Saudi Aramco World: May/June 2014 - page 18

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Saudi Aramco World
from below by this aquifer, which has allowed them to survive
the climatic changes so far.
Remember, however, our barchan dunes. Slow but relent-
less, they have coalesced into “megabarchans,” corridors of
dunes that extend for hundreds of kilometers. Long ago, they
moved into the two big original lakes in Ounianga Kebir and
Ounianga Serir, splitting them into the clusters of smaller lakes
we see today.
As we pass in the Land Cruiser, it is easy to see how the elon-
gated dunes separate the lakes in the basin, an action that con-
tinues, and that may, in time, obliterate them.
From the settlement at Ounianga Serir, one of the world’s most
remote, we take in the view of Lake Tili. It doesn’t have the mats
of reeds that cover the surface of the fresh lakes. Also in contrast
to them, Tili’s rapid evaporation rate makes it hyper-saline and
keeps its depth to only two to three meters (6 to 9½'); its surface
elevation is the lowest of all the lakes.
The difference in elevation drains a slow flow of water into
Lake Tili from adjacent lakes through the semipermeable dunes.
This “evaporation pump” causes the other lakes to draw con-
tinually from the aquifer below
to replace their own losses. To-
gether with depth and the par-
tial reed covers, this replenish-
ment works to keep the water
in the other lakes fresh.
Yet what we see today is but
a remnant of the two vast lakes
that once fed south into the
largest inland lake ever known
to have existed on Earth: the
Mega-Chad, once larger than today’s Caspian Sea.
“O
ur mission,” says Baba, “is to sample
diatomite
to see how the lake levels have
changed during the different periods.”
Diatomite is a light soil made up of the
microscopic skeletal remains of diatoms,
single-celled plants that have sunk to the bottom of lakes and
oceans. When the water of a lake recedes, diatomaceous earth
Leaders of two of northern Chad’s most remote communities, Aahmat Moussa Thozi,
left,
of Ounianga
Kebir, and Nebi Guett,
right,
of Ounianga Serir, link their legendary tribal histories to today’s science.
“There was a very ancient population that lived next to this lake. We believe that our grandfather came
out of the lake,” says Thozi. Not far from here, the 2001 and 2002 Toumai skull discoveries show that
hominid and human habitation in this region may date back seven million years.
Right and opposite,
top:
The Office Tchadien du Tourisme, led by Director-General Younous Mahmoud, promotes adventure travel
to the lakes that Guett says have “become a source of pride, and we plan for a bright future.”
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