Subglacial basins: Their origin and importance in glacial systems and landscapes

Closed topographic basins are found beneath contemporary ice masses and within the footprint of former ice masses in all glaciated regions. We present the first integrated review of subglacial basin occurrence and formation and the implications of such basins for glaciological processes and the evolution of landscape. Our purpose is to motivate research in areas where understanding of basin origin and process significance is weak. Basins on the order of 10–102 m deep and 102–103 m long are produced by glacial erosion of subglacial rock and/or sediment and are known as ‘overdeepenings’. Outlet and valley glaciers can ‘overdeepen’ their beds far below sea level or local fluviatile base level. Larger basins, typically in ice sheet contexts, may have a pre-glacial (usually tectonic) origin. Subglacial basins are important glaciologically because they require ice, water and sediment to ascend an adverse subglacial slope in order to exit the glacial system, the efficiency of which is dependent upon the gradient of the adverse slope and that of the ice surface. Basins thus influence subglacial drainage system morphology and transmissivity, the thickness and distribution of basal ice and sediment layers, and the mechanisms and dynamics of ice flow. Adverse gradients that exceed 11 times that of the ice surface may even permit the formation of subglacial lakes. We speculate that, in comparison to ice masses with few or no subglacial basins, those with numerous or very large basins may respond to climatic changes with unexpected vigour. In addition, erosion rates and transport pathways of water and sediment through the glacial system, and the expression of these processes in the sediment and landform record, may be unexpectedly complex. Further, our review shows that, in a warming climate, ice masses resting on adverse slopes will be vulnerable to rapid and potentially catastrophic retreat; new lakes in subglacial basins exposed by mountain glacier retreat will present an increasing hazard; and subglacial lakes may drain catastrophically. On even longer time scales, we speculate that the glacial excavation and post-glacial filling of basins in mountainous regions should contribute importantly to climate-related changes in isostasy and relief. Although the controls on overdeepening and their influence on other glacial and landscape processes remain uncertain, we hypothesise that overdeepened glacial systems reflect an equilibrium ice–bed geometry that maximises the efficiency of ice discharge. Improved understanding of overdeepening processes, especially overdeepened-bed hydrology, is therefore necessary to understand fully the dynamic behaviour of valley and outlet glaciers, and thus the fate of Earth's largest ice masses.