On the Granular Stress-Geometry Equation

Using discrete calculus, we derive the missing stress-geometry equation for rigid granular materials in two dimensions, in the mean-field approximation. We show that (i) the equation imposes that the voids cannot carry stress, (ii) stress transmission is generically elliptic and has a quantitative relation to anisotropic elasticity, and (iii) the packing fabric plays an essential role.Comment: 6 page

The role of subtemperate slip in thermally driven ice stream margin migration

The amount of ice discharged by an ice stream depends on its width, and the widths of unconfined ice streams such as the Siple Coast ice streams in West Antarctica have been observed to evolve on decadal to centennial timescales. Thermally driven widening of ice streams provides a mechanism for this observed variability through melting of the frozen beds of adjacent ice ridges. This widening is driven by the heat dissipation in the ice stream margin, where strain rates are high, and at the bed of the ice ridge, where subtemperate sliding is possible. The inflow of cold ice from the neighboring ice ridges impedes ice stream widening. Determining the migration rate of the margin requires resolving conductive and advective heat transfer processes on very small scales in the ice stream margin, and these processes cannot be resolved by large-scale ice sheet models. Here, we exploit the thermal boundary layer structure in the ice stream margin to investigate how the migration rate depends on these different processes. We derive a parameterization of the migration rate in terms of parameters that can be estimated from observations or large-scale model outputs, including the lateral shear stress in the ice stream margin, the ice thickness of the stream, the influx of ice from the ridge, and the bed temperature of the ice ridge. This parameterization will allow the incorporation of ice stream margin migration into large-scale ice sheet models

Drainage through Subglacial Water Sheets

Subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed. Here, we study the flow of water in thin, macroporous sheets between ice and bed. Previous work shows that small perturbations in depth of a nearly parallel-sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof. We argue that in the presence of bed protrusions bridging a water sheet, downward motion of the ice roof can stabilize this sheet. Stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation. We therefore modify existing theory to include protrusions that partially support the overlying glacier. Differences in the pressure on protrusions relative to water pressure drive roof closure. The mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the icebed gap. In order to account for multiple protrusion sizes along the bed (for instance, resulting from an assortment of various-sized sediment grains), we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet. Partitioning is dependent on the amount of ice protrusion contact and, therefore, water depth. This method allows prediction of roof closure rates. We then investigate stable, steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures, as is appropriate for ice streams. Moreover, we find that multiple steady sheet thicknesses exist, raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

Kapitalmarktwahrnehmung der integrierten nichtfinanziellen Berichterstattung in Deutschland

In meiner Dissertation habe ich die Kapitalmarktwahrnehmung der integrierten nicht-finanziellen Berichterstattung (IR) in Deutschland analysiert. Die durchgeführten Analysen leisten einen Beitrag zur aktuellen Forschung, indem sie einen Kontext mit hoher Berichtsqualität in Bezug auf nicht-finanzielle Aspekte berücksichtigen, den inkrementellen Vorteil gegenüber einer eigenständigen nicht-finanziellen Berichterstattung bestimmen und den Einfluss der einzelnen angewandten Rahmenwerke unterscheiden. Zur Überprüfung der Hypothesen über die Auswirkungen auf die Unternehmensbewertung durch den Kapitalmarkt wird ein modifiziertes Ohlson-Modell verwendet. Datengrundlage ist eine bereinigte Datenstichprobe von 811 Unternehmensjahresbeobachtungen von CDAX-Unternehmen im Untersuchungszeitraum von 2001 bis 2018. Die Ergebnisse zeigen, dass die Veröffentlichung eines IR in Deutschland zu einem signifikanten Anstieg der Unternehmensbewertung durch den Kapitalmarkt führt. Hinsichtlich der Rahmenwerke zeigt sich, dass die Anwendung des Deutschen Nachhaltigkeitskodex (DNK) und des Integrated Reporting Frameworks keinen signifikanten Effekt, der United Nations Global Compact einen schwach signifikanten negativen Effekt und die Standards der Global Reporting Initiative (GRI) einen schwach signifikanten positiven Effekt auf die Unternehmensbewertung in Deutschland haben. Weiterhin zeigt sich, dass die Veröffentlichung eines IR in nachhaltigkeitssensiblen Branchen einen signifikant höheren positiven Effekt auf die Unternehmensbewertung durch den Kapitalmarkt hat als für Unternehmen in nicht nachhaltigkeitssensiblen Branchen. IR ist somit geeignet, Informationsasymmetrien im Sinne der Principal-Agent-Theorie abzubauen, wenngleich nur einzelne Rahmenbedingungen eine effektive und effiziente Berichterstattung ermöglichen

