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An der Basis der Eisschilde, die Grönland und die Antarktis bedecken, finden sich Kanalstrukturen. Sie stellen einen wichtigen Teil des subglazialen hydrologischen Systems der Eisschilde und der Schelfeise dar. Mithilfe eines numerischen Modells der viskoelastischen Rheologie von Eis wird die zeitliche Entwicklung dieser Strukturen in dieser Arbeit untersucht. Ein Abfall des Wasserdrucks innerhalb eines subglazialen Kanals an der Basis eines gegründeten Gletschers führt zu einer Verringerung seiner Querschnittsfläche. Die Geschwindigkeit, mit der diese Verringerung erfolgt, hängt wesentlich vom Wasserdruck und der initialen Form des Kanals ab. Ein Teil der Querschnittsflächenverringerung erfolgt instantan durch die elastische Deformation des viskoelastischen Eises. Am Support-Force Gletscher in der Westantarktis findet sich ein Schmelzkanal an der Unterseite des Filchner Schelfeises. Aufgrund des Schmelzkanals ist das hydrostatische Gleichgewicht des Schelfeises gestört. Der Kanal beginnt sich durch ein Fließen des Eises zu schließen. Die Geschwindigkeit dieses Eisflusses wird wesentlich durch die nichtlineare Viskosität des Eises beeinflusst. Basales Schmelzen und oberflächliche Akkumulation stellen zusätzliche Einflussfaktoren auf die Entwicklung des Eiskörpers dar. Der elastische Anteil der viskoelastischen Deformation von Eis trägt in signifikantem Maß zur Verringerung der Querschnittsfläche subglazialer Kanäle großer Breite und geringer Höhe bei. In der Reaktion des Schelfeises auf eine zyklische Belastung spiegelt sich die viskoelastische Rheologie des Eises ebenfalls wider. Zukünftige Betrachtungen der untersuchten Strukturen mithilfe numerischer Modelle sollten daher die viskoelastischen Materialeigenschaften des Eises mit berücksichtigen.

Jakobshavn Isbrae is a major marine terminating outlet glacier of the western Greenland Ice Sheet, which has been undergoing widespread acceleration and strong mass loss since the disintegration of its floating ice tongue in the late 1990s. The underlying mechanisms are poorly understood despite a wealth in observational and modelling studies. This doctoral thesis analyses the dynamic changes of Jakobshavn Isbrae using the Ice Sheet System Model (ISSM), a state-of-the-art finite-element ice flow model. Two missing model features for 1) the modelling the polythermal regime of glaciers and ice sheets, and 2) the dynamic evolution of its horizontal calving front position are designed and implemented into ISSM. A three-dimensional, thermodynamically coupled model of Jakobshavn Isbrae is set up and calibrated using modern observational data products. Low basal drag in the trough under the ice stream requires that its high driving stress is balanced by lateral drag in the shear margins, which allows for high flow velocities, as the ice viscosity is strain-rate-dependent. The developed modules are applied to the glacier model, which captures 90% of the observed changes from 1985 to 2015. Analysis of the model results reveals that calving front retreat is able to trigger widespread inland acceleration due to a rheological ice viscosity drop in the shear margins. Thermal feedbacks contribute 5 to 10% to the total acceleration. The study shows that Jakobshavn Isbrae will continue to contribute to eustatic sea level rise for at least the next century due to ongoing geometry adjustment to the new calving front position. Future fields of research include deriving a suitable calving rate parametrisation for large-scale ice flow models, a material law for temperate ice with a microscopic water content larger than 1%, and technical refinements of the modules developed for this thesis.

