We use a physical model to investigate how changes in the distribution of subglacial hydrology affect ice motion of Antarctic ice streams. Ice streams are modeled using silicone polymer placed over a thin water layer to mimic ice flow dominated by basal sliding. Dynamic similarity between modeled and natural ice streams is achieved through direct comparison of the model force balance and the observed force balance of Whillans Ice Stream (WIS). The WIS force balance has evolved over time due to increased basal resistance. We test two hypotheses: 1) the subglacial water distribution influences the ice flow speed and thus the force balance and; 2) shear margins are locations where transitions in water layer thickness occur. The velocity and force balance are sensitive to pulsed water discharge events and changes in lubrication associated with sticky spots, and model shear margins tend to overlie water lubrication boundaries. Local changes in basal lubrication near margins (possibly as a result of the presence of sticky spots or subglacial lakes) influences the stability of the margin position and may be responsible for large and rapid shifts in margin location. / text

No description available.

The presence and configuration of subglacial water and sediment maintain the fast flow of arterial ice streams in Antarctica and airborne ice-penetrating radar data represent a potential resource of information about the ice-bed interface. In this thesis an original contribution to the exploration of Antarctic subglacial environments is made through the analysis of airborne surveys from Evans, Institute and Möller Ice Streams, West Antarctica. The primary approach employed is the derivation of bed-returned power (BRP), a proxy for ice-bed reflectivity, which is strongly influenced by the presence of liquid water. Estimating radar englacial attenuation (EA) accurately is a critical part of BRP analysis and a modelled approach is primarily used. BRP is derived across Evans Ice Stream and shows large-scale patterns relating to hypothesised hydrological and geological contrasts at the ice-bed. These results are developed to investigate the influence of: (1) adopted EA correction; (2) the influence of assigned ice dielectric properties in modelled EA; (3) subglacial roughness and (4) the spatial scale over which BRP is derived. Some areas of high basal drag can be detected with BRP analysis, indicating that variations in subglacial hydrology are responsible for their existence. The widely-used empirical method of estimating EA by relating ice thickness to uncorrected BRP is shown to be unreliable where ice properties change along a transect. Monte Carlo error analysis of modelled EA shows that poorly constrained ice dielectric properties also result in significant BRP uncertainty. BRP beneath Institute and Möller Ice Streams is derived on catchment- and local-scales over hypothesised subglacial features. Bungenstock Ice Rise is marked with a clear BRP signal but the locations of "active" lakes, as delineated by satellite altimetry, do not. The sensitivity of idealised flow paths to surface change and grid size are investigated. Potential future research directions regarding BRP analysis are discussed.

Indiana University-Purdue University Indianapolis (IUPUI) / Tills from major ice streams (Institute, Foundation, Academy, Recovery, and Slessor) of the Weddell Sea Embayment contain detrital zircons with distinct U-Pb age populations that can be used as a provenance tool to better understand ice stream dynamics. U-Pb ages of detrital zircons were measured in 21 samples of onshore till, erratics, and bedrock of potential source rocks, and 12 samples of offshore till. Grains were analyzed by LA-ICPMS at the University of Arizona (n=5447). Relative probability U-Pb age density plots of till in moraines along the Institute Ice Stream have dominant Grenville (1070 Ma) and secondary Ross/Pan-African peaks (560 Ma, 630 Ma). The Foundation and Academy show prominent Ross/Pan-African peaks (500-530 Ma and 615-650 Ma). The Recovery transports zircons with prominent 530 Ma and 635 Ma peaks along the southern margin, and 1610 and 1770 Ma along the northern margin. The Slessor carries zircons with prominent populations at 1710 Ma and secondary 2260-2420 Ma. U-Pb ages in zircons from offshore till samples show a general trend of fewer Mesozoic ages from west to east. The western most core, PS 1423, has dominant Jurassic populations while cores 1197 and 1278 have a high proportion of early Ross/Pan-African ages relative to Grenville ages. The similar zircon age distributions between PS 1278 and the Foundation Ice Stream tills suggest that the Foundation switched to an easterly flow path around Berkner Island (BI) at some point during the LGM. In the eastern Weddell Sea (PS 1400), there was a near absence of Proterozoic zircon age populations carried by the Slessor and northern side of the Recovery. Another unexpected find was a lack of Grenville ages in PS 1423 relative to the Institute tills. The U-Pb data in this study provides a basis for two possible LGM ice flow reconstructions. In the first, the Institute flowed west around the unnamed isolated bedrock highs, deposited tills between PS 1423 and PS 1197, providing a westerly flow path around BI for the Foundation. In the second, the Institute flows over the subglacial topography and deposited till closer to PS 1197, forcing the Foundation east around BI.

