The Last Ice Sheet in the North Sea

Parts of this article are modified from Andy Emery’s PhD thesis.

What was the latest ice sheet in the North Sea? | How do we know about the latest ice sheet in the North Sea? | How big was the latest ice sheet in the North Sea? | What happened to the latest ice sheet in the North Sea? | Were there previous ice sheets in the North Sea?

What was the latest ice sheet in the North Sea?

The North Sea, now a large body of water, was once underneath the British-Irish and Scandinavian Ice Sheets1,2. During the Last Glacial Maximum, sea level was much lower around the globe. This meant that the bed of the southern North Sea was above sea level. Ice, which accumulated in the British-Irish and Scandinavian ice sheets, flowed down into the relatively flat, low basin of the North Sea bed.

Figure 1. Coverage of the latest ice sheet in the North Sea at the Last Glacial Maximum. Compiled from Sejrup et al., 200529, Bradwell et al., 200832 and Sejrup et al., 201610. 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 1 shows the approximate extent of the latest ice sheet in the North Sea. This phase of ice occupying the North Sea began around 32,000 years ago3. It reached its maximum extent around 27,000 years ago1,4. Ice had gone from the North Sea by 17,000 years ago5, leaving the North Sea basin exposed as land. Archaeologists call this low-lying area that was land when sea level was lower “Doggerland”6.

How do we know about the latest ice sheet in the North Sea?

Much like shelf-edge glaciation, advances in marine surveying technology slowly revealed more and more information, and our understanding of the ice sheet in the North Sea slowly grew7. Figure 2 shows how our estimates of the extent of the ice sheet evolved as more data became available. Initially, it was assumed that the British-Irish Ice Sheet was relatively small, and did not join the Scandinavian Ice Sheet7. As regional marine mapping programs, such as that run by the British Geological Survey8, and industry acquired more data, fresh evidence allowed us to discover that the two ice sheets had coalesced to form the Eurasian Ice Sheet complex2,7.

Figure 2. “Spider trail” diagram, a compilation of all the published estimates of the extent of the British-Irish Ice Sheet at the Last Glacial Maximum. References8,10,13,23-33. Taken from Emery (2020) PhD Thesis. 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 big was the latest ice sheet in the North Sea?

Despite understanding more and more about the latest ice sheet in the North Sea, we still don’t know exactly how far south in reached during the Last Glacial Maximum9. Figure 3 shows some of the reconstructions of the extent relative to Dogger Bank, the shallowest point of the North Sea. The reason this section of the ice sheet is still poorly constrained is that not enough detail exists within marine datasets to pinpoint the southern margin.

Figure 3. Compilation of extents proposed as the southern margin of the latest British-Irish Ice Sheet in the North Sea, modified after Emery et al. (2019)9. References4,8,10,29,30,34. 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).

Studies such as Sejrup et al. (2016)10 and Phillips et al. (2018)11 argue that ice covered Dogger Bank and extended further south, into the flat area known as Oyster Ground. However, based on evidence seen in high-resolution offshore windfarm site data, close to the southern slope of Dogger Bank, Emery et al. (2019)9 place the margin at the southern edge of Dogger Bank. More high-resolution data is required from south of Dogger Bank to find the exact margin.

Another uncertainty is the thickness of the latest ice sheet in the North Sea. Knowing how thick it was, and where it was thickest, would allow us to understand more how the ice sheet evolved. Some attempts to model ice thickness in the North Sea have given estimates of ice thickness12–15, but these models do not match the extent of the ice sheet we know from geomorphological evidence, so the thicknesses they estimate may be inaccurate.

What happened to the latest ice sheet in the North Sea?

After reaching its maximum extent at around 27,000 years ago, the latest ice sheet in the North Sea remained relatively stable for a few thousand years4. At Dogger Bank, the southern margin of the ice sheet retreated northwards by around 24,000 years ago, but then its retreat slowed once it was off Dogger Bank9. The ice sheet remained stable in the North Sea until about 22,000 years ago, until it began to unzip from the Scandinavian Ice Sheet16, separating completely by 20,000 years ago17.

