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  • 1
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    Production and Transport of Supraglacial Debris: Insights From Cosmogenic 10Be and Numerical Modeling, Chhota Shigri Glacier, Indian Himalaya (2021)
    Details
    Publication Date: 2021-04-20
    Description: Plain Language Summary High and steep mountain ranges are currently undergoing changes due to increasing temperatures. These changes include rapidly shrinking glaciers as well as thawing permafrost, which together destabilize rock walls that surround valley glaciers. In consequence, slope failures and thus erosion rates in these environments are expected to increase. However, quantifying rock wall erosion in alpine landscapes is difficult and estimates of background erosion rates that are unaffected by Global Warming are rare. Here we estimate rock wall erosion rates above the Chhota Shigri Glacier, Indian Himalaya, by studying rocky debris from the glacier surface. This debris is sourced from the surrounding topography and we use geochemical tools to measure its residence time at the Earth surface. We combine our geochemical observations with a computer model of the glacier that allows us to explore the effect of Global Warming on the evolution of the glacier and the debris on its surface. Our results suggest recent changes in rock wall erosion rates that may be related to glacier retreat and an increase in the erosion of rock walls that were previously ice covered.
    Description: Key Points 10Be‐derived headwall erosion rates are ~0.5–1 mm year−1 on average and apparently increasing toward the present We use ice modeling to explore the effects of transience and spatial variability in erosion rates and source areas on 10Be concentrations Potential explanations for the observed trend in 10Be concentrations include enhanced erosion of recently deglaciated areas
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Description: EC | H2020 | H2020 Priority Excellent Science | H2020 European Research Council (ERC) http://dx.doi.org/10.13039/100010663
    Keywords: 551.31 ; debris‐covered glaciers ; cosmogenic nuclides ; ice flow modeling ; erosion ; glacial landscapes
    Type: article
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  • 2
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    Pleistocene Evolution of a Scandinavian Plateau Landscape (2018)
    American Geophysical Union
    Details
    Publication Date: 2018-12-01
    Print ISSN: 2169-9003
    Electronic ISSN: 2169-9011
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 3
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    A new methodology to simulate subglacial deformation of water-saturated granular material (2015)
    Copernicus
    Details
    Publication Date: 2015-11-20
    Description: The dynamics of glaciers are to a large degree governed by processes operating at the ice–bed interface, and one of the primary mechanisms of glacier flow over soft unconsolidated sediments is subglacial deformation. However, it has proven difficult to constrain the mechanical response of subglacial sediment to the shear stress of an overriding glacier. In this study, we present a new methodology designed to simulate subglacial deformation using a coupled numerical model for computational experiments on grain-fluid mixtures. The granular phase is simulated on a per-grain basis by the discrete element method. The pore water is modeled as a compressible Newtonian fluid without inertia. The numerical approach allows close monitoring of the internal behavior under a range of conditions. Our computational experiments support the findings of previous studies where the rheology of a slowly deforming water-saturated granular bed in the steady state generally conforms to the rate-independent plastic rheology. Before this so-called critical state, deformation is in many cases accompanied by volumetric changes as grain rearrangement in active shear zones changes the local porosity. For previously consolidated beds porosity increases can cause local pore-pressure decline, dependent on till permeability and shear rate. We observe that the pore-water pressure reduction strengthens inter-granular contacts, which results in increased shear strength of the granular material. In contrast, weakening takes place when shear deformation causes consolidation of dilated sediments or during rapid fabric development. Both processes of strengthening and weakening depend inversely on the sediment permeability and are transient phenomena tied to the porosity changes during the early stages of shear. We find that the transient strengthening and weakening in turn influences the distribution of shear strain in the granular bed. Dilatant strengthening has the ability to distribute strain during early deformation to large depths, if sediment dilatancy causes the water pressure at the ice–bed interface to decline. Oppositely, if the ice–bed interface is hydrologically stable the strengthening process is minimal and instead causes shallow deformation. The depth of deformation in subglacial beds thus seems to be governed by not only local grain and pore-water feedbacks but also larger-scale hydrological properties at the ice base.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
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    A new methodology to simulate subglacial deformation of water saturated granular material (2015)
    Copernicus
    Details
    Publication Date: 2015-07-13
