Palaeo-ice stream landsystem

By Jacob Bendle – Last updated 22/06/2020 tagged ice stream, Post-16

Ice streams are corridors of fast-flowing ice within ice sheets that are flanked on either side by slowly moving ice¹. Palaeo-ice streams are ice streams that existed in former ice sheets^2,3, such as the continental ice sheets that grew during the last Ice Age. Glaciologists know that these palaeo-ice streams existed as they left a clear imprint on the landscape over large parts of North America⁴, Scandinavia⁵, and Britain⁶.

Landsat 7 ETM+ satellite image of Byrd Glacier, an ice stream in West Antarctica. Ice flow is towards the top of the image. Note how flow converges into the main ice stream trunk. Also, note the sharp boundary between fast- and slow-flowing ice. Image: NASA.

Why are ice streams important?

Ice streams in Greenland and Antarctica are the main control on ice sheet mass balance and discharge to the world’s oceans^7. Understanding how ice streams behave and change over time, therefore, is important for predicting and managing the impacts of future climate change.

But this is easier said than done…

Firstly, records of modern-day ice stream activity only cover the most recent ~50 years (the length of the satellite record), which is not enough to confidently predict how they may change in the future.

Secondly, it is almost impossible for glaciologists to study the processes that occur at ice stream beds – which control fast ice flow and, ultimately, ice stream discharge to the oceans¹ – owing to the great thickness (up to ~3 kilometres) of ice sheets.

Fast-flowing ice streams (blue and white areas) drain the interior of the East and West Antarctic ice sheets, controlling ice sheet mass balance and discharge to the oceans. Image: Jonathan Bamber

Why study palaeo-ice streams?

Therefore, the landforms and sediments left behind by palaeo–ice streams in areas like North America, Scandinavia, and Britain, are very important.

Firstly, they allow glaciologists to study how ice streams have evolved over thousands to tens of thousands of years, through important stages, such as ice sheet build-up, at a glacial maximum, and during deglaciation^2,3,8,9.

Secondly, the landform record offers a window into the processes that occurred at former ice stream beds, allowing researchers study how they flowed, shifted, turned ‘on’ and ‘off’, and interacted with the landscape².

The palaeo–ice stream record, therefore, can be used to better understand how ice streams change over long timescales and under different climate conditions, in order to improve predictions of future ice sheet change.

The palaeo–ice stream landsystem

Ice streams have three important characteristics that are reflected in the landforms they create^10,11,12. First, they flow very rapidly – orders of magnitude faster than a typical valley glacier¹³ – by a combination of internal deformation, sliding, and subglacial deformation^1,10. Second, they have convergent onset zones^1,10 (onset zones are areas where ice flow changes from slow- [sheet flow] to fast-moving [stream flow] at the head of an ice stream). Third, their lateral margins are very sharp^1,10.

Characteristics of an ice stream (fast-flowing ice, a convergent onset, and sharp lateral margins) displayed at Byrd Glacier, West Antarctica. Image: NASA.

Fast ice flow

Mega-scale glacial lineations are the most striking landforms created by fast ice flow in palaeo–ice streams^14,15. They are streamlined sediment ridges formed at the bed in the main ice stream trunk zone¹⁶. You can think of these landforms as ‘stretched’ out flutes or drumlins, as they are similar in shape, but much larger and more elongate^14,15.

In size, mega-scale glacial lineations are between 10–100 kilometres long and 200–1300 metres wide¹¹, making it difficult to identify them on the ground. Instead, they are most easily mapped from satellite images (see below). When viewed from space, it is also obvious that mega-scale glacial lineations are not isolated features, but occur together in large groups. Within these groups, they run parallel to one another over great distances^11,14,15.

Mega-scale glacial lineations formed at the bed of the Duawnt Lake palaeo-ice stream in Canada (see ref. 23). Note how individual lineations are highly elongate and closely parallel each other. In this example, the palaeo-ice stream flowed from right to left. Image: Google Earth.

