[meteorite-list] When the Sahara was wetter (relevant to yourinterests)

Sun Aug 17 16:46:30 EDT 2008

Hi Sterling and all,

And here is another Wikipedia article you might want to look at:

_http://en.wikipedia.org/wiki/Garamantes_ 
(http://en.wikipedia.org/wiki/Garamantes)   

Only 2000 years ago, central-southern Libya was a prosperous area, a  big 
supplier of grain to the Roman Empire. And a lot of meteorites have  been found 
in the area.

You could also look uo:  Timbuktu, Gao and the other African  Kingdoms.

Any body else interested by archaeology, and how it relates to  meteorites?

Anne M. Black
_www.IMPACTIKA.com_ (http://www.IMPACTIKA.com) 
_IMPACTIKA at aol.com_ (mailto:IMPACTIKA at aol.com) 
Vice-President of  IMCA
_www.IMCA.cc_ (http://www.IMCA.cc)  

In a message dated 8/17/2008 2:14:04 PM Mountain Daylight Time,  
sterling_k_webb at sbcglobal.net writes:
Hi, Darren, Herman,  List,

The Sahara appears to have cycled back  and
forth many times, from extremely dry to quite
wet over the last  hundreds of thousands of
years. There's even a theory that says  these
climatic changes are responsible for major human
movement out of  Africa, the Sahara Pump  Theory:
http://en.wikipedia.org/wiki/Sahara_Pump_Theory

The question is water, that is, rainfall. And that
depends on the monsoon  winds which are caused
by summer heating. Air over the center of the  continent
becomes warmer and rises, pulling in cool wet air from
the  ocean, which causes rain. Paradoxically, the Sahara
was wetter when it  received more solar insolation in the
summer. And that summer solar  insolation  is affected
by the cyclically changing orbital parameters of  the
Earth and its present Ice Age.

So, the present  desertification of the southern
Sahara is the result of the fact that monsoon  winds
don't reach it anymore, since about 3400 BC, and
that is the result  of the Earth long-term Global Cooling
ever since 4000 BC. These dates seem to  me to
correspond pretty well with the ones in the article
Darren  cited.

If you stand back (like off the planet completely) and
look at  it, Africa is a block of continental crust 5000
miles by 4600 miles and  raised about 2000 feet above
sea level with bordering mountains and plateaus  along
its east edge and some of its north edge.

South America, on the other hand, is a similarly
situated block of  continental crust with bordering
mountains and plateaus along its west edge  and some
of its north edge. Because of this completely different
pattern  of "rain-shadows," the fact that the dry Sahara
and sopping wet Amazonia  occupy functionally identical
places on the planet climate-wise does not  produce
the same results at all! No Sahara-like desert in South
America,  except west of the mountains.

"Desertification" is not  restricted to deserts. The
decline in rainfall during these last 6000 years  of Global
Cooling (hotter means wetter, remember?) has caused
drying up of  one of the greatest inland lakes of all time,
which occupied the present  Congo River Basin and was
350,000 square miles in area, more than twice the  size
of the Caspian "Sea" (now the largest lake) and over
four times the  size of Lake Superior. (The former Congo
"Sea" was circular and 600-700 miles  across.)

When the Congo "Sea" was fullest, it overflowed
through the  Shari River into Lake Chad (or Tchad,
if you like), and made it almost as big  as itself, at the
end of the last glaciation. The Chad "Sea" extended
far  north (and south and east and west) of its present
location, covering 150,000  to 250,000 square miles.
The Lake was at its largest 6000 years ago at the  peak
of climatic warmth (and the strongest  monsoon).

All this talk of water, water everywhere  when we're
talking about the Great Sand Sea of the Sahara, which
contains  almost one million cubic kilometers of sand,
may seem a little odd, but sand  and water are involved
in a critical interaction. Saharan sand is generated  largely
by aeolian processes. This creates the possibility of a
positive  feedback cycle. If wind-generated sand is not
removed from the landscape,  then it is available as a
suitable abrasive for making more sand, which  then
makes even more...

What removes sand is rain,  a good abrupt torrential
downpour (like the monsoon) washes it into the  riverbeds
and lake bottoms and eventually out to sea. Without rain,
sand  just piles up! Wind won't remove it; wind just uses
sand to make more sand.  So, enough water means no sand
and not enough water means ever-increasing  sand and...
That's the way sand deserts grow.

An  enterprising species, given to planetary management,
could reverse the  process.

The Congo "Sea" and the Chad "Sea" could  easily be
restored. The Congo "Sea" existed even though there was
drainage  of the Congo river through the gorges of the
Chenal. The Chenal has an  energetic drainage, falling 800
feet in less than 200 miles, so the river has  cut itself so deeply
as to drain the Congo "Sea" away and shallowing it so  that
evaporation could get the upper hand. Dam the Chenal gorges
and the  Congo Basin fills up again, overflows the Shari, fills Lake
Chad, which would  then drain through the wadi's to the NW,
curve to the east, and empty into  the Mediterranean in Tunisia!

Rainfall would go up to  many times the current rate, sand
would start to wash out, and in a  (geologically) short time, sand
would be hard to find! The Sahara would be  navigable by boat,
and Moroccan merchants (I have no doubt) would sail down  to
the Chad Sea and trade along its 2000-mile-long shore of  many
prosperous farm towns. There would in fact be a freshwater
"sea"  waterway through the interior of Africa from the edge of
the Mediterranean as  far south as the edges of Zambia and
Angola.

The  Earth, just like any other planet, can be "terraformed,"
you  know.

Sterling K.  Webb
---------------------------------------------------------------------------
-----  Original Message ----- 
From: "Darren Garrison"  <cynapse at charter.net>
To:  <meteorite-list at meteoritecentral.com>
Sent: Sunday, August 17, 2008  2:26 PM
Subject: Re: [meteorite-list] When the Sahara was wetter (relevant to  
yourinterests)

On Sun, 17 Aug 2008 15:13:18 EDT, you  wrote:

>  I too have considered the effects of a wetter sahara  and how it  affected
>meteorites.It is unreal how many meteorites  comes out of that desert  and 
>all
>the others seem to only  give up a few.I am glad they do come out and  give 
>us  a
>chance to study and compare to the pitiful little earth rocks we   find.

My meteorite collecting (and, really, awareness that there even is  a 
meteorite
collecting market) began after the Sahara "gold rush"  started, and my 
collection
strongly reflects that.  How old those  really weathered meteorites coming 
out of
NWA are is interesting (and a  data point in determining fall rate).  I 
already
knew that the  Sahara was much wetter in the past-- but a data point that it 
was
much  wetter as recently as 4,500 years ago is interesting-- and I would 
think  it
would push the possible age for the weathered meteorites forwards (some,  I
believe, have been estimated to be 10s of thousands of years  old).
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