Microstructures in a shear margin: Jarvis Glacier, Alaska

Christopher Gerbi,Robert Hawley,Renée Clavette,Seth Campbell,David Clemens-Sewall,Ian Lee,Steven Bernsen,Stephanie Mills,Karl Kreutz,Kate Hruby

doi:10.1017/jog.2021.62

Christopher Gerbi, Robert Hawley + Show 8 more

Open Access

https://doi.org/10.1017/jog.2021.62

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Abstract

AbstractMicrostructures, including crystallographic fabric, within the margin of streaming ice can exert strong control on flow dynamics. To characterize a natural setting, we retrieved three cores, two of which reached bed, from the flank of Jarvis Glacier, eastern Alaska Range, Alaska. The core sites lie ~1 km downstream of the source, with abundant water present in the extracted cores and at the base of the glacier. All cores exhibit dipping layers, a combination of debris bands and bubble-free domains. Grain sizes coarsen on average approaching the lateral margin. Crystallographic orientations are more clustered and withc-axes closer to horizontal nearer the lateral margin. The measured fabric is sufficiently weak to induce little mechanical anisotropy, but the data suggest that despite the challenging conditions of warm ice, abundant water and a short flow distance, many aspects of the microstructure, including measurable crystallographic fabric, evolved in systematic ways.

Highlights

The glaciological community has only medium confidence in the prediction of ice discharge (Vaughan and others, 2013), in part because of uncertainty in the viscous rheology of streaming ice that constitutes glaciers and is responsible for draining most of the ice sheets
We used a Geophysical Survey Systems Incorporated (GSSI) SIR-4000 ground-penetrating radar (GPR) control unit coupled with a GSSI 100 MHz antenna to select appropriate ice core sites and place micro-structural observations from ice cores into a broader context
Our investigation of three ice cores in the margin of Jarvis Glacier, two of which reached the bed, reveals that microstructural properties are more consistent within cores than between cores

Summary

Introduction

The glaciological community has only medium confidence in the prediction of ice discharge (Vaughan and others, 2013), in part because of uncertainty in the viscous rheology of streaming ice that constitutes glaciers and is responsible for draining most of the ice sheets. We recognize how temperature, water content and microstructure affect ice flow (e.g. Barnes and others, 1971; Duval, 1977; Lile, 1978; Weertman, 1983; Castelnau and others, 1996, 1998; Goldsby and Kohlstedt, 2001; Cuffey and Paterson, 2010; Minchew and others, 2018; Haseloff and others, 2019), we do not have a robust understanding of how these factors are distributed in natural settings, in locations of high mechanical significance These critical locations include the lateral and basal margins of ice streams and glaciers, which deform under different conditions than do the fairly well studied ice beneath divides. Knowledge of the magnitude and distribution of those parameters are critical given that margins, whether frozen or sliding, commonly provide the majority of the resistance to flow (cf. Raymond and others, 2001)

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