Stonehenge and the Ice Age: Rising and sinking coasts; isostatic and eustatic effects

Thursday 30 August 2018

Rising and sinking coasts; isostatic and eustatic effects

Inishowen, County Donegal, Ireland. A rising coastline.......

Another big and important paper which will help us to elucidate what went on during the Late Devensian.

Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum
by Ian Shennan, Sarah L. Bradley, Robin Edwards
Quaternary Science Reviews 188 (2018) 143-159

Shennan et al 2018 QSR GB&Ireland RSL.pdf

ABSTRACT

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 ~+40 to -55 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 syn- chronous coastal advance and retreat across different regions, -100-500 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.

This is a very impressive data collection and analytical exercise, and this is the key map:

Click to enlarge. What this map shows is five different RSL regions, all affected by the same eustatic sea-level rise since the end of the last glaciation, but all with different histories of isostatic and tectonic adjustment. The brown dots (sampling areas) mark the area of greatest isostatic uplift associated with the melting of the Celtic Ice Sheet; this was the "core area" and the last to melt. Overall, in this area, RSL has fallen -- see the diagram at top left. The area with the yellow dots was also heavily inundated by ice and shows a similarly complex RSL history, but with isostatic recovery rates more or less in step with the eustatic sea level rise. The area with the green dots experienced some isostatic uplift (the ice was thinner and not so persistent) but an overall rise in RSL until the levelling off which started around 7.000 BP. In the area with blue dots, although there was intense glaciation in places, the ice cover was relatively thin and short-lived, so that the amount of isostatic depression was quite small, leading to a complex interaction between eustatic and isostatic effects up to about 10,000 BP and a relatively straightforward RSL rise since then. The area with the black dots was mostly outside the range of the Devensian glaciation, so although there were some isostatic "forebulge" and other effects, the RSL rise has been relatively straightforward and gradual since 20,000 years ago. This can all be seen in the graphs attached to the map.

This is another usefil diagram, showing the predicted eustatic sea-level curve for the last 20,000 years, commencing at about -120 m and rising irregularly since then, with an additional kink about 7,000 years ago for continuing Antarctic ice melt.

The top curve on the graph is a proxy for the climate change curve. The bottom line with the black dots shows the predicted eustatic (global) sea level rise, and the other thin lines show the RSL curves for the 86 regions studies and marked on the map.

And one more very useful diagram:

This one separates out the isostatic and eustatic components. It shows the same sea-level curve, but shows the range of isostatic recoveries demonstrated for the 86 regions -- as we can see, the amount of isostatic adjustment varies between zero and about 150m. For West Wales, the amount of recovery is probably less than 50m, and this is suggestive of relatively thin ice. I must get the actual figure from the authors of this paper..........

Overall, very useful indeed. The paper will be much cited.