The Perfect Dust Storm
A desolate basin on the southern edge of the Sahara Desert supplies 45 million tons of dust that naturally fertilizes the Amazon rainforest each year.
Scientists have recently investigated how such an amazing amount of dust arises.
A remarkable combination of geography and climate makes the Bodélé depression in northern Chad the world's dustiest spot. Unusual jet winds flowing close to the ground form the engine behind this perfect dust machine.
Two mountain ranges in north-central Africa, Tibesti and Ennedi, sit aligned to precisely aim prevailing northeasterly winds through a narrow pass. The pass creates a bottleneck that accelerates the air flow and sends it hurtling at the Bodélé depression. Wind speeds average 13.2 metres per second (29.5 miles per hour) for eight hours at a time on 40% of the days from November to March.
The Bodélé depression is now a dry, remote desert, but long ago was a large lakebed. Left behind is a ground cover of diatomite, the silica and mineral remains of diatom algae that once inhabited the lake.
Diatomite is ideally suited to long-distance airborne travel. The material's light weight allows wind to readily pick it up. Diatomite flakes are also quite fragile and disintegrate into fine particles when bumped around.
Besides wind and diatomite, the third crucial piece of the Bodélé dust apparatus serves to pulverize the raw material. The depression's landscape of scattered sand dunes acts as a huge grinding mill. Giant barchan dunes create air turbulence that whips the airborne diatomite flakes against each other and the dunes, shattering the pieces into a fine flour.
Each eight-hour pulse of perfect dust storm picks up over 700,000 tons of dust, giving rise to a discrete cloud travelling westward. The dust clouds average 370 by 700 kilometres (230 x 435 miles) in area, extensive enough to show up clearly on satellite photos.
When scientists tracked the dust's massive migration by satellite during the winter and spring of 2004, they estimated Bodélé emitted 58 million tons of material that was still aloft 1300 kilometres (800 miles) downwind. Most of this, 45 million tons continued across the Atlantic Ocean on westward trade winds.
These numbers show that the Bodélé depression alone supplies 56% of the dust carried from Africa to Brazil.
Diatomite dust raining down on the Amazon's watershed contains nutrients that feed the rich rainforest ecosystems. The relatively compact area northeast of Lake Chad is a giant fertilizer factory for the Amazon's lush vegetation.
Ilan Koren, Yoram J. Kaufman, Richard Washington, Martin C. Todd, Yinon Rudich, J. Vanderlei Martins and Daniel Rosenfeld. 2006. The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest. Environmental Research Letters. 1 014005.
Andrew Warren, Adrian Chappell, Martin C. Todd, Charlie Bristow, Nick Drake, Sebastian Engelstaedter, Vanderlei Martins, Samuel M'bainayel and Richard Washington. 2007. Dust-raising in the dustiest place on earth. Geomorphology. 92(1-2): 25-37.