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

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 17 Aug 2008 15:13:29 -0500
Message-ID: <05e901c900a5$b72ddd90$f34ce146_at_ATARIENGINE>

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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Received on Sun 17 Aug 2008 04:13:29 PM PDT