Click to enlarge the graphs.
In trying to work out what might have happened in SW England during the Devensian glaciation, here is a reminder of some of the data in the influential paper by Dr Alun Hubbard and colleagues published in 2009. The graphs above might look complicated, but they show how the climate oscillated between about 40,000 years ago and 10,000 years ago. In the middle part of the Devensian it is now thought that the climate cooled in Great Britain sufficiently for ice to form in the uplands, and for several pulses of glacier expansion to occur prior to 27,000 BP. But in this period there were also 6 interstadials or warmer episodes with greatly increased melting followed by glacier retreat. The British - Irish ice sheet almost melted away around 27,000 BP, and then (because there was a relatively prolonged period of increased accumulation without any major interruptions) the ice sheet grew and grew until it reached its maximum volume and extent around 20,000 BP. There was a phenomenal rate of growth over 7,000 years, followed by a catastrophic collapse. The authors refer, over the whole lifetime of the ice sheet, to "binge-purge cycles" of gradual accumulations followed by rapid ice evacuations.
Beyond the ice edge, for example in periglacial SW Wales or SW England, what we see in the period 40,000 BP to 27,000 BP is a series of relatively short-lived oscillations between hard permafrost and boreal conditions, with some of these episodes lasting no more than 500 years. Then, from 27,000 to 20,000 BP the climate was continuously cold -- and I surmise that conditions must have been just right for small ice cap formation over Exmoor, Dartmoor and other uplands, and for periglacial processes to continue to operate unabated over the lower-lying areas including Salisbury Plain.
The challenge is to see whether this model actually fits with the sediment accumulations and characteristics of coastal and inland sites in Cornwall, Devon, Somerset and Wiltshire.
Quaternary Science Reviews
Volume 28, Issues 7-8, April 2009, Pages 758-776
Quaternary Glaciodynamics
Volume 28, Issues 7-8, April 2009, Pages 758-776
Quaternary Glaciodynamics
Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet
6 comments:
Brian,
These charts contain very important information, but they are also very difficult to read. I am looking at the upper graph of the first diagram in your post.
I am assuming that the higher the graph gets, the warmer the temperatures, while the lower the graph gets the colder the temperatures. With this understanding, from 15,000 to about 10,000 BP what the upper graph shows is that there was a spike in temperatures around 14,000 BP (the highest in the entire graph) with presumably thawing and ice melting followed by a period of sustained cold temperatures (the coldest showing in the graph) between 14,000 to about 12,000 BP. Then a steady rise in temperatures and consequent thawing and melting of ice from 12,000 BP continuing onto the present. Am I correct?
If so, then what the graph tells us is that there was a rapid melting of the glacier ice around 14,000BP followed by a sustained deep freeze that lasted for some 2,000 years from 14,000 to 12,000 BP. Such rapid melting of the glacier ice would naturally have resulted in the formation of various small lakes (as you explained in another post) with ice caps remaining (not forming) in the higher elevations. These lakes would naturally freeze over solidly during the period of deep sustained freezing (the lowest temperatures in the entire graph!) from 14,000 to 12,000 BP.
Brian, these are the conditions I am hypothesizing in my paper “The un-Henging of Stonehenge”. This explains Stonehenge!
Constantinos Ragazas
Kostas, the evidence of these climate oscillations does nothing whatsoever to explain Stonehenge! The Older Dryas cold phase - Allerod warm phase - Younger Dryas cold phase sequence is pretty well known, and is supported by evidence from many fields. There may well have been lots of meltwater lakes in permafrost areas during the short-lived cold phases, but they would have melted (at least partly) during the summer months. Some of the periglacial experts think that Salisbury Plain and other parts of Southern England will have had some permanent snowbeds, and discontinuous permafrost, with a lot of slope mobility and accumulation of debris in valleys and hollows. In the Stonehenge area there would have been a treeless tundra landscape.
Brian,
Does my reading of the graph agrees with your reading? I am not sure this comes across clearly in your last response.
You write,
“There may well have been lots of meltwater lakes in permafrost areas during the short-lived cold phases, but they would have melted (at least partly) during the summer months.”
How do you know these solidly frozen lakes would have melted during the summers? We have a period of some 2,000 years of deep freezing temperatures – the coldest temperatures shown in the graph for the past 50,000 years!
Constantinos Ragazas
Kostas, you are misreading the graph. The Older Dryas was not the coldest episode in the last 50,000 years -- there were long periods during which annual mean temperatures were lower. And even with low annual means, summer temperatures here in the mid-latitudes would always have been well above zero. Interstadial (warm phase) July temperatures would have been above 15 deg C, and maybe 10 deg C during the period you are talking about. Summer temperatures rise above zero even in the middle of the Greenland ice sheet, and in the continuous permafrost areas of Siberia and Arctic Canada. In those circumstances, many lakes thaw and refreeze every year. Even those that do not become free of ice in the summer remain unfrozen at depth during the winter -- the winter ice is a seasonal cap. That is why fishes and other creatures are able to survive!
Brian,
You write, “...you are misreading the graph”.
You are right. Just to clarify this point, however, I am looking at that part of the upper graph of the first diagram between 15,000 to 10,000 BP. In this part of the graph I see a sharp spike of rising temperatures around 14,000 BP (the highest such spike in all the graph prior to 10,000BP, and very short lived) followed by a period of some 2000 years (from 14,000 to 12,000BP approximately) with the lowest temperatures or nearly so for all the previous 10,000 years. I will concede to you (after closer examination of the graph) that around 24,000BP and at various periods prior to that the temperatures were even colder. This alone is not too consequential to my overall argument, however.
In seeking the truth about Stonehenge, we must not leave any stone unturned. This is what I am doing, keeping an open mind about any final explanations while not being afraid to consider any and all ideas. Though the details of my reading of the graph were wrong (you are right), the larger reading of what the graphs are showing may still be correct. Namely, that there was a rapid rise in temperatures around 14,000BP; followed by a drop of temperatures (the coldest to date since 24,000BP) that lasted for some 2,000 years between 14,000 and 12,000BP approximately; followed by a steady temperature increase from 10,000BP and on.
Do we agree with this assessment of what the graph is showing?
Where I think we disagree are in our conclusions of the natural consequences of these events. What I am suggesting is that during the rapid period of warm temperatures the ice melted and the meltwater formed various small lakes throughout the area. When the temperature dropped to the coldest since 24,000BP for some 2,000 years, these 'meltwater lakes' froze. Whether they froze solidly or not, or whether they froze in the winter but thawed in the summer is an open question. You like to use current examples to make your argument that these 'meltwater lakes' thawed during the summer months (Robert would really like that!) but I like to argue that they remained frozen throughout the year. I have reasons to believe they remained frozen, but I need not go into that here.
Constantinos
As I said, the Older Dryas -- Allerod warm interval - Younger Dryas sequence is well known -- lots of evidence from all over the world. OK -- you want lots of meltwater lakes which then freeze. But in territory like Salisbury Plain most of the meltwater would have escaped in the river system. This was a very different type of landscape from that shown in the photos of Glama and Dranga -- you can see lots of lakes there, on the plateau, because the plateau is just an undulating surface with no clear stream patterns distinguishable. Salisbury Plain had a well developed stream network -- or pattern of dry valleys used by ephemeral streams when there was permafrost preventing infiltration into the underlying chalk. The only lakes I can imagine might have been in enclosed small hollows or where combes might have been blocked by snowbanks or debris slides.
Sorry Kostas -- just a look at the topography is enough to rule out these extensive lakes you want, and no matter how much you might want permanently frozen ice masses all over the place, there is just no evidence to support you.
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