BRITICE LGM reconstruction -- did Devensian glacier ice reach Somerset?

 Modelling matched with empirical data suggests that Pembrokeshire and Gower must have been glaciated in the Late Devensian, and that a fast-flowing stream of ice from St Georges Channel affected the outer part of the Bristol Channel.  Some of the model runs showed an ice margin some way beyond the limit shown on this reconstruction.

 The zones designated by the BRITICE team for more detailed studies.  Our area of interest is T4 - Irish Sea West.  It could also have been designated as Celtic Sea East........

Congratulations to the BRITICE-CHRONO team for the vast multi-authored concluding paper (60 pages long) that wraps up the project that has been running for many years, attempting to reconstruct the waxing and waning of the British - Irish ice sheet between 31,000 and 15,000 years ago.  

Here is the essential info:

https://onlinelibrary.wiley.com/doi/epdf/10.1111/bor.12594

Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE-CHRONO reconstruction

Chris D. Clark et al
Boreas, 07 September 2022

https://doi.org/10.1111/bor.12594

Abstract

The BRITICE-CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet-wide reconstructions consistent with these findings and using retreat patterns and dates for the inter-transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand-year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine-influenced sectors collapsed rapidly. First order controls on ice-sheet demise were glacio-isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector-scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well-constrained and a valuable data-rich environment for improving ice-sheet modelling.

The paper is too big to deal with in one post, but it is noteworthy for its attempt to draw together vast amounts of data from published reports of fieldwork with new modelling of ice sheet behaviour.  inevitably, there are mismatches between field data and modelled reconstructions, and the authors are honest enough to report anomalies and enigmas where they occur.  Of course, those of us who know particular bits and pieces of the Devensian glaciated area will complain that "they have got it wrong" -- but this is progress, and eventually all will be revealed.

Initially, it is a pity that this study takes a rather strange line running approx from St David's Head to the Isles of Scilly as the eastern edge of the Irish Sea Ice Stream, based on very little field evidence.  There is powerful evidence -- in the published literature -- of Devensian ice affecting all of the western coasts of Pembrokeshire, and for ice wrapping around the Isles of Scilly.  In addition, the proposed ice edge, as I have pointed out many times, defies the rules of ice physics relating to ice flow on unconstrained margins.

That having been said, as the paper goes on, the authors modify this "starting assumption" and have to accept that Devensian ice in reality pressed much further to the east. This is the map they publish for ice limits as derived from "empirical " observations in the field:

Ice sheet reconstruction based on "empirical date" for 26 ka -- the time of maximum ice extent in the Celtic Sea arena.

Note that the minimum extent is shown as leaving the bulk of Pembrokeshire ice free -- with a rather strange enclave or spur of ice-free land pushing far into the area of relatively thick streaming ice.  I have to take my share of the blame for this, since I have based many posts on this blog to the idea of "ice free South Pembrokeshire".  Those posts were based on the great difficulty we have all had in working out how old the glacial deposits of mid and south Pembrokeshire really are......

But over the past few years I have accumulated so much evidence of fresh glacial deposits along the south Pembrokeshire coast as far east as Caldey Island (and maybe even Ragwen Point) and inland in the Landshipping area and around Narberth - Penblewyn that I now think that the WHOLE of Pembrokeshire was affected by Devensian ice, presumably around 26,000 years ago. 

Interestingly enough, this brings the ice edge into the "optimum" area in the BRITICE empirical reconstruction, and the "maximum position" deemed to be possible from examinations of field evidence extends all the way to the Somerset coast -- so the authors of this vast report are being rather more daring than I have been thus far.........

Then we come to the numerical modelling, based on a vast database and on a multitude of parameters.  This gets even more interesting, since the model pushes a distinct ice stream well into the Bristol Channel:

At last we have streamlines flowing in a direction that makes glaciological sense........

These two modelling maps show that the "South Pembrokeshire ice free enclave" is a somewhat absurd proposition from a glaciological point of view, and that the modellers have found it almost impossible to accommodate in their modelling work.  So it has to be dumped.  It follows that this map is also incorrect, with regard to West Wales:

In the map, north Pembrokeshire is shown as being ice free (following deglaciation) for 26,000 years and mid and south Pembrokeshire for c 130,000 years.  This should be easy enough to check, given the range of dating methods now available to researchers.

Finally here is an interesting extract from the discussion part of the paper:

An egregious mismatch occurred at 26 ka, where the modelled grounding line position of the Irish Sea Ice Stream fell 150 km behind that indicated by the geological evidence (minimum reconstruction, Fig. 6; Scourse et al. 2021). We experimented with various nudges to the model to try and prevent this underrun but they resulted in significant overruns of other parts of the ice sheet, notably with the SW Peninsula of England (Cornwall and Devon) and much of the English Midlands becoming glaciated. The final choice of model run (Fig. 6) is therefore a compromise that mostly fits the wider empirical limits, but underruns in the Celtic Sea. The alternative choice of accepting a model run that reached the shelf edge in the Celtic Sea but also glaciated the SW Peninsula and parts of the English Midlands would have interesting implications for the long-standing hypothesis that some of the stones of Stonehenge may have been transported, at least partway as glacial erratics (e.g. Judd 1902; Scourse 1997; John 2018; Pearson et al. 2019). Although this may indeed be accommodated by earlier more extensive glaciations, we suggest however, that a process, forcing, or feedback is missing in the numerical modelling we present here and that once discovered would enable sufficient ice advance in the Celtic Sea without exceeding ice limits elsewhere. For example, Scourse et al. (2021) suggest that a non-steady oscillation (surge) arose as a release from the build-up of ice behind a topographic constriction in the Irish Sea. We further speculate that the drainage of lakes on the ice stream surface, or floods from subglacial lakes, may have temporarily facilitated faster ice flow and ‘over-extension’ of the ice margin by locally increasing basal lubrication. For the Irish Sea Ice Stream an important challenge remains, to develop new ingredients in process-based numerical modelling so that simulations can reconcile with the geological record.

This is very interesting, since the authors of the paper bring the transport of the Stonehenge bluestones into a debate about DEVENSIAN glaciation.  They also suggest that the Irish Sea Ice Stream might have affected Devon and Cornwall. That is something I have never dared to do, in spite of my reputation -- in certain quarters -- as an unreliable witness!!  The most I have ever dared to suggest is that local ice caps on the SW Peninsula uplands were in contact with ice belonging to the ice stream, somewhere inland of the Bristol Channel coast.

Interesting times.  Watch this space........ and bear in mind that the Somerset Levels embayment is a perfect conduit for ice being pushed in from the west.