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Abrasion acts to smooth glacial terrains and leaves behind linear scratch-like features (striations) on bedrock landscapes. Striations are often used as measures of glacier flow directions, but their morphology can also provide information about the subglacial stress conditions that produced the features. While striations are often abundant in the field, the processes that create them can be opaque and hard to examine in situ because they occur under thick layers of flowing ice. To alleviate that difficulty and provide information for interpretation of the populations of striations that are observed in the field, we conducted a set of laboratory experiments in which a ring of temperate debris-laden ice was slid atop a planar marble bed under various contact force conditions that led to the creation of hundreds of striations. During the experiment, numerous glaciological properties were continuously measured, including the resistive drag. Following the completion of the experiments, the marble beds were extracted, and the striations were measured for length and categorized by morphological type, and a subset was measured using a highresolution white-light profilometer. These experiments showed that, similar to field observations, type 2 striations were initially the most abundant; however, we found that type 3 striations became the most abundant at large displacements. We found good correlation between the abundance of striations as a function of displacement and measured drag as a function of displacement. When taken together, these results suggest that, in natural settings, ice flow around small roughness elements in glacier beds can “reset” the basal debris field, causing striations to become more abundant in their wake. As roughness is linked to quarrying, abrasion rates may increase in areas of increased quarrying. [ABSTRACT FROM AUTHOR]

Ice shelf calving constitutes roughly half of the total mass loss from the Antarctic ice sheet. Although much attention is paid to calving of giant tabular icebergs, these events are relatively rare. More frequent, smaller-scale calving events likely play an important role in the ice shelf frontal dynamics. Here, we investigate the role of bending stresses at the ice shelf front in driving calving on the scale 100 m – 1 km, perpendicular to the ice edge. We focus in particular on how buoyant underwater 'feet' that protrude beyond the above-water ice cliff may cause tensile stresses at the base of the ice and ultimately lead to fracture. Indirect and anecdotal observations of such feet at the Ross Ice Shelf front suggest that this process may be widespread. We consider satellite observations, together with an elastic beam model and a parameterization of frontal wave erosion to estimate the size and frequency of such calving events. Our results suggest that foot-induced mass loss at Ross Ice Shelf may cause up to 25 % of the total frontal ablation. However, stresses induced through this process are likely not sufficient to initiate crevassing but rather act to propagate existing crevasses. In addition, the relatively strong ice thickness dependence of the frontal uplift suggests an important role for internal bending moments due to temperature gradients in the ice. The highly variable environment, irregularity of pre-existing crevasse spacing, and complex rheology of the ice continue to pose challenges in better constraining the drivers behind the observed deformations and resulting calving rates. [ABSTRACT FROM AUTHOR]

Sea-level projections depend sensitively on the parameterization used for basal slip in glacier flow models. During slip over rock-beds, ice-bed separation increases with slip velocity and basal water pressure. We present a method for using these variables and measured bed topography to estimate the average bed slope in contact with ice, ${\bar m}$. Three-dimensional numerical modeling of slip over small areas of former beds has shown that changes in ${\bar m}$ with increasing slip velocity and water pressure mimic changes in basal drag. Computed values of ${\bar m}$ can thus provide the form of the slip law that relates drag to velocity and water pressure, avoiding computationally expensive numerical modeling. The method is applied to 618 sections from four former glacier beds. Results generally show an increase in ${\bar m}$ , and hence inferred basal drag, with slip velocity up to a limiting value, consistent with a regularized Coulomb slip law. [ABSTRACT FROM AUTHOR]

Glacier motion responds dynamically to changing meltwater inputs, but the multi-decadal response of basal sliding to climate remains poorly constrained due to its sensitivity across multiple timescales. Observational records of glacier motion provide critical benchmarks to decode processes influencing glacier dynamics, but multi-decadal records that precede satellite observation and modern warming are rare. Here we present a record of motion in the ablation zone of Saskatchewan Glacier that spans seven decades. We combine in situ and remote-sensing observations to inform a first-order glacier flow model used to estimate the relative contributions of sliding and internal deformation on dynamics. We find a significant increase in basal sliding rates between melt-seasons in the 1950s and those in the 1990s and 2010s and explore three process-based explanations for this anomalous behavior: (i) the glacier surface steepened over seven decades, maintaining flow-driving stresses despite sustained thinning; (ii) the formation of a proglacial lake after 1955 may support elevated basal water pressures; and (iii) subglacial topography may cause dynamic responses specific to Saskatchewan Glacier. Although further constraints are necessary to ascertain which processes are of greatest importance for Saskatchewan Glacier's dynamic evolution, this record provides a benchmark for studies of multi-decadal glacier dynamics. [ABSTRACT FROM AUTHOR]

