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Saturday, 4 December 2010

Isostatic adjustments in Southern England



We cannot get a reliable answer to the question"Was there ever any glacier ice on Salisbury Plain?" without fully understanding the relative positions of land and sea during past glacial and interglacial episodes.  For the Anglian Glaciation (if that's really the one we are looking at) we have already concluded that the only way to explain the emplacement of giant erratics along the south coasts of Cornwall and Devon -- and further to the east -- is to assume that the isostatic depression of the Channel Coasts must have been considerable at a time when ice floes and icebergs were carrying around piles of glacial debris from the north and west.  That means Southern England was a depressed area rather than a "forebulge" area -- that means ice in the southern counties, whether we like it or not.


What about the evidence from Southern England during the Late Devensian and Holocene (post-glacial) episodes?  What can we glean from the studies of eustasy and isostasy in the literature?  There is a huge literature -- and I think we can say that the eustatic side of the equation is fairly well known.  Previous posts contain the best sea level curves we can find.  I have also added an excellent summary below from Prof Simon Haslett.


The maps above come from the following fascinating presentation:

Locating the Centre of Holocene Glacio-isostatic Uplift in Scotland  -- poster presentation by
C.R.Firth, D.E.Smith and J.Rose

Thje general pattern of isobases ties in, as one would expect, with the pattern of Devensian glaciation and the thickness and weight of ice.  As Prof Haslett says, there has been c 250m of isostatic depression and then recovery beneath the thickest part of the UK ice sheet on the western side of Scotland.  As suggested in the map on the last post, to the south of this area of more or less concentric isobases there is a zone where current uplift is around zero (called the fulcrum or hinge area in somer
literature) and then south of that there is another area (including SW England) where it is assumed that there is current depression of around 1mm per year or 1m per millennium.  In the extreme SW of England it is assumed that the rate of sinking is around 2m per millennium.  There is another major area of sinking or depression in the North Sea, which may be as high as 1.5m per millennium.  This is attributed to recent high sedimentation rates, to the loading of water subsequent to the flooding of the area in post-glacial times, and to the forebulge or balancing effect partly related to the viscosity of the crust.  There may also be a tectonic sinking at play right across the North Sea - Southern England - English Channel area which may owe little or nothing to isostatic factors.

Complicated?  Yes, it is -- and we must remember that many of the maps are based on modelling exercises, backed up by as many coastal observations as possible, for processes that are very slow indeed.

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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.
Eustasy and isostasy
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.
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.

Devensian glaciation
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.
Forebulging 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.
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.

Holocene rising
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 mm per year.
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 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 cm per year, 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.


(Prof Simon Haslett, University of Wales, Newport)

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