Glacial Sediment Stores and Their Reworking

Abstract

In the light of heightened geomorphological activity associated with progressive deglaciation in alpine regions, the storage of sediments within and flux of sediments through the proglacial zone represents an increasingly important area for contemporary geomorphological and sedimentological study. The term ‘paraglacial’ is used to describe this heightened geomorphological activity, and the general model of paraglacial basin sediment yield is one of initial increase as deglaciation commences, followed by progressive decline during and after deglaciation. Against the backdrop of this paraglacial model, in this chapter we consider storage, release and reworking of sediments from lateral and forefield slopes in the proglacial zone. The propensity of lateral slope units to stand stable at steep angles might indicate that such units represent a long-term store of sediments. However, their genetic complexity and geomorphological evidence of sediment reworking, in the form of deep gullies and associated debris cones and fans, would suggest that such units can yield sediments dependant on multiple climatic, geomorphical and biogeomorphical factors. Similarly, the extent to which the generally lower-angled forefield slopes act as a source or sink of sediments is subject to conjecture. Although the forefield is regarded as the most dynamic component of the alpine sediment flux system, largely due to the efficacy of fluvial action, the great diversity of forms and associated genetic complexity, together with the operation of time-variant processes, will likely add great variability to long-term patterns of basin sediment yield. In addition to fluvial activity and associated sediment mobilisation and redistribution, processes such as permafrost degradation, melt of forefield buried ice and associated slumping and debris flowage offer additional sediment release mechanisms that may punctuate the often-assumed uni-directional decline in sediment yield associated with progressive deglaciation. As meltwater discharge declines, after the so-called ‘deglaciation discharge dividend’ peaks, progressive eluviation of fines and consequent armouring of previously abundant sediment supply areas will likely lead to overall declining sediment yields with time, enhanced by progressive vegetation colonisation. However, this net decline will inevitably be punctuated by stochastic geomorphological events. While uncertainty therefore exists concerning the detailed timescales of sediment release associated with deglaciation, contemporary progressive deglaciation offers an unparalleled opportunity to directly observe the genesis of deglaciation landforms, their modification and associated sediment fluxes and fluctuations in basin-scale sediment storage and release.