The influence of overdeepenings on sediment flux from an Alpine glacier

Overdeepenings, closed topographic depressions in glacier beds with reversed slopes at their down valley end, could play a key role in controlling the connection of glacially eroded sediment to a glacier's portal and hence subglacial sediment export. Here we investigate this role using suspended-sediment and bedload provenance analyses of samples collected in the Gornergletscher catchment, Switzerland, combined with long-term records of sediment export and meltwater discharge measured at the catchment outlet downstream of the glacier. Subglacial provenance mapping reveals that fine sediments are primarily sourced from an overdeepening under the glacier's main trunk and deglaciated tributaries, but coarser sediments are dominated by well-connected lithological units connected to the portal downstream of the overdeepening. Suspended sediment export is associated with spring-dominated peaks and episodic storm-driven events, consistent with conceptual models of distributed subglacial drainage on adverse slopes. In contrast, bedload export appears to be associated with history of sediment supply from tributary glaciers that are connected to the proglacial river downstream of the overdeepening. Thus, there is a pronounced grain-size dependent decoupling of sediment transport pathways where overdeepenings facilitate efficient evacuation of finer material but limit coarse sediment transfer. Overdeepenings actively restructure sediment fluxes in retreating glacier systems and likely complicate simple relationships between changing glacial erosion rates, hydrological forcing and sediment export from glaciers. Given the widespread nature of overdeepenings in glaciated environments, ongoing climate warming and continued glacier retreat, integrated approaches will be needed that combine hydrology, sediment provenance, and glacier dynamics to predict sediment delivery from alpine catchments.