Dust to gust

December 28, 2006

More than half of the dust needed for fertilizing the Brazilian rainforest is supplied by a valley in northern Chad, according to an international research team headed by Dr. Ilan Koren of the Institute's Environmental Sciences and Energy Research Department. In a study published recently in Environmental Research Letters, the scientists have explained how the Bodélé valley's unique features might be responsible for making it such a major dust provider.

It has been known for more than a decade that the existence of the Amazon rainforest depends on a supply of minerals washed off by rain from the soil in the Sahara and blown across the Atlantic by dust. By combining various types of satellite data, Dr. Koren and colleagues from Israel, the United Kingdom, the United States, and Brazil have now for the first time managed to obtain quantitative information about the weight of this dust. Analyses of dust quantities were performed near the Bodélé valley itself, on the shore of the Atlantic and at an additional spot above the ocean.

The data revealed that some 56 percent of the dust reaching the Amazon forest originates in the Bodélé valley. They also showed that a total of some 50 million tons of dust make their way from Africa to the Amazon region every year, a much higher figure than the previous estimates of 13 million tons. The new estimate matches the calculations on the quantity of dust needed to supply the vital minerals for the continued existence of the Amazon rainforest.

The researchers suggest that the Bodélé valley is such an important source of dust due to its shape and geographic features: it is flanked on both sides by enormous basalt mountain ridges, which create a cone-shaped crater with a narrow opening in the northeast. Winds that "drain" into the valley focus on this funnel-like opening similarly to the way light is focused by an optical lens, creating a large wind tunnel of sorts. As a result, gusts of surface wind that are accelerated and focused in the tunnel lift the dust from the ground and blow it toward the ocean, allowing the Bodélé valley to export the vast amount of dust that makes a life-sustaining contribution to the Amazon rainforest.
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Dr. Ilan Koren's research is supported by the Samuel M. Soref and Helene K. Soref Foundation; and the Sussman Family Center for the Study of Environmental Sciences.

American Committee for the Weizmann Institute of Science

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