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. 2019 Jul 10;5(7):eaaw5406.
doi: 10.1126/sciadv.aaw5406. eCollection 2019 Jul.

Sliding dominates slow-flowing margin regions, Greenland Ice Sheet

Nathan Maier  1 , Neil Humphrey  1 , Joel Harper  2 , Toby Meierbachtol  2
Affiliations

Sliding dominates slow-flowing margin regions, Greenland Ice Sheet

Nathan Maier et al. Sci Adv. .

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.

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Figures

Fig. 1
Fig. 1. Study area and field site setting.
(A) Three-dimensional representation of surface topography, bed topography (21), borehole locations (vertical colored lines), and GPS locations (black surface markers). (B) Regional surface velocity map showing winter flow conditions in 2010 (2, 43). Gray contours indicate bed topography in meters above sea level (m a.s.l.) (21). Left inset shows location of study region. (C) Surface map of study location showing GPS stations (black triangles, text—mean winter velocities) with flow vectors and borehole locations (colored circles: 14W, purple; 14N, blue; 14Sa, green; 14Sb, red; 15Ca, gray; 15Cb, cyan; 15S, pink; 15E, orange; 15N, dark gray). Surface strain rate tensor calculated across the GPS network: ε˙xx = 0.0006 a−1, ε˙yy = 0.0037 a−1, ε˙xy = 0.0014 a−1 .
Fig. 2
Fig. 2. Englacial measurements of temperature and ice flow.
(A) Englacial temperature profiles measured at each borehole location. (B) Site mean deformation profile (black markers), plotted with strain rate measurements in background. Pink envelope encompasses all the shear strain rate observations. Calculated deformation boundaries (gray envelope) given in situ ice temperatures and an estimated range of driving stress (76 to 107 kPa). (C) Velocity profile (blackline) derived by integrating site mean deformation profile. The surface velocity (us) is partitioned into basal sliding (ub) and ice deformation (ud) components.
Fig. 3
Fig. 3. Observed and modeled basal tilt rates.
The tilt rates (5 day) for the basal inclinometers in 15Cb (A), 15S (B), and 14Sa (C). Top panel for each hole is the tilt rates for the bottom five inclinometers plotted versus basal distance traveled during the 2015–2016 winter. The line color corresponds to the inclinometer installation height in meters above the bed (mab), as plotted in the bottom panel. The bottom panel shows the same data plotted as interpolated surface. The right column shows the best-match modeled tilt rates (D to F) for inclinometers sliding across beds with different roughness characteristics. The model type and roughness are indicated in the plot title. The top panel shows modeled inclinometer tilt rates plotted against basal distance, and the bottom panel shows the magnitude of the modeled tilt rates adjacent to the bed and the flow paths (colored lines) for the modeled basal inclinometers. The line colors in the top panel correspond to the inclinometer flow paths in the bottom panel. Each model scenario plotted is the best match to the adjacent observed tilt rate patterns. Model outputs for all model scenarios are plotted in fig. S5.
Fig. 4
Fig. 4. Driving stress across the western margin of the GrIS.
Driving stress calculated for the ablation zone of the western margin of the GrIS. Dots indicate field locations with direct measurements of deformation and sliding. Red dot is our field location, and cyan dots show other field locations with direct partitioning of surface motion.
Fig. 5
Fig. 5. Driving stress versus surface velocity, western GrIS.
Driving stress calculated across the western margin of the GrIS plotted against collocated, InSAR-derived surface velocities collected between 1 December 2008 and 28 February 2009 (2). Gray dots show all the measurements, while the red circle marker is measured from the field location and the blue dots represent those measured from the other locations with direct partitioning of surface motion (17, 18, 26).
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