Relative Sea Level
There is only a limited amount of water on the planet, and when the last ice age was dumping snow on land, it built many kilometre thicknesses of ice sheets on polar regions, and on mountainous areas.
The placing of ice sheets on land had two effects. The first was that the water that had been dumped on the land, frozen, reduced the amount of water in the global sea.
The second was that the weight of the ice on the land placed tremendous pressure on the geology that it rested on. That pressure forced the land that was under the ice-sheet, down into core of the planet.
It is these two characteristics that have had a major impact on post-glacial sea levels in the northern hemisphere, but as I will explain its not that simple.
Global sea-level rise
There is a limit to the extent of our ability to go back in time to measure sea levels in the Quaternary period. In many places records in the geology, for various reasons, only go back 10,000 years. Some go back 20,000 years, but that's about the limit.
This graph gives an impression of the rate and depth of sea level change in that period.
All the coastlines that are sampled above are in regions that were not capped with ice, (as far as I know!)
North Sea, sea-level rise
The sea-level data for four countries, having coastlines bordering on the North Sea. Some also border on the Atlantic Ocean.
Germany
Here, Hijma sequences sampled deposits from a wide scatter of locations, and the graph indicates that depth of the seabed in the southeast North Sea is not more than 50 metres deep, and was fully flooded just before 10,000BP.
Holland
In the above series of graphs, the sea level indicators for each of the areas sampled are charted separately. The result demonstrates that some of the locations were not inundated by the sea until quite recently in prehistoric times.
Norway
Charts for the change in relative sea-levels on the Norwegian Coasts are shown in the following series
(At the moment I have strong doubts about the theory i am putting forward here, in reference to Norway, and Roger Creel's work, and would welcome feedback. Good research involves getting it wrong , and then getting it right, or more righter, anyway!)
Charts 1,2,3, and 4 are for samples on the north coast of Norway. They all indicate that the mass of Norway rose after the loss of ice cover at around 14,000BP
Sites 5,6,7,8 and 9 show a continuation of ice sheet removal along the north coast of Norway.
A slight rise in water level in 4 out of the 5 charts at around 7000BP indicates that the considerable amount of meltwater locked up in ice filled valleys is being released, sending meltwater to increase the local sealevel.
Sites 10,11,12 and 13 continue the influences of rising land and meltwater in local sealevel rise.
Sites 14,15,16 and 17, are as above, but in sites 18, and 19 a pulse of meltwater raises local sealevel at about 12,000BP.
This pulse is emerging from the Norwegian Channel, and is continuous along the Norwegian Coast to the Skaggerak. The pulse of meltwater is supported, along the channel by a bank of sediments running along its' west side. The height of this bank that supports the pulse is likely to be in the region of 50 metres below our sea level, but it may have been much shallower. The material makeup of the west bank of the Norwegian Channel was removed during the Neolithic period of human prehistory.
This is followed by a short drop in sea level which is coincident with a short return to cold climate. Meltwater is no longer being released.
After 7000BP though the melting of valley glaciers causes local sealevel to rise again. (See also Middle and Late Holocene relative sea level changes and coastal
development at Rugard, Denmark MARIE HOLST RIIS, et al)
Sites 20 to 26 all have the meltwater pulse at 12,000BP, and, after a short dip for climate chilling, melting of the valley glaciers resumes, raising local sealevel.
Sites 27 and 28 show the continuation of plots from site 26, but Sites 29,30,31 and 32 lose this detail in the dominantly raised sealevel caused by both rising land and massive meltwater falling from the Skaggerak.
The meltwater running into the Skaggerak came from the whole of Sweden, and included blocks of ice, rocks, and sludge. These materials dropped off the southern edge of Norway gouging out the overdeepened Skaggerak. Full deglaciation of the icesheet took place within a period not much more than 300 years.
I estimate that the quantity of meltwater running through the Skaggerak from Norway, Sweden, and the Baltic Sea would been in the region of a cubic kilometre.
