Surging vs. streaming: the dual fast ice flow response to variations in efficiency of the subglacial drainage landsystem

Abstract

Observation and modelling have long contributed to associate surging and streaming of glaciers with glacier thermal regime, variations in meltwater availability and pressure and mechanical coupling at their beds. Using experimental modelling and palaeoglaciological mapping, we explore how the development of subglacial drainage landsystems controls variations in drainage efficiency and ice flow velocities for terrestrial-based ice lobes on flat horizontal beds. We observe that the achievement or not of efficient subglacial drainage landsystems determines the fast-flow regime of these glaciers. In the surge regime, rapid increase of drainage efficiency through development of tunnel valleys and their tributaries reduces the duration of ice flow speed-up events by lowering water pressures and increasing ice-bed coupling. Tunnel valleys connected to ice lobe margins, submarginal thrust moraines, reduced ice lobe extensions and ephemeral shear margins are the most distinctive characteristics of this regime. In the stream regime, disconnected channels of smaller dimensions develop, but they are unable to evacuate all the meltwater: this prolonged drainage inefficiency leads to sustained high ice flow velocity and steady shear margins. Small and rectilinear meltwater channels devoid of tributaries, often disconnected from ice lobe margins and lineation swarms are diagnostic of this regime.