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Monday, 11 December 2017

Ice in the Bristol Channel -- the latest modelling

This is a comprehensive and important paper which draws together a vast amount of information on the Devensian Eurasian ice sheet complex.There is a huge reference list -- including many articles that can be open-accessed as PDFs.

Henry Patton, Alun Hubbard, Karin Andreassen, Amandine Auriac, Pippa L.Whitehouse, Arjen P. Stroeven, Calvin Shackleton, Monica Winsborrow, Jakob Heymane, Adrian M. Hall. (2017)
"Deglaciation of the Eurasian ice sheet complex"
Quaternary Science Reviews
Volume 169, 1 August 2017, Pages 148-172


Here is the Abstract:

Abstract

The Eurasian ice sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea-level lowering. Whilst recent terrestrial and marine empirical insights have improved understanding of the chronology, pattern and rates of retreat of this vast ice sheet, a concerted attempt to model the deglaciation of the EISC honouring these new constraints is conspicuously lacking. Here, we apply a first-order, thermomechanical ice sheet model, validated against a diverse suite of empirical data, to investigate the retreat of the EISC after 23 ka BP, directly extending the work of Patton et al. (2016) who modelled the build-up to its maximum extent. Retreat of the ice sheet complex was highly asynchronous, reflecting contrasting regional sensitivities to climate forcing, oceanic influence, and internal dynamics. Most rapid retreat was experienced across the Barents Sea sector after 17.8 ka BP when this marine-based ice sheet disintegrated at a rate of ∼670 gigatonnes per year (Gt a−1) through enhanced calving and interior dynamic thinning, driven by oceanic/atmospheric warming and exacerbated by eustatic sea-level rise. From 14.9 to 12.9 ka BP the EISC lost on average 750 Gt a−1, peaking at rates >3000 Gt a−1, roughly equally partitioned between surface melt and dynamic losses, and potentially contributing up to 2.5 m to global sea-level rise during Meltwater Pulse 1A. Independent glacio-isostatic modelling constrained by an extensive inventory of relative sea-level change corroborates our ice sheet loading history of the Barents Sea sector. Subglacial conditions were predominately temperate during deglaciation, with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network. Moreover, the maximum EISC and its isostatic footprint had a profound impact on the proglacial hydrological network, forming the Fleuve Manche mega-catchment which had an area of ∼2.5 × 106 km2 and drained the present day Vistula, Elbe, Rhine and Thames rivers through the Seine Estuary. During the Bølling/Allerød oscillation after c. 14.6 ka BP, two major proglacial lakes formed in the Baltic and White seas, buffering meltwater pulses from eastern Fennoscandia through to the Younger Dryas when these massive proglacial freshwater lakes flooded into the North Atlantic Ocean. Deglaciation temporarily abated during the Younger Dryas stadial at 12.9 ka BP, when remnant ice across Svalbard, Franz Josef Land, Novaya Zemlya, Fennoscandia and Scotland experienced a short-lived but dynamic re-advance. The final stage of deglaciation converged on present day ice cover around the Scandes mountains and the Barents Sea by 8.7 ka BP, although the phase-lagged isostatic recovery still continues today.

Most of this paper is concerned with the wastage phase of this vast ice sheet complex -- following on from the modelling done by Patton et al in 2016.  We have discussed that work before, here:

Henry Patton's early modelling was intriguing since it showed a Devensian ice margin far beyond the conventional one in the Celtic Sea arena -- with an ice-filled Bristol Channel, Irish Sea ice impinging on the coasts of Cornwall and Devon, and pressing well inland in Somerset too.  This was the 2016 published model for the Devensian maximum ice margin:

The modelled Devensian ice extent, as determined by Patton et al in 2016.  The red line is the older assumed ice limit.  Note that in the Midlands and in the Bristol Channel area, modelling suggests that ice could have progressed more than 100 km further south than assumed.  Ground truthing holds the key......

The modelling work published in 2017 is based on a vastly increased amount of data from many sources, and the segment of the ice sheet complex incorporating the British Isles (which the authors refer to as the "Celtic Ice Sheet") looks like this:

 
The extent of the Devensian glaciated area is very similar indeed, adding to our confidence that it fits well with glaciological, climatic, topographical, sea-level and many other parameters.    The garish green colour used here relates to the rate of ice edge retreat.  Solid green indicates rapid retreat, yellow suggests approximate stillstands or slow retreat, and red suggests a more prolonged stillstand. So the map shows a relatively prolonged stillstand of the ice edge along the coast of Cardigan bay and on the northern flank of Preseli -- I can live with that, since it is supported by rather a lot of evidence on the ground.

