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Friday 5 July 2019

The Celtic Sea and its piedmont glacier

From a NASA animation of Antarctic ice flow.  Note the streamlines and the wide ice streams (coloured blue) transporting glacier ice towards the ice shelves.  All of these streams exist within landscapes almost completely submerged beneath glacier ice.


The Celtic Sea was affected by thick, dynamic and very extensive glacier ice during the Late Devensian.  But the ice was NOT contained within an ice stream.......

I’m returning to this one — the fascinating 2019 paper by James Scourse and many colleagues relating to the LGM advance of the Irish Sea Ice Stream (ISIS) between 27,000 and 24,000 years BP right out to the edge of the continental shelf and 150 km further our than previously estimated from marine floor sediments and bedforms.  The work is fastidious and the evidence presented by the authors looks solid enough — in line with former published work by Praeg and others.  So what this means is a substantial and rapid advance of ice across the Celtic Sea which was very early, and dramatically out of phase with events on other segments of the British and Irish Ice Sheet (BIIS).

One other point, to which I have not given much attention until now, is the conclusion that the advance (culminating around 25,000 yrs BP) took place at a time of falling global sea-level  but relatively stable relative sea-level on the shelf edge — and that retreat of the ice edge was triggered by "high relative sea-levels driven by significant glacio-isostatic depression, consistent with greater ice loads over Britain and Ireland than previously considered.”  That mean a very high crustal ice load in SW Britain — and that is not at all consistent with the idea that the ISIS was a narrow lobe pushing southwards from St George’s Channel towards the SW.  I have addressed that issue in many posts before……….

A much-reproduced eustatic sea-level curve covering the last two glacial episodes.  It's accepted that the lowest eustatic sea-level -- maybe below -130m -- occurred around 20,000 years ago.  This means that in the period around 27,000 - 25,000 yrs BP sea-level might have dropped by 3m. Could the crust in the Celtic Sea arena have been depressed isostatically by the weight of ice at an even greater rate, leading to a relative sea-level rise?

Then there is the name.  Was this really an ice stream in the normal sense of the word?  Scourse et al refer to it as having a width of c 100km, and they say: "A width of the order of 100 km places ISIS at the wider end of the phenomena (Margold et al., 2015) and comparable to the Hudson Strait Ice Stream of the Laurentide Ice Sheet and the Thwaites Ice Stream in Antarctica.”  But as I will never tire of saying, these and other ice streams flow, or flowed,  within landscapes completely inundated by glacier ice.  They are constrained laterally by flanking uplands, and their edges are seen to coincide with zones of shearing or intense crevassing on the contact between stagnant or slow-moving ice and the fast-moving ice within the ice stream.  The situation was nothing like that in the Celtic Sea — there are no topographic controls which might have prevented or constrained lateral spreading of the ice.  To the north of St George’s Channel, yes, that might have been the case, with an ice stream flanked by the Irish ice cap to the west and the Welsh ice cap to the east.  But to the south of St George’s Channel, no…….. in an “open landscape” with no deep troughs to direct the ice south-westwards, the ice MUST have spread laterally, and it MUST have spread well into the Bristol Channel and pressed against the coasts of Devon and Cornwall.  

It’s interesting that in the Ballum’s Bay article just published in QN, John Hiemstra and his colleagues now seem to accept that point.  And other authors are doing the same.  In the maps recently published by Glasser et al (2018) and Jenkins et al (2018) lobes are shown pressing well into Carmarthen Bay.  There is abundant empirical evidence in support of Devensian glacier ice affecting much of the south Pembrokeshire coast and also flowing across Caldey Island, as I have shown in many posts on this blog.. 

Glasser et al (2018).  An ice stream probably existed where the words are placed on the map, but the "outer" 400 km of the ice mass should be given a different label.  This was a vast Devensian piedmont glacier, covering 80,000 sq km.  Note that ice is shown pushing eastwards into the outer reaches of the Bristol Channel.

