Provenance of Suspended Sediment in Subglacial Drainage Systems

Swift, Darrel A (2002) Provenance of Suspended Sediment in Subglacial Drainage Systems. PhD thesis, University of Glasgow.

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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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Peter Nienow
Keywords: Geomorphology, Hydrologic sciences
Date of Award: 2002
Depositing User: Enlighten Team
Unique ID: glathesis:2002-75766
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 18:14
Last Modified: 19 Nov 2019 18:14

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