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Evacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesised to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelised flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock-floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice-bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates., Earth Surface Processes and Landforms, 46 (11), ISSN:0197-9337, ISSN:1096-9837

Proglacial icings (also known as naled or aufeis) are frequently observed in the forefields of polar glaciers. Their formation has been ascribed to the refreezing of upwelling groundwater that has originated from subglacial melt, and thus the presence of icings has been used as evidence of polythermal glacier regime. We provide an updated analysis of icing occurrence in Svalbard and test the utility of icings as an indicator of thermal regime by comparing icing presence with: (1) mean glacier thickness, as a proxy for present thermal regime; and (2) evidence of past surge activity, which is an indicator of past thermal regime. A total of 279 icings were identified from TopoSvalbard imagery covering the period 2008-2012, of which 143 corresponded to icings identified by Bukowska-Jania and Szafraniec (2005) from aerial photographs from 1990. Only 46% of icings observed in 2008-2012 were found to occur at glaciers with thicknesses consistent with a polythermal regime, meaning a large proportion were associated with glaciers predicted to be of a cold or transitional thermal regime. As a result, icing presence alone may be an unsuitable indicator of glacier regime. We further found that, of the 279 glaciers with icings, 63% of cold-based glaciers and 64% of transitional glaciers were associated with evidence of surge activity. We therefore suggest that proglacial icing formation in Svalbard may reflect historical (rather than present) thermal regime, and that icings possibly originate from groundwater effusion from subglacial taliks that persist for decades following glacier thinning and associated regime change.

Surface energy-balance models are commonly used in conjunction with satellite thermal imagery to estimate supraglacial debris thickness. Removing the need for local meteorological data in the debris thickness estimation workflow could improve the versatility and spatiotemporal application of debris thickness estimation. We evaluate the use of regional reanalysis data to derive debris thickness for two mountain glaciers using a surface energy-balance model. Results forced using ERA-5 agree with AWS-derived estimates to within 0.01 ± 0.05 m for Miage Glacier, Italy, and 0.01 ± 0.02 m for Khumbu Glacier, Nepal. ERA-5 data were then used to estimate spatiotemporal changes in debris thickness over a ~20-year period for Miage Glacier, Khumbu Glacier and Haut Glacier d'Arolla, Switzerland. We observe significant increases in debris thickness at the terminus for Haut Glacier d'Arolla and at the margins of the expanding debris cover at all glaciers. While simulated debris thickness was underestimated compared to point measurements in areas of thick debris, our approach can reconstruct glacier-scale debris thickness distribution and its temporal evolution over multiple decades. We find significant changes in debris thickness over areas of thin debris, areas susceptible to high ablation rates, where current knowledge of debris evolution is limited.

Glaciers are highly effective agents of erosion that have profoundly shaped Earth’s surface, but there is uncertainty about how glacial erosion should be parameterised in landscape evolution models. Glacial erosion rate is usually modelled as a function of glacier sliding velocity, but the empirical basis for this relationship is weak. In turn, climate is assumed to control sliding velocity and hence erosion, but this too lacks empirical scrutiny. Here, we present statistically robust relationships between erosion rates, sliding velocities, and climate from a global compilation of 38 glaciers. We show that sliding is positively and significantly correlated with erosion, and derive a relationship for use in erosion models. Our dataset further demonstrates that the most rapid erosion is achieved at temperate glaciers with high mean annual precipitation, which serve to promote rapid sliding. Precipitation has received little attention in glacial erosion studies, but our data illustrate its importance., Glaciers have profoundly shaped Earth’s surface, but glacial erosion models lack a strong empirical basis. Cook et al. have compiled a dataset that illustrates how the speed at which glaciers move controls the rate at which they erode, and that climate is crucial in modulating glacier sliding speed and erosion rates.