Glacier-bed characteristics that are poorly known and modeled are important in projected sea-level rise from ice-sheet changes under strong warming, especially in the Thwaites Glacier drainage of West Antarctica. Ocean warming may induce ice-shelf thinning or loss, or thinning of ice in estuarine zones, reducing backstress on grounded ice. Models indicate that, in response, more-nearly-plastic beds favor faster ice loss by causing larger flow acceleration, but more-nearly-viscous beds favor localized near-coastal thinning that could speed grounding-zone retreat into interior basins where marine-ice-sheet instability or cliff instability could develop and cause very rapid ice loss. Interpretation of available data indicates that the bed is spatially mosaicked, with both viscous and plastic regions. Flow against bedrock topography removes plastic lubricating tills, exposing bedrock that is eroded on up-glacier sides of obstacles to form moats with exposed bedrock tails extending downglacier adjacent to lee-side soft-till bedforms. Flow against topography also generates high-ice-pressure zones that prevent inflow of lubricating water over distances that scale with the obstacle size. Extending existing observations to sufficiently large regions, and developing models assimilating such data at the appropriate scale, present large, important research challenges that must be met to reliably project future forced sea-level rise. [ABSTRACT FROM AUTHOR]

Drumlin shape has been hypothesized to correlate with ice‐flow duration and slip speed, but modern‐day analogues and the Coulomb nature of till render the basis of these correlations in question. The evolution of flow‐parallel subglacial landforms is of importance for ice flow because the form drag they provide may be a dominant factor in regulating glacier slip speeds. Here we examine the relationship between drumlin shape and cumulative slip displacement (i.e. time‐integrated slip speed) as a dominant glaciological control on drumlin shape. First, a new method is developed that allows slip speed to be estimated for deformable bedded glaciers along a flow line from an ice surface profile. Then, reconstructed surface profiles for ice margin chronologies of the Green Bay Lobe (GBL) are used to construct and estimate the spatially varying cumulative slip displacement for use in comparison with drumlin elongation ratios. We focus on a sector of the GBL near the central flow line where the geology is simple and glaciological controls are likely to dominate bedform development. Using Bayesian statistical analysis, a positive and statistically robust relationship between cumulative slip displacement and drumlin elongation ratio is found. Our analysis indicates that drumlin shape could be used to infer palaeo glacier slip speeds if time under the ice can be well constrained and geologic influences are minimal. These findings also suggest that drumlin‐supplied drag could decrease with increased cumulative slip displacement in the absence of rigid geologic features that fix drumlin positions. [ABSTRACT FROM AUTHOR]

Theory and experiments indicate that ice–bed separation during glacier slip over 2-D hard beds causes basal shear stress to reach a maximum at a particular slip velocity and decrease at higher velocities. We use the sliding theory of Lliboutry (1968) to explore how friction between debris particles in sliding ice and a rock bed affects this relationship between shear stress and slip velocity. Particle–bed contact forces and associated debris friction increase with increasing slip velocity, owing to increased rates of ice convergence with up-glacier facing surfaces. However, debris friction on diminished areas of the bed counteracts this effect as cavities grow. Thus, friction from debris alone increases only slightly with slip velocity, and for sediment particles larger than ~60 mm in diameter, debris friction peaks and decreases with increasing slip velocity. The effect on the sliding relationship is to steepen its rising limb and shift its shear stress peak to a slightly higher velocity. These results, which exclude the effect of debris friction on cavity size and debris concentrations above ~15%, indicate that the effect of debris in ice is to increase basal shear stress but not significantly change the form of the sliding relationship. [ABSTRACT FROM AUTHOR]