground subsidence observed on andesitic volcanoes before extrusion.

Future sea level rise associated to global warming is one of the greatest societal and environmental challenges of tomorrow. A large part of the contribution comes from glaciers and ice sheets discharging ice and meltwater into the ocean and the recent worldwide increase is worrying. Future predictions of sea level rise try to encompass the complex processes of ice dynamics through glacier modelling but there are still large uncertainties due to the lack of observations or too coarse parameterisation, particularly for processes occurring at the glacier interfaces with the bed (sliding) and with the ocean (calving). This thesis focuses on modelling these processes from two marine-terminating glaciers in Svalbard, Kronebreen and Tunabreen. By inverting three years of high temporal resolution time-series of surface velocities on Kronebreen, basal properties are retrieved with the ice flow model Elmer/Ice in Paper I. Results suggest that surface melt during the summer greatly influences the dynamics of the following season and that sliding laws for such glaciers should be adapted to local and global processes changing in space and time. The subglacial drainage system, fed by the surface melt, is modelled in Paper II during two melting seasons. Results show different configurations of efficient and inefficient drainage systems between years and the importance of using a sliding law dependent on spatio-temporal changes in effective pressure. The interaction with the ocean is incorporated in Paper III by combining a series of models, including an ice flow model, a plume model and a particle model for discrete calving and compares the output with observations. Results show the importance of glacier geometry, sliding and undercutting on calving rate and location. However, more observations and analytic methods are needed. Time-lapse imagery placed in front of Tunabreen have been deployed and a method of automatic detection for iceberg calving is presented in Paper IV. Results show the influence of the rising plume in calving and the front destabilisation of the local neighbourhood.

Les structures de déformation pré-lithification s’enregistrent dans les sédiments meubles. Ces structures sont abondantes dans les domaines sous-glaciaires et marins profonds en raison du développement de surpressions de fluide. A partir de cas d’études choisis dans ces deux environnements, leurs implications sur les (1)reconstructions paléoenvironnementales, leurs impacts sur la (2) morphologie glaciaire, et sur les (3) propriétés pétrophysiques ont pu être définis.(1) L’analyse de ces structures de déformation a permis de mieux contraindre les paléoenvironnements sédimentaires. Ces structures de déformation ont été utilisées comme des «proxy » permettant d’estimer les variations de la vitesse d’écoulement, de l’épaisseur de glace, de la production d’eaux de fonte et de la position de la marge glaciaire.(2) Les séries sédimentaires des vallées tunnels ordoviciennes enregistrent la mise en place de nombreuses structures de déformation liées aux surpressions de fluides. L’analyse de ces structures a permis de proposer un nouveau modèle de creusement des vallées tunnels induits par des pressions de fluides élevées. Ce modèle de creusement, lié aux surpressions de fluide, est favorisé dans les zones d’inter ice-stream. A l’inverse, sous les ice-stream, l’écoulement des eaux de fonte se produit à l’interface glace-substrat et favorise d’autres modèles de formation des vallées tunnels.(3) Les processus de remobilisation sédimentaire ont un impact sur les propriétés pétrophysiques des réservoirs sableux. Les études pétrophysiques menées sur des grès déposés en environnements marins profonds et glaciaires ont permis de mettre en évidence l’impact des surpressions de fluides sur les propriétés réservoirs des sables. Les processus de fluidisation sont capable de créer de bons réservoirs, tandisque les processus d’élutriations ont tendance a réduire la porosité/perméabilité. / Soft-sediment deformation structures (SSDs) occur in unconsolidated sediments, during or shortly after deposition. SSDs are abundant in subglacial and deep-marine environments because of the development of fluid overpressure. Case studies of these two sedimentary environments were used (1) to reconstruct palaeoenvironments from SSDS, and (2) to define the impacts of SSDS on glacial morphologies and (3) petrophysical properties.(1) Analyses of strain regimes, deformation mechanisms, and chronologies in SSDs served to improve palaeoenvironmental reconstructions. These structures were used as proxys to estimate variations of ice flow velocities, ice thickness, meltwater production, and position of the ice margin.(2) The sedimentary series of ordovician tunnel valleys record numerous SSDs induced by fluid overpressure. A new model of tunnel valley formation controlled by the increase of porewater pressure in the bed is proposed. This model of formation occurs in inter-ice stream zones, where meltwater is transferred to the substratum. In ice-stream corridors, meltwater circulates at the ice-bed interface and promotes the formation of tunnel valleys controlled by meltwater processes.(3) Remobilisation processes triggered by the increase of fluid pressure have an impact on the granular framework and on the geometry of reservoirs. Petrophysical studies of subglacial and deep-marine sandstones demonstrated the impact of fluid overpressure on reservoir properties. Processes of fluidisation are responsible for the increase in porosity/permeability, while elutriation processes lead to a decrease in these petrophysical properties.

No description available.