Modelled Subglacial Floods and Tunnel Valleys Control the Life Cycle of Transitory Ice Streams
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The Cryosphere, 12, 2759–2772, 2018 https://doi.org/10.5194/tc-12-2759-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Modelled subglacial floods and tunnel valleys control the life cycle of transitory ice streams Thomas Lelandais1, Édouard Ravier1, Stéphane Pochat2, Olivier Bourgeois2, Christopher Clark3, Régis Mourgues1, and Pierre Strzerzynski1 1Laboratoire de Planétologie et Géodynamique, UMR 6112, CNRS, Le Mans Université, Avenue Olivier Messiaen, 72085 Le Mans CEDEX 9, France 2Laboratoire de Planétologie et Géodynamique, UMR 6112, CNRS, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes CEDEX 3, France 3Department of Geography, University of Sheffield, Sheffield, UK Correspondence: Thomas Lelandais ([email protected]) Received: 5 April 2018 – Discussion started: 8 May 2018 Revised: 30 July 2018 – Accepted: 9 August 2018 – Published: 27 August 2018 Abstract. Ice streams are corridors of fast-flowing ice that 2015). The mass transfer from these ice sheets to the ocean is control mass transfers from continental ice sheets to oceans. spatially heterogeneous: approximately 80 % of the ice dis- Their flow speeds are known to accelerate and decelerate, charge is focused in a finite number of ice streams, which act their activity can switch on and off, and even their locations as preferential drainage pathways for meltwater (Bamber et can shift entirely. Our analogue physical experiments reveal al., 2000; Bennett, 2003). that a life cycle incorporating evolving subglacial meltwater Modern and ancient ice streams are typically hundreds of routing and bed erosion can govern this complex transitory kilometres long and a few kilometres to tens of kilometres behaviour. The modelled ice streams switch on and acceler- wide, with ice velocities of the order of 102 to 104 m yr−1. ate when subglacial water pockets drain as marginal outburst Despite the fact that they occurred and occur in all for- floods (basal decoupling). Then they decelerate when the lu- mer and modern ice sheets, their initiation and the controls bricating water drainage system spontaneously organizes it- on their dynamics and evolution remain debated. Numeri- self into channels that create tunnel valleys (partial basal re- cal modelling suggests that ice flow might self-organize into coupling). The ice streams surge or jump in location when regularly spaced ice streams as a consequence of thermo- these water drainage systems maintain low discharge but they mechanical feedbacks within ice (Payne and Dongelmans, ultimately switch off when tunnel valleys have expanded to 1997; Hindmarsh, 2009) or because of inherent instability develop efficient drainage systems. Beyond reconciling pre- of thin subglacial meltwater films (Kyrke-Smith et al., 2014). viously disconnected observations of modern and ancient ice Numerous observations, however, have highlighted preferen- streams into a single life cycle, the modelling suggests that tial location of ice streams at sites of specific bed properties tunnel valley development may be crucial in stabilizing por- such as in topographic troughs, over areas of soft sedimen- tions of ice sheets during periods of climate change. tary geology, zones of higher geothermal heat flux or in ar- eas where subglacial meltwater is routed (Winsborrow et al., 2010; Kleiner and Humbert, 2014). These viewpoints might not be mutually exclusive if self-organization into regularly 1 Introduction spaced streams is the primary control but it is strongly medi- ated by local bed templates (e.g. troughs) or events (meltwa- Continental ice sheets currently store the equivalent of a 65 m ter drainage) that initiate or anchor streams in certain loca- thick global water layer and have been major contributors tions. Exploring this hypothesis using numerical modelling to the nearly 85 mm in global sea level rise measured be- has not yet been achieved because of uncertainties in how to tween 1993 and 2017 (Vaughan et al., 2013; Beckley et al., Published by Copernicus Publications on behalf of the European Geosciences Union. 