Mass changes of polar ice sheets have an important societal impact, because they affect global sea level. Estimating the current mass budget of ice sheets is equivalent to determining the balance between the surface mass gain through precipitation and the outflow across the grounding line. In Antarctica, the latter is mainly governed by oceanic processes and outlet glacier dynamics.In this thesis, we assess the mass balance of a part of eastern DronningMaud Land via an input/output method. Input is given by recent surface accumulation estimations of the whole drainage basin. The outflow at the grounding line is determined from the radar data of a recent airborne survey and satellite-based velocities using a flow model of combined plug flow and simple shear. We estimate the regional mass balance in this area to be between 1.88±8.50 and 3.78±3.32 Gt a−1 depending on the surface mass balance (SMB) dataset used. This study also reveals that the plug flow assumption is acceptable at the grounding line of ice streams.The mass balance of drainage basins is governed by the dynamics of their outlet glaciers and more specifically the flow conditions at the grounding line. Thanks to an airborne radar survey we define the bed properties close to the grounding line of the West Ragnhild Glacier (WRG) in the Sør Rondane Mountains. Geometry and reflectivity analyses reveal that the bed of the last 65 km upstream of the grounding line is sediment covered and saturated with water. This setting promotes the dominance of basal motion leading to a change in the flow regime: in the interior flow is governed by internal deformation while its relative importance decreases to become driven by basal sliding.Subsequently we present the results of the reconstruction of the SMB across an ice rise through radar data and inverse modelling. The analysis demonstrates that atmospheric circulation was stable during the last millennium. Ice rises induce an orographic uplift of the atmospheric flow and therefore influence the pattern of the SMB across them, resulting in an asymmetric SMB distribution. Since the geometry of the internal reflection horizons observed in radar data depends on the SMB pattern, the asymmetry observed in radar layers reveals the trajectories of air masses at the time of deposit. We present an original and robust method to quantify this SMB distribution. Combining shallow and deep radar layers, SMB across Derwael Ice Rise is reconstructed. Two methods are employed as a function of the depth of the layers: i.e. the shallow layer approximation for the surface radar layers and an optimization technique based on an ice flow model for the deeper ones. Both methods produce similar results. We identify a difference in SMB magnitude of 2.5 between the flanks and the ice rise divide, as well as a shift of ≈4 km between the SMB maximum and the crest. Across the ice rise, SMB exhibits a very large variability, ranging from 0.3 to 0.9 mw.e. a−1. This anomaly is robust in time.Finally we draw a comprehensive description of the Sør Rondane Mountains sector. The glacial system is close to the equilibrium and seems stable but evidences suggest that it is a fragile equilibrium. The proximity of the open ocean certainly favours the interaction between warm water and the ice shelf cavity conducting to potential important melting. The thinning associated with this melting can detach the ice shelf from pinning points. This will reduce the buttressing from the ice shelf, outlet glaciers will accelerate and mass transfer toward the ocean will increase. Therefore, the future of Antarctic Ice Sheet directly depends on the changes affecting its boundaries and assessing the sensitivity of the ice sheets is essential to quantify and anticipate the future variation of mass balance., Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Glubokoye bureniye na stantsii NEEM v severo-zapadnoy Grenlandii v 2008-2010 gg. bylo vypolneno glavnym obrazom dlya polucheniya dannykh, otnosyashchikhsya k predposlednemu mezhlednikov'yu (Eemian), imevshemu mesto okolo 115-130 tys. l.n. Dlya pravil'noy interpretatsii izotopnogo ryada na osnove izucheniya kerna l'da neobkhodimo otdelit' v nom «klimaticheskiy» signal ot «neklimaticheskogo». Prisutstviye neklimaticheskikh komponent obuslovleno vliyaniyem kolebaniy vysoty poverkhnosti lednika i advektsiyey l'da iz oblastey, raspolozhennykh vyshe po techeniyu, shirotnym kontrastom v raspredelenii d18O i drugimi prichinami. Razobrat'sya v etom