A look at how sea-level has changed since the last glacial period around 10,000 years ago.

1. Holocene sea-level change Professor Simon K. Haslett Centre for Excellence in Learning and Teaching Simon.haslett@newport.ac.uk 1st July 2010

2. Introduction The Holocene (or Flandrian in Britain) is the most recent Epoch of the Quaternary Period. It is an interglacial that has followed the last Pleistocene glaciation, known in Britain as the Devensian Glacial stage. Upon the melting of the Devensian ice sheets, sea-levels changed through a combination of eustasy and isostasy to achieve their present levels. This presentation describe how sea-level has changed since the last ice age, and also how records of sea-level change are established; through the construction of sea-level curves.

4. The main controlling factor of both eustasy and isostasy is the expansion and contraction of continental ice sheets over successive glacial/interglacial cycles.

5. Because of this, global sea-level change that results from the repeated extraction of water from the oceans, and its subsequent return on melting, is referred to as glacio-eustasy.

7. Devensian glaciation 1 At the height of the Devensian ice age (the last cold stage) at around 18 ka, enough water had been removed from the oceans by expanding ice sheets to reduce global sea-level by ~130m. This glacio-eustatic lowering was accompanied by the glacio-isostatic depression of Fennoscandia, northern Britain and Canada through GLACIAL LOADING. Forbulging occurs where ice loading depresses the crust, which is then compensated beyond the ice perimeter by a bulging of the crust. Following the melting of the Northern Hemisphere ice sheets, which began around 16 ka, global sea-level rose steadily while melting of the continental ice sheets resulted in rapid glacio-isostatic recovery.

8. Devensian glaciation 2 Shorelines that formed around the margins of the melting ice sheets were progressively raised above sea-level as glacio-isostatic rebound outpaced glacio-eustatic sea-level rise. Detailed analysis of raised shorelines and associated features provides evidence of the extent of glacio-isostatic rebound since deglaciation. In eastern Scotland, for example, isobase maps (maps showing lines of equal rebound or subsidence) indicate that over 250 m of rebound has occurred since deglaciation, and the amount of rebound further west near the centre of the Devensian ice sheet on Rannock Moor was even greater. Rannock Moor, Scotland (UK).

9. Holocene rising 1 In all glacially depressed areas, the process of land emergence has continued throughout the Holocene. In Scotland, glacio-isostatic rebound is still incomplete, and raised shoreline data indicate that in the inner Forth, Clyde and Tay valleys’ current rates of rebound range from 1.8 to 2 mmyrˉ1. In southern Britain and the southern North Sea isostatic depression has continued throughout the Holocene, producing submerged forests. Repeated rebound and subsidence results in a see-saw effect around a fulcrum line. In the North Sea, the Dogger Bank was submerged beneath the rising Holocene sea by 8.7 Ka BP, and the Straits of Dover were breached just before 8 Ka BP. The present configuration of the coastline of southern Britain was more or less established by 7.5 – 7.8 Ka BP.

10. Holocene rising 2 Holocene sea-level rise in the Bristol Channel area rose from -35 m OD at 9.5 Ka BP to 2-5 m OD at 5 Ka BP at the following rates: After around 6 Ka BP marine incursion into coastal areas of northwest Europe took place more slowly. The configuration of the British coastline was similar to the present day, except it was more indented due to the drowning of wetlands and estuaries which have subsequently silted up.

11. Absolute sea-level change The pattern of absolute sea-level change at the end of the ice age has been difficult to establish, principally because it is difficult to find stable coasts. However, new approaches using oxygen isotopes from deep-sea cores are beginning to provide an indication of global sea-level trends during the period of ice melting. The data suggest that at 14.5 Ka BP, sea-levels stood around -100 m, but a rapid rise of 40 m occurred up to 13 Ka BP at a rate of 3.7 m per century. A second major melting phase at 11 Ka BP raised eustatic sea-level to around – 40 m by the beginning of the Holocene (10 Ka BP) at a rate of 2.5 m per century, by which time global ice volumes had been reduced by over 50%.

12. Isostatic recovery During the late glacial period, however, rapid glacio-isostatic recovery in NW Europe outpaced sea-level rise and therefore the shorelines formed during that period now stand well above the present shorelines. In Scotland, the highest late glacial shorelines, dated at 13 ka BP now stand 50 and 41 mOD on the east and west coasts respectively. In the early Holocene, however, eustatic rise at rates of 1 cmyrˉ¹, began to exceed isostatic recovery in many areas. This resulted in a major marine transgression around the coastline of Scotland between 8.5 and 6.5 BP. After 6 ka BP in Scotland isostatic recovery once again outpaced eustatic rise.

