[meteorite-list] intriguing Question

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 11 Jan 2008 21:55:11 -0600
Message-ID: <017601c854ce$ee835640$795fe146_at_ATARIENGINE>

Hi, Svend. Pat, Jason,

> I fully second Sterling's post on this subject
> but take the liberty to add, that desertification
> in the Sahara may not be such an irreversible
> process as suggested.

    I may have been a bit dramatic is saying for
"millions of years to come," but that's the sand
issue. The climate issue is more complicated
(than I knew). 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 land 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 the 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
(sorry, Capt.) ever since 4000 BC.

    Svend is quite right about the importance of the
"wind shadow" effect. If you stand off the planet 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, the fact that the
Sahara and Amazonia occupy functionally identical
places on the planet climate-wise does not produce
the same results at all!

    "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.

> A single acacia growing on a hammadah or on an erg-surface
> accumulates soil in a radius of 25 - 40 meters. This soil layer
> can reach up to 1 m height during the lifetime of the plant.
> A climate change involving a noticeable growth of humidity,
> triggered by a change of e.g. ocean currents, would soon lead
> to the spreading of Savannah vegetation into the Saharan voids.

    The presence of that acacia is also critically dependent
on water also. (Everything in the Sahara is critically dependent
on water!) But I don't think changes in ocean currents play
that much of a role without a monsoon to move the evaporated
water inland. Warmer currents would help but only marginally.

> may not be ... irreversible

    You want to reverse it?

    Well, 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.

    Mars isn't the only planet that can be "terraformed."



Sterling K. Webb
-------------------------------------------------------------------------
----- Original Message -----
From: <info at niger-meteorite-recon.de>
To: <meteorite-list at meteoritecentral.com>
Sent: Friday, January 11, 2008 3:27 AM
Subject: Re: [meteorite-list] intriguing Question


Pat, Sterling, others,

moving or resting sand is not conserving meteorites but is
one of the major threats to the preservation of meteorites.
Most if not all meteorite aggregation fields in the Sahara have
one thing in common: they are free of sand. In fact they are
shielded against the sandflow by ridges or mountain chains,
e.g. the Djebel al Aswad is bordering the east of the Dar Al Ghani.

These natural barriers that prevent the sand from entering
certain areas are commonly situated in the east or northeast
of the aggregation field. And the winds on the African continent
tend to blow from the northeast during the whole year since
at least ten thousand years.

The surface horizons of the meteorite concentration areas such
as the HAH and the DAG are mostly composed of pebbles and
top soil and are free of quartz sand.

Terrestrial lifetimes of meteorites decrease as soon as quartz
sand is involved due to the quick mechanical abrasion.

It is true that meteorites found in aggregation areas in the
Sahara were sedimented and protected by soils from weathering
before the recent wind erosion uncovered them. This process
started about 12 - 10,000 years ago and continues until present.
But these sediments that are being eroded today were not
composed of quartz sand.

There is a good paper on the mechanisms responsible for
meteorite concentration in the dense aggregation areas by
Schlueter et al. :

J. Schl?ter et al. (2002) The Dar al Gani meteorite field,
Meteoritics & Planetary Science 37, 1079-1093.

I fully second Sterling's post on this subject but take the
liberty to add, that desertification in the Sahara may not
be such an irreversible process as suggested.

A single acacia growing on a hammadah or on an erg-surface
accumulates soil in a radius of 25 - 40 meters. This soil layer
can reach up to 1 m height during the lifetime of the plant. A
climate change involving a noticeable growth of humidity,
triggered by a change of e.g. ocean currents, would soon lead
to the spreading of Savannah vegetation into the Saharan voids.

Cheers
Svend

www.meteorite-recon.com













----- Original Message -----
From: "Pat Brown" <radio_ranch at yahoo.com>
To: "Jason Utas" <meteoritekid at gmail.com>; "Meteorite-list"
<meteorite-list at meteoritecentral.com>
Sent: Friday, January 11, 2008 7:41 AM
Subject: Re: [meteorite-list] intriguing Question


