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Saturday, 28 January 2017

New dates confirm age of Scilly Isles LGM



New dates for rock surfaces and sediments within the assumed LGM ice margin.  
From Fig 2 of the new paper.


I've received a copy of a new paper on the Isles of Scilly, which concentrates on the dating of LGM-related sediments and also rock surfaces within the assumed LGM limit (parts of which are of course in the wrong position).  It's an interesting and detailed piece of work, and I go along with almost all of it.  Abstract and other info below.

The most interesting point to come out of the paper us that the LGM here was a bit earlier than previously supposed -- ie around 24,000 years ago rather than 20,000 years ago.  By 20,000 years BP, the authors assume that the retreat from the maximum ice edge was well under way.

I would have liked the authors to have done the following:

1.  Consider whether the ice margin shown on the map is indeed accurate, by testing some "control" rock surfaces and sediments outside the assumed glacial maximum.  It's rather extraordinary that they didn't do that.........

2.  Give greater consideration to the classification of the sediments above the Scilly Till.

3.  Give more thought to the shape and other characteristics of the "Celtic Sea surge lobe" which is shown pushing well to the south of the Isles of Scilly.  It seems to me to disobey the laws of physics in that there is hardly any lateral spreading shown on Figure 1, and no calving bay where the lobe hits the grounding line.  I would have liked more consideration to be given to a powerful ice stream pressing south-eastwards from Southern Ireland and coalescing with the Irish Sea ice stream.

Another interesting thing to come out of the paper is the presence of a quatrzite erratic on Tresco that appears to have come from Anglesey or the east coast of Ireland.

 All in all, another valuable contribution to our understanding of the late Devensian glaciation.

=============================

New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly
R. K. SMEDLEY, J. D. SCOURSE, D. SMALL, J. F. HIEMSTRA, G. A. T. DULLER, M. D. BATEMAN, M. J. BURKE, R. C. CHIVERRELL, C. D. CLARK, S. M. DAVIES, D. FABEL, D. M. GHEORGHIU, D. MCCARROLL, A. MEDIALDEA and S. XU
JOURNAL OF QUATERNARY SCIENCE (2017) 32(1) 48–62  
DOI: 10.1002/jqs.29224
November 2016
http://onlinelibrary.wiley.com/doi/10.1002/jqs.2922/epdf

ABSTRACT:    The southernmost terrestrial extent of the Irish Sea Ice Stream (ISIS), which drained a large proportion of the last British–Irish Ice Sheet, impinged on to the Isles of Scilly during Marine Isotope Stage 2. However, the age of this ice limit has been contested and the interpretation that this occurred during the Last Glacial Maximum (LGM) remains controversial. This study reports new ages using optically stimulated luminescence (OSL) dating of outwash sediments at Battery, Tresco (25.5+/-1.5ka), and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on Scilly Rock (25.9 +/-1.6 ka), which confirm that the ISIS reached the Isles of Scilly during the LGM. The ages demonstrate this ice advance on to the northern Isles of Scilly occurred at c 26 ka around the time of increased ice-rafted debris in the adjacent marine record from the continental margin, which coincided with Heinrich Event 2 at c 24 ka. OSL dating (19.6 +/- 1.5 ka) of the post-glacial Hell Bay Gravel at Battery suggests there was then an  approx 5-ka delay between primary deposition and aeolian reworking of the glacigenic sediment, during a time when the ISIS ice front was oscillating on and around the Llyn Peninsula, c 390 km to the north. 

Copyright 2017 The Authors. Journal of Quaternary Science Published by John Wiley & Sons, Ltd.


2 comments:

Dave Maynard said...

I have my editing head on, what is LGM? Last Glacial Maximum?

Dave

BRIAN JOHN said...

Sorry Dave -- yes, correct!