Ice sheets are among the key controls on global climate and sea level. A detailed understanding of their dynamics is crucial to make accurate predictions of their future mass balance. Ice streams are the dominant negative component in this balance, accounting for up to 90% of the Antarctic ice flux into ice shelves and ultimately into the sea. Despite their importance, our understanding of ice-stream dynamics is far from complete. A range of observations associate ice streams with meltwater. Meltwater lubricates the ice at its bed, allowing it to slide with less internal deformation. It is believed that ice streams may appear due to a localisation feedback between ice flow, basal melting and water pressure in the underlying sediments. This thesis aims to address the instability of ice-stream formation by considering potential feedbacks between the basal boundary and ice flow. Chapter 2 considers ice-flow models, formulating a model that is capable of capturing the leading-order dynamics of both a slow-moving ice sheet and rapidly flowing ice streams. Chapter 3 investigates the consequences of applying different phenomenological sliding laws as the basal boundary condition in this ice-flow model. Chapter 4 presents a model of subglacial water flow below ice sheets, and particularly below ice streams. This provides a more physical representation of processes occurring at the bed. Chapter 5 then investigates the coupled behaviour of the water with the sediment, and Chapter 6 the coupled behaviour of the water with the ice flow. Under some conditions this coupled system gives rise to ice streams due to instability of the internal dynamics.

This thesis is focused on studies of glacial and slope morphology in the high Arctic of western Greenland shelf and the Molloy Hole seafloor spreading area, based on high-resolution acoustic methods and other geophysical data. The main purpose is to improve our understanding of glacial dynamics and associated processes in the marginal region of a large marine-terminating ice sheet. Newly acquired data, together with existing datasets have been compiled to create bathymetric models, which were used to study the seafloor landscape and its preserved record of glacial and sedimentary processes. The new bathymetric models were used with novel processing tools combined with seismic profiles, sub-bottom profiles and overlays of geological- and gravimetric maps to describe the observed landforms and interpret causal relationships. The main conclusions are: 1)   The underlying geology is an important control on the cross-shelf trough (CST) dimensions in western Greenland. This is likely due to the influence of underlying geology to the frictional resistance of the ice flow over the basement rock. Our observations show that ice streaming in areas with basaltic bed-types cause minimal over-deepening of the main trunk of the trough, which also has weaker lateral boundaries allowing the ice stream to shift flow direction more easily. CSTs on the Cenozoic-Mesozoic sedimentary basins indicate a stronger eroding and more focused paleo-ice streams. 2)   Bedrock lithology has an important part in controlling the location of the head-to-trough transition in CSTs of western Greenland. The areas where the head’s network of channels converges to form the main trunk of the trough are mostly located on the boundary from crystalline to sedimentary bedrock. These areas are also marked by distinct over-deepenings. 3)   Preglacial conditions such as faults/fractures and lithological properties of the basement rocks in western Greenland served as an important control on the erosional potential of the glacial processes, particularly on a local scale. Faults and fractures have led to the topographic steering of the ice flow that causes further excavation and erosion of the bed, while uneven erosion patterns, based on differences in glacial morphological features, is observed between areas of adjacent bedrocks with different lithology. 4)   The occurrence of trough mouth fans is suggested to be controlled mainly by the shelf width, which governs the glacial flow length along available sediment sources. It is also controlled by the continental slope steepness, which may be too steep for sediment fans to accumulate, or may cause slope failure which eventually transports the sediments to the deep basin. 5)   The maximum ice extent in west Greenland extended towards the shelf edge. Geomorphological evidence of ice margin standstills and slow retreat (grounding zone wedges and transverse moraines) in some areas reveal a multi-stage deglaciation process. 6)   The view of a highly dynamic paleo-Greenland ice sheet is supported by the presence of a large number of CSTs which hosted ice streams, and evidence of ice stream flow-switching throughout one or several glaciations. 7)   The influence of glacial sedimentary processes extends into the deepest areas of the Arctic Ocean. A submarine landslide, here termed the Molloy Slide, has been described in the Molloy Hole in the Davis Strait between Greenland and Svalbard. This slide was likely caused by massive glacial sediment deposition along the west Svalbard margin. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 2: Manuscript.</p>

