Deflation in the dustiest place on Earth: The Bodélé Depression, Chad

Abstract

The Bodélé Depression in Chad is the largest single source of dust on Earth because of a combination of strong near surface winds and a large reservoir of readily eroded and easily entrained, low density sediment. During the Holocene and Late Pleistocene, the Bodélé Depression was host to a great lake–palaeolake Megachad–that is now completely desiccated leaving the lake bed sediments exposed to deflation. Deflation in the Bodélé Depression is revealed in erosional remnants of lake bed deposits exposed as yardangs on the lake floor. Erosional remnants indicate that locally up to 4 m of sediment has been removed by deflation. Optically-stimulated luminescence dating shows that this occurred in less than 2400 years, giving a rate of deflation of at least 1.6 mm yr − 1 . A range of other geomorphological features indicate that in some areas rates of deflation may be greater than or equal to 10 mm yr − 1 . Deflation appears to be greatest in an arc around the northeastern margins of the lake basin where the diatomite deposits have been removed and revealed an underlying network of distributary channels preserved as gravel capped ridges. The ridges, which are part of an inverted topography, are interpreted as deltaic distributary channels from an inland delta similar to the Okavango Delta. Examples of barchan dunes exist that were flooded by the palaeolake and which have now been exhumed from beneath layers of diatomite. The distribution of extensive deflation around the northeasterly, upwind end of the Bodélé Depression probably results from a combination of large wind velocities, an upwind source of abrasive sand, and thinner diatomite deposits around the lake margin. Satellite image analysis shows that outcropping diatomite covers 24,049 km 2 of the Bodélé Depression. Furthermore, the area from which diatomite has been removed by deflation is around 14,000 km 2. Combining this information with the ages and rates of deflation mentioned above indicates that some 61,000 km 3 of diatomite has been eroded from the Bodélé Depression during the past 1000 years.