More than half the dust needed to fertilize the Brazilian rain forest originates on a dry lakebed in Africa, according to a team of researchers.
About 50 million tons of dust are transported by the wind from Africa to Brazil's Amazon basin each year, and 56% of the dust originates in the Bodele Depression on the southern edge of the Sahara Desert, new satellite measurements indicate.
Researchers concluded a decade ago that minerals in African dust reached the Amazon, but they had estimated that 13 million tons crossed the ocean each year -- only a quarter of the amount necessary.
Environmentalists wondered where the rest came from. A team led by Ilan Koren of the Weizmann Institute of Science in Rehovot, Israel, solved the riddle and revealed why the depression -- which is 200 times smaller than the Amazon basin -- has such a remarkable output.
The Bodele Depression was once part of Lake Chad, which in the 1960s was the size of Lake Erie. But drought and irrigation have reduced the lake to about 5% of its former size, exposing massive areas of sediment.
The depression, the team noted in a recent issue of the journal Environmental Research Letters, is flanked by mountain ridges that form a cone-shaped crater with a narrow opening in the northeast -- a large natural wind tunnel. Gusts are accelerated in the tunnel, lifting the dust and blowing it west.