Subglacial water plays an important role in the regulation of an ice sheet's mass balance. It may be the dominant control on the velocities of ice streams and outlet glaciers on scales of months to millennia. Recent satellite observations of ice surface elevation changes have given researchers new insights into how subglacial water is stored and transported. Localized uplift and settling of the ice surface implies that lakes exist beneath the ice sheet that are being filled and drained on relatively short time scales. %At the base of an ice sheet water can be transported through a variety of drainage networks or stored in subglacial lakes. Here, a numerical investigation of the mechanisms of transport and storage of subglacial water and the associated time scales is presented. Experiments are carried out using a finite element model of coupled ice and water flow. The first experiment seeks to understand the relationship between the depth of a basal depression and the area over which the feature affects basal water flow. It is found that as the perturbation to a topographic depression's depth is increased, water is rerouted in response to the perturbation. Additionally it is found that the relationship between perturbation depth and the extent upstream to which its effects reach is nonlinear. The second experiment examines how the aspect ratio of bed features (prolate, oblate, or equidimensional) influences basal water flow. It is found that the systems that develop and their interactions are mediated by both the topography and the feedbacks taken into account by the coupling of the systems in the model. Features oriented parallel to ice and water flow are associated with distributed fan systems that develop branches which migrate laterally across the domain and interact with one another on monthly and yearly timescales. Laterally oriented features develop laterally extensive ponds. As the ratio of longitudinal to lateral dimension of the topography is increased, a combination of these two water distributions is seen.

Indiana University-Purdue University Indianapolis (IUPUI) / In January 2007, 26 samples of till from 6 different moraines along the Reedy Glacier, East Antarctica were collected with the goal of differentiating between these samples and till collected from the base of the Whillans, Kamb, and Bindschadler Ice Streams of West Antarctica. The ability to differentiate between East and West Antarctic ice will allow us to constrain ice flow into the central Ross Sea during the Last Glacial Maximum (LGM), which has implications for more accurate reconstructions of the Ross Ice Sheet and its behavior. Moraines sampled from the head of Reedy Glacier give insight to the geology beneath the EAIS, and may be representative of what the glacier is eroding from its bed. Samples along the trunk of the glacier capture representative rock types eroded along the length of Reedy Glacier. At each moraine 3 replicate sub-sites were selected for collection to represent the diversity of material within each moraine. Comparisons are based on the composition of pebbles, particle size distributions, and sand petrography. Analysis of the pebble fraction shows that each sub-site contains similar rock types, however, the concentration of each rock type varies as much as 25-35%. Similar variation is also seen within the sub-site sand fraction. Both the pebble and sand fraction reflect the mapped bedrock geology. The dominant pebble types are coarse-grained felsic and intermediate igneous rocks, as well as quartzite. Similarly felsic igneous grains, quartzite, quartz, and feldspar characterize the sand fraction. Particle size analysis shows that v Reedy Glacier till averages 85% sand. The subglacial West Antarctic samples contain approximately 30% sand, and equal amounts of silt and clay, approximately 35% each. An observation of the sand fraction from beneath the West Antarctic Ice Streams shows composition similar to tills from Reedy Glacier. However, tills from the base of the West Antarctic Ice Streams contain up to 75% polymict grains, and in contrast, these grains are absent in the tills from Reedy Glacier. These sand-sized polymict grains dominate material from the base of Whillans and Bindschadler Ice Streams, whereas material from the base of Kamb Ice Stream contains grains of felsic igneous, quartz, feldspar, and few to no polymict grains. In addition to the polymict grains, the sand fraction in the ice stream cores contains trace fragments of sedimentary, and volcanic rocks, both of which are absent from the Reedy Glacier sand fraction. However, polymict grains are believed to represent a process occurring beneath the ice sheet, rather than indicate provenance. It is difficult to differentiate between the two tills, as both contain high concentrations of felsic-intermediate igneous lithics, quartz, and feldspar. The central Ross Sea contains sediment similar in rock type and mineralogy as seen within sediments from both Reedy Glacier, and the base of the ice streams of West Antarctica.