Hypothesised lobe‐like flow of a temperate glacier in southeast Iceland, proposed from an analysis of ice surface crevassing patterns, is appraised from both empirical and theoretical perspectives. The hypothesis comprises the migration of individual lobes (or ‘pulses’) of ice through the glacier body, with central lobes migrating more rapidly along a narrow, central, ‘axial flow corridor’. Our alternative hypothesis is that crevasse patterns at this glacier instead reflect simple surface ice responses to stresses caused by flow over uneven bed topography. To substantiate our rejection of the lobe‐like, pulsed axial flow hypothesis, we provide: (a) evidence for a prominent transverse foliation that exhibits no evidence of shear of the required magnitude to support the hypothesis; and (b) an analysis of ice surface displacement, obtained by feature tracking, that shows a uniform flow field throughout the glacier tongue. We argue that caution needs to be exercised when interpreting glacier flow solely from crevasse patterns and observations of minor displacements along near‐surface fractures and other features.

Understanding the geomorphology left by waxing and waning of former glaciers and ice sheets during the late Quaternary has been the focus of much research. This has been hampered by the difficulty in dating such features. Luminescence has the potential to be applied to glacial sediments but requires signal resetting prior to burial in order to provide accurate ages. This paper explores the possibility that, rather than relying on light to reset the luminescence signal, glacial processes underneath ice might cause resetting. Experiments were conducted on a ring-shear machine set up to replicate subglacial conditions and simulate the shearing that can occur within subglacial sediments. Luminescence measurement at the single grain level indicates that a number (albeit small) of zero-dosed grains were produced and that these increased in abundance with distance travelled within the shearing zone. Observed changes in grain shape characteristics with increasing shear distance indicate the presence of localised high pressure grain-to-grain stresses caused by grain bridges. This appears to explain why some grains became zeroed whilst others retained their palaeodose. Based on the observed experimental trend, it is thought that localised grain stress is a viable luminescence resetting mechanism. As such relatively short shearing distances might be sufficient to reset a small proportion of the luminescence signal within subglacial sediments. Dating of previously avoided subglacial sediments may therefore be possible.

Continuity of sediment transfer through glacial systems is essential to maintain\ud subglacial bedrock erosion, yet transfer at temperate glaciers with overdeepened beds, where\ud subglacial fluvial sediment transport should be greatly limited by adverse slopes, remains poorly\ud understood. Complex multiple transfer processes in temperate overdeepened systems has been\ud indicated by the presence of large frontal moraine systems, supraglacial debris of mixed\ud transport origin, thick basal ice sequences, and englacial thrusts and eskers. At Svínafellsjökull,\ud thrusts comprising decimetre-thick debris-rich bands of stratified facies ice of basal origin, with\ud a coarser size distribution and higher clast content than that observed in basal ice layers,\ud contribute substantially to the transfer of subglacial material in the terminal zone. Entrainment\ud and transfer of material occurs by simple shear along the upper surface of bands and by straininduced\ud deformation of stratified and firnified glacier ice below. Thrust material includes\ud rounded and well-rounded clasts that are also striated, indicating that fluvial bedload is\ud deposited as subglacial channels approach the overdeepening and then entrained along thrusts.\ud Substantial transfer also occurs within basal ice, with facies type and debris content dependent\ud on the hydrological connectedness of the adverse slope. A process model of transfer at glaciers\ud with terminal overdeepenings is proposed, in which the geometry of the overdeepening\ud influences spatial patterns of ice deformation, hydrology, and basal ice formation. We conclude\ud that the significance of thrusting in maintaining sediment transfer continuity has likely been\ud overlooked by glacier sediment budgets and glacial landscape evolution studies.

