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Wednesday 21 October 2020

The Late Devensian ice edge and the inconvenient bones

"Mission control -- we have a problem..."

A few days ago I did a post suggesting that the LGM ice cover across South Wales was far more extensive that previously suggested by a host of researchers over the past 100 years.  I also suggested that the ice-free enclave of south Pembrokeshire, shown on many maps, did not exist, since apparently fresh glacial deposits are just as widespread as they are in North Pembrokeshire.  So far so good...

But I have chased after a great many people on this blog for ignoring "inconvenient" evidence -- so I must be honest and admit that the hypothesis is challenged by the evidence of human and other mammal bones found on Caldey Island, at Paviland Cave and elsewhere.  The problem is that the dating of the LGM in western Britain is conventionally placed at 25,000 - 20,000 yrs BP  -- and if the landscape was covered by glacier ice at that time, how come there are a number of radiocarbon dates for bones (mostly found in caves) suggesting ice-free conditions at exactly the same time period?

This is a rather delicious dilemma. Is my hypothesis shown to be incorrect?  Or could the dating of the LGM be faulty?  Or are the radiocarbon dates for bone samples incorrect, and subject to adjustment in the light of recent research on methodology and correction factors?

One thing at a time.  First, what about the dating of the LGM?  Well, it appears that it is being pushed back in time by modern research:

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

It now appears that the ice reached its maximum extent in the Celtic Sea arena around 27,000 years ago -- somewhat earlier than many of us have assumed in the past.  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. On the other hand Jenkins et al (2018) suggest that there was an ice edge running across St George's Channel to the Wexford area at about 25,000 yrs BP.  Then the ice edge retreated northwards during a period of catastrophic ice wastage, over a period of c 2,000 years.  The Welsh Ice Cap lasted longer, and started its major retreat around 20,000 years ago. So the two ice masses were seriously out of synchroneity -- if the cosmogenic and other dating results are to be believed:

Late Devensian deglaciation of south-west Wales from luminescence and cosmogenic isotope dating
ISSN 0267-8179.
DOI: 10.1002/jqs.3061 

There are a host of issues here, still to be resolved.  The Celtic Sea ice edge at c 25,000 yrs BP is variously placed at the shelf edge and in St George's Channel -- one postulated position is almost 500 km from the other!  No matter how catastrophic the collapse of the ISIS (Irish Sea Ice Stream) was, somebody has the dating all wrong.  Also, we have problems of the ice build-up, which is always much slower than ice wastage.  So when was west Wales first affected by the ice of the ISIS?  It must have been well before 30,000 yrs BP.  So should we start thinking about Early and Middle Devensian ice edge positions?  (Of course, there is a school of thought that says Lundy was ice-covered during an Early Devensian glacial episode.)  And what about the binge-purge scenario postulated by Alun Hubbard and his colleagues some years ago?  Was there a massive surge, as suggested by James Scourse and others?  And if so, what was the footprint of the ISIS at the time?  Many questions and no adequate answers..........

Quaternary Science Reviews Volume 28, Issues 7-8, April 2009, Pages 758-776
Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet
Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper and Martyn Stoker

On the evidence currently available, it seems most likely that the land surface of Pembrokeshire was covered by ice for around 5000 years (c 30,000 yrs BP - 25,000 yrs BP) and maybe longer, and that there was then a catastrophic ice edge retreat with a transformation from glacial to tundra conditions.

Source:  A Mid-Upper Palaeolithic human humerus from Eel Point, South Wales, UK.  
Rick J Schultinga, Erik Trinkaus, Tom Higham, Robert Hedges, Michael Richards, Bernice Cardy,  Journal of Human Evolution, 2005

On to the animals and the bones. 

I have done a number of posts on this before, focussing on Caldey Island and Paviland.  

Ogof yr Ychen radiocarbon dating:

Sadly, most of the research on animal and human bone samples from the limestone caves is now rather old, meaning that the radiocarbon dates cited by Schulting, David and others need to be recalibrated.  In Chapter 1 of Vol 1 of the Pembrokeshire County History (2016), Elizabeth Walker seeks to unravel the assorted bits of evidence relating to the animal occupation of caves and the human occupation as well.    She suggests that there was human use of the Coygan Cave near Laugharne between 66,000 yrs BP and 38,000 yrs BP, and that mammoth, woolly rhinoceros, reindeer, bison and brown bear all roamed about in a tundra landscape at the time.  Evidence from Hoyle's Mouth near Tenby, suggests human presence around 32,000 yrs BP, and animal remains have been dated to the period 35,000 - 29,000 yrs BP.   The Paviland Cave evidence suggests human occupation between 28,000 and 21,000 yrs BP, and the famous radiocarbon date on the "Red Lady" skeleton is now put at 24,490 yrs BP.  The radiocarbon date from a human bone found at Eel Point on Caldey Island was dated by Schulting et al as 24,470 yrs BP, again suggesting the presence of human hunters around Carmarthen Bay at the peak of the last glaciation locally, when ice cover must have been very extensive. There are other animal remains from caves dated around 22,350 yrs BP (our "rhino date" from Ogof yr Ychen) and 22,800 yrs BP (barnacle geese bones from Little Hoyle cave) -- but the general assumption among archaeologists is that there was a "human settlement hiatus" between 24,000 and 12,600 yrs BP all around the South Wales coasts.

The Eel Point human humerus, dated as 24,470 yrs BP.  But it might not actually have come from Eel Point......

A plot, by Schulting et al (2005) of the radiocarbon dated bone samples from South Wales in the period 17,000 - 30,000 yrs BP.

This is all very confusing, made even more confusing by the current opinion that the "Eel Point human bone" did not come from Eel Point at all, and the assumption that nearly all of these radiocarbon dates need recalibration -- which tends to push dates back in time by highly variable amounts.  Also, some of the studies in which these dates are published do not accurately show the cave stratigraphy which might enable us to identify episodes of environmental change.  Which bone caves, for example, were covered by Devensian ice and which ones were not? 

There is no problem at all over the ability of both grazing and carnivorous animals  and human beings to exist and indeed thrive very close to an active ice edge -- and indeed at the present day musk-oxen, Arctic hares, wolves, foxes and reindeer move about freely very close to the edge of the Greenland ice sheet and even cross glaciers when necessary. 

Muskoxen and morainic ridge, close to Russell Glacier, NE Greenland.

But there are substantial dating issues here, with uncertainties and debates around the precise dating of glacial events and around the precise timing of animal and human occupation of this glaciated landscape.

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