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Thursday, 25 April 2019

More on the Devensian Celtic Sea ice edge

My reconstruction of the outermost position of the ISIS in the Celtic Sea, out on the shelf edge.  The inner red line is the limit as it was accepted a few years ago, prior to the new marine research programme of the BRITICE-CHRONO team.

There is an important new paper from the BRITICE-CHRONO gang, confirming the earlier ideas about the location of the Devensian Irish Sea Glacier ice front out on the Celtic Sea shelf edge.  There is a lot of new information in it -- much of it very detailed and impenetrable for non-specialists.  But I have extracted some of the key info for reproduction below.

The key points are as follows:

1.  The ice reached its maximum extent between 27,000 and 24,000 years ago -- somewhat earlier than many of us have assumed in the past.

2.  Scilly lay on the eastern lateral flank of the Irish Sea Ice Stream  (ISIS) rather than being in a terminal position.  This is exactly the point I made in my short paper on the Isles of Scilly, supported by evidence which James Scourse did not much like.  But it's all very good that we are now in harmony on this!

3.  After a rapid initial retreat of the ice edge from the shelf edge dated at around 25,000 years ago, there was a stabilisation of the ice front in St George's Channel which lasted from around 24,000 - 22,000 years ago.  This interests me a lot, since this will -- if correct -- have influenced the course of deglaciation in Pembrokeshire.  More on this in due course......

4.  The suggestion that there was a FALLING relative sea-level in St George's Channel around 21,000 years ago is rather interesting, since phases of ice-sheet collapse are normally associated with episodes of relative sea-level rise.  However, this phenomenon, shown by the dotted line in the graph below, may be partly related to a lack of synchroneity with global events and partly with isostatic unloading on the flanks of the Channel, associated with rapid ice melting.  In such a case isostatic uplift might, for a while, have outpaced the eustatic sea level rise, leading to a relative sea-level fall..........

5.  At long last, following a great deal of prodding from me and many others, the authors of this paper are beginning to address (in the final para reproduced below) the matter of the ISIS footprint.  As  I have never tired of pointing out, a narrow glacier tongue, not much more than 100 km wide, extending from St George's Channel out to the shelf edge almost 500 km away, does not make glaciological sense.  The authors explain this hypothetical narrow tongue thus: Our chronology of rapid advance and withdrawal is consistent with this (ie very rapid growth and rapid collapse), as are findings of numerical modelling investigations that struggle to simulate a steady state ice stream of this scale.  I disagree with that.  No matter how rapid the advance and then the collapse of the ice stream might have been,  it must have spread laterally more or less as shown in the map at the head of this post.  The authors clearly have a problem with ice impacting on the coasts of Devon and Cornwall  -- but at least they are now confronting this issue, and we look forward to seeing where the new modelling work will lead.

All in all, a very useful new paper.


Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent

James Scourse et al, Marine Geology, Vol 412, June 2019, pp 53-68

The new radiocarbon data from lithofacies package LF8, indicating deposition between 27 and 24 cal ka BP, are consistent with the single determination published by Praeg et al. (2015) of 24.3 cal ka BP, and confirm the timing of the initial advance phase into the Celtic Sea. Geochronological analysis has recently been undertaken by BRITICE-CHRONO of the terrestrially-exposed retreat sequences along the ISIS retreat axis, in the Isles of Scilly using new combined optically-stimulated luminescence (OSL) and terrestrial cosmogenic nuclide (TCN) rock-exposure ages. Results indicate ISIS reached the islands at 25.5 ± 1.5 ka (Smedley et al., 2017b), slightly earlier than suggested by the legacy Bayesian analysis (Chiverrell et al., 2013) and the timing of the deep-sea Celtic Sea IRD signal (Haapaniemi et al., 2010) but consistent with the offshore ages presented here and by Praeg et al. (2015). The timing of the advance into the Celtic Sea is now tightly constrained by both offshore and onshore geochronological data. The distribution of facies offshore and the presence of a clear ice limit on Scilly (Scourse, 1991; Hiemstra et al., 2006) indicate that Scilly lay on the eastern lateral flank of the ISIS advance, and not in a terminal position.

The initial retreat of ISIS from the Celtic Sea was rapid. In eastern Ireland, new OSL and TCN data register the slowing of the retreat rate from 300 to 600 m yr−1 across the Celtic Sea to stabilisation (3 m a−1) between 24.2 and 22.1 ka during Greenland Interstadial 2 in St George’s Channel (Small et al., 2018). The timing of retreat indicated by a Bayesian analysis of the new eastern Irish data indicates the primacy of topographic and internal glaciological controls over external climatic (atmospheric and oceanographic) forcings in controlling ice stream behaviour within the axis of ISIS (Jamieson et al., 2012; Joughin et al., 2014; Mosola and Anderson, 2006; Small et al., 2018), but these controls cannot be responsible for the initiation of ice retreat from the shelf break at ~25 ka. However, the initiation of ice retreat from the continental shelf west of Ireland has also been dated to ~25 ka (Ó Cofaigh et al., 2019). This synchrony may suggest deglaciation driven by rising/high relative sea level as a function of glacio-isostatic depression of theouter shelf during peak BIIS volume. If correct, this implies greater iceloads and glacio-isostatic depression over Britain and Ireland than hitherto considered likely (cf.Bradley et al., 2011). Palaeotidal simulations for the outer Celtic Sea (Ward et al., 2016) indicate high tidal amplitudes during full glacial conditions (Scourse et al., 2018) which would have accentuated deglaciation as a result of high relative sealevel (Fig. 10). Available glacial isostatic adjustment simulations, which do not incorporate the revised ice extent and loads presented here,indicate falling relative sea level in the vicinity of St Georges Channel at 21 ka (Bradley et al., 2011) associated with very low tidal amplitudes (Ward et al., 2016; Fig. 10). Falling relative sea level combined with low tidal amplitudes likely decrease the rate of deglaciation and icesheet retreat (Scourse et al., 2018) and these external factors may have combined with topographic and internal glaciological controls to determine the decrease in retreat rate as the ice front moved northwards out of the Celtic Sea into St George's Channel. The growth and decay phases of ISIS significantly precedes (by 3 ka) the other major ice stream in the system, the Norwegian Channel Ice Stream, most probably due to differences in the timing of ice sheet build up or that one or the other arises from internal glaciological instabilities.

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 simulatea 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.

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