    Description: The dynamics of glaciers are to a large degree governed by processes operating at the ice–bed interface, and one of the primary mechanisms of glacier flow over soft unconsolidated sediments is subglacial deformation. However, it has proven difficult to constrain the mechanical response of subglacial sediment to the shear stress of an overriding glacier. In this study, we present a new methodology designed to simulate subglacial deformation using a coupled numerical model for computational experiments on grain-fluid mixtures. The granular phase is simulated on a per-grain basis by the discrete element method. The pore water is modeled as a compressible Newtonian fluid without inertia. The numerical approach allows close monitoring of the internal behavior under a range of conditions. The rheology of a water-saturated granular bed may include both plastic and rate-dependent dilatant hardening or weakening components, depending on the rate of deformation, the material state, clay mineral content, and the hydrological properties of the material. The influence of the fluid phase is negligible when relatively permeable sediment is deformed. However, by reducing the local permeability, fast deformation can cause variations in the pore-fluid pressure. The pressure variations weaken or strengthen the granular phase, and in turn influence the distribution of shear strain with depth. In permeable sediments the strain distribution is governed by the grain-size distribution and effective normal stress and is typically on the order of tens of centimeters. Significant dilatant strengthening in impermeable sediments causes deformation to focus at the hydrologically more stable ice–bed interface, and results in a very shallow cm-to-mm deformational depth. The amount of strengthening felt by the glacier depends on the hydraulic conductivity at the ice–bed interface. Grain-fluid feedbacks can cause complex material properties that vary over time, and which may be of importance for glacier stick-slip behavior.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 5
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    The periglacial engine of mountain erosion – Part 1: Rates of frost cracking and frost creep (2015)
    Copernicus
    Details
    Publication Date: 2015-04-22
    Description: With accelerating climate cooling in the late Cenozoic, glacial and periglacial erosion became more widespread on the surface of the Earth. The resultant shift in erosion patterns significantly changed the large-scale morphology of many mountain ranges worldwide. Whereas the glacial fingerprint is easily distinguished by its characteristic fjords and U-shaped valleys, the periglacial fingerprint is more subtle but potentially prevailing in some landscape settings. Previous models have advocated a frost-driven control on debris production on steep headwalls and glacial valley sides. Here we investigate the important role that periglacial processes also play in less steep parts of mountain landscapes. Understanding the influences of frost-driven processes in low-relief areas requires a focus on the consequences of an accreting soil-mantle, which characterizes such surfaces. In this paper, we present a new model that quantifies two key physical processes: frost cracking and frost creep, as a function of both temperature and sediment thickness. Our results yield new insights to how climate and sediment transport properties combine to scale the intensity of periglacial processes. The thickness of the soil-mantle strongly modulates the relation between climate and the intensity of mechanical weathering and sediment flux. Our results also point to an offset between the conditions that promote frost cracking and those that promote frost creep, indicating that a stable climate can only provide optimal conditions for one of those processes at a time. Finally, quantifying these relations also opens the possibility of including periglacial processes in large-scale, long-term landscape evolution models, as demonstrated in a companion paper.
    Electronic ISSN: 2196-6338
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
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    Basal shear stress under alpine glaciers: Insights from experiments using the iSOSIA and Elmer/ICE models (2015)
    Copernicus
    Details
    Publication Date: 2015-10-07
    Description: Shear stress at the base of glaciers controls basal sliding and is therefore immensely important for glacial erosion and landscape evolution in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution. In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/ICE). We find that iSOSIA and Elmer/ICE predict similar first-order stress and velocity patterns, and that differences are restricted to local variations over length-scales on the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to suble changes in local topographic relief. Following these initial stress benchmark experiments, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.
    Electronic ISSN: 2196-6338
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
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    The periglacial engine of mountain erosion – Part 2: Modelling large-scale landscape evolution (2015)
    Copernicus
    Details
    Publication Date: 2015-10-06
    Description: There is growing recognition of strong periglacial control on bedrock erosion in mountain landscapes, including the shaping of low-relief surfaces at high elevations (summit flats). But, as yet, the hypothesis that frost action was crucial to the assumed Late Cenozoic rise in erosion rates remains compelling and untested. Here we present a landscape evolution model incorporating two key periglacial processes – regolith production via frost cracking and sediment transport via frost creep – which together are harnessed to variations in temperature and the evolving thickness of sediment cover. Our computational experiments time-integrate the contribution of frost action to shaping mountain topography over million-year timescales, with the primary and highly reproducible outcome being the development of flattish or gently convex summit flats. A simple scaling of temperature to marine δ18O records spanning the past 14 Myr indicates that the highest summit flats in mid- to high-latitude mountains may have formed via frost action prior to the Quaternary. We suggest that deep cooling in the Quaternary accelerated mechanical weathering globally by significantly expanding the area subject to frost. Further, the inclusion of subglacial erosion alongside periglacial processes in our computational experiments points to alpine glaciers increasing the long-term efficiency of frost-driven erosion by steepening hillslopes.
