Structural Glaciology of Juneau Icefield

This article is based on the followed accepted and published article about Juneau Icefield: Davies et al., 2022¹, which has been published in final form at: https://doi.org/10.1002/esp.5383. All data produced in this work, including shapefiles and an A0 poster of the icefield, are available as supplementary data with the final published version.

This article is part of a series on our work on Juneau Ice

Article authors: Bethan Davies, Jacob Bendle, Jonathan Carrivick, Robert McNabb, Christopher McNeil, Mauri Pelto, Seth Campbell, Tom Holt, Jeremy Ely, Bradley Markle

Summary

Juneau Icefield, satellite image from 2013

In this work, we look at the glacial structures and glacial geomorphology of Juneau Icefield (Alaska/British Columbia). We map the glacier extent in 2019, glacier lakes, ogives, crevasses, icefalls, and glacier disconnections across the icefield.

We found that the icefield’s outlet glaciers covered 3066.6 km²in 2019 AD, and altogether the 1053 glaciers in the study region covered 3816.4 km².

Critically, this plateau icefield is susceptible to non-linear and threshold behaviour that will accelerate its response to climate change, including ‘disconnections’ between glacier accumulation and ablation area, as ice thins.

You can read about the glacial geomorphology we have mapped around the icefield here.

You can read about how glacier structures, including glacier disconnections, can impact ice flow here.

Structural glaciology of Juneau Icefield

This new structural database includes 20,805 individual structures, with elevation data, including 16,352 crevasses in 2,387 heavily crevassed zones, evidence of ductile deformation in the form of deformed foliation and ogives, and limited supraglacial debris cover.

*Map of Gilkey Glacier, Juneau Icefield. Map produced by Bethan Davies.*

Primary Stratification

Primary stratification is common in the upper parts of the glaciers. Down the glacier trunk it is increasingly stretched and folded. You can see this clearly on Meade Glacier, for example. As the primary stratification is increasingly folded it becomes longitudinal foliation. It is most clear where several accumulation basins are tributaries on one key trunk.

Map of Meade Glacier, Juneau Icefield. — *Meade Glacier, Juneau Icefield. Map produced by Bethan Davies*

On Gilkey Glacier, large folds of the longitudinal foliation are clear on the glacier trunk. This is associated with looped medial moraines, a characteristic of previous surging activity.

Crevasses

Crevasses across the icefield include bergschrund, transverse, marginal, longitudinal, splaying, rifts and icefalls. They typically form in different places on the glacier, with bergschrunds at the top of the glacier. There were heavily crevassed zones on the trunks of many outlet glaciers, including Taku.

*Map of Mendenhall Glacier, Juneau Icefield. Map produced by Bethan Davies.*

Heavily crevassed zone on Hole in the Wall Glacier, 2022. Credit: Bethan Davies

Icefalls and Ogives

150 icefalls were mapped on 23 outlet glaciers, including 13 that drain from the main interconnected plateau. They are also common on peripheral valley and mountain glaciers. We mapped 4981 icefall-type crevasses in total, with a mean elevation of 1481 m.

On 11 outlet and 2 valley glaciers we mapped ogives below the icefalls, including on Gilkey, Battle, Denver, West and East Twin, Tulsequah, Field and Bacon Glacier.

*Map of Tulsequah Glacier, Juneau Icefield*

Field photographs of Juneau Icefield. — A. Icefall and ogives on Gilkey Glacier, taken by Austin Post in mid-20^th century (Wikimedia Commons). Note the ogives becoming increasingly deformed down-ice of the icefall. B. Vaughan Lewis Icefall (right) and Little Vaughan Lewis Icefall (left) on Gilkey Glacier, taken in 1955 by Austin Post (Wikimedia Commons). These are the same icefalls as observed in panel A. Little Vaughan Lewis Icefall is now disconnected from Gilkey Glacier. C. Photograph looking down-glacier from above the icefall on Gilkey Glacier, taken by Ron Clausen (Wikimedia Commons). D. The plateau accumulation area (1100 m asl) of Taku Glacier, with the Taku Towers nunatak. E. Terminus of Mendenhall Glacier in 2014. Credit: Robert McNabb. F. Terminus of Norris Glacier and Taku Glacier in 1975, showing the build-up of Grizzly Bar moraine, taken by Mauri Pelto.

The drone video below shows the ogives forming at the bottom of Vaughan Lewis Icefall on Gilkey Glacier.

Glacier disconnections

We are now seeing thinning at these icefalls, which is driving glacier disconnections and fragmentation. Read more here:

Glacier disconnections, Juneau Icefield

Methods

In this project, we edited the glacier outlines available in the Randolph Glacier Inventory from 2005 in line with satellite imagery from 2019.

We used 10 m Sentinel imagery and the ArcticDEM for elevation data. 2019 was especially suitable for this approach due to its high late-summer snowline and limited cloud cover. We used both composite true colour and black and white band 4 imagery for mapping.

Gilkey glacier juneau icefield. Sentinel imagery from 2019. — *Sentinel imagery (2019) of Gilkey Glacier, Juneau Icefield. Top: black and white band 4 imagery. Bottom: composite full colour image.*

Wider reading

References

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