Jenkins et al (2018).  A similar Late Devensian ice edge is shown, but the ice flow arrows in the Celtic Sea are far too generalised.  Ice always flows perpendicular to the ice edge in unconstrained situations, so there must have been ice flow eastwards into the Bristol Channel. If the ice edge really did retreat from the shelf edge to the north Pembrokeshire coast (over a distance of 400 km) in 2,000 years, that represents a retreat rate of 200m per year at a time when glaciers were apparently expanding almost everywhere else.......

Glacier ice grounded right across the vast area of the Celtic Sea also provides a better explanation for the substantial ice load now required by Scourse et al in order to explain the isostatic depression and stratigraphic and bedform features at the edge of the continental shelf.  A rising relative sea level, at a time of a eustatic drop in global sea level, requires a lot of glacier ice, of considerable thickness!

So - thick ice in the Celtic Sea arena.  For 400 km from the constriction of St George's channel, there must have been a continuous gradient towards the south-west.  So what was the surface altitude of the ice in the middle of the Channel between Pembs and the SE coast of Ireland?   Let's assume, as I have done on many occasions, that the gradient was shallow -- consistent with a surging glacier on old sea-floor sediments, as modelled by many authors.  The ice must still have had a surface altitude of over 1500m in mid-channel c 27,000 years ago.  But why, in those circumstances, did the ice not overwhelm the whole of Pembrokeshire at the same time?  Maybe it did, and maybe we have just got our ice edge mapping all wrong?  I have been struggling with this issue for a long time, without having the skills or the tools for effective modelling.

My old map showing proposed ice masses and directions of ice movement at the time of the Anglian Glaciation.  Should we now be thinking of a similar situation during the Late Devensian?

My suggested ice contour map for the Anglian Glaciation.  How different might the ice contours have been
 in the Late Devensian?

This is all a bit of a glaciological puzzle, and I keep on coming back to this old map, which is so old that it is covered in cobwebs.  It looks crude, but I have sneaking feeling that all of the ice directions assumed by generations of geologists and geomorphologists are all wrong, and that this is what they were really like:

Does anybody have any evidence that this map is fundamentally wrong?  As far as I can see, it is the only one that fits the field evidence from a multitude of different sites around the edges of the Celtic Sea.


Anyway, here is a suggestion.  Can we please abandon the ice of the term “Irish Sea Ice Stream” for the area to the south of St George’s Channel, and use the term “Celtic Sea Ice Lobe” or “Celtic Sea Piedmont Glacier” instead?  

Relevant info:

James Scourse et al.   2019.  Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent.  Marine Geology,  Volume 412, June 2019, pp 53-68



The dynamics of the British-Irish Ice Sheet (BIIS) during the Last Glacial were conditioned by marine-based ice streams, the largest of which by far was the Irish Sea Ice Stream (ISIS) which drained southwest across the Celtic shelf. The maximum extent and timing of the ISIS have been constrained by onshore evidence from the UK and Ireland, and by glacigenic sediments encountered in a small suite of vibrocores from the UK-Irish continental shelf, from which a single radiocarbon date is available. These data have long supported ice advance to at least the mid-shelf, while recent results suggest the ISIS may have extended 150 km farther seaward to the shelf edge. The glacigenic sequences have not been placed within a secure seismic-stratigraphic context and the relationship between glaciation and the linear sediment megaridges observed on the outer shelf of the Celtic Sea has remained uncertain. Here we report results of sedimentological, geochemical, geochronological and micropalaeontological analyses combined with a seismic-stratigraphic investigation of the glacigenic sequences of the Celtic Sea with the aims of establishing maximum extent, depositional context, timing and retreat chronology of ISIS. Eight lithofacies packages are identified, six of which correlate with seismic facies. Lithofacies LF1 and LF2 correlate to a seafloor seismic facies (SF1) that we interpret to record the postglacial and Holocene transgressive flooding of the shelf. Lithofacies LF10 (till), LF3, LF4 and LF8 (glacimarine) correlate to different seismic facies that we interpret to be of glacigenic origin based on sedimentological, geotechnical and micropalaeontological evidence, and their distribution, supported by geochemical evidence from lithofacies LF8 and LF10 indicate extension of ISIS as far as the Celtic Sea shelf break. New radiocarbon ages on calcareous micro- and macrofauna constrain this advance to be between 24 and 27 cal ka BP, consistent with pre-existing geochronological constraints. Glacimarine lithofacies LF8 is in places glacitectonically contorted and deformed, indicating ice readvance, but the nature and timing of this readvance is unclear. Retreat out of the Celtic Sea was initially rapid and may have been triggered by high relative sea-levels driven by significant glacio-isostatic depression, consistent with greater ice loads over Britain and Ireland than previously considered.