HL was funded by a NERC PhD studentship (NE/I528050/1) at Queen Mary University of London, the Queen Mary Postgraduate Research Fund, and an Arctic Field Grant from the Research Council of Norway through the University Centre in Svalbard (UNIS). SL acknowledges funding from the Westfield Trust. JCY was funded by a Carlsberg Foundation grant (2007/01/0383). A Royal Geographical Society Peter Fleming Award and a grant from the National Geographic Committee for Research and Exploration to Andy Hodson helped fund some of the fieldwork. Some surge-type glaciers on the High-Arctic archipelago of Svalbard have large glacitectonic composite ridge systems at their terrestrial margins. These have formed by rapid glacier advance into proglacial sediments during the active surge phase, creating multicrested moraine complexes. Such complexes can be formed during single surge advances or multiple surges to successively less-extensive positions. The few existing studies of composite ridge systems have relied on detailed information on internal structure and sedimentology to reconstruct their formation and links to surge processes. However, natural exposures of internal structure are commonly unavailable, and the creation of artificial exposures is often problematic in fragile Arctic environments. To compensate for these issues, we investigate the potential for reconstructing composite ridge system formation based on geomorphological evidence alone, focusing on clear morphostratigraphic relationships between ridges within the moraine complex and relict meltwater channels/outwash fans. Based on mapping at the margins of Finsterwalderbreen (in Van Keulenfjorden) and Grønfjordbreen (in Grønfjorden), we show that relict meltwater channels that breach outer parts of the composite ridge systems are in most cases truncated upstream within the ridge complex by an inner pushed ridge or ridges at their ice-proximal extents. Our interpretation of this relationship is that the entire composite ridge system is unlikely to have formed during the same glacier advance but is instead the product of multiple advances to successively less-extensive positions, whereby younger ridges are emplaced on the ice-proximal side of older ridges. This indicates that the Finsterwalderbreen composite ridge system has been formed by multiple separate advances, consistent with the cyclicity of surges. Being able to identify the frequency and magnitude of former surges is important as it provides insight into the past behaviour of surge-type glaciers and, if absolute dating is possible, allows for the assessment of surge-type glacier response to climate change on decadal to centennial timescales. Although further investigations into the internal structure of these deposits should be sought where possible, our study demonstrates that geomorphology could be an invaluable tool for reconstructing the formation of composite ridge systems. Postprint

Bed topography is a critical boundary for the numerical\ud modelling of ice sheets and ice–ocean interactions.\ud A persistent issue with existing topography products for the\ud bed of the Greenland Ice Sheet and surrounding sea floor is\ud the poor representation of coastal bathymetry, especially in\ud regions of floating ice and near the grounding line. Sparse\ud data coverage, and the resultant coarse resolution at the ice–\ud ocean boundary, poses issues in our ability to model ice\ud flow advance and retreat from the present position. In addition,\ud as fjord bathymetry is known to exert strong control\ud on ocean circulation and ice–ocean forcing, the lack\ud of bed data leads to an inability to model these processes\ud adequately. Since the release of the last complete Greenland\ud bed topography–bathymetry product, new observational\ud bathymetry data have become available. These data can be\ud used to constrain bathymetry, but many fjords remain completely\ud unsampled and therefore poorly resolved. Here, as\ud part of the development of the next generation of Greenland\ud bed topography products, we present a new method for constraining\ud the bathymetry of fjord systems in regions where\ud data coverage is sparse. For these cases, we generate synthetic\ud fjord geometries using a method conditioned by surveys\ud of terrestrial glacial valleys as well as existing sinuous\ud feature interpolation schemes. Our approach enables the capture\ud of the general bathymetry profile of a fjord in north-west\ud Greenland close to Cape York, when compared to observational\ud data. We validate our synthetic approach by demonstrating\ud reduced overestimation of depths compared to past\ud attempts to constrain fjord bathymetry. We also present an\ud analysis of the spectral characteristics of fjord centrelines using\ud recently acquired bathymetric observations, demonstrating\ud how a stochastic model of fjord bathymetry could be parameterised\ud and used to create different realisations.

