Provenance of Suspended Sediment in Subglacial Drainage Systems

The importance of subglacial sediment transport by fluvial processes has long been recognised, such that suspended sediment loads in proglacial streams have commonly been used to calculate glacier erosion rates. However, glaciers of similar type, bedrock lithology and location demonstrate large variations in sediment yield that cannot be adequately explained by estimates of glacier erosional potential. It is likely that seasonal and annual variations in suspended sediment yield that have complicated estimates of subglacial erosion reflect changes in the efficiency of subglacial fluvial processes. Such processes are important because the efficiency of basal sediment evacuation likely plays a critical role in maintaining glacier erosion rates and may explain large differences in rates and styles of glacier erosion and ice marginal sedimentation. Previous studies have failed to reliably link variations in proglacial suspended sediment transport with subglacial processes because; 1) independent observations of the dynamic nature of the glacial and fluvioglacial systems have rarely been obtained; and 2) suspended sediment quality has been infrequently used to infer sediment provenance. In this study, variation in proglacial suspended sediment transport and quality is investigated at Haut Glacier d'Arolla, Switzerland during the 1998 and 1999 melt seasons. The study was supported by a NERC project that provided data on ice motion and catchment hydroclimatological conditions against which variation in suspended sediment concentration and quality could be rigorously interpreted. Suspended sediment transport from the western subglacial catchment during the 1998 melt season demonstrates changes in both sediment availability and the efficiency of suspended sediment evacuation that are controlled by the evolution of glacial meltwater sources and pathways. Strong relationships between discharge and suspended sediment concentration for hydrologically defined sub-periods of the melt season demonstrate the importance of flow capacity in controlling suspended sediment concentration. Early in the season, relationships demonstrate relatively high sediment availability at low discharges and an approximately linear relationship between suspended sediment concentration and discharge. Increased flow through the distributed system due to rising supraglacial meltwater inputs are associated with periods of rapid forward glacier motion followed by episodes of channelisation. However, changes in suspended sediment concentration are commensurate with increased discharge, and early season 'spring events' result in the evacuation of only a small proportion of the annual sediment load. Later in the season, non-linear relationships between discharge and suspended sediment concentration reflect the rapid increase in flow velocity with discharge in hydraulically efficient, channelised subglacial drainage systems (Alley et al., 1997). Sediment availability during development of the channelised system is initially limited due to the concentration of supraglacially-derived meltwaters into channels located along preferential drainage axes. However, sediment availability increases during the melt season as the rate of extension of the channelised system declines. Increasing overpressurisation of channels due to the increasing peakedness of supraglacial runoff likely increases sediment availability by: 1) extra channel flow excursions at peak discharges over wider areas of the glacier bed; 2) the enhanced deformation of high-pressure subglacial sediments towards low-pressure channels; or 3) the winnowing of fines from sediments near to channels as bulk discharge declines (Hubbard et al., 1995). The net effect is such that suspended sediment evacuation demonstrates a 125x increase for only a 7.5x increase in discharge between periods representative of flow through predominantly distributed and channelised subglacial drainage configurations. Suspended sediment particle size during the 1998 melt season demonstrates variation in the finer fractions that likely reflects spatial variation in the access of meltwater to subglacial sediments. Suspended and in situ sediment size distributions are characterised by two principal modes that likely represent rock particles (~ 500-2000 mum) and their constituent mineral grains (? 5-50 ?m). Hubbard et al. (1995) have suggested that finer distributions will occur in basal sediments distal to subglacial channels due to a winnowing effect associated with the diurnally reversing hydraulic gradient. The in-phase flushing of fines with discharge in the proglacial stream is observed early in the melt season when supraglacial meltwaters are known to contribute to a predominantly distributed subglacial drainage system. Fines also suggest changes in sediment availability to be associated with increased flow through 'new' areas of the distributed system as the spatial pattern of supraglacial runoff evolves

Origin and significance of 'dispersed facies' basal ice: Svínafellsjökull, Iceland

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 Svfnafellsjokull, Iceland, and find that none fully account for dispersed facies characteristics at this location. Instead, dispersed facies physical, sedimentological and stable-isotope (5180, 8D) characteristics favour a predominantly tectonic origin that we suggest comprises the regelation and straininduced 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 Svfnafellsjokull 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

