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The genesis of overdeepenings in proglacial environments has been investigated within both the Quaternary and pre-Quaternary glacial-geologic record. These landforms are important as they can control the behaviour of glaciers and ice sheets through their impact upon subglacial meltwater routing and the glacial hydrological regime. The formation of such landforms has not been studied in detail at contemporary glacier margins, constituting a major gap in our understanding. This research aims to identify the processes that are responsible for the formation of overdeepenings at the margin of Skeiðarárjökull, south east Iceland, which is comparable to the outlet glaciers of the Quaternary and older glacial-geologic record. Through detailed sedimentological and geomorphological analysis, this thesis tests existing models for the formation of overdeepenings, and identifies a number of new processes which had not previously been attributed to the formation of these landforms within contemporary glacial environments. In addition to the erosive role of meltwater, this work has identified that sandur aggradation, ice stagnation, ice fracturing, variations in meltwater routing, and the formation of subglacial bedforms such as eskers can all play a key role in the formation of overdeepenings at contemporary glacier margins. The evidence presented within this thesis, which demonstrates that overdeepenings are not exclusively formed as a result of erosion, and that preferential deposition of sediments can lead to their genesis, is a huge leap from our current understanding. It has found that overdeepenings can form as a result of processes associated with the interactions of individual landforms within the wider proglacial landsystem, and that the development of the system as a whole is more important than individual processes.

The supraglacial environment is an integral part of the glacial ecosystem. It is known to harbour an active microbial community that is concentrated in cryoconite holes, unique freshwater environments formed when solar-heated dark debris melts down into the glacier ice. Cryoconite holes are potentially important sources of organic carbon and nutrients to the glacial system. Field measurements and laboratory analyses and experiments were undertaken to explore the chemical environment, to determine the inorganic carbon uptake rate and the interactions between photosynthesis and water chemistry, and to examine the role ot phosphorus as the limiting nutrient for the cryoconite community from Werenskioldbreen, a well-researched Svalbard glacier.

Recent research at Arctic latitudes has demonstrated the close association between glacier hydrology and ice dynamics and a resultant sensitivity of high-latitude ice masses to climatic forcing. However, both hydraulic architecture and functionality remain poorly characterised or constrained in these glaciers. Therefore, in order to investigate the structure, functioning and dynamics of an Arctic glacier's hydrological system, this research presents an integrated study of Midtre Lovénbreen, Svalbard, using a variety of analytical techniques for comprehensive hydrometeorological data collected during field campaigns in 2004 and 2005.

Measurements from Austre Brøggerbreen (Svalbard, 2009) and the Greenland Ice Sheet (GrIS, near Kangerlussuaq, 2010) are used to examine microbially mediated supraglacial carbon fluxes and feedbacks between these fluxes and the abiotic conditions at the ice surface. Linear relationships between mass and area of cryoconite deposits indicate constant sediment layer thicknesses at a range of Arctic locations. This is suggested to result from a tendency for cryoconite to form a layer of single grains, with the thickness determined by grain diameter. A thermodynamic mechanism of single grain layer (SGL) maintenance is proposed, in which holes expand laterally to accommodate increased sediment volumes. This is shown to reduce ice surface albedo and promote photosynthesis because the greatest possible surface area for irradiance of cryoconite is maintained. Since cryoconite only contributes to supraglacial carbon fluxes while it resides upon ice surfaces, two major mechanisms of sediment evacuation are examined: melt-out and hydraulic removal. Energy balance modelling indicates that melt out is unlikely unless high air temperature and low incident radiation persist for multiple days. Stream migration is proposed to be the most likely mechanism of sediment removal; however for the majority of holes, multi-year residence times can be expected. This thesis also provides new estimates of microbially mediated carbon fluxes from the GrIS. New models estimate carbon fluxes from a section of GrIS for which spatially variable parameter values were derived from point-to-point interpolation of field data. An algal ecosystem is included for the first time. The results indicate that cryoconite can fix about four times more carbon than previously predicted, and surface algal ecosystems fix about eleven times more carbon than cryoconite. Biologically mediated carbon fluxes on the GrIS are therefore shown to be much higher than previously thought. Further, the GrIS is shown to be in a relatively stable state of net autotrophy.

