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

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 km2 in 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.

Accelerated loss from Alaskan icefields

The large icefields of Alaska contain a huge volume of glacier ice; enough to raise global sea levels by a total of 46.4 mm if it all melted2. Alaska currently accounts for 25% of all ice loss from global glaciers, losing about 66.7 billion tonnes (gigatones, or Gt) of ice each year3. If that rate continues, all Alaskan ice will be gone in around 250 years. Unfortunately, there is evidence that ice loss from Alaska is accelerating3–5.

Plateau Icefields

Juneau is a large plateau icefield; that is, it has a fairly flat upper icefield6. This can make it quite sensitive to climate change. This is because as warming temperatures result in the snowline rising, the area of the icefield with permanent snow (accumulation zone) shrinks rapidly across its flat upper surface. The end of summer snowline lies at the Equilibrium Line Altitude (ELA), where over the course of the year, the mass gain is equal to the mass loss. Above the ELA, there is net gain over the course of the year. Below the ELA, there is net loss.

Rising ELAs across low-slope plateau icefields are problematic, as the low slope here means that there is a rapid and large loss of accumulation area6–9. These top-heavy glaciers are predicted to experience significant area loss over coming decades 6,10–12.

plateau icefield and mountain glacier equilibrium line altitudes
Glacier Equilibrium Line Altitudes (ELAs) on mountain/valley glaciers and plateau icefields.

Juneau Icefield

Juneau Icefield straddles the Alaska and British Columbia border. It is one of the largest icefields in the world, with glaciers reaching from 9 m to 2300 m above sea level.

Juneau Icefield in Alaska / british Columbia
Juneau Icefield (red). Glaciers in grey from the Randolph Glacier Inventory. Black line denotes the boundary of the Alaska region of the Randolph Glacier Inventory. Map produced by Bethan Davies.

The accumulation area is very low slope, at 1200 to 2300 m (1400 km2), drained by large outlet glaciers. It is surrounded by numerous valley glaciers, with defined accumulation areas, and smaller mountain glaciers and glacierets.

Glaciers, lakes and rivers of Juneau Icefield. Yellow area is icefield plateau.
Juneau Icefield, with named glaciers, Alaska/British Columbia. Glacier extent in 2019 is shown. Yellow/orange area is the interconnected, low-slope plateau. Map produced by Bethan Davies.

Glacier inventory of Juneau Icefield

We used the Randolph Glacier Inventory (date: 2005) as our basemap13. The corrected and updated RGI 2005 files comprised 1,113 glaciers, with a total area of 4,238.7 ± 47.6 km2. Mean glacier area was 3.8 km2 with a median area of 0.41 km2. Individual glacier area ranged from 0.012 to 736.07 ± 2.39 km2.

Glacier area

The icefield had shrunk by 2019, with 1050 glaciers and a mean area of 3.6 km2. The total area was 3816.43 ± 15.92 km2. Over the 14-year time period between the surveys, 63 glaciers disappeared and glacier area shrank by 422.3 km2 (10.0%), at a mean rate of 30.16 km2 a-1.

The icefield’s 40 outlet glaciers covered 2939.1 ± 4.2 km2 in 2019. The largest is Taku Glacier (728.6 ± 1.0 km2). Meade Glacier is the second-largest outlet glacier; it covered 423.8 ± 0.6 km2 in 2019 and calved into a proglacial lake (4.7 km2). On the other side of the icefield, the third-largest is Llewellyn Glacier (290.8 ± 0.4 km2 in 2019 and calving into a proglacial lake (11.9 km2) at 725 m asl). The icefield is surrounded by 145 valley glaciers, 584 mountain glaciers and 281 glacierets.

glaciology and geomorphology of Juneau Icefield.
Glaciological and geoomorphological map of Juneau Icefield. Map produced by Bethan Davies.

Glacial lakes of Juneau Icefield

We mapped 420 lakes, including 18 supraglacial, 28 ice-dammed, 47 proglacial ice-contact, 38 tarns in cirque basins, and 289 ice-distal lakes. This includes five moraine-dammed lakes. 12 of the outlet glaciers terminate in proglacial lakes. In total, the ice-contact proglacial lakes of Juneau Icefield cover 58.4 km2 with most of the water volume associated with outlet glaciers. Some of the larger proglacial lakes contain many icebergs.

lakes of Juneau icefield
Different kinds of lakes of Juneau Icefield. A, B, C: ice dammed lakes. D: Proglacial lake. E: Moraine-dammed ice-distal lake in a tarn. Figure produced by Bethan Davies.

