EGU2020-12602, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-12602
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Spontaneous Formation of Internal Shear Zone in Ice Flowing over a Topographically Variable Bed

Emma Weijia Liu1, Ludovic Räss2,3, Jenny Suckale1, Frédéric Herman4,5, and Yury Podladchikov5,6
Emma Weijia Liu et al.
  • 1Department of Geophysics, Stanford University, Stanford, CA 94305, USA
  • 2Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, Zurich, Switzerland
  • 3Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland
  • 4Institute of Earth Surface Dynamics, University of Lausanne, 1015 Lausanne, Switzerland
  • 5Swiss Geocomputing Centre, University of Lausanne, 1015 Lausanne, Switzerland
  • 6Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland

The transition from slow flow to rapid sliding is a noticeable feature of both ice sheets and outlet glaciers. Most existing models attempting to understand the complex physical transition processes assume an idealized model geometry with a flat bed. These models have shown that the onset of sliding entails basal refreezing, which in turn suppresses sliding. The theoretical difficulties in understanding sliding commencement in these process-based models contrast with the apparent ubiquity of the transition in the field. Here, we hypothesize that the presence of basal topography could resolve the inconsistency between model predictions and field observations.

We test our hypothesis by investigating the flow-to-sliding transition in a process-based model of ice flowing over bedrock with significant roughness. We assume that the bed is rigid and that the boundary condition at the bed is no-slip. We incorporate variations in basal topography into an iterative nonlinear Stokes solver for thermo-mechanically coupled ice deformation using the Immersed Boundary Method. This approach permits us to address the basal ice to bedrock transition with high accuracy and to study the impact of the shape of this transition zone.

Our results suggest that shear heating in the vicinity of pronounced roughness extends well into the bulk of the ice, leading to a spatially variable viscosity. These spatial variations in topography can therefore significantly impact the overall viscosity distribution in the ice. High shear strain rates localize at the tops of the bedrock topography. Thermo-mechanical feedback lead to the spontaneous formation of internal shear band over time, by connecting the topographic heights. The internal shear zone accommodates the majority of shear deformation, inducing a sliding motion of the upper part of the domain. Our results provide a process-based explanation of recently measured ice deformation data at the West margin of Greenland Ice Sheet (Maier et al. 2019). It is also consistent with the proposed existence of a radio-echo free zone located in the lowest hundreds of meters above bedrock (Drews et al. 2009, Fujita et al. 1999).

 

Maier, Nathan, et al. "Sliding dominates slow-flowing margin regions, Greenland Ice Sheet." Science advances 5.7 (2019): eaaw5406.

Drews, Reinhard, et al. "Layer disturbances and the radio-echo free zone in ice sheets." The Cryosphere 3 (2009): 195-203.

Fujita, Shuji, et al. "Nature of radio echo layering in the Antarctic ice sheet detected by a two‐frequency experiment." Journal of Geophysical Research: Solid Earth 104.B6 (1999): 13013-13024.

How to cite: Liu, E. W., Räss, L., Suckale, J., Herman, F., and Podladchikov, Y.: Spontaneous Formation of Internal Shear Zone in Ice Flowing over a Topographically Variable Bed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12602, https://doi.org/10.5194/egusphere-egu2020-12602, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-12602, Matt Trevers, 06 May 2020

    Hi Emma,

    This is a great presentation, and really impressive for your first year.

    The concept is very interesting. It reminded me of the regelation mechanism which is a component of Weertman sliding. The mechanism is completely different of course, but it's more efficient for a rougher bed, and from above looks much the same with the majority of deformation taking place in lower layers in the region dominated by bedrock bumps.

    Have you tried allowing the model to slip at the bed. If resistance to sliding is increased by greater bed roughness, but the deformation in lower layers is enhanced, I wonder if you'd end up with a sliding law very similar to Weertman, with a critical bedrock bump size that regulates the combined sliding and shearing velocity?

    • AC1: Reply to CC1, Emma Weijia Liu, 07 May 2020

      Hi Matt,

       

      Thanks very much for your comment!

       

      There are two differences between the shear band and regelation mechanism. First, the shear band is internal on top of the bedrock while regelation happens at the rock-ice interface with melt water flowing around the bumps. Second, regelation works for smaller bumps usually of order of 1 meter, internal shear band is on a larger scale with 100 meters or more.

       

      Right now we prescribe no slip boundary condition at the bed. I haven’t tried imposing a slip boundary condition, but my guess is that this will not change the results qualitatively. I wouldn’t say with full confidence that resistance to sliding is increased by greater bed roughness. Because once a shear band has formed and the upper part of the ice starts to slide, the shear band may actually accelerate instead of hinder the flow. When this happens, the ice above the shear band may no longer feel the fine bed structures below this band. Yes there should exist some critical parameters of the topography, which is the characteristic length scale I talked about in the presentation, above which a shear band will form. I have not investigated how an internal sliding velocity is related to this characteristic length scale. This may be one of the next steps. It would be possible to derive a sliding law that parametrizes the physical processes leading to the formation of an internal slip interface and the resulting ice speed on the surface. I would expect that to look different than Weertman's sliding law, though, since the physics controlling slip are different ones.

       

      Thanks,

      Emma

  • CC2: Comment on EGU2020-12602, Matt Trevers, 07 May 2020

    Hi Emma,

    Yes you're correct, I had it backwards. Regelation favours smaller bedrock bump wavelength while enhanced straining favours longer wavelength obstacles. There may still be some crossover wavelength under your mechanism.

    Do you have plans to publish your results soon?

    • AC2: Reply to CC2, Emma Weijia Liu, 07 May 2020

      Hi Matt,

      Yes, I'm currently writing up the paper, and hopefully it will soon get admitted and published!

      Best,

      Emma