Earth Sciences Forum

Andre · Last edited by Andre on Tue Aug 21, 2007 7:20 pm; edited 6 times in total

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.

This is the first one, one of my favorites (albeit not even the most compelling), the Meerfelder maar:

It's an old crater lake in the Volcanic Eifel in Germany. The Eifel area has been active during the last
glacial transition like many other areas. Due to the moist soils, volcanism here is thought to be
explosive due to the water explosive evaporating, leaving deep craters, which fill up with water, the
"maars". There are dozen of them, containing a marvellous record of geologic history in their muddy
sediments.

In those static waters sediments accumulate slowly. In dry periods, wind- blown dust settles on the
bottom (facies). In spring there usually a short bloom period of algae and other micro-organisms
(diatoms), the remains of which show a tiny white layer on the bottom. But on top of that there are pollen in summer,
which don’t decay for some reason. So you get separate layers accumulating for each season and as
long as nothing disturbs this annual “stratification”, you can count each individual year just like the ice
cores. Here is such a sediment core with the annual layers (varves)

The beauty of it is that what-you-see-is-what-you-get, unlike ice cores where you need interpreting
things with hypotheses and allow for all kind of “in situ” changes due to the dynamics of ice sheets.

Now after laborious work of the researches an inventory can be made of the annual pollen content of
such a core. Here is the Meerfelder maar in the Younger Dryas, a unique record:

Source:
Lücke, A. Brauer A., 2004. Biogeochemical and micro-facial fingerprints of ecosystem response to
rapid Late Glacial climatic changes in varved sediments of Meerfelder Maar (Germany).
Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 211, Issues 1- 2, 19 August.

In the next part we will analyse what we see. Is it temperature or is it moisture?

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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billiards · Posted: Wed Jul 25, 2007 9:39 pm Post subject:

Hi Andre, for those of us that don't know how to interpret this core data, what exactly is it telling us? I'd imagine that tree pollen going down in the Younger Drias is consistent with the number of trees declining, which is consistent with this cold spell in earth history. (c.f.climate diagrams on "mammoth thread".)

It will be fascinating to put this stuff up next to the ice core data from the same time and try to integrate it, has anybody to your knowledge done that yet?

Andre · Posted: Thu Jul 26, 2007 9:06 am Post subject:

Matt · Joined: 25 Jul 2007 Posts: 14

An interesting read. One thing isn't clear to me though: In the UK the younger dryas (aka. Loch Lomond stadial) is characterised by a small glacial readvance. This doesn't seem consistent with increased aridity and no decrease in temperature. Why would the glaciers grow if you were correct?

Andre · Posted: Fri Jul 27, 2007 9:18 am Post subject:

Bryn Thomas · Joined: 27 Jul 2007 Posts: 3

I look forward to your story unfolding, but for the present still have difficulty with the inference you draw of a correlation between aridity and warmth.

Alow me to play devil's advocate.

The author of the wikipedia entry about the YD <http://en.wikipedia.org/wiki/Younger_Dryas> notes:
"Other features seen include:
* More dust in the atmosphere, originating from deserts in Asia.
* Drought in the Levant, perhaps motivating the Natufian culture to invent agriculture."

But those are generalities, not necessarily applicable to the Eifel.

You are inferring from the greater abundance of grass pollen that the climate is drier, but the percentages of Pine and Birch are not insignificant. And the alga Pediastrum flourished well during at least the lower half of the YD. One has to be careful interpreting pollen diagrams. At any one horizon the percentages of all groups of pollen add up to 100. The apparent reduction in Pine and Birch is perhaps due to the addition of a herbaceous component, not a reduction in the tree population. The principle herbs are grasses (which you mentioned) and sedges (Cyperaceae) and daisy-like plants (Artemisia). The sedges typically like a moist climate (around the shore of the maar?), Artemisa lgenerally ike a dry to semi-dry habitat.

The region remained coated in a cool temperate flora. I envisage a more open forest, not a tundra, which apparently characterizes more "typical" YD assemblages, and certainly not a completely dry lake. I note there is no mention of Dryas itself, which is a point in your favour.

Also in your favour is the visual impression of the core through the Dryas section. I could be wrong, but the darker varves of the YD might have a higher organic content in the winter portion of the varve cycles. Some of that colour could, of course, be mineral matter (perhaps worn from the flanking tephra to the diatreme/maar?? If there is a significant organic component, it suggests greater organic productivity, perhaps a milder climate.

One more thought, the Eifel at 50degS is about mid-way between Scandinavia and the Alps, the two nearest glaciation centres. I don't think anyone has (or can) determined the extent of ice fields during the YD (other than the discredited association with Lake Agassiz). In that midway position, the influence of permanent ice sheets could have been minimal and the vegetation not changed unduly.

I hope these thoughts might be of interest.

Bryn
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Andre · Posted: Fri Jul 27, 2007 8:08 pm Post subject:

Thanks, Bryn.

A few notes.

The pollen count is the second from the right showing a dramatic reduction of all pollen. The other appear to be percentage's (when filled in black) or promilles (when not filled in). So for Pinus and Betula, in the Younger Dryas, both the percentage dropped and the total, but visually this effect is reduced. Should they have used absolute numbers then thedrop would be much more significant.

Then again, if you would run "dryer than before and after" against the pollen, it is consistent indeed with mix of an open forest and steppe and marshy conditions around the lake.

However if you would run "colder than before and after", you would have problems explaining the presence of Ephedra, Quercus, Helianthemun and Sanguisorba and indeed the absence of Empetrum and Dryas, which uses to be abundant in nordic pollen assemblages.

Please stay tuned indeed. I'd appreciate comment on exhibit #3 South Greenland. Furthermore I hope to zoom in on glaciations tomorrow.
_________________
Moderator of http://tech.groups.yahoo.com/group/climatesceptics/

"Don't worry about the world coming to an end today. It's already tomorrow in Australia."

Charley Brown

Bryn Thomas · Joined: 27 Jul 2007 Posts: 3

Thank you Andre
I should have looked at the significance of the other floral groups.

We both note the lack of Dryas itself. I think we are both viewing this as a 'warmer than' circumstance. This is where the geographic locations of the sample sites become significant. Is it valid to correlate events in the Eifel with a temperature gradient in Greenland?

But I await further installments with great interest. You are tackling a problematic period of time that has great significance globally.

Bryn
_________________
All the world's a stage,
And all the men and women merely players.
They have their exits and their entrances,
And one man in his time plays many parts,
His acts being seven ages.

Andre · Posted: Sat Jul 28, 2007 9:04 am Post subject:

Mk · Joined: 22 Jul 2007 Posts: 18

Andre

Mk · Joined: 22 Jul 2007 Posts: 18

Andre