Contemporary Glacier Processes and Global Change: Recent Observations from Kaskawulsh Glacier and the Donjek Range, St. Elias Mountains

With an extensive ice cover and rich display of glacier behaviour, the St. Elias Mountains continue to be an enviable natural laboratory for glaciological research. Recent work has been motivated in part by the magnitude and pace of observed glacier change in this area, which is so ice-rich that ice loss has a measurable impact on global sea level. Both detection and attribution of these changes, as well as investigations into fundamental glacier processes, have been central themes in projects initiated within the last decade and based at the Kluane Lake Research Station. The scientific objectives of these projects are (1) to quantify recent area and volume changes of Kaskawulsh Glacier and place them in historical perspective, (2) to investigate the regional variability of glacier response to climate and the modulating influence of ice dynamics, and (3) to characterize the hydromechanical controls on glacier sliding. A wide range of methods is being used, from ground-based manual measurements to space-based remote sensing. The observations to date show glaciers out of equilibrium, with significant ongoing changes to glacier area, volume, and dynamics. Computer models are being used to generalize these results, and to identify the processes most critical to our understanding of the coupled glacier-climate system.Grâce à leur importante couverture de glace et au riche étalage de comportement des glaciers, les monts St. Elias continuent de servir de laboratoire naturel enviable pour la recherche glaciologique. Des études récentes ont été motivées, en partie, par la magnitude et la vitesse des changements observés dans les glaciers de l’endroit, qui sont riches en glace au point que la perte de glace a une incidence mesurable sur le niveau général de la mer. La détection et l’attribution de ces changements de même que les recherches à l’égard des processus des glaciers ont servi de thème central à des projets qui ont été mis en oeuvre au cours de la dernière décennie à la station de recherche du lac Kluane. Les objectifs scientifiques de ces projets consistent (1) à quantifier les changements récents relativement à l’aire et au volume du glacier Kaskawulsh, puis à les mettre dans une perspective historique, (2) à faire enquête sur la variabilité générale de la réaction du glacier vis-à-vis du climat et de l’influence modulatrice de la dynamique de la glace, et (3) à caractériser le contrôle hydromécanique par rapport au glissement du glacier. Une vaste gamme de méthodes est employée pour parvenir à ces fins, allant des mesures manuelles sur le terrain à la télédétection spatiale. Jusqu’à maintenant, les observations indiquent que les glaciers ne sont pas en équilibre et que d’importants changements se produisent quant à l’aire, au volume et à la dynamique du glacier. Des modèles informatiques sont utilisés pour généraliser ces résultats ainsi que pour cerner les processus les plus critiques à notre compréhension du système couplé glacier-climat

Persistence and variability of ice-stream grounding lines on retrograde bed slopes

In many ice streams, basal resistance varies in space and time due to the dynamically evolving properties of subglacial till. These variations can cause internally generated oscillations in ice-stream flow. However, the potential for such variations in basal properties is not considered by conventional theories of grounding-line stability on retrograde bed slopes, which assume that bed properties are static in time. Using a flow-line model, we show how internally generated, transient variations in ice-stream state interact with retrograde bed slopes. In contrast to predictions from the theory of the marine ice-sheet instability, our simulated grounding line is able to persist and reverse direction of migration on a retrograde bed when undergoing oscillations in the grounding-line position. In turn, the presence of a retrograde bed may also suppress or reduce the amplitude of internal oscillations in ice-stream state. We explore the physical mechanisms responsible for these behaviors and discuss the implications for observed grounding-line migration in West Antarctica

Managing FAIR Tribological Data Using Kadi4Mat

The ever-increasing amount of data generated from experiments and simulations in engineering sciences is relying more and more on data science applications to generate new knowledge. Comprehensive metadata descriptions and a suitable research data infrastructure are essential prerequisites for these tasks. Experimental tribology, in particular, presents some unique challenges in this regard due to the interdisciplinary nature of the field and the lack of existing standards. In this work, we demonstrate the versatility of the open source research data infrastructure Kadi4Mat by managing and producing FAIR tribological data. As a showcase example, a tribological experiment is conducted by an experimental group with a focus on comprehensiveness. The result is a FAIR data package containing all produced data as well as machine- and user-readable metadata. The close collaboration between tribologists and software developers shows a practical bottom-up approach and how such infrastructures are an essential part of our FAIR digital future

Seismologically Observed Spatiotemporal Drainage Activity at Moulins

Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor