Subglacial export of coarse sediment from temperate Alpine glaciers by meltwater

Proglacial measurements of subglacial sediment export by meltwater are commonly used to estimate glacial erosion rates. Such estimates generally assume that subglacial meltwater flow is the dominant agent driving export and that eroded sediment is rapidly conveyed through subglacial drainage networks to the glacier outlet in efficient channels with large and generally-unsatisfied transport capacities. However, understanding of sediment transport processes under glaciers is limited, especially in the thin, snout-marginal zones of retreating temperate Alpine glaciers.

A growing body of field and model-based research from such systems challenges the theory that eroded sediment is rapidly evacuated. Conversely, Alpine glaciers develop intense diurnal and seasonal discharge variation leading to highly variable sediment transport competence and the moderation of sediment export by associated cycles of alluviation. This has important consequences for the timescales over which sediment export can be used as a reliable proxy for glacial erosion, though the problem remains that very little is known about when and under what conditions a glacier margin is capable of evacuating the sediment supplied to it.

This study attempts to elucidate sediment transport mechanisms and timescales in the main snout-marginal subglacial channel of glacier d’Otemma, Switzerland, by tracking 324 cobble-sized particles tagged with 433 MHz active radio transponders over the mid-to-late 2021 melt season. Tagged particles were injected into the channel via a 48 m deep borehole and were then tracked over a 350 m subglacial reach and a 150 m proglacial reach with a mobile antenna and stationary antenna arrays.

Only 86 particles (27%) were exported in 2021. Preliminary analyses indicate that cobble-sized particles generally have extended residence times in snout marginal zones (weeks to months), with no clear effect of particle size, shape or density on overall transport velocities. Ongoing work involves the daily localisation of particles using a signal strength-based algorithm, providing a unique record of the down-glacier transport of coarse particles in a subglacial channel. Repeat measurements will follow in 2022.