Plateau Icefields: Glacial geomorphology of Juneau Icefield

This article is based on the followed accepted and published article on Juneau Icefield geomorphology and glaciology: Davies et al., 20221, which has been published in final form at: https://doi.org/10.1002/esp.5383 

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

In this article, we present the glacial geomorphology of Juneau Icefield, which is a plateau icefield. We show the moraines, flutes, and trimlines around its margins. We find that these are characteristic of a temperate glacier landsystem. You can also read about the glaciology and crevasses of the icefield, and topographic controls on present-day recession. This article is part of a series on our work on Juneau Icefield.

Plateau Icefields

Plateau Icefields have a large, low-slope plateau, with a flat upper accumulation area. This means that they can be sensitive to climate change, as the accumulation area shrinks rapidly as the snowline rises with global warming.

Plateau Icefields can have a rapidly shrinking accumulation area as snowlines rise.

Juneau Icefield

Juneau Icefield is a plateau icefield in Alaska/British Columbia. In this region, there is widespread evidence of a Late Holocene neoglaciation, often termed the “Little Ice Age” 2–6.

Juneau icefield
Location of Juneau Icefield in Alaska/British Columbia. Map produced by Bethan Davies.

Juneau Icefield is one of the largest icefields in the world. It has a large, low-slope plateau area lying above 1500 m. Our study includes 1050 glaciers, with the main icefield being surrounded by valley and mountain glaciers. You can see our structural glaciological mapping of the icefield here, including the new process of Glacier disconnections.

map of glaciers and lakes of Juneau Icefield
Map of glaciers and lakes of Juneau Icefield. Map produced by Bethan Davies.

The “Little Ice Age” of Juneau Icefield

Glaciers advanced here in the middle 1700s to late 1800s, which were often the most extensive of the last 10,000 years3,7. Many glaciers in Alaska have been receding more or less continuously since this maximum5

The exception to this is Taku Glacier, which advanced since the late 19th century. It has since been shrinking but only since 20138. It calved until 1948, but after this time it built a shoal terminal moraine that protected the terminus from calving icebergs and the warm ocean water9. This reduced melt and was the reason for its delayed response to climate change.

In this study, we mapped Juneau Icefield geomorphology and glacial landforms using Sentinel-2 imagery and ArcticDEM. The well-defined, sharp-crested moraines that surround the icefield and its glaciers are assumed to date from the Little Ice Age. They form a clear and decisive marker with ice-scoured bedrock, diamicton or fluted till, and trimlines.

Glacial landforms of Juneau Icefield

We have mapped over 10,400 geomorphological landforms across the icefield. This includes 3620 moraine ridge crests, 642 flutes, 1778 bedrock lineations, 535 trimlines, 828 cirque headwalls, 410 rivers, and 351 polygons of glacial sediment. The map below shows Juneau Icefield geomorphology and glaciology.

geomorphology of Juneau Icefield
Glacial geomorphology and glaciology of Juneau Icefield. Map produced by Bethan Davies.

The main mountain range is drained by parabolic valleys. Close to the icefield, they contain outlet glaciers, but further afield they bear the signs of a larger icefield, with ice-scoured bedrock (roche moutonnée). The broader scale geomorphology has the key features common in glaciated landscapes, including steep-sided arêtes, pyramidal peaks and cirques.

The forefields of the glaciers often contain ice-scoured bedrock and glacigenic sediment, inside the Little Ice Age moraines. The ice-scoured bedrock is often characterised by smooth, polished bedrock and lineations (roches moutonnées) that point down-valley towards the terminal moraines.

Llewellwyn glacier Juneau Icefield
Llewellwyn Glacier, Juneau Icefield. Map produced by Bethan Davies

Trimlines are common around the margins of the valley and outlet glaciers, and sometimes connect up with the terminal moraines. This reflects ice-thickness during the last neoglaciation10.

Meade glcaier Juneau Icefield
Meade Glacier, Juneau Icefield. Map produced by Bethan Davies.

Moraine complexes are often made up of closely-spaced nested terminal moraines. There are few recessional moraines within the main moraine complexes, see Llewellwyn Glacier for examples.  Most glaciers have just one moraine complex with few recessional moraines.

Gilkey glacier juneau icefield
Gilkey Glacier, Juneau Icefied. Map produced by Bethan Davies

Sandur frequently form in front of the larger outlet glaciers, especially for the outlet glaciers terminating in large proglacial lakes like Tulsequah Glacier.

Tulsequah Glacier Juneau Icefield
Tulsequah Glacier, Juneau Icefield. Map produced by Bethan Davies.

