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  • 1
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    Band-pass-filtered ice-sheet surface elevation, thickness, velocity and derived data maps for the northern Greenland Ice Sheet (2018)
    PANGAEA
    In:  Supplement to: Bons, Paul D; Kleiner, Thomas; Llorens, Maria-Gema; Prior, David J; Sachau, Till; Weikusat, Ilka; Jansen, Daniela (2018): Greenland Ice Sheet: Higher Nonlinearity of Ice Flow Significantly Reduces Estimated Basal Motion. Geophysical Research Letters, 45(13), 6542-6548, https://doi.org/10.1029/2018GL078356
    Details
    Publication Date: 2023-02-06
    Description: In times of warming in polar regions, the prediction of ice sheet discharge is of utmost importance to society, because of its impact on sea level rise. In simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n=3. This exponent influences the softness of the modeled ice, as higher values would produce faster flow under equal stress. We show that the stress exponent, which best fits the observed state of the Greenland Ice Sheet, equals n=4, Our results, which are not dependent on a possible basal sliding component of flow, indicate that most of the interior northern ice sheet is currently frozen to bedrock, except for the large ice streams and marginal ice.
    Keywords: File content; File format; File name; File size; MULT; Multiple investigations; Northern_Greenland_Ice_Sheet; Uniform resource locator/link to file
    Type: Dataset
    Format: text/tab-separated-values, 75 data points
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    DATA PANGAEA
  • 2
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    Mountain-building under extension (2013)
    HighWire Press
    Details
    Publication Date: 2013-04-01
    Description: A mechanism is presented which explains how intra-continental rifting can cause large topographic uplift. The effect is sufficient to account for the uplift of rift flanks and the very high and strongly localized uplift of the Rwenzori horst in the Western Branch of the East African Rift System. We propose that the uplift is generated by crustal bending, which is caused by a misfit of the lateral tensile stress between the upper and middle crust. The misfit is a function of different yield mechanisms when the upper crust breaks whereas the middle crust flows.Two independent numerical schemes confirm the suggested uplift mechanism. Both models—a 2 and 2.5 D elastoplastic lattice-particle model and a multilayer beam model—were used to calculate the surface topography as a result of lateral uniaxial extension. Using the fault geometry of the Rwenzori area, we find that the amount of topographic uplift is controlled by the viscosity and elasticity of the crust. The extreme uplift of the Rwenzori horst is—at least to some extent—a function of its considerably high elastic stiffness. The stiffness unites the two rifts that bound the Rwenzori horst and leads to an extremely high topography and a high Moho uplift in the center of the two rifts where the Rwenzori mountains sit.
    Print ISSN: 0002-9599
    Electronic ISSN: 1945-452X
    Topics: Geosciences
    Published by HighWire Press on behalf of The American Journal of Science.
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    PAPER CURRENT
  • 3
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    Comment on “Exceptionally high heat flux needed to sustain the Northeast Greenland Ice Stream” by Smith-Johnsen et al. (2020) (2021)
    Copernicus Publications
    In:  EPIC3The Cryosphere, Copernicus Publications, 15, pp. 2251-2254
    Details
    Publication Date: 2021-05-25
    Description: Smith-Johnsen et al. (The Cryosphere, 14, 841–854, https://doi.org/10.5194/tc-14-841-2020, 2020) model the effect of a potential hotspot on the Northeast Greenland Ice Stream (NEGIS). They argue that a heat flux of at least 970 mW m−2 is required to have initiated or to control NEGIS. Such an exceptionally high heat flux would be unique in the world and is incompatible with known geological processes that can raise the heat flux. Fast flow at NEGIS must thus be possible without the extraordinary melt rates invoked in Smith-Johnsen et al. (2020).
