Dust From One African Valley Feeds Brazilian Rainforest

REHOVOT, Israel, January 3, 2007 (ENS) - Much of the dust needed for fertilizing the entire Brazilian rainforest originates in a single valley in the African country of Chad and travels thousands of miles across the Atlantic Ocean to South America, newly published research shows.

Based on satellite data, the study was conducted by an international research team headed by Dr. Ilan Koren of the Environmental Sciences and Energy Research Department at the Weizmann Institute of Science in Rehovot.

The data revealed that some 56 percent of the dust reaching the Brazilian Amazon rainforest comes from the Bodélé Valley in northern Chad.


Dust blows across Lake Chad on its way to Amazonia. (Photo courtesy NASA)
It has been known for more than a decade that the existence of the Amazon rainforest depends on a supply of minerals washed by rain from the soil in the Sahara and blown across the Atlantic as dust.

By combining various types of satellite data, Dr. Koren and colleagues from Israel, the United Kingdom, the United States, and Brazil have for the first time measured the weight of this dust.

They showed that a total of some 50 million tons of African dust is deposited upon the Amazon region every year, a much higher figure than the previous estimates of 13 million tons.

The new estimate matches calculations on the quantity of dust needed to supply the vital minerals for the continued existence of the Amazon rainforest, the scientists say.

They calculate that 56 percent of the 50 million tons of dust originates in the Bodélé Valley located northeast of Lake Chad.

To arrive at their conclusion, the scientists analyzed dust quantities near the Bodélé Valley itself, on the shore of the Atlantic and at an additional spot above the ocean.

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.

Dr. Koren says winds that "drain" into the valley focus on this funnel-like opening the way light is focused by an optical lens, creating a large wind tunnel.


Dr. Ilan Koren is a senior scientist at the Weizmann Institute of Science. (Photo courtesy WIS)
As a result, gusts of surface wind that are accelerated and focused in the tunnel lift the dust from the ground and blow it westward toward the Atlantic Ocean. This wind action allows the Bodélé Valley to export the millions of tons of dust that sustain life in the Amazon rainforest.

The Bodélé Valley study was published in the first issue of the new quarterly journal "Environmental Research Letters," dated October - December 2006.

In a commentary on the study, also published in the journal, NASA atmospheric scientist Dr. Lorraine Remer observes that on its journey across the populated regions of west Africa, the Bodélé Valley dust can be affected by smoke and urban pollution.

"Although Koren et al do not speculate on the chemical possibilities in their paper," writes Remer, "the interaction between the dust and the pollutants provides opportunity for acids to coat the dust particles and to mobilize the iron compounds, creating a highly efficient fertilizing agent for ocean phytoplankton and the biota of the Amazon forest."

Remer says the findings of Koren and his co-authors suggest that dust emission sources may be highly localized spots in the Earth's deserts that can be mapped precisely by satellites of moderate to fine resolution.

Co-authors of the study are Yoram Kaufman of the NASA Goddard Space Flight Center in Greenbelt, Maryland; Richard Washington or the Climate Research Lab, Oxford University Centre for the Environment, UK; Martin Todd of the Department of Geography, University College London, UK; Yinon Rudich of the Joint Center for Earth Systems Technology at the University of Maryland; J. Vanderlei Martins of the Institute of Physics, University of Sao Paulo, Brazil; and Daniel Rosenfeld of the Institute of Earth Sciences at The Hebrew University, Jerusalem.