Figure 4. Timeline of the advance and retreat of various sectors of the latest British-Irish Ice Sheet in the North Sea basin. Taken from Emery (2020) PhD thesis. References4,5,9,10,16,17

At the same time, ice began to flow quickly southwards down what is now the east coast of Britain, reaching as far south as North Norfolk by 22,000 years ago. This is called the North Sea Lobe (Figure 3). This lobe began to retreat around 20,000 years ago, and had retreated back to North Yorkshire, and out of the North Sea basin entirely, by 17,000 years ago18. The timeline in Figure 4 captures the complex pattern of ice advance and retreat from different areas.

Were there previous ice sheets in the North Sea?

Yes! The Last Glacial Maximum ice sheet was the latest in a series of ice sheets that go back at least 2.5 million years in the North Sea19. More recently, there have been two other large ice sheets in the North Sea, namely the Anglian/Elsterian (~450,000 years ago) and Wolstonian/Saalian (370,000-130,000 years ago) ice sheet20 (Figure 5). The evidence for multiple ice sheet episodes in the North Sea comes from 3D seismic reflection surveys, which allow us to look into the subsurface over a wide area. These surveys reveal many generations of tunnel valleys, valleys incised under an ice sheet.

Figure 5. The Elsterian/Anglian, Saalian/Wolstonian and Weichselian/Devensian ice sheet extents in the North Sea. The Elsterian/Anglian and Saalian/Wolstonian ice sheets were much larger than the Last Glacial Maximum Weichselian/Devensian ice sheet. From Eaton et al., 202035.

There are at least seven generations of tunnel valleys at different stratigraphic levels, which means there have been at least seven glacial cycles that form tunnel valleys, before the Last Glacial Maximum21,22. Although their maximum extent is not precisely understood, we know they were both bigger ice sheets than the Last Glacial Maximum British-Irish Ice Sheet because of tunnel valleys found further to the south of the latest ice sheet in the North Sea.

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.          Sejrup, H. P. et al. Quaternary glaciations in southern Fennoscandia: Evidence from southwestern Norway and the northern North Sea region. Quat. Sci. Rev. 19, 667–685 (2000).

4.          Roberts, D. H. et al. Ice marginal dynamics of the last British-Irish Ice Sheet in the southern North Sea: Ice limits, timing and the influence of the Dogger Bank. Quat. Sci. Rev. 198, 181–207 (2018).

5.          Evans, D. J. A. et al. Glacial Lake Pickering: Stratigraphy and chronology of a proglacial lake dammed by the North Sea Lobe of the British-Irish Ice Sheet. J. Quat. Sci. 32, 295–310 (2016).

6.          Coles, B. J. Doggerland: a Speculative Survey. Proc. Prehist. Soc. 64, 45–81 (1998).

7.          Phillips, E. R., Hodgson, D. M. & Emery, A. R. The Quaternary geology of the North Sea basin. J. Quat. Sci. 32, 117–126 (2017).

8.          Cameron, T. D. J. et al. United Kingdom offshore regional report: the geology of the southern North Sea. (HMSO, London, 1992).

9.          Emery, A. R. et al. Left High and Dry: Deglaciation of Dogger Bank, North Sea, Recorded in Proglacial Lake Evolution. Front. Earth Sci. 7, 1–27 (2019).

10.        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).

11.        Phillips, E. et al. Large-scale glacitectonic deformation in response to active ice sheet retreat across Dogger Bank (southern central North Sea) during the Last Glacial Maximum. Quat. Sci. Rev. 179, 24–47 (2018).

12.        Boulton, G. & Hagdorn, M. Glaciology of the British Isles Ice Sheet during the last glacial cycle: form, flow, streams and lobes. Quat. Sci. Rev. 25, 3359–3390 (2006).

13.        Hubbard, A. L. et al. Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British-Irish ice sheet. Quat. Sci. Rev. 28, 758–776 (2009).

14.        Patton, H. et al. Deglaciation of the Eurasian ice sheet complex. Quat. Sci. Rev. 169, 148–172 (2017).

15.        Kuchar, J. et al. Evaluation of a numerical model of the British-Irish ice sheet using relative sea-level data: implications for the interpretation of trimline observations. J. Quat. Sci. 27, 597–605 (2012).

16.        Merritt, J. W., Connell, E. R. & Hall, A. M. Middle to Late Devensian glaciation of north-east Scotland: implications for the north-eastern quadrant of the last British-Irish ice sheet. J. Quat. Sci. 32, 276–294 (2017).

