Reorganization of subglacial drainage processes during rapid melting of former ice sheets

Our ability to simulate the hydrology beneath rapidly retreating contemporary ice sheets is hampered by our limited understanding of subglacial conditions. To this end, sediment-based landforms deposited beneath former ice-sheets can provide crucial information regarding basal hydrology during rapid ice loss. Murtoos are a recently documented type of subglacial landform found concentrated in Finnish and Swedish portions of the former Fennoscandian ice sheet (FIS). The depositional environment and formation of murtoos are not yet predicted by existing models of subglacial landforms. However, excavation within these triangular forms as well as analysis of their spatial distribution suggests that murtoo genesis is associated with periods of rapid ice loss and specifically the spatial onset of channelised hydrological systems. Here, to investigate the conditions which may give rise to murtoos, we used the Glacier Drainage System (GlaDS) incorporated as part of the Ice Sheet System Model (ISSM). This coupled ice sheet-hydrology model was parametrised using high-resolution LiDAR digital elevation models (2 m/pixel) to constrain both basal topography and subglacial connectivity; ice surface elevation and basal ice velocity modelled using ISSM and bound by paleoclimatic evidence; and subglacial melt rates informed by geothermal heat flux/ice thickness. We applied this model first to a specific corridor of ice-flow in the Finnish Lake District where murtoos are densely concentrated and then to the entire FIS domain---comparing modelled distributed system water depth, channel cross-sectional area, water pressure, and discharge through both the distributed and channelised system to mapped murtoo density and their geomorphic characteristics. Our results support the emerging theory that murtoos are diagnostic indicators of a switch between distributed and channelised flow beneath retreating ice sheets, and highlight the value of applying such a model to paleoglacial regions in which hydrological outputs can be compared directly to geomorphological evidence.