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Friday 13 March 2020

Forest growth on a rising shoreline

Submerged forest exposure on the beach at Newgale.  Blue-grey sticky clay below, and a thin peat bed with tree remains above.  That indicates a sea-level regression, but how can it be explained at a time of sea level rise?

In some recent posts I have been trying to understand the stratigraphic relationships between the submerged forest which is ubiquitous in most of the embayments around the Welsh coast and the underlying blue clay with Scrobicularia shells in it.

The silty clay is suggestive of deposition on the lower part of an estuarine / beach system, closer to LWM than to HWM.  That means quite a sudden transformation from a marine environment to a freshwater environment (allowing peat growth) and to conditions dry enough for a mixed deciduous woodland to thrive.  I have suggested that maybe the changing configurations of offshore bars, sand dunes, etc might have been responsible for this apparent sea-level REGRESSION -- just at a time when the eustatic sea-level curve shows that there was a continuous TRANSGRESSION on stable coasts throughout the world, fed by the meltwater released from the melting of the northern hemisphere ice sheets.  There is also a suggestion that one can get a temporary or apparent marine regression during an overall transgression because of "salt-marsh autocyclicity", as discussed here:

But over and again I have come back to the significance of the dramatic slowing down of the rate of sea-level rise, from 1m per century to just 8 cms per century, around 7,000 years ago.  That's the magic date, give or take a few centuries to account for dating and calibration errors.  It's magic because it is a highly significant geomorphological event that must have affected coastal processes worldwide -- but it also seems to mark the beginning of peat and forest growth, as indicated by scores of radiocarbon dates.

Having given this much thought, I'm now convinced that this relatively short-lived 7 ka marine regression on the coasts of Wales can only be adequately explained by reference to residual isostatic rebound following the wastage of Late Devensian glacier ice.

Heyworth doctorate thesis:

Some relevant posts on isostatic rebound:

Click to enlarge. This map shows by coloured dots where similar relative sea level histories can be discerned from the records.  From Shennan et al 2018.

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

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 (including West Wales), 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.

As Prof Ian Shennan and his colleagues have noted, there is a large volume of research on current isostatic recovery rates around the British Isles, and there is a consensus that in the West Wales area there is approximate equilibrium, with very slight uplift to the north and very slight sinking to the south.  The modelling seems to match up pretty well with current RSL observations.  

But much less is known about the HISTORY of isostatic uplift in the West Wales / St Georges Channel area.  As Ian kindly pointed out to me in a letter:  ".........the St George's Channel area will be complicated - the relationship between uplift and ice thickness will reflect the advance and retreat of ice which never was in equilibrium with the mantle, and the area was at varying distances from a migrating ice front and changing ice mass. There would be, in theory, many combinations of ice mass histories (spatial pattern, thickness profiles and rate of advance and retreat) and earth model parameters that could generate the ~ 50 m uplift. We need the field data to help us decide which are the plausible combinations."

It's difficult to create isostatic uplift graphs because each site has a unique history, and because regional and local isostatic histories interact.  Nonetheless, from my own research (in Greenland, Iceland, and West Antarctica) I am quite convinced that glacio-isostatic recovery histories can be remarkably sensitive, relating to the deglaciation behaviour of local ice caps as well as the ice sheets that tend to attract most of the research attention.  Prof John Andrews and many others have worked out that with the onset of catastrophic ice wastage initial recovery rate may be in excess of 10m per century, then gradually decreasing on an exponential curve.  Complete crustal recovery may take 20,000 years -- from one equilibrium state to another.  But as Ian Shennan points out, if a glacial episode at a particular locality is short-lived (for example, less than 5,000 years) then the ideal scenario is disturbed.  Andrews recognized three stages of isostatic rebound:  restrained uplift while some ice load is still present; post-glacial or unrestrained uplift following the disappearance of the ice load; and residual uplift, being the anticipated amount of recovery still to come.   During overall deglaciation, short-lived glacial readvances can cause short-term coastal depression and hence local marine transgressions if isostatic recovery rates and eustatic sea-level rise rates are more or less in step.

See also:
Post glacial rebounds measure the viscosity of the lithosphere
Jozsef Garai

In the above graph constructed for Scandinavia by Garai, we can see how the rate of isostatic rebound decreases exponentially from onset to the present day.  In Scandinavia it is assumed that c 150 m of uplift happened in the "restrained uplift" period, prior to complete deglaciation, over a period of less than 2,000 years; that c 150 m of uplift was in the "unrestrained uplift period," lasting for c 8,000 years; and that there is slightly less than 50 m of uplift still to be achieved.

Other complicating factors in seeking to define isostatic responses include the "forebulge effect" in which depression of part of the crust may trigger a measurable bulge or crustal uplift some distance away;  the variable viscosity of the crust, in which vertical isostatic movements may be associated with lateral transfers of mass (as when you put a weight on a lump of dough and find that it spreads laterally); the isostatic effect of the sea flooding back into the Severn Estuary or Cardigan Bay in the late glacial period, giving rise to some crustal depression; and the crustal depression caused by sedimentation on a large scale in coastal estuaries at the end of the Late Devensian glaciation.  Even such things as small water temperature rises can lead to the expansion of the water mass and enhanced depression of the sea bed.

Those who try to model isostatic effects do not have an easy time of it!

