The African source of the Amazon's fertilizer.

In the winter months in the Northern Hemisphere, massive dust storms from the African Sahara waft southwest across the Atlantic to drop tons of vital minerals on the Amazon basin in South America. Now, scientists have pinpointed the source of many of those dust storms and estimated their dust content.

The Amazonian rainforest depends on Saharan dust for many of its nutrients, including iron and phosphorus (SN: 9/29/01, p. 200). "If it weren't for those nutrients, the Amazon would be a wet desert," says Ilan Koren, an atmospheric scientist at the Weizmann Institute in Rehovot, Israel.

Using satellite measurements of dust clouds, Koren and his colleagues estimate that 40 million tons of Saharan dust reaches South America each year. The images indicate that more than half of that dust originates from the Bodele depression, a now-dry basin on the southern edge of the Sahara that in wetter times held a body of water the size of Lake Erie.

Although the depression is only 0.2 percent of the Sahara's surface area, it's a prodigious dust source, the researchers report in the October-December Environmental Research Letters. Dust storms arise from the area on 40 percent of winter days. On average, the storms loft more than 700,000 tons of dust each day, says Koren.--S.P.