Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet

Abstract. Unknown basal characteristics limit our ability to simulate the subglacial hydrology of rapidly thinning contemporary ice sheets. Subglacial water is typically conceptualised as being routed through either distributed, inefficient, and high pressure systems, or channelised, efficient, and low-pressure systems, transitioning between the two as a function of discharge. Sediment-based landforms generated beneath Pleistocene ice sheets, together with detailed digital elevation models, offer a valuable means of parameterising and testing models of subglacial hydrology. However, previous work using geomorphology to inform modelling has concentrated on landforms relating to channelised drainage (e.g., eskers) while using inherently channelised models unable to capture transitions in drainage state. Landscapes relating to the distributed drainage system, and the hypothesised transitional zone of drainage between distributed and channelised drainage modes have therefore been largely ignored. To address this, we use the Glacier Drainage System model (GlaDS), a 2D finite element model capable of capturing the transition between distributed and channelised drainage, to explore the genesis of ‘murtoos’, a distinctive triangular landform found in murtoo fields throughout Finland and Sweden. Murtoos are hypothesised to form 40–60 km from the former Fennoscandian ice margin at the onset of channelised drainage where water pressure is at or exceeds ice overburden pressure. Concentrating within a specific ice lobe of the former Fennoscandian Ice Sheet and parameterised using digital elevation models with a simulated former ice surface geometry, we carried out a range of sensitivity testing to explore murtoo genesis and drainage transitions beneath the palaeo ice sheet. Our modelling supports many of the predictions for murtoo origin, including the location of water pressures equal to ice overburden, the onset of channelised drainage, and the predicted water depths in terrain surrounding murtoo fields. Modelled channels also closely match the general spacing, direction and complexity of eskers and mapped meltwater routes. Our results demonstrate that examining palaeo basal topography alongside subglacial hydrology model outputs holds promise for mutually beneficial analyses of palaeo and contemporary ice sheets to assess the controls of hydrology on ice dynamics and subglacial landform evolution.