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 4
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    Greenland Ice Sheet - Higher non-linearity of ice flow significantly reduces estimated basal motion (2018)
    AGU, Wiley
    In:  EPIC3Geophysical Research Letters, AGU, Wiley, 45(13), pp. 6542-6548, ISSN: 00948276
    Details
    Publication Date: 2018-07-30
    Description: In times of warming in polar regions, the prediction of ice sheet discharge is of utmost importance to society, because of its impact on sea level rise. In simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n=3. This exponent influences the softness of the modeled ice, as higher values would produce faster flow under equal stress. We show that the stress exponent, which best fits the observed state of the Greenland Ice Sheet, equals n=4. Our results, which are not dependent on a possible basal sliding component of flow, indicate that most of the interior northern ice sheet is currently frozen to bedrock, except for the large ice streams and marginal ice. Ice in the polar ice sheets flows towards the oceans under its own weight. Knowing how fast the ice flows is of crucial importance to predict future sea level rise. The flow has two components: (1) internal shearing flow of ice and (2) basal motion, which is sliding along the base of ice sheets, especially when the ice melts at this base. To determine the first component we need to know how "soft" the ice is. By considering the flow velocities at the surface of the northern Greenland Ice Sheet and calculating the stresses that cause the flow, we determined that the ice is effectively softer than is usually assumed. Previous studies indicated that the base of the ice is thawed in large parts (up to about 50%) of the Greenland Ice Sheet. Our study shows that that is probably overestimated, because these studies assumed ice to be harder than it actually is. Our new assessment reduces the area with basal motion and thus melting to about 6-13% in the Greenland study area.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 5
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    Influence of viscosity on growth of high pressure phases in computer experiments (2010)
    Universitätsverlag Göttingen
    Details
    Publication Date: 2021-03-29
    Description: The general aim of the project is the examination of microstructures that develop under HP conditions in computer experiments. Starting point is an interest in the dynamics of HP phase transitions, as for instance the probably catastrophic phase-change event of olivine to spinel in the upper mantle. This is either explained by large overpressure or failure during the development of micro-structures during the growth of the spinel phase. Experimental results on this subject are rare, and do not lead by themselves to a deeper insight into the complicated stress/strain/volumechange/ micro-crack relationships of the transition. We developed a central force spring model, where particles can undergo a phase change using parameters of olivine and spinel. The algorithm is capable of simulating the local growth of the mentioned phases on the basis of direction-dependant rate laws. In the current context newtonian viscosity is added to the previously solely elastic system, since under HP/HT conditions the viscous flow within the material will have a large influence on the distribution of elastic energies, which in turn have an important influence on the driving force of the transition. Thus we are dealing with a visco-elastic system, which will be subjected to timedependant strain.
    Description: conference
    Keywords: 551 ; VKA 200 ; VBE 000 ; VKA 110 ; VAE 120 ; Gefügekunde der Gesteine ; Modellierung von Prozessen in der Geosphäre ; Gesteinsbestimmung ; Methodik {Strukturgeologie} ; Hochdruckparagenese ; Viskosität ; Olivin ; Spinell ; Kristallisation ; Computersimulation
    Language: German
    Type: anthologyArticle , publishedVersion
    Format: application/pdf
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    CITATION GEO-LEO
  • 6
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    Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study (2019)
    Elsevier
    In:  EPIC3Journal of Structural Geology, Elsevier, 124, pp. 81-90, ISSN: 01918141
    Details
    Publication Date: 2020-01-02
    Description: Localisation of ductile deformation in rocks is commonly found at all scales from crustal shear zones down to grain scale shear bands. Of the various mechanisms for localisation, mechanical anisotropy has received relatively little attention, especially in numerical modelling. Mechanical anisotropy can be due to dislocation creep of minerals (e.g. ice or mica) and/or layering in rocks (e.g. bedding, cleavage). We simulated simple-shear deformation of a locally anisotropic, single-phase power-law rheology material up to shear strain of five. Localisation of shear rate in narrow shear bands occurs, depending on the magnitude of anisotropy and the stress exponent. At high anisotropy values, strain-rate frequency distributions become approximately log-normal with heavy, exponential tails. Localisation due to anisotropy is scale-independent and thus provides a single mechanism for a self-organised hierarchy of shear bands and zones from mm-to km-scales. The numerical simulations are compared with the natural example of the Northern Shear Belt at Cap de Creus, NE Spain.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 7
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    Heat flux and the North East Greenland Ice Stream (2021)
    In:  EPIC3EGU General Assembly 2021, Online, 2021-04-19-2021-04-30
    Details
    Publication Date: 2021-08-23