Convergent onset zones

Shorter subglacial bedforms, such as flutes and drumlins, form in palaeo–ice stream onset zones, where ice velocity is lower than in the ice stream trunk zone^11,17. These landforms are arranged in a fan-like pattern that flows in toward (or converges on) a narrower corridor of fast-flow landforms that include mega-scale glacial lineations.

Convergent flow in the onset zone of the Transition Bay palaeo-ice stream, Arctic Canada (see ref. 17). Also, note how this ice stream flow path (white) crosscuts an older ice stream flow path (black). Image: Google Earth.

Sharp ice stream margins

In modern ice streams, shear zones – areas of intense deformation several kilometres wide, marked by crevassing at the ice-surface¹⁸ – develop at the margins of ice streams, where fast- and slow-moving ice meet¹⁹.

Surface crevasses in a shear zone at Recovery Glacier ice stream in East Antarctica Image: NASA.

Ice stream shear margin moraines are sediment ridges deposited subglacially in the shear zone²⁰. At first glance, they look similar to mega-scale glacial lineations, but they are generally wider and longer²⁰. Shear margin moraines can be used to identify the edges (and thus lateral extent) of palaeo–ice streams^11,12.

Shear margin moraine (arrowed) with a fast-flow assemblage (e.g. drumlins, mega-scale glacial lineations) ‘inside’ the palaeo-ice stream flow path (right of shear moraine) and ice-stagnation landforms ‘outside’ the ice stream flow path (left of shear moraine). Example from the M’Clintock Channel palaeo-ice stream in Arctic Canada (see ref. 20). Image: Google Earth.

Flow-direction changes

Ice streams do not always follow the same flow pathway; they are capable of switching flow-direction over time owing to glaciological (e.g. ice thickness) or topographic (e.g. basin infilling) changes^9,21.

In the palaeo–ice stream landsystem, flow-direction changes can be mapped where one group of flow assemblages (e.g. drumlins) crosscuts another^11,12,14. It is usually possible to work out the relative order of flow changes by studying the pattern of crosscutting (see the Transition Bay palaeo-ice stream diagram above).

Ice stream shutdown

While the palaeo–ice stream landsystem is dominated by features relating to fast ice-flow (e.g. mega-scale glacial lineations), these may be overprinted by other landform assemblages. For example, during deglaciation, moraine ridges and ice-stagnation landforms may be deposited over the top of fast-flow landforms as the active ice-front moves back^2,11,12.

Similarly, ribbed moraines (transverse sediment ridges) may form over the top of glacial lineations²². Ribbed moraines are thought to form where ice-flow changes from an extensional (ice streaming) to a compressional regime. Where they lie on top of glacial lineations, therefore, they may record the slowing or shutdown of palaeo-ice streams during ice sheet deglaciation²².

Ribbed moraines lying on top of glacial lineations at the bed of the former Dubawnt Lake palaeo-ice stream. This ordering of landform assemblages records ice stream shutdown during deglaciation (see ref. 22).

Summary

Ice streams shape the land surface they flow over, leaving behind a distinctive landsystem¹¹that includes mega-scale glacial lineations, which record the passage of fast-moving ice¹⁴, convergent bedforms in onset zones, and shear margin moraines that mark their sharp lateral margins²⁰. In addition, the palaeo–ice stream landsystem often displays evidence of dynamic ice sheet changes^5,6, such as switches in flow-direction^9,21 (crosscutting landforms) and velocity.

References

1. Bennett, M.R., 2003. Ice streams as the arteries of an ice sheet: their mechanics, stability and significance. Earth-Science Reviews, 61, 309-339.

2. Stokes, C.R. and Clark, C.D., 2001. Palaeo-ice streams. Quaternary Science Reviews, 20, 1437-1457.

3. Livingstone, S.J., Cofaigh, C.Ó., Stokes, C.R., Hillenbrand, C.D., Vieli, A. and Jamieson, S.S., 2012. Antarctic palaeo-ice streams. Earth-Science Reviews, 111, 90-128.