Closed topographic basins are found beneath contemporary ice masses and within the footprint of former ice masses in all glaciated regions. We present the first integrated review of subglacial basin occurrence and formation and the implications of such basins for glaciological processes and the evolution of landscape. Our purpose is to motivate research in areas where understanding of basin origin and process significance is weak. Basins on the order of 10–102 m deep and 102–103 m long are produced by glacial erosion of subglacial rock and/or sediment and are known as ‘overdeepenings’. Outlet and valley glaciers can ‘overdeepen’ their beds far below sea level or local fluviatile base level. Larger basins, typically in ice sheet contexts, may have a pre-glacial (usually tectonic) origin. Subglacial basins are important glaciologically because they require ice, water and sediment to ascend an adverse subglacial slope in order to exit the glacial system, the efficiency of which is dependent upon the gradient of the adverse slope and that of the ice surface. Basins thus influence subglacial drainage system morphology and transmissivity, the thickness and distribution of basal ice and sediment layers, and the mechanisms and dynamics of ice flow. Adverse gradients that exceed 11 times that of the ice surface may even permit the formation of subglacial lakes. We speculate that, in comparison to ice masses with few or no subglacial basins, those with numerous or very large basins may respond to climatic changes with unexpected vigour. In addition, erosion rates and transport pathways of water and sediment through the glacial system, and the expression of these processes in the sediment and landform record, may be unexpectedly complex. Further, our review shows that, in a warming climate, ice masses resting on adverse slopes will be vulnerable to rapid and potentially catastrophic retreat; new lakes in subglacial basins exposed by mountain glacier retreat will present an increasing hazard; and subglacial lakes may drain catastrophically. On even longer time scales, we speculate that the glacial excavation and post-glacial filling of basins in mountainous regions should contribute importantly to climate-related changes in isostasy and relief. Although the controls on overdeepening and their influence on other glacial and landscape processes remain uncertain, we hypothesise that overdeepened glacial systems reflect an equilibrium ice–bed geometry that maximises the efficiency of ice discharge. Improved understanding of overdeepening processes, especially overdeepened-bed hydrology, is therefore necessary to understand fully the dynamic behaviour of valley and outlet glaciers, and thus the fate of Earth's largest ice masses.

Published version at http://doi.org/10.1016/j.quascirev.2016.07.012. License CC BY-NC-ND 4.0. Glacier bed overdeepenings are ubiquitous in glacier systems and likely exert significant influence on ice dynamics, subglacial hydrology, and ice stability. Understanding of overdeepening formation and evolution has been hampered by an absence of quantitative empirical studies of their distribution and morphology, with process insights having been drawn largely from theoretical or numerical studies. To address this shortcoming, we first map the distribution of potential overdeepenings beneath the Antarctic and Greenland ice sheets using a GIS-based algorithm that identifies closed-contours in the bed topography and then describe and analyse the characteristics and metrics of a subset of overdeepenings that pass further quality control criteria. Overdeepenings are found to be widespread, but are particularly associated with areas of topographically laterally constrained ice flow, notably near the ice sheet margins where outlet systems follow deeply incised troughs. Overdeepenings also occur in regions of topographically unconstrained ice flow (for example, beneath the Siple Coast ice streams and on the Greenland continental shelf). Metrics indicate that overdeepening growth is generally allometric and that topographic confinement of ice flow in general enhances overdeepening depth. However, overdeepening depth is skewed towards shallow values – typically 200–300 m – indicating that the rate of deepening slows with overdeepening age. This is reflected in a decline in adverse slope steepness with increasing overdeepening planform size. Finally, overdeepening long-profiles are found to support headward quarrying as the primary factor in overdeepening development. These observations support proposed negative feedbacks related to hydrology and sediment transport that stabilise overdeepening growth through sedimentation on the adverse slope but permit continued overdeepening planform enlargement by processes of headward erosion.

Understanding the future evolution of Himalayan glaciers is important in terms of runoff that provides an essential water source to local populations and has far-reaching downstream impacts. However, the climatic response of glaciers in High-Mountain Asia is complicated by ice stagnation and considerable supraglacial debris coverage, which insulates the ice from warming. Typical runoff modelling only crudely incorporates debris cover and there is currently no consensus on how significantly this may impact future glacier and runoff evolution. Here, a glacio-hydrological model is modified to incorporate fully distributed debris cover, using melt reduction factors that vary depending on debris thickness, and to redistribute mass losses according to observed surface elevation changes. A range of debris thickness data are implemented, including a remote-sensing survey and a modelled debris surface, to analyse the sensitivity of glacier evolution and runoff to possible future debris-cover changes in a series of experiments in the upper Khumbu catchment, Nepal. Simulations are undertaken using climate input data from Regional Climate Model simulations from CORDEX (Coordinated Regional Downscaling Experiment) which are further statistically downscaled using data from the Pyramid meteorological station. Results suggest that the accurate calibration of the model to volume change compensates for the inclusion of distributed debris cover but only if the climatic sensitivity of the calibration period (1999–2010) and the nature of the debris-covered surface remain constant during future simulations. Altering the nature of the debris surface has a significant impact on simulated ice volume, with melt rates under debris suppressed by up to 85 %. The sensitivity of runoff ranges from 60 to 140 million m3 yr-1, although there are considerable uncertainties relating to non-glacial snow melt. Moreover, incorporating locally enhanced melt at ice cliffs into the model also impacts upon volume loss and discharge, with a greater proportion of ice cliffs leading to enhanced volume losses compared to a homogeneous debris surface. Finally, using the most representative model configuration, the future evolution of Khumbu Glacier under various climate scenarios shows continued mass losses with a reduction in volume ranging from 60 % to 97 % by 2100. Runoff trends show an initial increase followed by an eventual decrease, with runoff in 2100 predicted to be 8 % lower than current levels.

Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502-3885.2010.00196.x. ISSN 0300-9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample-group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence-based techniques.

This special issue presents a collection of papers that address a wide range of important challenges and exciting advances in the field of glacial landscape evolution. Primarily, these papers reflect persistent uncertainty that surrounds the mechanisms and timescales of glacial landscape evolution. For example, estimates of the duration of glacial occupancy required for the evolution of characteristic glacial valley forms from previously fluvial landscapes range from 100 kyrs for landscapes beneath large ice sheets (Jamieson et al.) to ~ 400–600 kyrs for glaciated alpine terrains (Brook et al.). Further, the mechanisms of glacial erosion are debated through analyses of the importance of ice thickness (Brocklehurst et al.; van der Beek and Bourbon), ice surface steepness (Vieira) and, in the case of large ice sheets, the co-evolution of ice sheet thermal regime, dynamics, and subglacial topography (Kleman et al.; Swift et al.). Debate concerning the potential climatic impacts of landscape evolution in alpine terrains is represented by van der Beek and Bourbon, who infer a significant increase in relief as a direct result of glacial erosion, and by Brocklehurst et al. and Heimsath and McGlynn, who demonstrate respectively that glacial relief production can be surprisingly modest and that rates of glacial erosion may be lower than those for fluvial incision. Further confirmation that valleys beneath large ice sheets evolve through selective linear erosion comes from studies that have combined geomorphological evidence with cosmogenic nuclide (Briner et al.) and apatite (U–Th)/He thermochronometry (Swift et al.), and the resulting style of landscape evolution is demonstrated by the antiquity of fjords in East Greenland (Swift et al.) and of deep erosion zones and thick drift covered zones in Fennoscandia (Kleman et al.), although the location of areal scouring zones may be subject to major alteration during single glacial events (Kleman et al.). Another set of papers shows that analyses of glacial lineation systems continue to provide important data on the dynamics of glacial landscape evolution, whether the lineations are formed underneath ice streams (Bradwell et al.; Andreassen et al.) or not (Jansson and Glasser), and whether they indicate intricate patterns of landscape modification (Andreassen et al.) or preservation (Jansson and Glasser). The final three papers address rarely-reported issues relating to landscapes of glacial deposition, including moraine degradation (Putkonen et al.), proglacial hydrogeology (Robinson et al.), and the evolution of hummocky-till topography (Clayton et al.).

