2 research outputs found
An ice-sheet scale comparison of eskers with modelled subglacial drainage routes
Eskers record a time-integrated signature of channelised meltwater drainage during
deglaciation providing vital information on the nature and evolution of subglacial drainage. In this
paper, we compare the spatial pattern of eskers beneath the former Laurentide Ice Sheet with
subglacial drainage routes diagnosed at discrete time intervals from the results of a numerical icesheet
model. Perhaps surprisingly, we show that eskers predominantly occur in regions where
modelled subglacial water flow is low. Eskers and modelled subglacial drainage routes were found to
typically match for lengths <10 km, and most eskers show a better agreement with the routes close to
the ice margin just prior to deglaciation. This supports a time-transgressive esker pattern, with
formation in short (<10 km) segments of conduit close behind a retreating ice margin, and probably
associated with thin, stagnant or sluggish ice. Esker forming conduits were probably dominated by
supraglacially fed meltwater inputs. We also show that modelled subglacial drainage routes containing
the largest concentrations of meltwater show a close correlation with palaeo-ice stream locations. The
paucity of eskers along the terrestrial portion of these palaeo-ice streams and meltwater routes is
probably due to the prevalence of distributed drainage and the high erosion potential of fast-flowing
ice
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