Greenland Ice Sheet Surface Topography and Drainage Structure Controlled by the Transfer of Basal Variability

Abstract

Ice flow can transfer variations in basal topography and basal slipperiness to the ice surface. Recent developments in this theory have made it possible to conduct numerical experiments to predict mesoscale surface topographical undulations and surface relief on an ice sheet-scale. Focussing here on the contemporary Greenland Ice Sheet (GrIS), we demonstrate that the theory can be used to predict the surface relief of the ice sheet from bed topography, ice thickness and basal slip ratio datasets. In certain regions of the GrIS our approach overestimates, while in others underestimates, the observed surface relief. The magnitude and spatial pattern of these mismatches correspond with the theory's limitations and known uncertainties in the bed topography and basal slip ratio datasets. Our prediction experiment establishes that the first-order control on GrIS surface relief is basal topography modulated by ice thickness, surface slope and basal slip ratio. Additional analyses show that the surface relief, which is controlled by the bed-to-surface transfer of basal topography, preconditions the large scale spatial structure of surface drainage, with other factors such as surface runoff modulating the actual drainage system through influencing the temporal evolution of meltwater features. It follows that the spatial structure of surface drainage depends strongly on the transfer of basal topography to the ice surface. These findings represent an important step toward investigating and understanding the net long-term (>10 2 years) effect of surface drainage on ice sheet mass balance and dynamics during deglaciation events.

Supraglacial rivers on the northwest Greenland Ice Sheet, Devon Ice Cap, and Barnes Ice Cap mapped using Sentinel-2 imagery

“…In contrast, the southern section of the DIC and the low-elevation areas of the western sections of the DIC and the BIC have supraglacial river drainage patterns that are more dendritic, (Figure 8 and 9), similar to the southwest GrIS (Smith et al, 2015;King et al, 2016;Yang et al, 2016;Smith et al, 2017) and many terrestrial river systems (Dingman, 2015), indicating relatively large variations in surface relief and greater transfer of basal roughness and slipperiness (Crozier et al, 2018;Ignéczi et al, 2018). There is also considerable variation within some of the study sites.…”

“…A variety of supraglacial drainage patterns is revealed. On the northwestern GrIS, on high-elevation areas of the western sections of the DIC and the BIC, and on the eastern section of the BIC, supraglacial rivers are straight, subparallel, and densely distributed (Figure 7-9) indicating limited variations in surface relief (Karlstrom and Yang, 2016;Crozier et al, 2018;Ignéczi et al, 2018).…”

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

“…Lakes form in bed-topographically controlled surface depressions (Echelmeyer et al, 1991;Lampkin and VanderBerg, 2011;Sergienko, 2013;Ignéczi et al, 2018) during the melt season across much of the ablation area (McMillan et al, 2007;Selmes et al, 2011). Up to 55% of lakes drain rapidly, often in clusters (Box and Ski, 2007;Selmes et al, 2011) and progressively away from the ice sheet margin as the melt season progresses (Sundal et al, 2009;Morriss et al, 2013;Fitzpatrick et al, 2014).…”

“…The effect of bed topography on ice flow variations is not well understood Prescott, 2013;, not least because the bed topography itself is poorly constrained, despite major ongoing mapping efforts (Leuschen et al, 2010(Leuschen et al, , updated 2017Allen, 2013), and improved assimilation techniques (Morlighem et al, 2017). Bed topography affects: (i) subglacial channel location via hydropotential gradients (Shreve, 1972); (ii) surface topography (Gudmundsson, 2003;Ignéczi et al, 2018;Ng et al, 2018) and therefore surface strain rates, supraglacial water routing and lake formation and drainage (Karlstrom and Yang, 2016;Crozier et al, 2018;Ignéczi et al, 2018); (iii) sediment distribution, which may be concentrated in troughs (Bullard and Austin, 2011;Booth et al, 2012;Dow et al, 2013;Jezek et al, 2013;Harper et al, 2017), and; (iv) subglacial cavity geometry, which should affect the magnitude of ice displacement during cavity expansion (Cowton et al, 2016). Observations suggest that although ice flow during the early-melt season is typically fastest overlying deep troughs (Joughin et al, 2013) and spatial variations in ice flow relate to bed topography via its effect on the routing of surface water (Bartholomew et al, 2011b;Palmer et al, 2011), relative annual and inter-annual ice flow variability is broadly consistent over scales characterised by spatially varying bed topography (Tedstone et al, 2014(Tedstone et al, , 2015.…”

A new surface meltwater routing model for use on the Greenland Ice Sheet surface

“…However, to our knowledge, all models of meltwater routing over bare ice assume ice is impermeable and that Darcy's law is therefore not applicable (Arnold et al, 1998;Banwell et al, 2013;de Fleurian et al, 2016). Field studies do reveal that the bare-ice surface of ablating glaciers is often characterized by a porous ice layer termed "weathering crust" (Müller and Keeler, 1969;Fountain and Walder, 1998;Irvine-Fynn et al, 2011;Stevens et al, 2018), and lowdensity well-developed weathering crust has been observed in bare ice of the Rio Behar catchment (Cooper et al, 2018). Our results suggest that in contrast to current practice, principles of porous-media flow may be applied even in the bareice ablation zone if conditions of weathering crust and porous low-density bare ice are found.…”