Subglacial sediment flux will increase under retreating glacier scenarios

The discharge of sediment from glaciers holds considerable importance for downstream hydropower operators and ecosystems amongst others. Sediment discharge in the subglacial environment is a consequence of two processes: (1) bedrock erosion that creates sediment at the glacier bed, and (2) the capacity of subglacial water to transport this sediment. As glaciers retreat, ice dynamics and subglacial hydrology both evolve, resulting in a response in bedrock erosion and sediment discharge. On one hand, rising equilibrium line altitude not only result in larger melt water fluxes but also cause the subglacial hydrological system to be more active on the high elevation regions of the glacier. The combined effects of these processes likely increase the subglacial sediment transport capacity. On the other hand, a retreating glacier usually thins and slides at lower velocities. This diminishes its capacity to erode bedrock material and hence reduces the rate of sediment production and the ability of subglacial water to transport it. Here, these competing processes are captured by coupling models of surface mass balance, glacier dynamics, bedrock erosion, and subglacial sediment transport in order to determine the evolution of subglacial sediment discharge in mountain environments. For a 100-year of model scenario with accelerating climate warming, it is found that the sediment transport dominates over the bedrock erosion. This results in pronounced increases in subglacial sediment discharge in glacier retreating scenarios. The amplitude of the enhanced sediment discharge, however, is strongly modulated by the initial bedrock erosion rate and the distribution of sediment below the glacier. Furthermore, presence of complex bedrock topography can limit the amount of sediment available at the bed in some instances, which results in non-linear feedbacks between glacier retreat and sediment discharge.