> Hi Jasoon, Peter and the List,
>
> Some thoughts on why the Sahara is so productive:
>
> Another significant factor is the climate over the
> past several thousand years. The sahara and parts of
> the American southwest have been dry for a long time,
> this increases the terrestrial recognizability time of
> any meteorites that are there. The sahara has another
> interesting and significant factor having to do with
> the sand dunes; slowly moving dunes provide a soft
> landing and a protective environment as meteorites are
> covered by a slow moving dune. Later (sometimes much
> later) meteorites are uncovered in great shape with
> significant terrestrial age.
>
> Another factor is economic; most of us in the US that
> hunt meteorites do so on weekends and vacation time.
> Our motivation is to get out in nature and do a bit of
> 'big science'. Many of the hunters in the sahara are
> subsistance hunting, many are truely professional full
> time hunters. I get in 25 days of meteorite hunting
> tops per year, the locals in the sahara get in well
> above 100 days per year in many cases. They have in
> many cases spent their lives in the desert. I have
> only spent a total of 10 or 12 weeks in the areas
> where I hunt. They are better at seeing the odd rock.
>
> A few American hunters that are retired and in good
> health likely pass the 100 days per year in the field
> hunting mark. I hope to be one of them in 12 or 13
> years:)
>
> Pat
> --- Jason Utas <meteoritekid at gmail.com> wrote:
>
>> Hola Peter, All,
>> I know individual aspect of your questions have been
>> addressed, but
>> I'd like to as well...
>>
>> >I know the Sahara desert is about a galgillion
>> square miles.
>> Then there are the deserts in Calif., South America,
>> the Antarctic continent
>> and God only knows where else. Why don't I see any
>> meteorites from the
>> Gobi desert, or maybe the Mongolia desert.
>>
>> Well, yeah, it's big. But the main reason that
>> there are more
>> meteorites found in the Sahara than elsewhere is
>> because there are
>> more people looking. There are just as many martian
>> and lunar
>> meteorites elsewhere, but they tend to be harder to
>> recognize due to
>> harder hunting terrain (similar rocks, etc) - and
>> because there are
>> probably twenty or so meteorite hunters in the
>> southwest who get out
>> into the wilderness with some frequency - probably a
>> few more, since
>> Franconia became popular. Compared to the undefined
>> number of hunters
>> who have been scouring the Sahara for the past
>> decade, well, it's just
>> not much of a comparison.
>> Same goes for South America, though Antarctica has
>> seen a good deal of
>> thorough hunting, as results would suggest.
>>
>> >And then there is little dinky Roosevelt Co, NM at
>> just 2,455 sq
>> miles and it has a
>> staggering
>> 109 meteorites, which comes to one for every 22.5 sq
>> miles. What gives?
>>
>> This is due to, as has been said, the hunting of
>> Skip Wilson, who has
>> spent years in the area, hunting blowout after
>> blowout with remarkable
>> success. A good bit of hunting land paired with his
>> diligence has
>> turned up pretty spectacular results...
>> That said, the density of meteorites that actually
>> exist on the land
>> should be, at the very least, several per square
>> mile; yes, many have
>> been found, but there are still countless more
>> waiting to be
>> discovered.
>>
>> >They are of a wide variety of classifications, so
>> it can't be turning
>> every piece in
>> for classification. I can't speak for anyone else,
>> but I find this
>> very puzzling.
>>
>> I don't see why he couldn't be turning every stone
>> in for
>> classification; data gathered suggest that smaller
>> falls would be more
>> common than larger ones, and this would mean that
>> the majority of
>> falls would consist of small, individual stones.
>>
>> I also don't know how many of his finds have been
>> paired with one
>> another, but I have the feeling that if he has been
>> concentrating on
>> individual areas (as opposed to moving on whenever
>> he finds a
>> meteorite), it would stand to reason that he has
>> found at least a few
>> paired meteorites, whether or not they are listed as
>> such in the
>> catalogue.
>>
>> Regards,
>> Jason

Hi, All,

> The [S]ahara and parts of the American southwest
> have been dry for a long time...

The American SW, yes. The Sahara, no. It's desertification
began in the eastern Sahara about 14,000 years ago and
worked its way slowly west. Cyrene on the Egyptian-Libyan
border was a flourishing Greek city-state 2700 to 2200 years
ago, then dried up and blew away.

Western North Africa was the "breadbasket" of the Roman
Empire and was still producing agricultural surpluses for
Rome as late as 400 CE. Agriculture was practiced more than
200 miles further inland from the North African shore than
is possible today.

During the most recent glaciation, the Sahara was a bountiful
land with a substantial Neolithic population. Some 8000 to
10,000 years ago, most of the interior Sahara was grassy
plains with scattered forests, lakes and rivers. The present
progressive desertification of the Sahara is irreversible because
of runaway sand generation The Sahara will be a desert now
for millions of years to come, most likely.


Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: "Pat Brown" <radio_ranch at yahoo.com>
To: "Jason Utas" <meteoritekid at gmail.com>; "Meteorite-list"
<meteorite-list at meteoritecentral.com>
Sent: Friday, January 11, 2008 12:41 AM
Subject: Re: [meteorite-list] intriguing Question


Hi Jasoon, Peter and the List,

Some thoughts on why the Sahara is so productive:

Another significant factor is the climate over the
past several thousand years. The sahara and parts of
the American southwest have been dry for a long time,
this increases the terrestrial recognizability time of
any meteorites that are there. The sahara has another
interesting and significant factor having to do with
the sand dunes; slowly moving dunes provide a soft
landing and a protective environment as meteorites are
covered by a slow moving dune. Later (sometimes much
later) meteorites are uncovered in great shape with
significant terrestrial age.

