Holocene sea-level change Professor Simon K. Haslett Centre for Excellence in Learning and Teaching Simon.firstname.lastname@example.org 1st July 2010
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.
Eustasy and isostasy 1 Changing levels of land and sea reflect the interplay of two major elements: EUSTASY – global changes in sea-level. ISOSTASY – localised tectonic activity which results in vertical displacement of the land.
Isostasy refers to the state of balance that exists in the earth’s crust so that depression in one locality will be compensated for by a rise in the crust elsewhere.
The main controlling factor of both eustasy and isostasy is the expansion and contraction of continental ice sheets over successive glacial/interglacial cycles.
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.
Similarly, crustal deformation caused by loading of glacier ice is termed glacio-isostasy.
Eustasy and isostasy 2 Changes in sea-level that take place through the interplay of these factors are known as RELATIVE SEA-LEVEL CHANGES, and are usually local changes in the position of sea-level relative to the land. In tectonically stable areas, evidence for sea-level change should reflect only the eustatic component and such regions record ABSOLUTE SEA-LEVEL CHANGES. A raised sea arch in western Scotland (UK).
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.
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).
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.
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.
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%.
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.
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.
Constructing sea-level curves 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). The age, altitude, indicative meaning, and sea-level tendency must be known for each SLIP before it may be employed to construct a sea-level curve:
AGE can be ascertained by, for example:
Dated pollen sequences Radiocarbon (14C) dating of organic material (60 ka limit)
ALTITUDE – leveling to bench marks OD e.g. estuarine and tidal flats.
INDICATIVE MEANING refers to the position of a SLIP with reference to the contemporary tide level.
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.
INDICATIVE RANGE refers to the accuracy with which the indicative meaning is applied.
Calculating SLIPs 1 The altitude of sea-level index points (SLIPs) can be calculated according to Massey et al. (2008): SLIP (m MTL) = H - D - I + A + C
Where;H = height of the back-barrier marsh surface (m MTL) from survey (m OD) and the use of tide tables:
E.g. if marsh height is 4.32 mOD OD is 0.11 m above MTL H = 4.32 – 0.11 = 4.21 m MTL
D = sample depth below ground level (m) distance down-core to middle of sample slice e.g. 8.755 m.
Calculating SLIPs 2
I = sample indicative meaning (m MTL). (calculated from counts of fossil Foraminifera by a Transfer Function e.g. Massey et al. (2006a)). Indicative meaning is the height at which a sea-level indicator was deposited in relation to a reference tide level e.g. 2.1575 m.
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.
C = core compaction from percussion drilling (m) (determined by measuring the length of retrieved sediment in each 1 m core tube). E.g. 0.00 m.
Therefore, SLIP (m MTL) = 4.21 – 8.755 – 2.1575 + 0.35 + 0.0 = -6.3525 ± 0.29 m MTL (±0.29 m = RMSEP from modern analogue of dead Foraminifera).
When the altitude (y-axis) is determined the sample must be dated to give the x-axis co-ordinate.
Examples of types of sample that are commonly radiocarbon dated include: charcoal, wood, twigs and seeds; marine, estuarine and riverine shells; peat, and Foraminifera.
Indicative meaning 1 Example from Haslett (2008), p .142.
SLIP No. 2: 4 mOD – 5 mOD for palaeo sea-level
Therefore, sea-level was at -1 mOD 4000 years ago.
Indicative meaning 3 Establishing the indicative meaning of a potential sea-level index point can really only be achieved through using biological remains (fossils). Many organisms live at, and are restricted to particular tide levels. Recovering these tidally specific organisms from either sediments or erosional rock surfaces can tell us quite precisely the indicative meaning of a deposit or landform. Analysis of diatoms, foraminifera, amoebae, ostracods, molluscs, crustaceans, pollen and other plant remains have been used with some degree of success in determining indicative meaning.
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.
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.
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