Landscape generation by subglacial hydrology beneath the Fennoscandian Ice Sheet

Unknown basal characteristics limit our ability to simulate the subglacial hydrology of rapidly thinning contemporary ice sheets. Sediment-based landforms deposited beneath former ice sheets can provide crucial information about basal hydrology during rapid ice loss. Murtoos—low-relief (5–10 m) features with a distinct triangular morphology—have been identified throughout Finland and Sweden within terrain formerly occupied by the Fennoscandian Ice Sheet (FIS). The depositional environment and formation of murtoos are not yet predicted by existing models of subglacial landforms. Excavations have revealed that, distally, murtoos are composed of alternating facies of heterogeneous diamicton, with strong fabrics interbedded with sorted gravelly and sandy sediment. Proximally, murtoos exhibit glaciofluvial deposits, such as current ripples, transitional cross-bedding, and antidunal sinusoidal laminations reflecting alternating lower and upper flow regimes. Additionally, regional mapping has revealed a spatial association of murtoos with other meltwater features and a characteristic presence no closer than 40–60 km from the FIS margin at ~12 ka. Collectively, these indicate that murtoo deposition is accompanied by rapid increases in meltwater discharge—potentially within a single melt season—and is associated with areas of low effective pressure and the spatial onset of channelised drainage systems.

We used the Ice Sheet System Model (ISSM) implementation of the Glacier Drainage System (GlaDS) model to investigate murtoo genesis beneath the FIS. We parametrised GlaDS using digital elevation models (25 m/pixel) and estimations of ice surface elevation given by viscously relaxing initially parabolic ice profiles. Transient surface melt was introduced to a stable hydrological system over 10,000 days via moulins randomly distributed throughout the model domain. Moulin discharge rates were calculated using a positive degree day scheme forced by a depressed contemporary climate. Sensitivity testing was carried out for several poorly constrained parameters in GlaDS, as well as for the initial ice geometry and climatic inputs. 

We first applied GlaDS to a specific corridor of ice-flow within the relatively low-relief Finnish Lake District, where murtoos are densely concentrated, and then to a high-relief area of the Scandinavian Mountains towards which the FIS retreated prior to its demise. Murtoo density, as well as their gemorphic characteristics, was compared to the modelled sheet thickness, channel cross-sectional area, water pressure, and discharge rates through both the distributed and channelised system. Our modelling reproduces the hypothesised area of low effective pressure 40–60 km from the margin and supports the hypothesis that murtoos form in highly dynamic areas of the basal water system. This work highlights the value of applying GlaDS to glaciated regions in which hydrological outputs can be compared directly to geomorphological evidence.