Another factor is economic; most of us in the US that
hunt meteorites do so on weekends and vacation time.
Our motivation is to get out in nature and do a bit of
'big science'. Many of the hunters in the sahara are
subsistance hunting, many are truely professional full
time hunters. I get in 25 days of meteorite hunting
tops per year, the locals in the sahara get in well
above 100 days per year in many cases. They have in
many cases spent their lives in the desert. I have
only spent a total of 10 or 12 weeks in the areas
where I hunt. They are better at seeing the odd rock.

A few American hunters that are retired and in good
health likely pass the 100 days per year in the field
hunting mark. I hope to be one of them in 12 or 13
years:)

Pat
--- Jason Utas <meteoritekid at gmail.com> wrote:

> Hola Peter, All,
> I know individual aspect of your questions have been
> addressed, but
> I'd like to as well...
>
> >I know the Sahara desert is about a galgillion
> square miles.
> Then there are the deserts in Calif., South America,
> the Antarctic continent
> and God only knows where else. Why don't I see any
> meteorites from the
> Gobi desert, or maybe the Mongolia desert.
>
> Well, yeah, it's big. But the main reason that
> there are more
> meteorites found in the Sahara than elsewhere is
> because there are
> more people looking. There are just as many martian
> and lunar
> meteorites elsewhere, but they tend to be harder to
> recognize due to
> harder hunting terrain (similar rocks, etc) - and
> because there are
> probably twenty or so meteorite hunters in the
> southwest who get out
> into the wilderness with some frequency - probably a
> few more, since
> Franconia became popular. Compared to the undefined
> number of hunters
> who have been scouring the Sahara for the past
> decade, well, it's just
> not much of a comparison.
> Same goes for South America, though Antarctica has
> seen a good deal of
> thorough hunting, as results would suggest.
>
> >And then there is little dinky Roosevelt Co, NM at
> just 2,455 sq
> miles and it has a
> staggering
> 109 meteorites, which comes to one for every 22.5 sq
> miles. What gives?
>
> This is due to, as has been said, the hunting of
> Skip Wilson, who has
> spent years in the area, hunting blowout after
> blowout with remarkable
> success. A good bit of hunting land paired with his
> diligence has
> turned up pretty spectacular results...
> That said, the density of meteorites that actually
> exist on the land
> should be, at the very least, several per square
> mile; yes, many have
> been found, but there are still countless more
> waiting to be
> discovered.
>
> >They are of a wide variety of classifications, so
> it can't be turning
> every piece in
> for classification. I can't speak for anyone else,
> but I find this
> very puzzling.
>
> I don't see why he couldn't be turning every stone
> in for
> classification; data gathered suggest that smaller
> falls would be more
> common than larger ones, and this would mean that
> the majority of
> falls would consist of small, individual stones.
>
> I also don't know how many of his finds have been
> paired with one
> another, but I have the feeling that if he has been
> concentrating on
> individual areas (as opposed to moving on whenever
> he finds a
> meteorite), it would stand to reason that he has
> found at least a few
> paired meteorites, whether or not they are listed as
> such in the
> catalogue.
>
> Regards,
> Jason
>
> On Jan 9, 2008 7:01 PM, Peter A Shugar
> <pshugar at clearwire.net> wrote:
> > Hello,
> > I'm the newbie, so please explain this to me. This
> is an intriguing question.
> > I can't figure it out. I know the Sahara desert is
> about a galgillion square miles.
> > Then there are the deserts in Calif., South
> America, the Antarctic continent
> > and God only knows where else. Why don't I see any
> meteorites from the
> > Gobi desert, or maybe the Mongolia desert.
> > And then there is little dinky Roosevelt Co, NM at
> just 2,455 sq miles and it has a
> > staggering
> > 109 meteorites, which comes to one for every 22.5
> sq miles. What gives?
> > They are of a wide variety of classifications, so
> it can't be turning every piece in
> > for classification. I can't speak for anyone else,
> but I find this very puzzling.
> > Any thoughts, List?
> > Pete
> >
> > ______________________________________________
> > http://www.meteoritecentral.com
> > Meteorite-list mailing list
> > Meteorite-list at meteoritecentral.com
> >
>
http://six.pairlist.net/mailman/listinfo/meteorite-list
> >
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Received on Fri 11 Jan 2008 10:55:11 PM PST