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Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
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 Posted: Sat Jul 28, 2007 8:56 am Post subject: Exhibit #4, The Younger Dryas and glaciation |
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The objective of the "Exhibit-#n-The-Younger-Dryas-and .......,"-threads is to investigate for ourselfs what evidence is available to judge about the alleged cold conditions of the Younger Dryas.
It is assumed that:
| Quote: | | The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900 – 11,500 years before present (BP)[4] in sharp contrast to the warming of the preceding interstadial deglaciation. |
So let's check and see indeed if the Younger Dryas saw more glaciations than the preceding Bolling Allerod or?
This would get to be a somewhat larger thread let's start of with a fairy recent paper about glaciers in Scotland:
Golledge N.R. 2007, An ice cap landsystem for palaeoglaciological reconstructions: characterizing the Younger Dryas in western Scotland Pages Quartenary Science Reviews Volume 26, Issues 1-2 pp. 213-229 (January 2007)
this issue is -currently- free. But hurry. it changes.
Anyway we find:
| Quote: | | ...this landsystem tool predicts an extensive Younger Dryas ice cap with a maximum surface elevation of 900m above sea level, implying colder or wetter conditions than previously thought,.... |
So, is my assessment about a much warmer Younger Dryas debunked by this now? Let's see about the dates in the document. I only found these:
| Quote: | ..Complete disappearance of the Younger Dryas ice mass by 10.6–10.4 14C ka BP is inferred from dated basal organic sediments on Rannoch Moor (Lowe and Walker, 1976).
.....The timing of glaciation in the study area is bracketed by organic deposits buried beneath subglacial till at Croftamie, southeast of Loch Lomond, suggesting a maximum glacier extent at 10,560 +/- 160 14C ka BP, (Evans and Rose, 2003), and inferred ice-free conditions on Rannoch Moor by 10,660 +/- 240–10,390 +/- 200 14C ka BP, (Lowe and Walker, 1976). Although the dates from Rannoch Moor may be 'too old' (Sissons, 1979), the Croftamie dates nonetheless imply that the Younger Dryas ice reached its maximum southerly extent very late in the Stadial, and that subsequent deglaciation was very rapid indeed.... |
So we have only these few carbon dates to go by. Carbon dating before the times of AMS around 1980 is too unreliable to depend on. So we are left with the boldface dates, let's calibrate them.
"maximum glacier extent at 10,560 +/- 160 yrs 14C ka BP" gives us
10,400 BP = 12,360 Cal BP
10,720 BP = 12,800 Cal BP
consequently, 10,560 +/- 160 yrs 14C ka BP calibrates to 12,580 +/- 220 Calendar years before present.
Now if we look at the Meerfelder maar chronology (perhaps the most accurate):
we see the gray transition area from Bolling Allerod to Younger Dryas to be about centrered around ~12,665 "varve"-years BP, so in which period should we place this glacial advance, that reached it's maximum ~80 +/- 220 years into the Younger Dryas?
The Wiki statement is also about deglaciation in the Bolling Allerod, well let's see:
A fragment of the 'opus magnus' article I'm writing:
| Quote: | | Clark (2003) examines evidence from alpine glacial deposits in the American Cordillera and observes glacial retreats In the Sierra Nevada, between 17,000 and ~15,000 14C yr BP(~20,100-18,500 cal yr B.P. and in the North Cascades by ~17,000 36Cl yr BP; in southern Idaho at 13ka BP (15,3 ka Cal BP) He observes limited Glacier readvance respectively after 12,200 14C yr BP (14,000) cal yr. BP and ~14,000 and 13,000 36Cl yr BP and for southern Idaho ~12,800 – 11,000 14C yr BP (~14,100-12.900 cal yr BP), consequently during the alleged warm Bolling Allerod . After that The Sierra Nevada remained largely or entirely free of glacier ice, including during the Younger Dryas (YD) chronozone. |
Clark, D.H, 2003. Complex Timing and Patterns of Glaciation in the American Cordillera during Termination 1, Poster presentation XVI INQUA Congress, Paper No. 88-4, July 30.
