Glacier disconnections, Juneau Icefield

This article on the new and critical process of glacier disconnections 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.

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

In this work, we have mapped the structural glaciology of Juneau Icefield for 2019 AD. Glaciers in this area are receding rapidly, contributing substantially to global sea level rise. We provide the first systematic and icefield-wide map of the process of glacier disconnections, and show how this is affecting glacier recession. This article is part of a series on our work on Juneau Icefield.

What are glacier disconnections?

Glacier disconnections form when the ice thins in an area of steep and thin ice in the middle of a glacier, such as within an icefall. This can rapidly result in bare rock appearing within the glacier polygon. If this happens, then less ice is able to flow down-glacier to the glacier tongue, which accelerates stagnation and recession of the glacier tongue.Disconnections occur where the lower portions of the glacier become discontinuous with the higher elevations that formerly supplied ice2-4. They typically form over bedrock steps or in places with steep and heavily crevassed ice, such as in icefalls.

Separations occur where tributary glaciers have receded from a trunk glacier. this often happens on valley floors as glaciers shrink, resulting in discrete glacier snouts.

glacier disconnections of Juneau Icefield
Glacier separations and disconnections, Juneau Icefield. Figure produced by Bethan Davies.

Juneau Icefield is a plateau icefield, with a large, low-slope accumulation area. We mapped icefalls, crevasses, ogives and other structural features around the icefield. The icefield is surrounded by icefalls around the edge of the plateau. This may make it particuarly susceptible to glacier disconnections.

Furthermore, the icefield is rapidly thinning and receding. Between 2005 and 2019, Juneau icefield shrank, 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.

Glacier separations and disconnections

We mapped 59 separations and 281 disconnections across the icefield and its peripheral glaciers. Disconnections were observed on 164 glaciers, with multiple disconnections at some glaciers. Below, you can see some examples of the mapped disconnections around Meade Glacier.

Why are there disconnections happening on Juneau Icefield?

Across Juneau Icefield, glacier thinning is now reaching the elevation of the icefalls5. Glacier disconnections mostly occur in places with thin, steep and heavily crevassed ice, as a result of thinning of the ice here.

Glaciers associated with disconnections tend to have more debris on their glacier tongues than average. Denver, Laughton and Thiel are examples of valley glaciers with disconnections and hig amounts of debris on their tongues. Where there is at least one disconnection, there are inceased signs of stagnation; for examples, Eagle and Thiel glaciers have increased debris cover and deformed longitudinal foliation.

Eagle Glacier, Juneau Icefield
Map of Eagle Glacier, Juneau Icefield. Map produced by Bethan Davies

Topographic controls on glacial recession at Juneau Icefield

Juneau Icefield is rapidly shrinking; it has lost 63 glaciers and 422.3 km2 of ice from 2005-2019. There is particularly rapid down-wasting on the outlet glacier tongues, especially Tulsequah, Gilkey, Thiel, Eagle, Meade, Warm Creek, Ogive, East Twin, Battle and Field Glacier5. However, the thicker, larger glaciers such as Willison, Norris, Taku, Llewellyn and Mendenhall show lower rates of ice-surface lowering.  

The disconnections we have mapped have strong implications for the future mass wastage of the icefield. These processes increase fragmentation of the icefield and decrease nourishment in glacier tongues (cf. 4).

13 of the 40 outlet glaciers that draw ice directly from the plateau have ablation zones currently connected to the plateau via icefalls. Future disconnections in these outlet glaciers may accelerate the recession of the ablation portions of 1,325.7 km2 of the main Juneau Icefield. Additionally there are 11 valley glaciers immediately contiguous with the icefield with narrowing icefalls in this zone, with an additional area of 132.9 km2. We are already observing a narrowing of the icefall on East Twin Glacier, with rapid thinning below the icefall (data from Hugonnet et al., 2021).

These disconnections are occurring on steep slopes at a mean altitude of 1354 m above sea level. Glacier ELAs in this region (now 1172 m for Taku Glacier 6,7) now increasingly intersect the mean elevation of icefalls around the plateau (1481 m). As icefalls increasingly often fall within the ablation zone, thinning and disconnection of more flow units is likely.

Elevations of key structures on Juneau Icefield.

As glaciers thin, the bedrock control on ice surface slope and therefore crevassing increases. Thinning glaciers may therefore become increasingly crevassed. This can result in increased ice falls and more disconnections. Continued thinning and appearance of more bare rock within the icefield is therefore increasingly likely.

The glacier ELAs are now reaching the plateau area (1200 to 1500 m above sea level). As glacier ELAs reach this area, the low surface slope of the plateau means that future small rises in ELA will drive large losses of the snow cover in the accumulation area.

Finally, there is potential here for an altitude-mass balance feedback (cf. 8,9), as the ice here is very thick10. A thinning of the plateau ice cap will lower its ice surface, bringing more of the area into the ablation area. This will drive further rapid recession and fragmentation of the icefield, as it cannot readjust by receding back up-valley in the same way as a valley or mountain glacier11.

Altogether, this shows how topography is influencing a non-linear response of these glaciers to climate change, which has the potential to significantly accelerate glacier response to climate change.

Further reading

References

1.          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.          Jiskoot, H., Curran, C. J., Tessler, D. L. & Shenton, L. R. Changes in Clemenceau Icefield and Chaba Group glaciers, Canada, related to hypsometry, tributary detachment, length-slope and area-aspect relations. Ann. Glaciol. 50, 133–143 (2009).

3.          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).

4.          Rippin, D. M., Sharp, M., Van Wychen, W. & Zubot, D. ‘Detachment’ of icefield outlet glaciers: catastrophic thinning and retreat of the Columbia Glacier (Canada). Earth Surf. Process. Landforms 45, 459–472 (2020).

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

6.          McNeil, C. et al. Explaining mass balance and retreat dichotomies at Taku and Lemon Creek Glaciers, Alaska. J. Glaciol. 66, 530–542 (2020).

7.          McNeil, C. J., Campbell, S. W., O’Neel, S. & Baker, E. H. Glacier-Wide Mass Balance and Compiled Data Inputs: Juneau Icefield Glaciers (ver. 2.0, January 2022). U.S. Geological Survey data release. (2019). doi:https://doi.org/10.5066/P9YBZ36F

8.          Huss, M., Hock, R., Bauder, A. & Funk, M. Conventional versus reference-surface mass balance. J. Glaciol. 58, 278–286 (2012).

9.          Sass, L. C., Loso, M. G., Geck, J., Thoms, E. E. & McGrath, D. Geometry, mass balance and thinning at Eklutna Glacier, Alaska: an altitude-mass-balance feedback with implications for water resources. J. Glaciol. 63, 343–354 (2017).

10.        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

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).

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