Proglacial lakes were mapped in front of 47 glaciers, with large proglacial lakes in front of 12 outlet glaciers. As these outlet glaciers in the flat valley bottoms thin, they increasingly reach flotation, which can exacerbate calving, thinning and stretching of the glacier snout, and increased melt at the glacier terminus.

We mapped no icebergs at Mendenhall Glacier, suggesting that calving here is much reduced compared with 2004 14,15, as the glacier has receded into the shallow water at the end of the lake. Some of the terminus is in fact now on land.

Instead, we note substantial calving at other glaciers (Tulsequah, Meade, Gilkey, Field), which may now be prone to this process.

The new moats mapped in front of Taku Glacier are likely to grow as the ice thins here and recedes into the substantial over-deepening upstream. This may re-initiate calving at this glacier16, and accelerate recession.

Taku Glacier, JUneau Icefield
Taku Glacier, Juneau Icefield. Map produced by Bethan Davies.

Structural glaciology of Juneau Icefield

This new 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 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.

Mendenhall Glacier, Juneau Icefield
Map of Mendenhall Glacier, Juneau Icefield. Map produced by 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.

Tulsequah Glacier, Juneau Icefield
Map of Tulsequah Glacier, Juneau Icefield
Field photographs of Juneau Icefield.
A. Icefall and ogives on Gilkey Glacier, taken by Austin Post in mid-20th 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.

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.

Further reading

References

Davies, B. et al. Topographic controls on ice flow and recession for Juneau Icefield (Alaska/British Columbia). Earth Surf. Process. Landforms in press, (2022).

2.          Millan, R., Mouginot, J., Rabatel, A. & Morlighem, M. Ice velocity and thickness of the world’s glaciers. Nat. Geosci. (2022). doi:10.1038/s41561-021-00885-z

3.          Hugonnet, R. et al. Accelerated global glacier mass loss in the early twenty-first century. Nature 592, 726–731 (2021).

4.          McNeil, C. J. et al. Glacier-Wide Mass Balance and Compiled Data Inputs: USGS Benchmark Glaciers (ver. 6.0, January 2022): U.S. Geological Survey data release. (2016). doi:10.5066/F7HD7SRF

5.          Berthier, E., Larsen, C., Durkin, W. J., Willis, M. J. & Pritchard, M. E. Brief communication: Unabated wastage of the Juneau and Stikine icefields (southeast Alaska) in the early 21st century. Cryosphere 12, (2018).

6.          McGrath, D., Sass, L., O’Neel, S., Arendt, A. & Kienholz, C. Hypsometric control on glacier mass balance sensitivity in Alaska and northwest Canada. Earth’s Futur. 5, 324–336 (2017).

7.          Boston, C. M. & Lukas, S. Topographic controls on plateau icefield recession: insights from the Younger Dryas Monadhliath Icefield, Scotland. J. Quat. Sci. 34, 433–451 (2019).

8.          Barr, I. D. & Lovell, H. A review of topographic controls on moraine distribution. Geomorphology 226, 44–64 (2014).

9.          Oerlemans, J. On the response of valley glaciers to climatic change. in Glacier fluctuations and climatic change 353–371 (Springer, 1989).

10.        Åkesson, H., Nisancioglu, K. H. & Morlighem, M. Simulating the evolution of Hardangerjøkulen ice cap in southern Norway since the mid-Holocene and its sensitivity to climate change. Cryosph. 11, 281–302 (2017).

11.        Zekollari, H., Huybrechts, P., Noël, B., van de Berg, W. J. & van den Broeke, M. R. Sensitivity, stability and future evolution of the world’s northernmost ice cap, Hans Tausen Iskappe (Greenland). Cryosph. 11, 805–825 (2017).

12.        Ziemen, F. A. et al. Modeling the evolution of the Juneau Icefield between 1971 and 2100 using the Parallel Ice Sheet Model (PISM). J. Glaciol. 62, 199–214 (2016).

13.        Randolph Glacier Inventory Consortium et al. Randolph glacier inventory–a dataset of global glacier outlines: Version 6.0: technical report, global land ice measurements from space, Colorado, USA. Digit. Media (2017).

14.        Boyce, E. S., Motyka, R. J. & Truffer, M. Flotation and retreat of a lake-calving terminus, Mendenhall Glacier, southeast Alaska, USA. J. Glaciol. 53, 211–224 (2007).

15.        Motyka, R. J., O’Neel, S., Connor, C. L. & Echelmeyer, K. A. Twentieth century thinning of Mendenhall Glacier, Alaska, and its relationship to climate, lake calving, and glacier run-off. Glob. Planet. Change 35, 93–112 (2002).

16.        McNeil, C. et al. The Imminent Calving Retreat of Taku Glacier. Eos, Am. Geophys. Union (2021).

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