pomogayut metody matematicheskogo modelirovaniya. Dlya rekonstruktsii dinamicheskoy istorii Grenlandskogo lednikovogo shchita na protyazhenii dvukh poslednikh lednikovo-mezhlednikovykh tsiklov ispol'zovalas' trokhmernaya regional'naya model' techeniya l'da v ogranichennoy oblasti v severo-zapadnoy chasti Grenlandii mezhdu stantsiyami NEEM i NGRIP. Vozrast kerna l'da skvazhiny NEEM i koordinaty yego proiskhozhdeniya na poverkhnosti rasschityvalis' metodom obratnogo otslezhivaniya chastits l'da v model'nom pole skorosti. Rezul'taty raschotov pokazyvayut, chto izmerennaya raznitsa mezhdu sovremennym znacheniyem d18O v kerne i znacheniyem, sootvetstvuyushchim maksimumu potepleniya okolo 127 tys. l.n., dolzhna byt' uvelichena na 1,5o/oo. Stratigraficheskiye narusheniya v nizhney chasti kerna sushchestvenno zatrudnyayut interpretatsiyu dannykh bureniya. Translated in Eglish: Deep drilling at NEEM station in north- western Greenland in 2008-2010 . was carried out mainly to obtain data relating to the penultimate interglacial (Eemian), occurring approximately 115-130 kyr To correctly interpret the isotopic series based on the study of ice cores is necessary to separate it "climate " signal from the " non-climate ". The presence of non-climatic component due to effects of fluctuations in the height of the surface of the glacier and ice advection from areas upstream , in contrast to the latitudinal distribution of ?18O and other causes. Help to understand this mathematical modeling methods . For the reconstruction of the dynamic history of the Greenland ice sheet over the last two glacial- interglacial cycles used a three-dimensional model of regional ice flow in a bounded region in the northwestern part of Greenland between stations NEEM and NGRIP. Age NEEM ice core hole and the coordinates of its origin on the surface were calculated by tracing back the ice particles in the model velocity field . The calculation results show that the measured difference between the modern ?18O value in the core and the value corresponding to the maximum warming of about 127 kyr , shall be increased by 1,5 o/oo. Stratigraphic disturbances in the bottom of the core significantly complicate the interpretation of drilling data., The new ice core obtained from the NEEM site in the north-western Greenland (77.449˚N, 51.056˚W, 2447 m a.s.l) in 2008–2012 was expected to improve our knowledge about the Last interglacial (also known as the Eemian, ca. 115 to 130 kyr before present). A numerical modeling, the overview of which is given in this paper, aims at assistance in interpretation of the NEEM ice core. For this purpose, an area of 400 × 400 km was delineated in northwestern Greenland as the domain for the fine-scale model at 2.5 km resolution. Modeled present-day surface velocity was in good agreement with satellite and GPS measurements. The nested ice-sheet model was run over the last two glacial cycles to reconstruct the flow history relevant for interpreting the NEEM ice core. A Lagrangian backtracing procedure was applied to determine the particle trajectories of the ice drilled at the NEEM site. This provides the places of origin at the time of deposition of NEEM ice, from which the ice chronology and non-climatic biases of the records are determined. The latter biases arise from elevation changes of the ice sheet, advection of higher upstream ice, and from latitudinal contrasts in isotopical composition of the ice. These need to be separated from the isotope records to retrieve the climatic signal. In spite of initial expectations, the segment in the NEEM ice core between 2206 m and 2435 m turned out to be heavily disturbed. The presence of high isotope values below 2206 m depth however reveals that there is ice from the previous warm interglacial. Our model locates the EMBED Equation.DSMT4 maximum of the warmest Eemian ice at the right depth (2400 m). On this basis we suggest that the measured contrast between the present and the Eemian EMBED Equation.DSMT4 must be increased by about 1.5 ‰. Most of this non-climatic bias results from upstream advection over an estimated distance of ~175 km. Except for the disturbed section, we are still confident to be able to provide accurate model-based estimates of the ice chronology and non-climatic biases for the core down to ~87% of relative depth. This requires revision the palaeo-accumulation parameterization to better match thinner annual layer thicknesses observed in the ice core for the last glacial period.