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Wednesday, 19 May 2021

New dating for LGM Irish Sea Ice Stream


Maximum extension of the Irish Sea Ice Stream (ISIS) at the time of the Last Glacial Maximum, 
with dated retreat stages.

Here is a big new paper from the BRITICE-CHRONO project, reporting on new dates for the retreat (with occasional readvances) of the ISIS through St Georges Channel and the Celtic Sea and the "Irish Sea Glacier" which is deemed to have pressed through the Cheshire Gap and into the Midlands of England. This new terminology is a bit confusing -- but no matter. This is a very intreresting and useful paper.



ABSTRACT

The BRITICE-CHRONO Project has generated a suite of recently published radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically stimulated luminescence ages from adjacent onshore sites. All published data are integrated here with new geochronological data from Wales in a revised Bayesian analysis that enables reconstruction of ice retreat dynamics across the basin. Patterns and changes in the pace of deglaciation are conditioned more by topographic constraints and internal ice dynamics than by external controls. The data indicate a major but rapid and very short-lived extensive thin ice advance of the Irish Sea Ice Stream (ISIS) more than 300 km south of St George's Channel to a marine calving margin at the shelf break at 25.5 ka; this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel. The release event between 25 and 26 ka is interpreted to have stimulated fast ice streaming and diverted ice to the west in the northern Irish Sea into the main axis of the marine ISIS away from terrestrial ice terminating in the English Midlands, a process initiating ice stagnation and the formation of an extensive dead ice landscape in the Midlands.

In the dating, using a variety of techniques, there are a few anomalies (as one might expect) but the pattern and timing of ice retreat now seems pretty well established, following a rapid advance of thin ice right out to the Celtic Sea shelf edge at about 25,500 BP.  the authors say "this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel."  I'm not sure how strong the evidence is for this, and I'm not sure how this "plugging" might have worked, but that's a minor point.

Some of the dates now published are around a thousand years adrift from those published by the same team of researchers a couple of years ago, but there is now a much more sophisticated analysis of the data.   As we can see from the map, the ice edge retreat was remarkably rapid, progressing across terrain which is now mostly sea floor over a distance of more than 700 km from shelf edge to the Isle of Man in little more than 5,000 years.  That's extraordinary if correct -- a rate of around 7 km per year.  The key positions were:

Shelf edge  25,500 yrs BP
West of Scilly 25,300
Outer Bristol Channel  25,000
Outer St Brides Bay (off Pembrokeshire) 24,700
North Pembs coast  24,300
Cardigan Bay  24,000
North Cardigan Bay  22,700
Llyn Peninsula 21,000
Anglesey 20,900
Isle of Man c 19,000

One interesting thing is that all of these speculative ice edge positions are convex.  That means they are all interpreted as land-based.  If they had been calving bays with ice breaking free off a floating ice front, as suggested by Ed Lockhart, they would have been concave, like the ice fronts shown in the Irish Sea in the latter phases of deglaciation.  This means -- or so the current authors think --  that the ice occupying the Celtic Sea arena was not an ice shelf but a grounded glacier; and it must have followed the rules of ice physics, with a gradual (if shallow) ice gradient from source to ice front.  I have discussed this before on this blog:



This brings me to my next point, concerning the eastern edge of the Irish Sea Ice Stream.  As shown on the maps in this article, I don't think it makes sense.  it would have been nice to have more information on the interactions between the ice of the ISIS and that of the Welsh Ice Cap, both in Cardigan Bay and on the south Wales coast; maybe that will be forthcoming in future articles from the BRITICE-CHRONO group.   But let's look at the outer reaches of the Bristol Channel, where the authors show streaming ice travelling broadly NE >>SW from the constriction of St Georges Channel towards the shelf edge:


As I have pointed out to the researchers on this team many times before, this is not how ice flows when it is grounded.  Ice always flows perpendicular to the ice edge in situations unconstrained by topography, and if the ice surface was high enough (probably in excess of 350m) off the Pembrokeshire coast to maintain a flow all the way to the shelf edge at -200m, 400 km away, it must also have pushed ice much further to the east across Carmarthen Bay and up the Bristol Channel.  I have discussed this with respect to Ed Lockhart's thesis, here:


I'm gratified that the authors of this new paper have accepted my point that Caldey Island and the South Pembrokeshire coast were glaciated during the LGM, but they have not gone so far as to accept my contention that the ice reached the coasts of west Cornwall and west Devon, and that the Scilly Islands archipelago was a nunatak:





I repeat here that the Bristol Channel ice edge as shown by the authors in this paper is not supported by sound published evidence.  I would have liked more geomorphology in this paper.

But these large research teams are often very reluctant to abandon their working hypotheses.  They will get there in the end........


One of my recent reconstructions of the relationships between the ISIS and the Welsh Ice Cap.  
Here I am suggesting that Preseli was completely inundated by the ISIS; in reality 
it might be that there was a small cold-based ice cap on Preseli which protected it from 
intensive scouring or other glacial effects.  On the north flank of Preseli it is clear that there are traces of Irish Sea ice at least up to the 340m contour.






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