This thesis investigates the role that superglacial snowpack hydrology plays in mediating meltwater delivery to glacier systems. The movement of water through glaciers is of fundamental importance as a control on proglacial hydrograph amplitude and timing, subglacial and proglacial geomorphic processes, the hydrochemistry of glacial runoff, and glacier dynamics, and as such has been the subject of considerable research effort. Although studies in non-glacial environments have shown that meltwater waves are both dampened and delayed by passage through snow, the role of supraglacial snowcover in mediating water inputs to the rest of the glacier system ahs received limited attention in studies of glacier hydrology to date. It has been suggested, however, that the varying thickness, and ultimately removal, of the superglacial snowpack may play a role in controlling the timing and magnitude of ice velocity events. Despite this suggested importance there have been few field observations of the hydrological behaviour of supraglacial snowpacks or of the way in which this behaviour evolves during the melt season. A thorough assessment of the linkages between supraglacial snowpack conditions and glacier dynamic events has therefore not been possible. This study helps fill this gap in our knowledge by explicitly investigating the hydrological behaviour of the supraglacial snowpack at an alpine glacier and its evolution during the summer melt season. Field data was collected during two summer field seasons (2003 and 2004) at Haut Glacier d’Arolla, Valais, Switzerland. Dye tracing experiments were used as the primary method of obtaining information about water flow through the snowpack. Dye was used both qualitatively, to give a visual impression of flow patterns through the snowpack, and quantitatively, with return curves detected by fluorometer providing detailed information about rates of dye movement and dispersion through the snowpack. Physically-based modelling representations of water flow through snow also informed consideration of the characteristics of snowpack runoff. Experiments were designed to determine: i) the nature of water flow through the supraglacial snowpack; ii) if, and in what way, this evolves over the course of the melt season; and iii) what factors control water movement, and the importance of their roles. In order that links between supraglacial snowpack hydrology and other parts of the glacier system could be considered, season-long records of glacier dynamics, proglacial meltwater discharge, and water quality parameters indicating subglacial conditions were also collected.

The influence of seasonal influx of supraglacial meltwater on basal water pressures and consequent changes in ice surface velocity has been a focus of research spanning over three decades. With a need to better include glacial hydrology within models of ice sheet evolution, the ability to predict where and when meltwater reaches the subglacial system is paramount for understanding the dynamics of large Arctic ice masses. The response of ice velocities to melt production suggests efficient transmission of meltwater from the supraglacial to subglacial hydrologic systems, and it has been shown that build-ups of stored meltwater in supraglacial lakes can force crevasse penetration through hundreds of metres of ice. This thesis presents a new modelling routine for prediction of moulin formation and delivery of meltwater to the ice-bed interface. Temporal and spatial patterns of moulin formation and drainage of supraglacial lakes are presented, and quantitative controls on crevasse propagation are investigated through a series of sensitivity tests. _J .' . The model is applied to two glacial catchments: the Croker Bay catchment of the Devon Ice Cap in High Arctic Canada; and the Leverett glacier catchment of the Greenland Ice Sheet. Through model application to these sites, sensitivities to crevasse surface dimensions, ice tensile strength, ice fracture toughness and air temperatures are investigated. Model predictions of moulin formation and melt transfer are compared with field observations and remotely sensed data, including ice surface velocities, proglacial discharge, dynamic flow regimes, and visible surface features. The inclusion of spatially distributed points of meltwater delivery to the 'subglacial system is imperative to fully understand the behaviour of the subglacial drainage system. Furthermore, dynamic response to future climatic change and melt scenarios, and the evolution of ice masses, cannot be fully understood without first understanding the glacial hydrologic processes driving many of these changes.