Final section of text:

……………..While the existence of ISIS is well founded, its exact footprint remains ill-defined owing to few lateral geomorphological indicators such as a prominent edge to a field of mega-scale glacial lineations or shear margin moraines, and on the Celtic shelf there is a lack of obvious topography to constrain the margin. Nevertheless, a striking aspect is the large reconstructed width in comparison to other extant and palaeo ice streams from around the world. A width of the order of 100 km places ISIS at the wider end of the phenomena (Margold et al., 2015) and comparable to the Hudson Strait Ice Stream of the Laurentide Ice Sheet and the Thwaites Ice Stream in Antarctica. To maintain fast flow over such widths requires substantial feeder catchments that were probably unlikely from the BIIS. This suggests the ISIS could only exist as a transitory ice stream that rapidly drew down ice volumes. Our chronology of rapid advance and withdrawal is consistent with this, as are findings of numerical modelling investigations that struggle to simulate a steady state ice stream of this scale (Boulton and Hagdorn, 2006; Hubbard et al., 2009). It has also been a challenge for ice sheet models to simulate ice stream advance to the Isles of Scilly without building up ice elsewhere at locations more extensive than indicated by empirical evidence, such as mid and southern England (e.g. Patton et al., 2017). This problem is exacerbated by the Celtic Sea extent that we now report. Modelling investigations are underway to address this challenge. 

5. Conclusions 
Cored glacigenic sequences correlated to seismic stratigraphic units provide clear evidence of the advance and extent of the Irish Sea Ice Stream across the Celtic shelf during the Last Glacial. Overconsolidated subglacial diamicts (till) of ISIS origin have been recovered from close to the shelf break. Together with deformed and contorted proximal to distal glacimarine sequences containing distinctive cold water foraminiferal assemblages recovered from across the shelf, these diamicts indicate the advance of the ISIS into the Celtic Sea as far as the continental shelf break of the Irish and UK sectors of the Celtic Sea, and raises questions regarding the extent of ice into the French sector. The timing of this advance has been constrained by a series of new radiocarbon ages to between 27 and 24 cal ka BP. These ages are consistent with a single published radiocarbon determination from a glacimarine sequence recovered close to the shelf break, and with new geochronological data from the Isles of Scilly indicating ice advance at 25.5 ka. Comparison with ages for deglaciation farther north in the Irish Sea suggests that ice retreat across the Celtic shelf was initially rapid and then slowed, constrained by topographic controls, falling relative sea level and low tidal amplitudes in the vicinity of St George's Channel. Deglaciation on the outer shelf was probably initiated by high or rising sea level driven by glacio-isostatic depression during peak glaciation.


About ice streams (Bethan Davies):

Ice streams are corridors of fast flow within an ice sheet (ca. 800 metres per year). They discharge most of the ice and sediment from these ice sheets, flowing orders of magnitude faster than their surrounding ice. Their behaviour and stability is therefore essentially important to overall ice sheet dynamics and mass balance[1]. The Antarctic Ice Sheet currently discharges 90% of ice and sediment through ice streams. Antarctic Ice Streams are fed by complex tributaries that extend up to 1000 km into the interior of the ice sheet.

Ice streams can be constrained by topography or by areas of slow moving ice. They are called topographic ice streams or pure ice streams respectively. Both types show variations in behaviour (both through time and space), which indicates potential for instability and are therefore particularly interesting.

According to this definition, th 80,000 sq kn of ice that filled the Celtic Sea arena should NOT be referred to as an ice stream,

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