    Description: The prominent North East Greenland Ice Stream (NEGIS) is an exceptionally large ice stream in the Greenland Ice sheet. It is over 500 km long, originates almost at the central ice divide, and contributes significantly to overall ice drainage from the Greenland Ice sheet. Surface velocities in the inland part of the ice stream are several times higher inside NEGIS than in the adjacent ice sheet. Modelling NEGIS is still a challenge as it remains unclear what actually causes and controls the ice stream. An elevated geothermal heat flux is one of the factors that are being considered to trigger or drive the fast flow inside NEGIS. Unfortunately, the geothermal heat flux below NEGIS and its upstream area is poorly constrained and estimates vary from close to the global average for continental crust (ca. 60 mW/m2) to values as high as almost 1000 mW/m2. The latter would cause about 10 cm/yr of melting at the base of the ice sheet. We present a brief survey of global geothermal heat flux data, especially from known hotspots, such as Iceland and Yellowstone. Heat fluxes in these areas that are known to be among the hottest on Earth rarely, if ever, exceed 300 mW/m2. A plume hotspot or its trail can therefore not cause heat fluxes at the high end of the suggested range. Other potential factors, such as hydrothermal fluid flow and radiogenic heat, also cannot raise the heat flux significantly. We conclude that the heat flux at NEGIS is very unlikely to exceed 100-150 mW/m2, and future modelling studies on NEGIS should thus be mindful of implementing realistic geothermal heat flux values. If NEGIS is not the result of an exceptionally high heat flux, we are left with the exciting challenge to find the true trigger of this fascinating structure.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
    Format: application/pdf
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  • 8
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    How ice anisotropy contributes to fold and ice stream in large-scale ice-sheet models (2024)
    In:  EPIC3
    Details
    Publication Date: 2024-04-29
    Description: Satellite and airborne sensors have provided detailed data on ice surface flow velocities, englacial structures of ice sheets and bedrock elevations. These data give insight into the flow behaviour of ice sheets and glaciers. One significant phenomenon observed is large-scale folds (over 100 m in amplitude) in the englacial stratigraphy in the Greenland ice sheet. A large population of folds is located at ice streams, where the flow is distinctly faster than in the surroundings, such as the North-East Greenland Ice Stream (NEGIS). While there is no consensus regarding the formation of large-scale folds, unraveling the underlying mechanisms presents significant potential for enhancing our understanding of the formation and dynamics of ice streams. Ice in ice sheets is a ductile material, i.e., it can flow as a thick viscous fluid with a power-law rheology. Furthermore, ice is significantly anisotropic in its flow properties due to its crystallographic preferred orientation (CPO). Here, we use the Full-Stokes code Underworld2 (Mansour et al.,2022) for 3D modelling of the power-law and transversely isotropic ice flow, also in comparison with the isotropic ice models. Our simulated folds with anisotropic ice show complex patterns on a bumpy bedrock, and are classified into three types: large-scale folds (fold amplitudes 〉100 m), small-scale folds (fold amplitudes 〈〈100 m, wavelength 〈〈km) and recumbent basal-shear folds. Our results indicate that bedrock topography contributes to perturbations in ice layers, and that ice anisotropy due to the CPO amplifies these into large-scale folds in convergent flow by horizontal shortening. As for our ice stream model, we simulate convergent flow as initial condition, which subsequently initiates the development of shear margins due to the rotation of the ice crystal basal planes. As soon as the shear margins develop, the ice stream starts to propagate upstream in a short time and narrows in the upstream part. Our modeling shows that the anisotropic rheology of ice and CPO change play a significant role for large-scale folding and for the initiation of ice streams with distinct shear margins. Hence, we promote the implementation of ice anisotropy in large-scale ice-sheet evolution models as it holds the potential to introduce novel perspectives to the glaciological community on the dynamics of ice flow.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
    Format: application/pdf
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  • 9
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    Shear margins in upper half of Northeast Greenland Ice Stream were established two millennia ago (2024)
    Springer Science and Business Media LLC
    In:  EPIC3Nature Communications, Springer Science and Business Media LLC, 15(1), pp. 1193-, ISSN: 2041-1723
    Details
    Publication Date: 2024-04-29
    Description: Only a few localised ice streams drain most of the ice from the Greenland Ice Sheet. Thus, understanding ice stream behaviour and its temporal variability is crucially important to predict future sea-level change. The interior trunk of the 700 km-long North-East Greenland Ice Stream (NEGIS) is remarkable due to the lack of any clear bedrock channel to explain its presence. Here, we present a 3-dimensional analysis of the folding and advection of its stratigraphic horizons, which shows that the localised flow and shear margins in the upper NEGIS were fully developed only ca 2000 years ago. Our results contradict the assumption that the ice stream has been stable throughout the Holocene in its current form and show that upper NEGIS-type development of ice streaming, with distinct shear margins and no bed topography relationship, can be established on time scales of hundreds of years, which is a major challenge for realistic mass-balance and sea-level rise projections.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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