In West Wales, far from the centre of the BIIS (Celtic Ice Sheet) deglaciation happened far earlier than in the north of the British Isles, and recent research puts this onset of local ice wastage at around 20,000 yrs PB.   In fact, some dating work in recent years suggests that the Irish Sea Ice Stream or glacier reached its maximum extent and thickness much earlier than previously thought -- at around 27,000 years ago.  Its downwasting was well under way by 26,000 yrs BP.  In fact, Jenkins et al (2018) suggest that there was an ice edge running across St George's Channel to the Wexford area at about 25,000 yrs BP.  The Welsh Ice Cap lasted longer, and started its major retreat around 20,000 years ago.  So the two ice masses were seriously out of synchroneity -- if the cosmogenic and other dating results are to be believed:

Late Devensian deglaciation of south-west Wales from luminescence and cosmogenic isotope dating
ISSN 0267-8179.
DOI: 10.1002/jqs.3061

How thick was the ice over the north Pembrokeshire coast and in St George's Channel at the time of the LGM?  As we have seen in many posts on this blog, there are abundant traces of a Devensian ice edge around an altitude of 250m on the north face of Mynydd Preseli, and some evidence that the summit of Carningli might have been overwhelmed by Devensian ice -- that would fix an ice surface at an altitude of at least 347 m.  But then we have the "glaciological dilemma" of an Irish Sea Glacier front at the edge of the continental shelf, more than 400 km to the south-west.  Then we have accumulating evidence of Devensian glacial deposits wrapping around the south coast of Pembrokeshire at least as far east as Amroth -- suggesting that Carmarthen Bay and much of the Bristol Channel were filled with Irish Sea ice at the LGM.  I have long thought that the ice surface altitude in St George's Channel must have been at or near +1000 m in order to maintain forward momentum to the distant ice edge -- and even that would have involved an anomalously low long profile gradient.  I don't know of any hard evidence in West Wales that militates against that idea, given that not every glaciated area is plastered with glacial and fluvioglacial deposits.

Let's assume, for sake of argument, that the LGM ice in the centre of St Georges Channel was 600m thick, and 500m thick over north Pembrokeshire.  That means that in a state of equilibrium, there would have been up to 200m of isostatic depression of the crust.  Let's then assume that because the glacial incursion was relatively short-lived -- at maybe 10,000 years -- that equilibrium was NOT achieved, and the the amount of actual depression was not more than 150m.   In the first 2,000 years of isostatic rebound, maybe between 26,000 and 24,000 yrs BP, the rebound might have been 75 m.  All other things being equal,  most of the remaining (unrestrained) rebound might have happened before the onset of the Holocene at 10,000 yrs BP.  At 7,000 yrs BP there might still have been a small residual rebound of maybe 5 m, with the rate of uplift measured in just a few centimetres per century............

But then we have the complicating factor of the Welsh Ice Cap, which survived for much longer than the Irish Sea Glacier, and which must still have been associated with isostatic depression of the crust between 20,000 and 15,000 yrs BP before finally melting away around 14,000 years ago.  There are many articles in which attempts are made to characterise the ice cap, and the consensus view now seems to be that it covered even the highest peaks of wales at the time of the LGM.  Patton et al refer to a maximum ice thickness in excess of 1200m.

A number of researchers have noted that as melting set in, the ice surface was lowered to the extent that the high summits were emerging as nunataks around 20,000 - 19,000 yrs BP.  Nonetheless, it's a reasonable estimate that the average ice thickness across Wales might have been c 600m -- and that means it might have been responsible for c 200m of isostatic depression.

Pembrokeshire must certainly have felt the isostatic depression effects of the Welsh Ice cap, although it was c 40 km away from its southern section.  In North Wales there is still a small residual isostatic uplift effect, c 23,000 years after it reaches its LGM.  

It's too early to seek to create a "West Wales isostatic rebound model", but since there must have been substantial Late Devensian crustal depression during the LGM between 30,000 and 13,000 years ago,  there must also have been a substantial rebound afterwards.  Coming back to the enigmatic blue clay beneath the peat beds and the submerged forest, we have to ask ourselves what the rate of rebound might have been between 10,000 years ago and 5,000 years ago.  Was it faster, or slower, than the rate of eustatic sea-level rise?  I suggest that up to 7,000 years ago, the eustatic rate of sea-level rise, at around 1 m per century, was substantially faster than the residual isostatic rebound rate.  So the result was an unbroken marine transgression.  However, with the sudden slowing down of the sea-level rise to around 8 cm per century, around 7,000 years ago (at the 7 ka "event"), everything changed.  For maybe 2,000 years the isostatic rebound rate was faster than the sea-level rise, and so relative sea-level around the Welsh coast dropped, and the shoreline advanced.  The old intertidal zone was transformed into a salt-marsh, then into a freshwater landscape of lagoons and marshes with peat bed development, and then into a deciduous woodland similar to the woodland already established further inland on the coastal slopes. The development of sand bars and sand dunes might have assisted in this process.  But then, around 5,000 yrs BP, the isostatic uplift rate declined and was again overtaken by the eustatic sea-level rise -- and waterlogging and sphagnum moss growth gradually drowned or killed the forest.  So the "submerged forest" came into being.

At the moment this is just a theory than needs to be tested.  But it's my best attempt to explain the submerged forest itself and that enigmatic blue marine clay that lies beneath it........  as a complex interaction between eustasy and isostasy.


From Heyworth's thesis (1985):

Freshwater West
Leach(1918) describes a forest bed at the south end of the Bay, where trees are rooted in boulder clay. A radiocarbon date of 5960 ± 120 (Q. 530) was obtained herefrom a sample of woody fen peat at -2.0 m OD.

At Newport, occasional submerged forest trees have been seen in recent years, and C. Kidson obtained a radiocarbon date of 6370 ± 150 B.P. (H A,R S78) from a stump at -1.6m 0.0. (personal communication). Griffiths and Whiteside- Williams (1886) report submerged forest sightings at Abermawr (7 km south of Strmble Head), Whitesands Bay (St. David's Head) and Newgale Sands (St. Bride's Bay.)

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