Sliding dominates slow-flowing margin regions, Greenland Ice Sheet

Abstract

On the Greenland Ice Sheet (GrIS), ice flow due to deformation and sliding across the bed delivers ice to lower-elevation marginal regions where it can melt. We measured the two mechanisms of motion using a three-dimensional array of 212 tilt sensors installed within a network of boreholes drilled to the bed in the ablation zone of GrIS. Unexpectedly, sliding completely dominates ice motion all winter, despite a hard bedrock substrate and no concurrent surface meltwater forcing. Modeling constrained by detailed tilt observations made along the basal interface suggests that the high sliding is due to a slippery bed, where sparsely spaced bedrock bumps provide the limited resistance to sliding. The conditions at the site are characterized as typical of ice sheet margins; thus, most ice flow near the margins of GrIS is mainly from sliding, and marginal ice fluxes are near their theoretical maximum for observed surface speeds.

Basal traction mainly dictated by hard-bed physics over grounded regions of Greenland

“…where ( ) is a temperature dependent rate factor (calculated following Cuffey and Paterson 2010), is the flow law exponent equal to 3, and is the basal traction as dictated by the relationships in Figure 3. This corresponds to a 215 deformation fraction of 0.33 for the rate-strengthening range of the traction relationships, which is similar to that measured along the margins (Doyle et al, 2018;Lüthi et al, 2002;MacGregor et al, 2016;Maier et al, 2019;Ryser et al, 2014), and would approximate deformation where changes in flowline ice thickness are small, i.e. some troughbound outlet glaciers.…”

“…The surface velocity reflects the combined motion due to basal motion and internal ice deformation and thus interpretation of the derived relationships requires consideration of both components of motion. In Greenland, the rheologic uncertainty and complex thermal structure make it difficult to infer a location specific rheology to confidently calculate deformation across the ice sheet (Maier et al, 2019). However, direct measurements of basal 205 motion collectively show the prevalence of high fractions of basal motion [0.44 -0.96] across a wide range of glaciological environments in Greenland (Doyle et al, 2018;Lüthi et al, 2002;MacGregor et al, 2016;Maier et al, 2019;Ryser et al, 2014).…”

Sliding Relations for Glacier Slip With Cavities Over Three‐Dimensional Beds

“…Ice sheets and smaller glaciers rest directly on beds consisting of rigid rock (hard bed) or deformable sediment (soft bed). In either case, glaciers move primarily by slip over their beds if basal ice is at the melting temperature (Cuffey & Paterson, ; Maier et al, ). Slip of ice over hard beds has diverse consequences.…”

Ice-marginal lake hydrology and the seasonal dynamical evolution of Kennicott Glacier, Alaska

“…Sub-annual glacier surface velocity changes are driven by variable rates of basal motion (Willis, 1995, and references within). Basal motion often accounts for a large fraction () of glacier mass flux (Raymond, 1971; Truffer and others, 2000; Harrison and others, 2004; Amundson and others, 2006; Morlighem and others, 2013; Ryser and others, 2014b; Doyle and others, 2018; Maier and others, 2019) across a wide range of glaciologic settings (Maier and others, 2019), from valley glaciers to the ice sheets. Basal motion is possible under the large temperate fractions of the Greenland ice sheet () (MacGregor and others, 2016) and Antarctica () (Pattyn, 2010), making its understanding important for predicting ice fluxes to the global ocean and thus, sea level rise.…”