African Valley Dust Feeds Brazilian Rainforest

African Valley Dust Feeds Brazilian Rainforest





    
 
January 2007  - 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.