Britain and Ireland
Britain is an archipelago on the northwestern coast of Europe. As such, it has a small surface area, and a rather limited extent of mountainous terrain, compared to its' neighbouring areas, like Norway and the Alps of southern Europe.
The map and graph plots below shows the British Isles, with the locations of spot samples on the British coastline,
 |
| Shennan |
The plan is from:-
Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum, Ian Shennan , Sarah L. Bradley , and Robin Edwards
The abstract to the work reads:-
The new sea-level database for Britain and Ireland contains >2100 data points from 86 regions and records relative sea-level (RSL) changes over the last 20 ka and across elevations ranging from ~þ40 to 555 m. It reveals radically different patterns of RSL as we move from regions near the centre of the Celtic ice sheet at the last glacial maximum to regions near and beyond the ice limits. Validated sea-level index points and limiting data show good agreement with the broad patterns of RSL change predicted by current glacial isostatic adjustment (GIA) models. The index points show no consistent pattern of synchronous coastal advance and retreat across different regions, ~100e500 km scale, indicating that within-estuary processes, rather than decimetre- and centennial-scale oscillations in sea level, produce major controls on the temporal pattern of horizontal shifts in coastal sedimentary environments. Comparisons between the database and GIA model predictions for multiple regions provide potentially powerful constraints on various characteristics of global GIA models, including the magnitude of MWP1A, the final deglaciation of the Laurentide ice sheet and the continued melting of Antarctica after 7
ka BP.
The assembled drawing of all the sea-level plots and locations offers a reasonable interpretation of where the thickest ice of the icesheet had been. The brown dots suggest the deepest ice, and the black dots , the shallowest.
Assuming that the maximum thickness of the ice-sheet over Britain was 1000 metres, (though most authorities say it was 1500 metres), the thicknesses represented by the coloured dots would be as follows:-
Brown, 800 to 1000 metres
Yellow, 600 to 800 metres
Green, 400 to 600 metres
Blue, 200 to 400 metres
And
Black, zero to 200 metres
These dimensions are very simplistic, because the archipelago's proximity to the Atlantic Ocean had a profound effect on the location and thickness of snow on its' surface.
Above the sources of the datapoints are selected and grouped , charted together to provide a view across the study.
In Shennans work there are 5 types of data points, the marine limiting data points in blue circles, the terrestrial/freshwater limiting data points in red circles, those from basal peat, black crosses , those from within unconsolidated sediment sequences, termed intercalated data points, grey crosses, and those from lake basins, commonly called isolation basins.
For the purpose of this discussion I am considering only the marine data points and the freshwater data points.
Broadly speaking, the greater proportion of marine data points are present at dates that precede 15,000BP, and the mass of freshwater data points are from after that date.
This suggests to me that the freshwater data points represent meltwater from 15,000BP to after 10,000BP. Any coastline assessed for after 15,000BP is much more affected by melwater flowing off land than actual sea level rise.
The bottom right hand edge clearly emulates the pace of global sea level rise from 10,000BP to present day.
The marine data points rest at today's sea level, and they suggest that there is no significant deflection here from the weight of ice resting on Britain. (Though further detailed analysis of the sea level charts suggests fairly minor movements on the highland areas of Scotland).
Conclusion
Although a lot of ice was stored up on the British ice sheet it was not thick enough to compress the earth’s crust by more than a few metres. The volume of water that resulted from the melt of the ice was huge though.
Lack of correlation between the RSL for Norwegian Coasts and the RSL for British coasts tends to confirm that a ridge of land extended from Dogger Bank north along the west bank of the Norwegian Channel, dividing the North Sea into at least two areas of water.
This ridge was present in 12,000BP, and evidence elsewhere suggests that it remained into the Neolithic period of prehistory.
References
Postglacial Relative Sea Level Change in Norway by Roger C. Creel et al
Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum, by Ian Shennan et al
Global sea-level rise in the early Holocene revealed from North Sea peats" by Marc P. Hijma, et al
K. De Wit et al: HOLSEA NL: a holocene water level and sea level indicator for the Netherlands.
Views and opinions are my own
Jeffery Nicholls
Orkney
Jiffynorm@yahoo.co.uk
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