Now let's put some flowlines onto the map -- ice in ice sheets and ice caps always flows broadly perpendicular to the ice edge, so the patters will have been something like this.  (There will of course have been many internal irregularities caused by uplands, glacier discharge routes and changes in the surface topography and the bed conditions of the ice mass......as we have discussed many times regarding the interactions between ice from the Welsh ice cap and the ice of the Irish Sea Glacier or ice stream.)



This is in my view far more realistic than some of the maps published by the BRITICE team over recent years.  Since we are all interested in how the bluestones got to Stonehenge, note that the modelling shows -- yet again-- that Devensian ice COULD have reached Salisbury Plain.  I personally doubt that it did in reality, but since this modelling work will also hold pretty accurately for pre-Devensian glacial episodes, my view is reinforced that during the Anglian Glaciation (around 450,000 years ago) ice might well have carried those dearly beloved bluestones almost all the way from Preseli to Stonehenge.




14 comments:

cysgodycastell said...

Brian,

As far as i understand it the two major arguments against the theory that rocks were entrained within ice and at some point desposited across the Bristol channel and into south west England is that

a. the ice sheet didnt get that far and
b. there are no other west wales erratics found in the vicinity other than those at stonehenge and possibly the Boles Barrow stone

So if the previous accepted geographic extent of the ice sheets is now in serious doubt following this new research, is it just the case that we await the discovery of spotted dolerite between Pembrokeshire and Wiltshire to blow the human transportation argument out of the water? Or is this too simplistic?

BRIAN JOHN said...

Far too simplistic, I'm afraid. For a start, the obsession with spotted dolerite needs to be dumped -- there are many other rock types involved in this debate, including rhyolites, ashes and sandstones. It's true that no big "bluestone" erratics -- big enough to be called boulders or slabs or pillars -- have been found lying around on Salisbury Plain. The only ones we know about are at Stonehenge -- and the only other one is the one taken from Boles Barrow. Olwen Williams-Thorpe suggests that there may have been other ones too, but that they have been smashed up and or removed and buried during field clearance operations. That's not a substitute for hard evidence, of course! My own thesis is that when Stonehenge was being built the builders made a systematic search for stones of all types -- sarsens and bluestones -- over a wider and wider area until they could find no more that were of any use-- at which point they abandoned the project in an incomplete state. I haven't ever been given any convincing evidence that Stonehenge was completed.....

The new modelling work is rather convincing, suggesting that from a glaciological point of view the ice might well have reached Salisbury Plain on at least one occasion. The ground truthing problem is a big one. There is good evidence of ice reaching the Bristol and Bath area and extending well into Somerset -- but we need some bright young research students to do a systematic study of the sediments on and around the edges of Salisbury Plain.

Gordon said...

The Malvern Hills contain Dolerite intrusions,Rhyolite,Tuffs,Diorite and Old Red Sandstone.They are approximately 70 miles as the crow flies from Stonehenge.Are these stones so well known to be different from the stones at Stonehenge that they are not worth considering?

BRIAN JOHN said...

I believe they are very different, Gordon. Myris will have chapter and verse on all of that, and maybe he will enlighten us.......

Myris of Alexandria said...

Yes
M

Myris of Alexandria said...

Petrographical, geochemical and age differences are enormous.

It is worth reading the 1991 Thorpe et al paper they looked at all possible contenders.

When the pet rocks boys first started (as flaxen- haired, ripped godlins) they flirted with the Wrekin with visions of orthostats floating g down the Severn.
Alas not to be.Craig Rhosyfelin came along.
Trust us, we are doctors, any rock you can think of anywhere has been tested.

M

Jon Morris said...

I don't think I've read the Thorpe paper Myris

In general Stonehenge is symmetrical about the North East axis. The symmetrically arranged stones would, from an architectural point of view, ideally have been sourced from the same material. The larger sarsens, sourced probably from the same location, have two symmetrical structures. The bluestones also have two symmetrical structural arrangements.

The source of these two material sets would appear to have been located in Marlborough and Preseli. The oddity is the Altar Stone, which does not need to form part of either of the two sets of two symmetrical structures: This one stone appears to be have a source that is neither Preseli nor Marlborough.