Thick exposures of debris-rich ice at various Icelandic glaciers are central to the debate over the prevalence of glacial sediment transfer by glaciohydraulic supercooling. We present physical analyses of ice and debris at Kviarjokull, a temperate glacier in southeast Iceland with a terminal glacier-bed overdeepening, where stratified debris-rich ice forms up to metre-thick transverse englacial bands. Our results are not consistent with debris-rich ice formation predominantly by supercooling because: (1) 137Cs was absent from sediment filtered from debris-rich ice; (2) isotopic analysis (δD and δ18O) demonstrated no clear pattern of isotopic enrichment of debris-rich ice with respect to englacial ice; and (3) melt-out debris from debris-rich ice included large striated clasts from both fluvial and basal sources. We support transverse englacial debris-rich ice band formation by the thickening and elevation of basal materials in a region of longitudinally compressive ice flow situated between the reverse slope of the overdeepening and the base of an ice fall. Debris band form and distribution are likely to be controlled by thrusting along transverse englacial foliae associated with the formation of band ogives on the glacier surface.

Statistical classification of hydrograph form is used to elucidate the controls on diurnal runoff cycle evolution at Haut Glacier d'Arolla, Switzerland during the 1998 and 1999 melt seasons. Hydrographs are objectively grouped using statistical techniques into four principal types that are qualitatively interpreted as rising, falling, peaked-falling and peaked hydrographs. Peaked hydrographs are further grouped on the basis of the magnitude of their bulk flow, baseflow and diurnal flow components. Comparison with the evolution of meltwater sources and pathways demonstrates that runoff cycles evolve systematically during the melt season in response to removal of the seasonal snowpack from the ablation area. Peaked hydrographs predominate following the onset of snowpack removal and demonstrate an increasing and progressively earlier diurnal peak, but also an unusually low baseflow component that is probably due to surface melt mainly contributing direct to subglacial channels. Runoff cycle evolution has potentially significant geomorphic implications because peaked surface runoff cycles result in the formation of hydraulically efficient, channelised subglacial drainage and a significant increase in the gradient of the relationship between suspended sediment transport and discharge. Increasingly peaked diurnal cycles also result in increased basal sediment availability, most likely related to high diurnal water pressure variation within subglacial channels that may also have enhanced rates of basal sliding and hence subglacial erosion. Differences in runoff cycle form and evolution therefore have the potential to significantly influence glacial erosion rates and sediment yields.

Proglacial suspended sediment transport was monitored at Haut Glacier d'Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub-seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice-bed separation leading to extra-channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low-pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less-efficient drainage. Copyright © 2005 John Wiley & Sons, Ltd.

Previously reported low luminescence of basal glacial sediment has raised the possibility that processes operating at the ice–bedrock interface have the potential to reset (or ‘bleach’) natural luminescence signals (Swift et al., 2011). This finding indicates that certain types of glacial sediment (for example, sub-glacial diamicts) might be amenable to dating using luminescence-based techniques. Using a purpose-built ring-shear apparatus situated in a light-controlled environment, we have investigated the potential for mineral grains to be reset when subjected to conditions similar to those experienced by sediment that has undergone transport at the ice–bedrock interface. Reported here are the preliminary results of an initial experiment that used medium quartz sand with a naturally-acquired palaeodose of ∼4.3 Gy that had been obtained from a relict dune system. Incremental sampling during the shearing experiment and measurements were made to track changes in the luminescence properties of the sand as strain/shearing increased. The results indicate that increased strain/shearing resulted in an increase in the number of zero-dose grains and evolution of the De distribution from unimodal to multimodal. In light of the very much longer shearing distances that sub-glacial sediment would endure in nature, these results would appear to suggest that geomechanical processes at the ice–bed interface of glaciers and ice sheets may be a viable mechanism for resetting sediment.

