Search

Modelling paleo-glacier networks in mountain ranges on the millennial timescales requires ice flow approximations. Hybrid models calculating ice flow by combining vertical shearing (shallow ice approximation) and longitudinal stretching (shallow shelf approximation) have been applied to model paleo-glacier networks on steep terrain, yet their validity has not yet been assessed quantitatively. Moreover, hybrid models consistently yield higher ice thicknesses than Last Glacial Maximum geomorphological reconstructions in the European Alps. Here, we compare results based on the hybrid Parallel Ice Sheet Model (PISM) and the Stokes model Elmer/Ice on the Rhine Glacier, a catchment of the former European Alpine Icefield. For PISM, we also test two magnitudes of flux limitation in a scheme that reduces shearing velocities. We find that the flux limitation typically used in PISM yields significantly reduced shearing speeds and increases ice thicknesses by up to 500 m, partly explaining previous overestimations. However, reducing the ice flux limitation allows the hybrid model to minimize this mismatch and captures sliding speeds, ice thicknesses, ice extent and basal temperatures in close agreement with those obtained with the Stokes model. [ABSTRACT FROM AUTHOR]

At the Last Glacial Maximum (LGM), the Rhine Glacier complex (Rhine and Linth glaciers) formed large piedmont lobes extending north into the Swiss and German Alpine forelands. Numerous overdeepened valleys there were formed by repeated glaciations. A characteristic of these overdeepened valleys is their location close to the LGM ice margin, away from the Alps. Numerical models of ice flow of the Rhine Glacier indicate a poor fit between the sliding distance, a proxy for glacial erosion, and the location of these overdeepenings. Calculations of the hydraulic potential based on the computed time-dependent ice surface elevations of the Rhine Glacier lobe obtained from a high-resolution thermo-mechanically coupled Stokes flow model are used to estimate the location of subglacial water drainage routes. Results indicate that the subglacial water discharge is high and focused along glacial valleys and overdeepenings when water pressure is equal to the ice overburden pressure. These conditions are necessary for subglacial water to remove basal sediments, expose fresh bedrock, and favor further erosion by quarrying and abrasion. Knowledge of the location of paleo-subglacial water drainage routes may be useful to understand patterns of subglacial erosion beneath paleo-ice masses that do not otherwise relate to the sliding of ice. Comparison of the erosion pattern from subglacial meltwater with those from quarrying and abrasion shows the importance of subglacial water flow in the formation of distal overdeepenings in the Swiss lowlands. Kurzfassung: Während des letzteiszeitlichen Maximums (LGM) kam es zur Bildung von grossen Vorlandloben des Rheingletschersystems (Rhein- und Linthgletscher), die sich nordwärts in das Schweizer und deutsche Alpenvorland erstreckten. Durch wiederholte Vergletscherungen wurden dort zahlreiche übertiefte Täler ausgeschürft. Ein Merkmal dieser übertieften Tälern ist deren Lage in der Nähe des LGM-Eisrands weit weg von den Alpen. Jedoch zeigen numerische Modellierungen des Eisfliessens des Rheingletschers eine schlechte Übereinstimmung der Gleitdistanz, Proxyindikator für glaziale Erosion, mit der Lage dieser Übertiefungen. Deshalb werden Berechnungen des hydraulischen Potenzials basierend auf zeitabhängigen Höhen der Eisoberfläche der Rheingletscherlobe, welche von einem hoch aufgelösten thermo-mechanisch gekoppelten Modell für Stoke'sches Fliessen resultieren, benutzt, um die Lage der Entwässerungsrouten unter dem Eis abzuschätzen. Die Resultate deuten darauf hin, dass der subglaziale Wasserabfluss gross ist und entlang glazialen Tälern und Übertiefungen geführt wird, wenn der Wasserdruck dem Eisüberlagerungsdruck entspricht. Dies sind notwendige Bedingungen, unter denen basale Sedimente wegtransportiert werden und frischer Fels freigelegt wird, um weitere glaziale Erosion zu begünstigen. Somit ist die Kenntnis der Lage von subglazialen Paläo-Entwässerungsrouten nützlich, um die Erosionsmuster unter Paläo-Gletschern zu verstehen, die nicht mit der Gleitbewegung des Eises in Verbindung gebracht werden können. Ein Vergleich der durch subglaziale Schmelzwässer erzeugten Erosionsmuster mit jenen, die durch direkte Gletschererosion entstanden sind, zeigt die Wichtigkeit der subglazialen Schmelzwasserflüsse für die Entstehung von Übertiefungen im alpenfernen Schweizer Vorland auf. [ABSTRACT FROM AUTHOR]

