Shelf-edge margins of the British-Irish Ice Sheet

What is the shelf-edge margin of the British-Irish Ice Sheet? | How do we know the British-Irish Ice Sheet reached the shelf edge? | How did the British-Irish Ice Sheet retreat from the shelf edge? | Why do we study shelf-edge margins of the British-Irish Ice Sheet?

What is the shelf-edge margin of the British-Irish Ice Sheet?

At its maximum extent, around 27,000 years ago1, the British-Irish Ice Sheet covered most of Britain, all of Ireland, and extended all the way to the edge of the continental shelf2. The continental shelf is the shallow part of the ocean that surrounds most land, before the continental slope drops into the deep oceans. Around Britain and Ireland, this continental shelf extends 100-150 km from the present-day coastline. It is generally shallower than 120 m below present-day sea level. The maximum extent of the British-Irish Ice Sheet, crossing over the continental shelf, is shown in Figure 1.

Figure 1. The maximum extent of the British-Irish Ice Sheet, extending over the continental shelf (green) to reach the ocean at the shelf edge (blue). Trough-mouth fans are shown in yellow outlines, taken from the BRITICE v2 dataset7. Bathymetry data are from the GEBCO 2016 dataset (https://www.gebco.net/data_and_products/gridded_bathymetry_data/) and land elevation from the European Environment Agency EU-DEM (https://www.eea.europa.eu/data-and-maps/data/eu-dem)

Figure 2 shows a cross-section that runs from the northwest to the southeast from the deep ocean of the Rockall Trough through to the Cairngorm mountains of Scotland. The ice sheet margin is located at the shelf edge because in deeper water, the ice floats and becomes an ice shelf. Thick ice, which built up in the accumulation zones of the Outer Hebrides and Scottish domes1, spread out across the flat topography of the continental shelf to reach the margin when the ice sheet was at its greatest volume.

Figure 2. A cross-section from the deep ocean to the mountains, showing the relationship of the British-Irish Ice Sheet to the continental shelf. Bathymetry data are from the GEBCO 2016 dataset (https://www.gebco.net/data_and_products/gridded_bathymetry_data/) and land elevation from the European Environment Agency EU-DEM (https://www.eea.europa.eu/data-and-maps/data/eu-dem)

How do we know the British-Irish Ice Sheet reached the shelf edge?

Evidence for the ice sheet reaching the shelf edge comes from marine geological surveys. These geophysical surveys tell us the shape of the seabed, and the pattern of sediment layers under the seabed. Such surveys reveal glacial geomorphology all over the continental shelf, right out to the shelf edge3. At the shelf edge, large trough-mouth fans are present (Figure 3), which tell us that large volumes of sediment were carried to the shelf edge by ice, then tipped over the side and down the continental slope. The style of these fans, the type of glacial geomorphology, and the depth of the shelf edge, tell us that the British-Irish Ice Sheet ended in the ocean. This means the British-Irish Ice Sheet was marine-based, much like the West Antarctic Ice Sheet is today.

Figure 3. Trough-mouth fans formed at the margin of the British-Irish Ice Sheet. Trough-mouth fans are shown in yellow outlines, taken from the BRITICE v2 dataset7. Bathymetry data are from the GEBCO 2016 dataset (https://www.gebco.net/data_and_products/gridded_bathymetry_data/) and land elevation from the European Environment Agency EU-DEM (https://www.eea.europa.eu/data-and-maps/data/eu-dem)

How did the British-Irish Ice Sheet retreat from the shelf edge?

Figure 4. Ice streams of the British-Irish Ice Sheet and their relation to trough-mouth fans and the shelf-edge margin. Background image from Gandy et al., 20196.

Along the shelf edge, the British-Irish Ice Sheet had numerous ice streams (Figure 4). These ice streams moved vast amounts of ice rapidly from the accumulation zone of the ice sheet out to the shelf edge. Ice streams are vulnerable to rapid retreat when they are marine-terminating, a process known as marine ice sheet instability. This instability can cause rapid, irreversible retreat, and is suggested to have caused the demise of the Minch Ice Stream4. The removal of ice shelves and retreat of ice streams can cause a debuttressing effect5, which caused the retreat of the ice sheet between the ice streams. De Geer moraines are moraines that form underwater, and are present on the continental shelf north of Scotland (Figure 5). These moraines tell us that the sectors of ice between the ice streams, known as interstreams, retreated more slowly as sea level rose3, as ice from the accumulation zones flowed more slowly out to an ice margin, which was calving icebergs straight into the sea1.

Figure 5. Glacial geomorphology offshore Orkney, showing retreat moraines and De Geer moraines, based on Bradwell et al. (2008)3 and Bradwell and Stoker (2015)8. Bathymetry data are from the GEBCO 2016 dataset (https://www.gebco.net/data_and_products/gridded_bathymetry_data/) and land elevation from the OS Terrain 5 dataset (https://www.ordnancesurvey.co.uk/business-government/products/terrain-5)

Why do we study shelf-edge margins of the British-Irish Ice Sheet?

The marine ice sheet instability theory could mean that present day, marine-based ice sheets, such as the West Antarctic Ice Sheet, may retreat rapidly4. That means studying past marine-based ice sheets, such as the British-Irish Ice Sheet, becomes important to understand what may happen in the future. Theories on the effect of marine ice sheet instability can be tested using models based on actual ice sheet data, such as in the Minch Ice Stream4. These models tell us that ice streams may be vulnerable to marine ice sheet instability6, but interstream areas may be more dependent on calving, and topography beneath the ice sheet, and less susceptible to runaway retreat1,3.

References

1.          Clark, C. D., Hughes, A. L. C., Greenwood, S. L., Jordan, C. & Sejrup, H. P. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quat. Sci. Rev. 44, 112–146 (2012).

2.          Hughes, A. L. C., Gyllencreutz, R., Lohne, Ø. S., Mangerud, J. & Svendsen, J. I. The last Eurasian ice sheets – a chronological database and time-slice reconstruction, DATED-1. Boreas 45, 1–45 (2016).

3.          Bradwell, T. et al. The northern sector of the last British Ice Sheet: Maximum extent and demise. Earth-Science Rev. 88, 207–226 (2008).

4.          Gandy, N. et al. Marine ice sheet instability and ice shelf buttressing of the Minch Ice Stream, northwest Scotland. Cryosph. 12, 3635–3651 (2018).

5.          Sejrup, H. P., Clark, C. D. & Hjelstuen, B. O. Rapid ice sheet retreat triggered by ice stream debuttressing: Evidence from the North Sea. Geology 44, 355–358 (2016).

6.          Gandy, N. et al. Exploring the ingredients required to successfully model the placement, generation, and evolution of ice streams in the British-Irish Ice Sheet. Quat. Sci. Rev. 223, 105915 (2019).

7.          Clark, C. D. et al. BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British-Irish Ice Sheet. Boreas 47, 11-e8 (2018).

8.          Bradwell, T. & Stoker, M. S. Asymmetric ice-sheet retreat pattern around northern Scotland revealed by marine geophysical surveys. Earth Environ. Sci. Trans. R. Soc. Edinburgh 105, 297–322 (2015).