Doesn't look good for Wikipedia, does it?
See also
The Younger Dryas exhibits
Exhibit #1, The Younger Dryas and the Meerfelder maar
Exhibit #2, The Younger Dryas and Mediterranean region
Exhibit #3, The Younger Dryas and South greenland
Exhibit #4, The Younger Dryas and Glaciation
Exhibit #5, The Younger Dryas and the Mystery interval
Exhibit #6, The Younger Dryas and North America
Exhibit #7, The start of the Younger Dryas
Exhibit #8, The Younger Dryas and the Siberian Steppes
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Last edited by Andre on Tue Aug 21, 2007 7:19 pm; edited 1 time in total |
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Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
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| Posted: Thu Aug 09, 2007 7:55 pm Post subject: |
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Another fragment:
| Quote: | | Bondevik and Mangerud 2002 find in west Norway small cirque glaciation very early in the Younger Dryas basing this on a BA/YD boundary of 12,800 years, which, however would coincide with the last Allerød spike (Lücke and Brauer 2002). Furthermore, they attempt to circumference the carbon dating complications and use the Vedde Ash tephra layer (11,980+/-80 Greenland Ice core years, Grönvold et al 1995) in a sediment core and assume a constant sedimentation to estimate the date of a glacial silt deposit. They assume a maximum glaciation between 11,600 and 11,700 years ago, Using the same sedimentation time scale they infer the glacial retreat around 12,500 years ago. The rapid increase of Betula pollen is interpreted as the Younger Dryas boundary, hence around the Holocene transition. It appears however that the dating error margin, induced by assuming constant sedimentation rate, would allow for glaciation at the Younger Dryas-preboreal transition, due to the increase in precipitation, also suppressing the betula population. This all, would be consistent with aridness during the Younger Dryas, between two moist periods. |
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Andre
Joined: 21 Jul 2007
Posts: 298
Location: Germany - The Nederlands
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| Posted: Sun Aug 12, 2007 10:52 am Post subject: |
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Another fragment of the paper:
| Quote: | Pienitz et al 2000, conducted Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequence in the Yukon and find in interval I (c. 11 000–10 500 yr BP) that combined evidence from all stratigraphic markers suggests either deposition in a basin fed by glacial meltwaters from the receding Laurentide Ice Sheet or deposition during a cool and dry climatic episode corresponding to the Younger Dryas. However the indicated timeframe calibrates to 12,900 - 12.590 and coincides with the last spike of the Allerød, not the Younger Dryas.
Porter (1978) identifies a late-glacial advance of the Cordilleran Ice Sheet in the Fraser Lowland between about 12,000 and 11,000 14C years ago (which calibrates to 13,800 to 11,900 cal years BP).
Denton, and Hendy (1994) date the advance of the Franz Josef Glacier in the Southern Alps of New Zealand at 11,050 C 14 years before present and assessed it to be Younger Dryas. However INTCAL04 (Reimer et al 2004) yields a date of 12,940 Ca years BP dating this event in the Bølling Allerød instead. |
The latter sort of exposes what went wrong. Many glacial advances dated around 11,000 carbon years ago. We now know that this is 12,910 Cal years BP . But look at the meerfelder maar pollen diagram, that's clearly before the Younger Dryas, during the last spike of the Allerod interval. And since early research suggested massive glacier advance AT 11,000 carbon years BP and AT 10,000 carbon years BP, the YD became quickly known as the interval between 11,000 and 10,000 years ago with massive glacier advance. NOT!
Pienitz et al 2000 also define an “interval II (c. 10 500–8100 yr BP)” for the Yukon area and contend about that:
| Quote: | | the early Holocene period showed initially high diatom-inferred salinity values, probably driven by low effective moisture and higher temperatures. |
However 10,500 yr BP calibrates to 12,590 Cal years BP. Consequently their interval II includes most of the Younger Dryas, low effective moisture and higher temperatures.
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