[meteorite-list] Sahara Paleoclimate was "When the Sahara was wetter (relevant to your interests)"

[Paul]

David Garrison wrote:

" http://i45.photobucket.com/albums/f79/bougainvillea1/Relevant_to_interests_hedgehog.jpg

How wet and for how long and how recently the Sahara 
was wet of course is a determining factor in the ages 
of the older Saharaian meteorites. (of course the 1,000 
years ago is an error on the part of the article writer

http://www.msnbc.msn.com/id/26203952/

Remains of cemetery found in Sahara A thousand 
years ago, the now-barren desert was moist and 
green By Randolph E. Schmid updated 6:28 p.m. 
ET, Thurs., Aug. 14, 2008"

The dissication of the Sahara Desert is discussed in 
"Climate and environmental history of the Sahara: the 
last 6000 years" by Dr.Patrick Honecker at:

http://www.pressoffice.uni-koeln.de/1651+M5f856bfc5ae.html

He states:

"The results of this work document a progressive drying 
of theregional terrestrial ecosystem between 5600 and 
2700 years ago, in response to gradually decreasing 
tropical monsoon rainfall. This drying followed a logical 
ecological sequence starting with tropical grassland trees 
and herbs being replaced by typical Sahel vegetation, 
followed by loss of grass cover and establishment of
the modern desert plant community that is largely 
restricted to oases."

and

"In summary, this new environmental reconstruction from 
within the Sahara proper strongly contrasts with the 
generally accepted hypothesis that the ‘green Sahara’ 
which existed between 10,000 and ~6000 years ago had ended 
abruptly."

The paper ("work") discussing the paleoclimatology of the 
Sahara Desert is:

Francus, P.,  J.-P. Cazet, M. Fagot, B. Rumes, J. M. Russell, 
F. Darius, D. J. Conley, M. Schuster, H. von Suchodoletz, and 
D. R. Engstrom, 2008 Climate-Driven Ecosystem Succession in 
the Sahara: The Past 6000 Years. Science. vol. 320, no. 5877, 
pp. 765-768.

The abstract to this paper can be found at:

http://www.sciencemag.org/cgi/content/abstract/320/5877/765
http://www.ncbi.nlm.nih.gov/pubmed/18467583?dopt=Abstract

Their paleoenvironmental reconstruction of the pollen from a
continuous core covering 6000-year from northern Chad indicates 
a "progressive drying of the regional terrestrial ecosystem" 
that resulted in strong reductions in tropical trees and then 
Sahelian grassland cover" and "large-scale dust mobilization" 
starting about 4300 calendar years before the present. They 
concluded that "today's desert ecosystem and regional wind 
regime were established around 2700 calendar years before 
the present."

A PDF file of this paper can be found at:

http://www.old.uni-bayreuth.de/departments/geomorph/docs/Kroepelinetal_2008.pdf

A discussion of this paper can be found in "Study: 
Sahara Gradually Dried Up Over 6,000 Years" at:

http://www.npr.org/templates/story/story.php?storyId=90289718

Another recent paper about the paleoclimatology of the 
Sahara Desert is;

Bubenzer1, O., and H. Riemer, 2007, Holocene Climatic 
Change and Human Settlement Between the Central Sahara 
and the Nile Valley: Archaeological and Geomorphological 
Results. Geoarchaeology, vol. 22, no. 6, 607–620.

For the eastern Sahara, they conclude:

"The evidence derived from archaeological excavations and 
surveys coupled to nearly 500 14C dates (Figure 2) suggests 
that the Holocene wet phase lasted from approximately 
9500–6000 B.P. (9000–5000 cal. B.C., calibration: dispersion 
calibration program, Cologne 2001, www.calpal.de). After 
the hyper-arid Pleistocene, the tropical summer rain front 
moved about 700–1000 km northward (e.g., Haynes, 1987;
Neumann, 1989a; Pachur and Hoelzmann, 2000), which 
initiated more humid conditions in the Eastern Sahara."

Notice that the Pleistocene before 9,500 BP was hyper-arid 
and the "wet" Sahara was only from 9500–6000 B.P.

An interesting web page on this topic is "Africa During the 
Last 150,000 Years" by Jonathan Adams at:

http://www.esd.ornl.gov/projects/qen/nercAFRICA.html
http://www.esd.ornl.gov/projects/qen/new_africa.html
http://www.esd.ornl.gov/projects/qen/refs.html

This is what he has to say about the Sahara:

"(dates in Guo et al are given in 14C years ago on the 
left, approximate calibrated of 'real' dates are given 
on the right)

Moist 9,500-8,200 14C ya (10,400-9,100 ya)
Slight drying 8,200-8,000 14C ya (9,100-8,900 ya)
Moist 8,000-7,000 14C ya (8,900-7,900 ya)
Moderately dry 7,000-5,700 14C ya (7,900-6,500 ya)
Moist 5,700-4,000 14C ya (6,500-4,500 ya)
Very dry - as dry as at present - 4,000-3,800 14C ya (4,500-4,100 ya)
Slightly moister than present 3,800-3,500 14C ya (4,100-3,700 ya)
After 3,500 14C ya (3,700 ya). Remaining about as dry as at present"

Yours,

Paul H.