2760 T. Lelandais et al.: The life cycle of transitory ice streams formulate basal ice flow in relation to bed friction, and due to Here, we describe the results of a physical experiment per- challenges of including all potentially relevant processes, es- formed with an innovative analogue modelling device that pecially for subglacial water flow (Flowers, 2015). provides simultaneous constraints on ice flow, subglacial Observations of spatial and temporal variations in the ac- meltwater drainage, subglacial sediment transport and sub- tivity of ice streams against fluctuations in their subglacial glacial landform development (Lelandais et al., 2016; Fig. 1). hydrology indicate that the style and flux of water drainage We propose that the location and initiation of ice streams is a major component driving change. Examples include re- might arise from subglacial meltwater pocket migration and organization of subglacial drainage systems (Elsworth and drainage pathways and that the evolution of ice stream dy- Suckale, 2016), subglacial water piracy (Vaughan et al., namics is subsequently controlled by subglacial drainage re- 2008; Carter et al., 2013), and development and migration organization and tunnel valley development. This study rec- of transient subglacial water pockets (Gray et al., 2005; Pe- onciles into a single story several detached inferences, de- ters et al., 2007; Siegfried et al., 2016). However, these vari- rived from observations at different timescales and at differ- ations have been observed or inferred independently, at dif- ent places on modern and ancient ice streams. ferent places and on yearly timescales, thus limiting our un- derstanding of the true role of the subglacial hydrology as a primary or secondary driver of ice stream changes. In this 2 Experimental ice stream model paper, we circumvent the challenge of numerically modelling ice stream initiation and dynamics, including subglacial wa- Ice stream dynamics are controlled by various processes that ter drainage, by exploiting a physical laboratory approach act at different spatial and temporal scales; they also in- that simultaneously combines silicon flow, water drainage volve several components with complex thermo-mechanical and bed erosion. behaviours (ice, water, till, bedrock) (Paterson, 1994). Con- Connections between ice stream activity and subglacial sidering all these processes and components simultaneously, hydrology are supported by the occurrence of geomorphic together with processes of subglacial erosion, is thus a chal- markers of meltwater drainage on ancient ice stream beds lenge for numerical computational modelling (Fowler and (e.g. meltwater channels, tunnel valleys, eskers) (Patterson, Johnson, 1995; Marshall, 2005; Bingham et al., 2010). Some 1997; Margold et al., 2015; Livingstone et al., 2016). Among attempts in analogue modelling have been made to improve these markers, tunnel valleys deserve specific attention be- our knowledge on subglacial erosional processes by melt- cause they have high discharge capacities and, as such, may water (Catania and Paola, 2001) or gravity current instabili- be major contributors to the release of meltwater and sed- ties produced by lubrication (Kowal and Worster, 2015). To iment to the ocean and may promote ice sheet stability by combine ice flow dynamics and erosional aspects in a sin- reducing the lubricating effect of high basal water pressure. gle model, we designed an alternative experimental approach These valleys are elongated and overdeepened hollows, rang- that allows simultaneous modelling of ice flow, subglacial ing from a few kilometres to hundreds of kilometres long, hydrology and sedimentary–geomorphic processes. With all from hundreds of metres to several kilometres wide and from the precautions inherent in using analogue modelling, our ex- metres to hundreds of metres deep. Their initiation is gener- periments reproduce morphologies and dynamics that com- ally attributed to subglacial meltwater erosion but their devel- pare well with subglacial landforms and ice stream dynamics opment processes (in time and space) and their relationship despite some differences in spatial and temporal scales and a to ice streaming are still debated. Indeed, ice streams com- number of active processes (e.g. Paola et al., 2009). monly operate because of high basal water pressure while the development of a tunnel valley system generally leads 2.1 Experimental apparatus to enhanced drainage efficiency and basal water pressure re- duction (Engelhardt et al., 1990; Marczinek and Piotrowski, The model is set in a glass box (70 cm long, 70 cm wide and 2006; Kyrke-Smith et al., 2014). 5 cm deep) (Fig. 1). A 5 cm thick, flat, horizontal, perme- Several field studies have already suggested a connection able and erodible substratum, made of sand (d50 D 100 µm) between catastrophic glacial outburst floods at ice sheet mar- saturated with pure water and compacted to ensure homoge- −3 gins and a suite of events involving ice streaming, tunnel val- neous values for its density (ρbulk D 2000 kg m ), porosity ley development and stagnation of the ice margin (Jørgensen (8 D 41 %) and permeability (K D 10−4 m s−1), rests on the and Piotrowski, 2003; Alley et al., 2006; Hooke and Jen- box floor. The ice sheet portion is modelled with a 3 cm thick nings, 2006; Bell et al., 2007). Such outburst floods can pro- layer of viscous (η D 5×104 Pa s) and transparent but refrac-