    Print ISSN: 2196-6311
    Electronic ISSN: 2196-632X
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 8
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    The periglacial engine of mountain erosion – Part 1: Rates of frost cracking and frost creep (2015)
    Copernicus
    Details
    Publication Date: 2015-10-06
    Description: With accelerating climate cooling in the late Cenozoic, glacial and periglacial erosion became more widespread on the surface of the Earth. The resultant shift in erosion patterns significantly changed the large-scale morphology of many mountain ranges worldwide. Whereas the glacial fingerprint is easily distinguished by its characteristic fjords and U-shaped valleys, the periglacial fingerprint is more subtle but potentially prevails in some mid- to high-latitude landscapes. Previous models have advocated a frost-driven control on debris production at steep headwalls and glacial valley sides. Here we investigate the important role that periglacial processes also play in less steep parts of mountain landscapes. Understanding the influences of frost-driven processes in low-relief areas requires a focus on the consequences of an accreting soil mantle, which characterises such surfaces. We present a new model that quantifies two key physical processes: frost cracking and frost creep, as a function of both temperature and sediment thickness. Our results yield new insights into how climate and sediment transport properties combine to scale the intensity of periglacial processes. The thickness of the soil mantle strongly modulates the relation between climate and the intensity of mechanical weathering and sediment flux. Our results also point to an offset between the conditions that promote frost cracking and those that promote frost creep, indicating that a stable climate can provide optimal conditions for only one of those processes at a time. Finally, quantifying these relations also opens up the possibility of including periglacial processes in large-scale, long-term landscape evolution models, as demonstrated in a companion paper.
    Print ISSN: 2196-6311
    Electronic ISSN: 2196-632X
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 9
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    The periglacial engine of mountain erosion – Part 2: Modelling large-scale landscape evolution (2015)
    Copernicus
    Details
    Publication Date: 2015-04-22
    Description: An increasing number of studies point to a strong periglacial control on bedrock erosion in mountain landscapes. Periglacial processes have also been suggested to control the formation of block-fields on high-elevation, low-relief surfaces (summit flats) found in many alpine landscapes. However, to which degree periglacial processes took part in accelerating global erosion rates in response to Late Cenozoic cooling still remains as an unanswered question. In this study, we present a landscape evolution model that incorporates two periglacial processes; frost cracking and frost creep, which both depend on the mean annual temperature (MAT) and sediment thickness. The model experiments allow us to time-integrate the contribution of periglacial processes to mountain topography over million-year time scales. It is a robust result of our experiments that periglacial frost activity leads to the formation of smooth summit flats at elevations dominated by cold climatic conditions through time periods of millions of years. Furthermore, a simplistic scaling of temperatures to δ18O values through the late-Cenozoic indicates that many of the highest summit flats in mid- to high-latitude mountain ranges can have formed prior to the Quaternary. The model experiments also suggest that cooling in the Quaternary accelerated periglacial erosion by expanding the areas affected by periglacial erosion significantly. A computational experiment combining glacial and periglacial erosion furthermore suggests that landscape modifications associated with glacial activity may increase the long-term average efficiency of the frost-related processes.
    Electronic ISSN: 2196-6338
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 10
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    Basal shear stress under alpine glaciers: insights from experiments using the iSOSIA and Elmer/Ice models (2016)
    Copernicus
    Details
    Publication Date: 2016-02-02
    Description: Shear stress at the base of glaciers exerts a significant control on basal sliding and hence also glacial erosion in arctic and high-altitude areas. However, the inaccessible nature of glacial beds complicates empirical studies of basal shear stress, and little is therefore known of its spatial and temporal distribution. In this study we seek to improve our understanding of basal shear stress using a higher-order numerical ice model (iSOSIA). In order to test the validity of the higher-order model, we first compare the detailed distribution of basal shear stress in iSOSIA and in a three-dimensional full-Stokes model (Elmer/Ice). We find that iSOSIA and Elmer/Ice predict similar first-order stress and velocity patterns, and that differences are restricted to local variations at length scales of the order of the grid resolution. In addition, we find that subglacial shear stress is relatively uniform and insensitive to subtle changes in local topographic relief. Following the initial comparison studies, we use iSOSIA to investigate changes in basal shear stress as a result of landscape evolution by glacial erosion. The experiments with landscape evolution show that subglacial shear stress decreases as glacial erosion transforms preglacial V-shaped valleys into U-shaped troughs. These findings support the hypothesis that glacial erosion is most efficient in the early stages of glacial landscape development.
    Print ISSN: 2196-6311
    Electronic ISSN: 2196-632X
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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