13. Presently? Eustatic sea-level is still rising. Tide gauge data from numerous localities suggest that over the course of the last century, global sea-levels have risen by 10-15 cm, and this may partly reflect the anthropogenically induced global warming.

15. INDICATIVE MEANING refers to the position of a SLIP with reference to the contemporary tide level.

16. TENDENCY OF SEA-LEVEL MOVEMENT – A positive tendency of sea-level movement represents an increase in marine influence at the site e.g. transgression, and a negative tendency indicates a decrease. Tendencies of sea-level movement are defined for each dated sample.

18. A = sediment autocompaction (m) (determined by geotechnical correction (Massey et al., 2006b). This is the amount of post-depositional compression of the sediment e.g. 0.35 m.

21. Mean High Water Spring Tide (MHWST) = 6 mOD

22. Mean High Water Neap Tide (MHWNT) = 5 mOD

25. SLIP No. 2: 4 mOD – 5 mOD for palaeo sea-level

27. Summary Sea-level change has been dynamic throughout the Holocene epoch. There is a balance between the absolute level of the sea and vertical movement of the land. Glacio-isostatic recovery is responsible for land emergence and reclamation due to the unloading of weights (land ice). The British coastline ~6000 Ka was similar to how it appears today. Absolute sea-level change is difficult to discern. Sea-levels continue to rise, possibly as a consequence of global warming. The record of sea-level change is established through the creation of sea-level curves based on data-points known as Sea-Level Index Points (SLIPs). To construct a sea-level curve, the age, altitude, indicative meaning, and sea-level tendency must be known for each SLIP. Indicative meaning describes the relationship of a SLIP to a contemporaneous tide level.

28. References Haslett, S.K. 2008. Coastal Systems (2nd ed.). Routledge, 240pp. Haslett, S. K., Davies, P., Curr, R. H. F., Davies, C. F. C., Kennington, K., King, C. P. and Margetts, A. J. 1998. Evaluating late-Holocene relative sea-level change in the Somerset Levels, southwest Britain. The Holocene, 8: 197-207. Lambeck, K. 1995. Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound. Journal of the Geological Society, 152: 437-448. Lowe, J. J. and Walker, M. J. C. 1997. Reconstructing Quaternary Environments (2nd ed.). Longmans, 472pp. Massey, A.C. 2004. Holocene sea-level changes along the Channel coast of south-west England. Unpublished PhD Thesis. University of Plymouth, Plymouth, Devon, UK. 330 pp. Massey, A.C., Gehrels, W.R., Charman, D.J. and White, S.V. 2006a. An intertidal Foraminifera-based transfer function for reconstructing Holocene sea-level change in southwest England. Journal of Foraminiferal Research, 36(3): 215–232. Massey, A.C., Paul, M.A., Gehrels, W.R. and Charman, D.J. 2006b. Autocompaction in Holocene coastal back-barrier sediments from south Devon, southwest England, UK. Marine Geology, 226(3-4): 225–241. Massey, A.C., Gehrels, W.R., Charman, D.J., Milne, G.A., Peltier, W.R., Lambeck, K. and Selby, K.A. 2008. Relative sea-level change and postglacial isostatic adjustment along the coast of south Devon, United Kingdom. Journal of Quaternary Science, 23, 415-433. Preece, R. C. (ed.). 1995. Island Britain: a Quaternary perspective. Geological Society Special Publication, No. 96, 274pp. Shennan, I., Innes, J. B., Long, A. J. and Zong, Y. 1994. Late Devensian and Holocene relative sea-level at Loch nanEala, near Arisaig, northwest Scotland. Journal of Quaternary Science, 9: 261-282.

29. This resource was created by the University of Wales, Newport and released as an open educational resource through the 'C-change in GEES' project exploring the open licensing of climate change and sustainability resources in the Geography, Earth and Environmental Sciences. The C-change in GEES project was funded by HEFCE as part of the JISC/HE Academy UKOER programme and coordinated by the GEES Subject Centre. This resource is licensed under the terms of the Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales license (http://creativecommons.org/licenses/by-nc-sa/2.0/uk/). All images courtesy of Professor Simon Haslett. However the resource, where specified below, contains other 3rd party materials under their own licenses. The licenses and attributions are outlined below: The name of the University of Wales, Newport and its logos are unregistered trade marks of the University. The University reserves all rights to these items beyond their inclusion in these CC resources. The JISC logo, the C-change logo and the logo of the Higher Education Academy Subject Centre for the Geography, Earth and Environmental Sciences are licensed under the terms of the Creative Commons Attribution -non-commercial-No Derivative Works 2.0 UK England & Wales license. All reproductions must comply with the terms of that license.