4. Margold, M., Stokes, C.R., Clark, C.D. and Kleman, J., 2015. Ice streams in the Laurentide Ice Sheet: a new mapping inventory. Journal of Maps, 11, 380-395.

5. Kleman, J., Hättestrand, C., Borgström, I. and Stroeven, A., 1997. Fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model. Journal of glaciology, 43, 283-299.

6. Hughes, A.L., Clark, C.D. and Jordan, C.J., 2014. Flow-pattern evolution of the last British Ice Sheet. Quaternary Science Reviews, 89, 148-168.

7. Rignot, E., Velicogna, I., van den Broeke, M.R., Monaghan, A. and Lenaerts, J.T., 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, 38 (5).

8. Stokes, C.R., Margold, M., Clark, C.D. and Tarasov, L., 2016. Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation. Nature, 530, 322-326.

9. Ó Cofaigh, C., Evans, D.J. and Smith, I.R., 2010. Large-scale reorganization and sedimentation of terrestrial ice streams during late Wisconsinan Laurentide Ice Sheet deglaciation. GSA Bulletin, 122, 743-756.

10. Clark, C.D., 1999. Glaciodynamic context of subglacial bedform generation and preservation. Annals of Glaciology, 28, 23-32.

11. Clark, C.D and Stokes, C.R. 2003. Palaeo-ice stream landsystem. In Evans, D.J.A. (Ed.) Glacial Landsystems. Hodder–Arnold, UK.

12. Stokes, C.R. and Clark, C.D., 1999. Geomorphological criteria for identifying Pleistocene ice streams. Annals of Glaciology, 28, 67-74.

13. Rignot, E., Mouginot, J. and Scheuchl, B., 2011. Ice flow of the Antarctic ice sheet. Science, 333, 1427-1430.

14. Clark, C.D., 1993. Mega‐scale glacial lineations and cross‐cutting ice‐flow landforms. Earth Surface Processes and Landforms, 18, 1-29.

15. Stokes, C.R. and Clark, C.D., 2002. Are long subglacial bedforms indicative of fast ice flow? Boreas, 31, 239-249.

16. King, E.C., Hindmarsh, R.C. and Stokes, C.R., 2009. Formation of mega-scale glacial lineations observed beneath a West Antarctic ice stream. Nature Geoscience, 2, 585-588.

17. Angelis, H.D. and Kleman, J., 2008. Palaeo‐ice‐stream onsets: examples from the north‐eastern Laurentide Ice Sheet. Earth Surface Processes and Landforms, 33, 560-572.

18. Raymond, C., 1996. Shear margins in glaciers and ice sheets. Journal of Glaciology, 42, 90-102.

19. Schoof, C. 2004. On the mechanics of ice-stream shear margins. Journal of Glaciology, 50, 208-218.

20. Stokes, C.R. and Clark, C.D., 2002. Ice stream shear margin moraines. Earth Surface Processes and Landforms, 27, 547-558.

21. Winsborrow, M.C., Stokes, C.R. and Andreassen, K., 2012. Ice-stream flow switching during deglaciation of the southwestern Barents Sea. GSA Bulletin, 124, 275-290.

22. Stokes, C.R., Lian, O.B., Tulaczyk, S. and Clark, C.D., 2008. Superimposition of ribbed moraines on a palaeo‐ice‐stream bed: implications for ice stream dynamics and shutdown. Earth Surface Processes and Landforms, 33, 593-609.

23. Stokes, C.R. and Clark, C.D., 2003. The Dubawnt Lake palaeo‐ice stream: evidence for dynamic ice sheet behaviour on the Canadian Shield and insights regarding the controls on ice‐stream location and vigour. Boreas, 32, 263-279.