Of the two sets of arrangements, the sarsens would take as much or more effort to transport than the bluestones; even accounting for the greater distance of transport (and regardless of whether or not the glacial transport hypothesis is true). In particular, the effort needed to transport the larger sarsens far exceeds the effort needed to transport the Altar Stone.

At about six tons, the effort needed to transport the Altar Stone from Wales could easily be dwarfed by the effort required to transport one of the larger the larger sarsens from Marlborough. However, the Altar Stone is given a prime location of its own, which indicates that it was probably more important than, or equally important to, any other individual stone.

If one completely rejects the glacial transport hypothesis, then this stone could have been sourced from anywhere at all: Any additional effort required to transport that particular stone is already evident from the location in which it is set.

Did the Thorpe paper cover the “anywhere at all” scenario Myris?

Gordon said...

Thanks to Myris and his mention of the Wrekin i now know of Uriconio.This is i believe another link to the Auroch (Uri,Ure,Uru) and its connection to ancient enclosures and stone circles.I am now aware of Urswick in the South lakes,Inverurie,Urchfont and the river Ure.Does anyone know of other places?I have also just read for the first time "Uriconium an ode"by Wilfred Owen.Cheers M.

chris johnson said...

I would have thought from the evidence at Avebury that we should accept that the people of that time were able to move and organise much heavier stones than those from Preseli.
Many would readily accept that the motive for choosing materials at Stonehenge is not "architectural". The decision to use different materials is deliberate surely, the big sarsens in the outer ring, the spotted dolorites on the inner bluestone ring and the other bluestones on the outer. Then the Altar Stone mystery: a fourth material, not local, alone. Had the motive been "architectural" then they would have likely built in wood.

Jon Morris said...

Out of interest, Tim Daw's posted up some comment on the research here:

http://www.sarsen.org/2017/11/stonehenge-glacial-transport-of.html

I can't comment on Tim's site (been like that for a year or two.. probably something to do with the site settings), so wondered what the team's thoughts were on a partial transport hypothesis (by glacier to, for example, the Borders area): The quarry hypothesis appears to be weak given what's been published to date, so it seems to me that without positive confirmation of some sort of "proto-Stonehenge", the obituary may be somewhat premature (but I'm not a geological expert so find it difficult to be certain).

Myris of Alexandria said...

The altar stone is perhaps the most 'important'of the stones (my fav. is 32e of course)and may be a link to the Secondary Neolithic Heimat.

Read the paper, as a mean 'umble plodder in the dinted steps of those heated sods I must defer to that paper.

Suffice to say the provenance of the bluestones, all of them, have exercised geologists for 140 years and this has included some of the finest geological minds. All simple sources have been explored-
The pet rock boys did something very grand -they started from scratch, believed no-one, went to the original material and acted like Victorian Divines and were incredibly lucky.
(As I used to tell my students "In Victorian days most geologists were divines, but where can you find a divine geologist these days".)

Cryf is a once in a lifetime find.

For the Altar stone you can read Ixer and Turner 2008? Ixer et al 2017 (On academia edu)but must also await the forthcoming Pet Roc Boys Antiquity paper. Thomas was not a divine but a professional.

All this talk of divines and altar is so very Adventual must go and decorate the tree for Rosie and wrap her presents.

Tomorrow is the Feast day of the Blessed Brez Nesti. (Patron saints of thirsty travellers), they gave the Holy Family a drink at a well. (This came to me in a dream/vision years ago)and we celebrate the day with cards and drinks).

M

Myris of Alexandria said...

Oh course the blessed sisters, Brezh and Nesti could be the patron saints of sacred wells or indeed of this blog.
Oh well I may as well wish that all your wells remain filled and unpolluted. Have a drink, even a mug of dead dog.
M

Jon Morris said...

Abstract seems clear Myris "The Altar Stone and a sandstone fragment (excavated at Stonehenge) are from two sources within the Palaeozoic of south-west Wales". But the full text of paper isn't something that's easily accessible so it's difficult for the lay reader to be sure that there's not an underlying assumptive error.

Why Secondary Neolithic Heimat? Wouldn't that be primary?

BRIAN JOHN said...

Jon -- you mentioned tim's blog and the post claiming that the glacial transport theory is dead -- sadly based on a map of the wring glacial episode. I have twice sent comments in response to that post, but it appears that Tim is disinclined to be corrected.