Glacial landsystems of Juneau Icefield

The Juneau Icefield landsystem is dominated by the large upland plateau surrounded by deep glacial valleys. The large plateau sustains and nourishes the glaciers that reach low altitudes, with the largest, Taku, reaching sea level.

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

Most of the outlet glaciers end in narrow valleys, but some, like Taku and Llewellwyn, spill out onto low flat areas as piedmont lobes. Other glaciers also did this at times of previous glacier advances, such as the East and West Twin Glaciers, forming moraines with a piedmont shape.

Outlet and valley glaciers are surrounded by moraines, with ice-moulded bedrock and roche moutonnées visible within recently exposed areas.

These moraines are mostly within one moraine complex, surrounding an area with few moraines. This is very different to the continuous sequences of recessional moraines seen in Scotland, for examples, associated with the Younger Dryas/Loch Lomond Stadial.

Some of the outlet glaciers have retreated into proglacial lakes (e.g. Gilkey), which may have drowned the moraines. Some smaller moraines may have been removed by rivers. Flutes indicates continuous recession without readvances, and indicates that the glaciers had continued forward velocity as they receded.

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

This landform assemblage is typical of warm-based, temperate glaciers, with evidence for glacial transport and deposition. Ice-scoured bedrock was observed even at high elevations, suggesting active erosion even on parts of the plateau.

Changing glacial landsystems

There is evidence of glacier disconnection across Juneau Icefield, where bare rock is increasingly becoming visible within the glacier polygon as ice thins in areas of steep, thin ice, often in icefalls. This decreases nourishment down-glacier and is likely to drive a change in the landsystem. If glacier tongues become disconnected from their upper parts they stagnate, becoming increasingly debris covered, with less forward motion and enhanced thinning.

The down-wasting of these tongues leads to a different landsystem, with ice-cored moraine, not moraine and flute formation11. This is already in front of Eagle and Thiel glaciers, for example. As glacier recession continues, this switch will be increasingly evident in the landform record.

Eagle and Thiel Glacier Juneau Icefield
Eagle and Thiel Glaciers, Juneau Icefield

Conclusions

We have produced a detailed icefield-wide geomorphological assessment, with 1050 glaciers and >10,400 geomorphological features. This includes 3620 moraine ridge crests, 642 flutes, 1778 bedrock lineations, 535 trimlines, 828 cirque headwalls, 410 rivers, and 351 polygons of glacial sediment.

Together, these landforms are characteristic of temperate glaciers and a temperate landsystem, with active glaciers (with continuous forward momentum) draining the icefield plateau.

The Little Ice Age landform record indicates a landsystem and glacier motion similar to that indicated by modern structural mapping.

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.          Porter, S. C. GLACIATIONS |
Neoglaciation in the American Cordilleras. in (eds. Elias, S. A. & Mock, C.
J. B. T.-E. of Q. S. (Second E.) 269–276 (Elsevier, 2013).
doi:https://doi.org/10.1016/B978-0-444-53643-3.00127-8

3.          Koch, J. & Clague, J. J. Extensive
glaciers in northwest North America during Medieval time. Clim. Change 107,
593–613 (2011).

4.          Wiles, G. C., Barclay, D. J. &
Calkin, P. E. Tree-ring-dated ‘Little Ice Age’ histories of maritime glaciers
from western Prince William Sound, Alaska. The Holocene 9, 163–173
(1999).

5.          Molnia, B. F. Late nineteenth to early
twenty-first century behavior of Alaskan glaciers as indicators of changing
regional climate. Glob. Planet. Change 56, 23–56 (2007).

6.          Motyka, R. J. Little Ice Age
subsidence and post Little Ice Age uplift at Juneau, Alaska, inferred from
dendrochronology and geomorphology. Quat. Res. 59, 300–309
(2003).

7.          Calkin, P. E. Holocene glaciation of
Alaska (and adjoining Yukon Territory, Canada). Quat. Sci. Rev. 7,
159–184 (1988).

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

9.          Kuriger, E. M., Truffer, M., Motyka,
R. J. & Bucki, A. K. Episodic reactivation of large‐scale push moraines in
front of the advancing Taku Glacier, Alaska. J. Geophys. Res. Earth Surf.
111, (2006).

10.        Rootes, C. M. & Clark, C. D. Glacial
trimlines to identify former ice margins and subglacial thermal boundaries: A
review and classification scheme for trimline expression. Earth-Science Rev.
210, 103355 (2020).

11.        Bradwell, T., Sigurđsson, O. &
Everest, J. Recent, very rapid retreat of a temperate glacier in SE Iceland. Boreas
42, 959–973 (2013).

 

 

 

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