Origin and significance of dispersed facies basal ice: Svínafellsjökull, Iceland

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

Mid- to Late-Quaternary evolution of the Wilderness Barrier dunes, South Africa

Barrier dunes represent potentially long-term, but complex, archives of coastal evolution. The examples occupying the Wilderness embayment, on the southern Cape coast of South Africa, form a regionally unique system of three shore-parallel barriers reaching up to 200 m in height and extending up to ~32 km alongshore. This research combines chronological and sediment provenance analyses to reconstruct the emplacement and evolution of the Wilderness barrier dunes through the Mid- to Late-Quaternary. Thirty-six new luminescence ages collected from ten sites across the three Wilderness barriers are presented, and are combined with a compilation of dates from the literature to produce a high-resolution chronology of barrier accumulation. The record spans at least the last two glacial-interglacial cycles, with notable phases between 245-217 ka, 155-143 ka, 128-121 ka, 91-86 ka and post-6 ka. Analysis of trace element geochemistry, heavy minerals, particle size, carbonate content and offshore topographic evidence all combine to indicate the provenance of the barrier sands has remained constant throughout their formation, and must involve marine transport pathways. The hypothesis that barrier accumulation at Wilderness during periods of low sea level was sustained by terrestrial aeolian activity is thus disproven, and evidence for a regional pre-MIS 5 marine transgression is provided. The terrestrially derived fraction of the barrier sands predominantly comprises quartzitic material derived from Table Mountain Group (TMG) rocks, most likely sourced from the Gouritz River ~75 km west of Wilderness. In addition to sediment from the TMG, the barrier sands also contain contributions of material derived from local geology, of material recycled from previous generations of aeolianite, and of authigenic marine sediment. The extensive coversand deposits inland of the Wilderness embayment, dated to >1.6 Ma using isothermal thermoluminescence, are demonstrated not to have made any significant input of sediment to the barriers. The Wilderness barriers record a complex history of erosion, as well as deposition through the Mid- to Late-Quaternary, and the preserved record clearly reflects the influence of local nearshore bathymetry on the rate of sea-level regression. The importance of previous generations of aeolianite in both fixing the position of subsequent depositional episodes, and protecting them from erosion, is also evident. The barriers exhibit similar behaviour to deposits on tectonically stable coastlines elsewhere, and contrast with the more complete and widely spaced barrier records present on uplifting coasts.EThOS - Electronic Theses Online ServiceUniversity of Sheffield StudentshipGBUnited Kingdo

Origin and significance of dispersed facies basal ice: Svínafellsjökull, Iceland

Dispersed facies basal ice (massive ice with dispersed debris aggregates) outcrops at the margins of many ice masses and is important to glaciologists because of the information it provides about the nature of subglacial conditions and processes in the deep interior of glaciers and ice sheets. There has been little agreement, however, about how it forms with possible mechanisms including regelation and water flow through the intercrystalline vein network, strain-induced metamorphism of firnified glacier ice, shearing of basal debris-rich ice, freeze-on of subglacial water, and incorporation of surface debris into glacier ice. We test these established hypotheses at the temperate glacier Svínafellsjökull, southeast Iceland, and show that none fully account for dispersed facies characteristics here. From analysis of physical, sedimentological and stable isotope ( 18O and D) characteristics we suggest that dispersed facies forms from a combination of regelation and strain-induced metamorphism of debris-laden ice originally entrained by tectonic processes at the base of an icefall. We suggest that a terminal overdeepening may serve to further thicken dispersed facies as the glacier flows against a prominent reverse bedslope. There may also be a lack of subglacial drainage across the overdeepening which further allows dispersed facies to survive in thicknesses up to 20m despite the temperate location. Our results demonstrate that, despite its low sediment content ( 1.6%), the thick layer of dispersed facies contributes a higher annual sediment flux than other more debris-rich basal ice types. Hence dispersed facies and the processes that create it should not be overlooked in assessments of glacial sediment budgets

Proglacial icings as indicators of glacier thermal regime : ice thickness changes and icing occurrence in Svalbard

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

Terminal zone glacial sediment transfer at a temperate overdeepened glacier system

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

Subglacial basins:Their origin and importance in glacial systems and landscapes

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