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Friday, 29 September 2017

Where was the Anglian ice sheet edge?



This is a topic to which we have returned many times, and will no doubt return again.......

I have been taking a fresh look at the highly influential paper by Patton et al (2016) which applies a more sophistcated modelling technique than ever before to the Eurasian ice sheet complex -- including the Celtic Ice Sheet (otherwise called the British / Irish Ice Sheet or BIIS).  Although the work relates to the climatic and other conditions for the Late Weichselian (let's say 40,000 BP to 15,000 BP as being relevant to SW Britain) the modelling is in some respects more applicable to the earlier glaciations, since ground truthing suggests that the Celtic Ice Sheet never did develop to the full extent as predicted by the model.

 The map above shows the most commonly assumed Devensian ice limit (red line) for the British Isles, which is clearly inadequate in many respects.  The area defined by the wavy dark blue line shows the maximum extent of the modelled ice sheet, based on an analysis of a vast range of different parameters.  The orange areas are the areas of assumed highest erosion -- the dark blue area shows the ice shed location, stretching from the Lake District across to the Isle of Man and thence towards SW Ireland.  Note that the ice is shown impinging well into the counties of Devon, Cornwall and Somerset, onto Salisbury Plain, and across the Midlands towards The Wash, incorporating a large area to the north of the Thames Basin assumed to have been unglaciated in the Devensian.

Take a look at these maps as well:

The upper map shows the positions of the ice divide around 27,000 BP (red), according to the run of the model, and around 22,000 BP (black).  An ice divide running across Ireland from NE to SW must have forced ice flow on one flank towards the SE and on the other flank towards the NW.  As far as St George's Channel and the Celtic Sea are concerned,  this supports my thesis of a Celtic Sea ice mass made up of ice from the Irish Sea Glacier, but with a powerful mass of ice coming from central Ireland, and forcing the ice to flow eastwards, as shown on this map:


The similarity between these maps is remarkable, given that my map is based largely on ground evidence, and theirs is based on computer-based glacier modelling.

So far so good -- but the intriguing thing is that my map shows my assumed Anglian glacial scenario, and theirs shows a theoretical Late Weichselian scenario!   I need advice from the experts.  Watch this space........


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Henry Patton, Alun Hubbard, Karin Andreassen, Monica Winsborrow, Arjen P. Stroeven. 2016.
The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing. Quaternary Science Reviews, Volume 153, 1 December 2016, Pages 97–121

http://dx.doi.org/10.1016/j.quascirev.2016.10.009
http://www.sciencedirect.com/science/article/pii/S0277379116304498

Note:   Late Weichselian = Late Devensian

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Abstract
The Eurasian ice-sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum (LGM), after the Antarctic and North American ice sheets. Despite its global significance, a comprehensive account of its evolution from independent nucleation centres to its maximum extent is conspicuously lacking. Here, a first-order, thermomechanical model, robustly constrained by empirical evidence, is used to investigate the dynamics of the EISC throughout its build-up to its maximum configuration. The ice flow model is coupled to a reference climate and applied at 10 km spatial resolution across a domain that includes the three main spreading centres of the Celtic, Fennoscandian and Barents Sea ice sheets. The model is forced with the NGRIP palaeo-isotope curve from 37 ka BP onwards and model skill is assessed against collated flowsets, marginal moraines, exposure ages and relative sea-level history. The evolution of the EISC to its LGM configuration was complex and asynchronous; the western, maritime margins of the Fennoscandian and Celtic ice sheets responded rapidly and advanced across their continental shelves by 29 ka BP, yet the maximum aerial extent (5.48 × 106 km2) and volume (7.18 × 106 km3) of the ice complex was attained some 6 ka later at c. 22.7 ka BP. This maximum stand was short-lived as the North Sea and Atlantic margins were already in retreat whilst eastern margins were still advancing up until c. 20 ka BP. High rates of basal erosion are modelled beneath ice streams and outlet glaciers draining the Celtic and Fennoscandian ice sheets with extensive preservation elsewhere due to frozen subglacial conditions, including much of the Barents and Kara seas. Here, and elsewhere across the Norwegian shelf and North Sea, high pressure subglacial conditions would have promoted localised gas hydrate formation.

2 comments:

Dave Maynard said...

Out of curiosity, what are the factors leading to a larger spread of ice in the different ice ages?

Duration, intensity of cold, appropriate precipitation, solar variation?

Probably a huge range of answers. can the same be said about the median temperature in the various interglacials?

Dave

BRIAN JOHN said...

A huge range of parameters, Dave. Solar radiation, mean temperatures, annual temperature variations, wind directions, extent of permafrost, sea level position, sea temperatures, precipitation and distribution through the year, ocean current oscillations, topographic details, altitude of land surface, ice thickness and bed conditions, etc etc etc. Modelling is hugely complex, especially when they do it in 100 year (or shorter) slices....... in both the glacials and the interglacials there are oxygen isotope controls. Often these come from polar ice sheet cores -- especially from Greenland and the Antarctic.