Low bedload mobility in an Alpine subglacial channel

The rate at which Alpine glaciers erode their bedrock and evacuate eroded sediment to downstream environments is a fundamental control on landscape evolution, ecosystem dynamics and water quality. However, directly measuring subglacial erosion and sediment evacuation processes is extremely difficult since they occur under tens to hundreds of meters of ice. In general, Alpine glaciers are thought to be efficient at evacuating subglacially eroded sediment via meltwater flow in subglacial channels with very high transport capacities. However, recent field observations and hydrological modelling work suggests that coarse sediment transport may be inhibited near glacier termini in the presence of unpressurised subglacial channels. Accumulation of coarse sediment may substantially affect subglacial erosion and hydrology. Extended lags between coarse sediment production and eventual evacuation may also limit the timescales over which sediment export measurements in proglacial rivers can be used to estimate glacial erosion rates. Here, we apply a recently developed method for tracking radio-tagged particles as they are transported through subglacial channels to assess coarse sediment mobility. We deployed 324 pebbles and cobbles tagged with active RFID tags directly into an unpressurised, ice-marginal subglacial channel at the Otemma Glacier, southwestern Switzerland and tracked their motion from the glacier surface using a system of roving and stationary antennas (350 m channel reach). We report very low movement and prolonged storage of coarse subglacial sediment. Particle motion remained low despite exceptionally high meltwater discharge during the mid to late melt season. Only 16% of particles were transported out of the glacier, indicating significant inter-annual storage of coarse sediment. These results reveal that some glaciers do not efficiently evacuate coarse sediment, suggesting that sediment export records from proglacial rivers may be better suited for inferring erosion rates over extended timescales (decadal or longer) rather than shorter, typical observation periods.