La modélisation des calottes polaires est importante pour reconstruire l'état passé des calottes, comprendre l'état présent, et prévoir son évolution dans le contexte du réchauffement climatique et de l'élévation du niveau des mers. Les mécanismes qui interviennent dans la dynamique des calottes de glace et qui dépendent du climat sont nombreux, mais pour l'Antarctique il y a deux mécanismes très importants qui s'opposent : L'augmentation de la température qui est supposée entraîner une augmentation de la précipitation et un épaississement de la calotte, et l'intensification de l'écoulement de la glace qui tend à amincir la calotte. Pour étudier ces deux mécanismes, nous avons suivi deux approches : caractériser la calotte à partir des observations directes (c.-à-d. topographie de la surface et les vitesses d'écoulement de glace) ou indirectes (c.-à-d. Flux de bilan). Et la modéliser avec GRISLI (GRenoble Ice Shelf and Land Ice), en prenant en considération la dynamique des fleuves de glace et leurs localisations précises, pour mieux comprendre les mécanismes actifs qui interviennent dans la calotte. Le sujet de la thèse est l'amélioration de la modélisation de la calotte Antarctique à partir des données disponibles. Notamment celles basées sur la première et la deuxième dérivées de la surface (pente et courbures respectivement) pour faire des liens avec le drainage de la glace, et les structures de vitesses de bilan. Ces informations nous ont permis entre autres de développer différentes méthodes pour autoriser les fleuves de glace, qui ensuite ont été introduites dans GRISLI. Nous avons ensuite fait plusieurs études de sensibilité de la calotte sur les localisations des fleuves de glace, les données du flux géothermique et des paramètres qui contrôlent le glissement et la déformation de la glace. Enfin nous avons fait des comparaisons entre les structures observées et modélisées de la calotte, et nous avons vu que le modèle n'est pas loin de reproduire les structures observées. / Modelling of the polar ice sheets is important to reconstruct its past, understand its current state and predict its evolution in the context of the global warming and rising sea levels. There are numerous mechanisms involved in the dynamics of ice sheets and these are climate-dependent. In particular there are two very important opposing mechanisms: the increase in the temperature which is supposed to lead to increased precipitation and thickening of the ice, and increased melting of the ice, which tends to reduce the mass of the ice sheets. To study these two mechanisms, we followed two approaches: characterize the ice sheets from direct observations (i.e., surface topography and ice flow velocities) or indirect observations (i.e., flow balance), and model it with GRISLI (Grenoble Ice Shelf and Land Ice), taking into account the dynamics of ice streams and their precise locations, to better understand the active mechanisms involved in the ice sheets. The subject of this thesis is to improve the modelling of ice sheets from the available data, especially those based on the first and second derivatives of the surface (slope and curvature respectively), and to link with the ice drainage, and velocity structure outcome. This data has allowed us to develop methods to model ice flows, subsequently introduce them into the GRISLI. We then made several sensitivity studies of the ice sheets, to localize ice streams, geothermal data flow and the parameters that control the slip and deformation of the ice. Finally, we compare the observed and modelled structures of the ice sheets, and show that model results are not far from the actual observed structures.

Les vallées tunnels sont les structures de drainage d'eau de fonte les plus imposantes de l'environnement sous-glaciaire. L'inaccessibilité des milieux sous-glaciaires actuels limitent nos connaissances des mécanismes impliqués dans leurs formations, des paramètres contrôlant leur morphologie et de leurs influences sur la dynamique glaciaire. Ce travail présente une nouvelle approche expérimentale visant à mieux contraindre la formation et le fonctionnement des systèmes de vallées tunnels. Cette approche repose sur le développement d'un nouveau dispositif expérimental simulant la circulation d'eau pressurisée au sein d'un substrat poreux et perméable sous une couverture visqueuse. Les résultats des expériences menées avec ce dispositif ont permis de déterminer des relations étroites entre les paramètres du substrat et les modalités de l'écoulement d'eau sur la formation et la morphologie des vallées tunnels. Les résultats issus des expériences démontrent que ce dispositif permet de recréer des systèmes de vallées tunnels. L'étude de ces vallées expérimentales suggèrent que la topographie du substrat et la production d'eau de fonte joue un rôle primordial sur la genèse des vallées tunnels et sur leurs morphologies. Deux morphotypes de vallées tunnels ont pu être identifiés avec des morphologies et des mécanismes de formation indépendants. L'analyse de la dynamique de la calotte sus-jacente a permis de mettre en évidence un lien étroit entre le développement des vallées tunnels et la dynamique des "ice streams". L'évolution de la capacité de drainage des vallées tunnels semble contrôler la dynamique glaciaire en régulant le flux de glace transitant dans les "ice streams". / Tunnel valleys are major components of the subglacial meltwater drainage system. The inaccessibility of modern subglacial environments reduces our knowledge on the mechanisms involved in tunnel valleys formation, the parameters controlling their morphology and their influence on ice-sheet dynamics. This work presents a new experimental approach aiming to better assess the processes of tunnel valleys development. This approach relies on the development of a new experimental device simulating a pressurized water flow within a porous and permeable substratum underneath a viscous layer simulating the ice-sheet. The main results of the experiments conducted with this device have demonstrated the influence of both substratum properties and meltwater drainage on tunnel valleys formation and morphology. Using the device, we first manage to reproduce tunnel valley systems experimentally. Analyses conducted on these valleys experimental valleys suggest that the substratum topography and meltwater production play a key role on tunnel valleys genesis and morphology. Two tunnel valleys morphotypes have been identified, each one being charaterized by a unique morphology and mechanism of formation. Monitoring of the experimental ice sheet during tunnel valley formation shows close relationship between tunnel valleys development and "ice streams" dynamics. The evolution of tunnel valley drainage capacity seems to have a strong influence on ice sheet stability by regulating ice flux within "ice stream corridors".