At the Last Glacial Maximum (LGM), the Rhine glacier in the Swiss Alps covered an area of about 16 000 km2. As part of an integrative study about the safety of repositories for radioactive waste under ice age conditions in Switzerland, we modeled the Rhine glacier using a thermodynamically coupled three-dimensional, transient Stokes flow and heat transport model down to a horizontal resolution of about 500 m. The accumulation and ablation gradients that roughly reproduced the geomorphic reconstructions of glacial extent and ice thickness suggested extremely cold (TJuly ~ 0°C at the glacier terminus) and dry (~ 10% to 20% of today's precipitation) climatic conditions. Forcing the numerical simulations with warmer and wetter conditions that better matched LGM climate proxy records yielded a glacier on average 500 to 700m thicker than geomorphic reconstructions. Mass balance gradients also controlled ice velocities, fluxes, and sliding speeds. These gradients, however, had only a small effect on basal conditions. All simulations indicated that basal ice reached the pressure melting point over much of the Rhine and Linth piedmont lobes, and also in the glacial valleys that fed these lobes. Only the outer margin of the lobes, bedrock highs beneath the lobes, and Alpine valleys at high elevations in the accumulation zone remained cold based. The Rhine glacier was thus polythermal. Sliding speed estimated with a linear sliding rule ranged from 20 to 100ma-1 in the lobes and 50 to 250ma-1 in Alpine valleys. Velocity ratios (sliding to surface speeds) were > 80% in lobes and ~ 60% in valleys. Basal shear stress was very low in the lobes (0.03-0.1MPa) and much higher in Alpine valleys (> 0:2MPa). In these valleys, viscous strain heating was a dominant source of heat, particularly when shear rates in the ice increased due to flow constrictions, confluences, or flow past large bedrock obstacles, contributing locally up to several watts per square meter but on average 0.03 to 0.2Wm-2. Basal friction acted as a heat source at the bed of about 0.02Wm-2, 4 to 6 times less than the geothermal heat flow which is locally high (up to 0.12Wm-2). In the lobes, despite low surface slopes and low basal shear stresses, sliding dictated main fluxes of ice, which closely followed bedrock topography: ice was channeled in between bedrock highs along troughs, some of which coincided with glacially eroded overdeepenings. These sliding conditions may have favored glacial erosion by abrasion and quarrying. Our results confirmed general earlier findings but provided more insights into the detailed flow and basal conditions of the Rhine glacier at the LGM. Our model results suggested that the trimline could have been buried by a significant thickness of cold ice. These findings have significant implications for interpreting trimlines in the Alps and for our understanding of ice-climate interactions. [ABSTRACT FROM AUTHOR]

Geomorphological evidence shows that during the last glacial period glaciers reached far intothe foreland of the Alps. How frequent such advances took place, how long the glaciersremained in advanced position and how fast they advanced or retreated, however, is difficultto reconstruct from geomorphology. Models of glacier flow are increasingly used to assess icedynamics during the last glacial period. Driving such models relies on a series ofestimates, including ice rheology, the forcing climate and glacier mass balance.Thereby, the latter strongly depends on climate and the equations used to describe theirconnection. Here, we focus on reconstructing surface melt of the Rhine glacier (∼16,000 km2) at thelast glacial maximum (LGM). Given large uncertainties in our knowledge of the LGMclimate, we base our estimates on (i) analogies from Arctic present-day climate and massbalance and on (ii) theoretical considerations. Thereby we investigate which are the keyclimatic characteristics that need to be specified to simulate LGM glacier melt. We find thatchanges in the amplitude of annual air temperature substantially influence simulatedablation-area mass balance gradients db∕dz, and thus glacier-wide mass balance. While thisinfluence has been shown in earlier research, it has not yet been considered in reconstructionsof glacier mass balance during the last glacial. Paleoclimatic evidence from Central Europe suggests that during the LGM annualamplitude of air temperature was larger than today. We show that computing LGM glaciermass balance while assuming no change in amplitude compared to present-dayclimate can result in an overestimation of db∕dz (∼0.4 m w.e. (100 m)−1 yr−1 vs.∼0.55 m w.e. (100 m)−1 yr−1). It remains to be explored how these differing db∕dz couldinfluence simulated glacier dynamics during the last glacial. [ABSTRACT FROM AUTHOR]

Ice flow modelling studies targeting the last glaciation of the European Alps have so far relied on present-day precipitation and temperature pattern and seasonality. These studies yield interesting insight to the dynamics of this former ice cap. However, the modelled maximum ice extent is inconsistent with geomorphological reconstructions based on terminal moraines. More precisely, the ice flow models systematically overestimate the ice extent in the north-east of the Alps and at the same time underestimate it in the south-west. Furthermore, the modelled flow directions are inconsistent with the depositional locations of erratic boulders with known origin, in particular those carried by the Valais Glacier. Recently, output of a global climate model run for Last Glacial Maximum (LGM, 20,000 years before present) conditions has been downscaled to a resolution of 2x2 km over the European Alps with a regional climate model. This offers, for the first, time the opportunity to drive an ice flow model with a high-resolution climate forcing that is truly representative for this period. Here we present some simulations of the Alpine Ice Sheet around the LGM utilizing this novel climate forcing. To model the ice flow and mass balance of the ice sheet, we use the Parallel Ice Sheet Model (PISM), which is a state-of-the-art ice sheet model. As only the climate at the LGM is available, we emulate a transient climate by applying a time-dependent temperature offset deduced from the EPICA ice core. We compare the modelled ice sheets with geological evidence such as terminal moraines and trajectories of erratic boulders. Preliminary results show that the new LGM climate forcing yields notable improvements between modelled and reconstructed ice extents. In particular, the ice distribution between south-west and north-east is significantly improved when compared to former simulations employing a climate forcing derived from present-day climate. [ABSTRACT FROM AUTHOR]