The Antarctic Ice Sheet is underlain by numerous subglacial lakes, which comprise a significant and active component of its hydrological network. These lakes are widespread and occur at a range of scales under a variety of conditions. At present much glaciological research is concerned with the role of modern subglacial lake systems in Antarctica. Another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets. This is challenging, both conceptually, in identifying whether and where subglacial lakes may have formed, and also distinguishing the signature of former subglacial lakes in the geological record. In this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo-ice sheets. This includes a discussion of the formative mechanisms that could trigger onset of (or drain) subglacial lakes. These concepts provide a framework for discussing the probability that subglacial lakes exist(ed) beneath other (palaeo-)ice sheets. Indeed we conclude that the former mid-latitude ice sheets are likely to have hosted subglacial lakes, although the spatial distribution, frequency and type of lakes may have differed from today's ice sheets and between palaeo-ice sheets. Given this possibility, we propose diagnostic criteria for identifying palaeo-subglacial lakes in the geological record. These criteria are derived from contemporary observations, hydrological theory and process-analogues and provide an observational template for detailed field investigations.

Dispersed facies basal ice – massive (i.e. structureless) ice with dispersed debris aggregates – is present at the margins of many glaciers and, as a product of internal glacial processes, has the potential to provide important information about the mechanisms of glacier flow and the nature of the subglacial environment. The origin of dispersed facies is poorly understood, with several hypotheses having been advanced for its formation, and there is disagreement as to whether it is largely a sedimentary or a tectonic feature. We test these established hypotheses at the temperate glacier Svínafellsjökull, Iceland, and find that none fully account for dispersed facies characteristics at this location. Instead, dispersed facies physical, sedimentological and stable-isotope (δ18O, δD) characteristics favour a predominantly tectonic origin that we suggest comprises the regelation and strain-induced metamorphism of debris-rich basal ice that has been entrained into an englacial position by tectonic processes operating at the base of an icefall. Further thickening of the resultant dispersed facies may also occur tectonically as a result of ice flow against the reverse bed slope of a terminal overdeepening. Lack of efficient subglacial drainage in the region of the overdeepening may limit basal melting and thus favour basal ice preservation, including the preservation of dispersed facies. Despite the relatively low sediment content of dispersed facies ( ∼1.6% by volume), its thickness (up to 25 m) and ubiquity at Svínafellsjökull results in a significant contribution to annual sediment discharge (1635–3270 m3 a−1) that is ∼6.5 times that contributed by debris-rich stratified facies basal ice.

International audience; The long-term evolution of the East Greenland fjord region is investigated using geomorphological and stratigraphical evidence to: (1) assess the nature of landscape modification caused by late Cenozoic ice sheet glaciation; and (2) relate patterns of glacial landscape modification to first-order (i.e. continent-margin scale) topography and geology. Geomorphological and stratigraphical evidence demonstrates evolution of the first-order topography and incision of at least part of the present first-order fjord system by 55 Ma. This hypothesis is tested using apatite (U­Th)/He ages for samples from two bedrock profiles near Kong Oscar Fjord. The thermochronology supports landscape evolution before 55 Ma, followed by relative tectonic stability, because it indicates rapid denudation around the time of rifting that occurred prior to continental breakup (i.e. between 75 and 55 Ma). The nature of landscape modification caused by late Cenozoic glacial erosion appears to have been controlled by first-order topography and geology, with selective ice sheet erosion in areas of high-elevation Caledonian basement and apparently little glacial erosion of low-elevation Mesozoic sedimentary strata. Nevertheless, fjord morphometry demonstrates systematic evolution of the first-order fjord system from confined and overdeepened fjords in Caledonian basement to wider and disproportionately larger fjords in Mesozoic strata. The latter indicates that changes in lithological strength enabled the development of more efficient fjord morphology under full glacial conditions that may have promoted fast ice flow.

The evolution of Earth's largest hidden landscape beneath the East Antarctic Ice Sheet is poorly understood. Analyses of offshore sediments confirm that the ice incised deep troughs that host fast-flowing ice streams today, while older landscape features have been preserved.