Bodélé Depression
Bodélé Depression
North Central Africa
Few thousand years
Drying up of Lake Chad
The Bodélé Depression (pronounced [bɔ.de.le]), located at the southern edge of the Sahara Desert in north central Africa, is the lowest point in Chad. It is 500 km long, 150 km wide and around 160 m deep.[1] Its bottom lies about 155 meters above sea level. The dry endorheic basin is a major source of fertile dust essential for the Amazon rainforest.
Dust storms from the Bodélé Depression occur on average about 100 days per year,[2] one typical example being the massive dust storms that swept over West Africa and the Cape Verde Islands in February 2004.[3][4] As the wind sweeps between the Tibesti and the Ennedi Mountains in Northern Chad, it is channeled across the depression. The dry bowl that forms the depression is marked by a series of ephemeral lakes, many of which were last filled during wetter periods of the Holocene.
Diatoms from these fresh water lakes, once part of the prehistoric Mega-Lake Chad, now make up the surface of the depression and are the source material for the dust,[2] which, carried across the Atlantic Ocean, is an important source of nutrient minerals for the Amazon rainforest.
In February 2005 the first field experiment, the Bodélé Dust Experiment or BoDEx 2005, was carried out in the Bodélé Depression. The experiment measured the surface winds and near surface winds, dust concentrations, and the influence of dust on the radiation budget in the Bodélé Depression for the first time.[2] This work coincided with a major dust emission event during which the Bodélé Low Level Jet sustained surface wind speeds of around 16 m/s. The core of the Bodélé Low Level Jet was also mapped for the first time from the wind data, and was shown to undergo a very marked diurnal cycle with peak winds occurring mid morning. During night time, the Bodélé Low Level Jet flows over a near-surface inversion, but quickly mixes down to the surface a few hours after sunrise once the intense surface heating induces turbulence in the lowest layers.
Dust from the Bodélé may be seen as a simple coincidence of two key requirements for deflation: strong surface winds and erodible sediment. But recent research has argued that long-term links exist between topography, wind, deflation and dust, and that topography acts as the controlling agent ensuring the long term maintenance of this source. The spatial co-location of strong winds and dust is not simply fortuitous, but results from a set of processes. Specifically:
Wind conditions which deflate the erodible sediment now may have created the depression necessary for generating the erodible diatomite in the past. Instead of a simple coincidence of nature, the world's largest dust source results from a system of processes operating over paleo timescales.[12]
The largest town associated with the Bodélé dust source is Faya-Largeau (17°55′00″N 19°7′00″E / 17.91667°N 19.11667°E), located just to the northeast of the depression.[13]