Elmer/Ice News

Modeling Ice-Crystal Fabric as a Proxy for Ice-Stream Stability

lilien2021Glacier ice is composed of individual grains, or crystals. These grains can have different orientations, similar to how a pile of snow has many flakes all pointing in different directions. When ice accumulates as snowfall, the grains point in approximately random directions. However, as glaciers move, the orientation of the grains is changed, leading to characteristic "fabrics," where the grains tend to point in distinctive directions. Here, we try to understand how one could work backwards, from the orientation of the grains, to determine how the ice flowed in the past. We use an ice-flow model to understand whether instabilities in ice streams, "rivers" of fast flowing ice in Greenland and Antarctica, would be recorded in the fabric. We find that changes in ice-stream flow could indeed be seen in the fabric for thousands or tens of thousands of years, depending on the exact type of change. We then show that these changes to fabric are large enough that they could be measured in ice cores or with specialized ice-penetrating radars. This helps lay the groundwork for better understanding of long-term changes to ice flow, which is important but hard-to-measure context for modern ice-stream retreat.

Read more: Lilien, D. A., N. M. Rathmann, C. S. Hvidberg and D. Dahl-Jensen, 2021. Modeling ice-crystal fabric as a proxy for ice-stream stability. Journal of Geophysical Research: Earth Surface, 126, e2021JF006306, doi: 10.1029/2021JF006306

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Do Existing Theories Explain Seasonal to Multi-Decadal Changes in Glacier Basal Sliding Speed?

gimbert2021Basal sliding is an important component of glacier motion. However, our knowledge of the physics that controls basal sliding is incomplete. This causes large uncertainties in the contribution to sea-level rise predicted for ice sheets over the coming century. Here, we test our understanding of basal sliding against particularly unique observations, made via a rotating bicycle wheel that has been continuously measuring glacier basal motion over three decades within excavated tunnels under the Argentière Glacier in the French Alps. Due to stress changes from significant glacier thinning over the multi-decadal period we are able to establish an observationally derived sliding law and compare it with expectations from theory. We report many observational features that are in striking agreement with theoretical predictions from glacier sliding over bedrock beds. However, we also observe an undocumented behavior of stress stabilization during the melting period at a specific stress state known as the Iken's limit. This behavior causes long term sliding velocities to follow a simple power law scaling with bed shear stress. This finding has the potential of strongly simplifying and reducing uncertainty on predicting glaciers response to climate change.

Read more: Gimbert F., A. Gilbert, O.  Gagliardini, C.  Vincent and L. Moreau, 2021.  Do existing theories explain seasonal to multi-decadal changes in glacier basal sliding speed? Geophysical Research Letters,  48, e2021GL092858. https://doi.org/10.1029/2021GL092858

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A new calving rate parameterization for ice-cliffs

Picture of deforming/calving ice-cliff  (from Nature Com.)Using HiDEM and Elmer/Ice to investigate the modes through which ice cliffs can structurally fail, this article contributes a conservative ice-cliff failure retreat rate parameterization that can easily be included in ice-sheet models. Results show the importance of viscous deformation, shear-band formation, and brittle-tensile failure in the context of marine ice-cliff instability. The findings also include possible mechanisms to inhibit a runaway situation by two major contributions, namely, the backforce exhibited by iceberg mélange as well as viscous flow deformation changing the cliff's shape.

Read more: Crawford, A.J., Benn, D.I., Todd, J, Åström, J.A, Bassis, J.N. and Zwinger, T., 2021. Marine ice-cliff instability modeling shows mixed-mode ice-cliff failure and yields calving rate parameterization. Nat Commun 12, 2701, doi:10.1038/s41467-021-23070-7


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