17.        Bradwell, T. et al. Pattern, style and timing of British–Irish Ice Sheet retreat: Shetland and northern North Sea sector. J. Quat. Sci. jqs.3163 (2019) doi:10.1002/jqs.3163.

18.        Livingstone, S. J. et al. Late Devensian deglaciation of the Tyne Gap Palaeo-Ice Stream, northern England. J. Quat. Sci. 30, 790–804 (2015).

19.        Rea, B. R. et al. Extensive marine-terminating ice sheets in Europe from 2.5 million years ago. Sci. Adv. 4, eaar8327 (2018).

20.        Graham, A. G. C., Stoker, M. S., Lonergan, L., Bradwell, T. & Stewart, M. A. The Pleistocene Glaciations of the North Sea Basin. in Quaternary Glaciations – Extent and Chronology, 2nd edition (eds. Ehlers, J. & Gibbard, P. L.) 261–278 (Elsevier, 2011). doi:10.1016/B978-0-444-53447-7.00021-0.

21.        Stewart, M. A. & Lonergan, L. Seven glacial cycles in the middle-late Pleistocene of northwest Europe: Geomorphic evidence from buried tunnel valleys. Geology 39, 283–286 (2011).

22.        Stewart, M. A., Lonergan, L. & Hampson, G. 3D seismic analysis of buried tunnel valleys in the central North Sea: Morphology, cross-cutting generations and glacial history. Quat. Sci. Rev. 72, 1–17 (2013).

23.        Jansen, J. H. F., Van Weering, T. C. & Eisma, D. Late Quaternary Sedimentation in the North Sea. in The quaternary history of the North Sea 175–187 (Acta Universitatis, Symposium Universitatis Usaliensis Annum Quingentesinum Celebrantis, 1979).

24.        Bowen, D. Q., Rose, J., McCabe, A. M. & Sutherland, D. G. Correlation of quaternary glaciations in England, Ireland, Scotland and Wales. Quat. Sci. Rev. 5, 299–340 (1986).

25.        Sejrup, H. P. et al. Quaternary stratigraphy of the Fladen area, central North Sea: a multidisciplinary study. J. Quat. Sci. 2, 35–58 (1987).

26.        Long, D. et al. The Sedimentary Record of Climatic Variation in the Southern North Sea [and Discussion]. Philos. Trans. R. Soc. B Biol. Sci. 318, 523–537 (1988).

27.        Hall, A. M. & Bent, A. J. A. The limits of the last British ice sheet in northern Scotland and the adjacent shelf. Quat. Newsl. 61, 2–12 (1990).

28.        Ehlers, J. & Wingfield, R. The extension of the Late Weichselian/Late Devensian ice sheets in the North Sea Basin. J. Quat. Sci. 6, 313–326 (1991).

29.        Sejrup, H. P. et al. Pleistocene glacial history of the NW European continental margin. Mar. Pet. Geol. 22, 1111–1129 (2005).

30.        Carr, S. J., Holmes, R., van der Meer, J. J. M. & Rose, J. The Last Glacial Maximum in the North Sea Basin: Micromorphological evidence of extensive glaciation. J. Quat. Sci. 21, 131–153 (2006).

31.        Ballantyne, C. K. Extent and deglacial chronology of the last British-Irish Ice Sheet: Implications of exposure dating using cosmogenic isotopes. J. Quat. Sci. 25, 515–534 (2010).

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

33.        Brooks, A. J. et al. Postglacial relative sea-level observations from Ireland and their role in glacial rebound modelling. J. Quat. Sci. 23, 175–192 (2008).

34.        Hjelstuen, B. O., Sejrup, H. P., Valvik, E. & Becker, L. W. M. Evidence of an ice-dammed lake outburst in the North Sea during the last deglaciation. Mar. Geol. 402, 0–1 (2017).

35.        Eaton, S., Hodgson, D. M., Barlow, N. L. M., Mortimer, E. & Mellett, C. L. Palaeogeographic changes in response to glacial-interglacial cycles, as recorded in Middle and Late Pleistocene seismic stratigraphy, southern North Sea. J. Quat. Sci. 1–16 (2020) doi:10.1002/jqs.3230.