A total of some 50 million tons of African dust is deposited upon the Amazon basin every year.More than half of the dust needed to fertilize the entire Amazon rainforest is blown thousands of miles across the Atlantic Ocean from a small valley in the African country of Chad, according to new research by an international team of scientists led by Israeli Dr. Ilan Koren.
The research, published in the first issue of the new quarterly journal Environmental Research Letters, reveals that some 56 percent of the dust reaching the Brazilian rainforest originates from the Bodele Valley in Northern Chad. The mineral-laden dust is washed by rain from the soil and blown across the Atlantic to sustain life in the Amazon.
For over a decade scientists were aware that the Amazon rainforest, which continually washes away its own resources through frequent rainfall, depends on a regular supply of minerals from the Sahara for its existence, but nobody was certain exactly where this dust came from, or how much of it was reaching the rainforest.
By combining various types of satellite data, Koren, a faculty member of the Environmental Sciences and Energy Research Department at the Weizmann Institute of Science in Rehovot, and colleagues from Israel, the UK, 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 basin every year, a much higher figure than the previous estimates of 13 million tons, and that 56 percent of that dust originates in the dry lakebed of the Bodele Valley, which is about 200 times smaller than the Amazon basin.
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.
Dust is a very important supply of minerals and nutrients. It interacts with clouds and radiation, and is linked to the production of plankton. “Dust is the first link in the ocean’s food chain,” Koren told ISRAEL21c.
Koren, who received his PhD at Tel Aviv University, first became fascinated by the dust emissions from the Bodele Valley during his post doctorate research on clouds at the NASA Goddard Space Flight Centre, in Greenbelt, Maryland.
“I saw this spot on satellite pictures and it puzzled me,” he explains. “It was a huge package of dust that covered hundreds of kilometers with a thick blanket, all leaving from one small location on the earth. It was something very unique and unusual that you don’t see happening anywhere else around the world.”
He began researching this dust package, but could find very little available information.
“Here was a very important source of dust with unique properties and basically there was nothing known about it,” says Koren.
In 2004, Koren and the late Yoram Kaufman of the NASA Goddard Space Flight Center, published a paper on the Bodele Valley dust emissions, showing that they traveled across the Atlantic Ocean at high velocity and that the emissions only occurred during the Northern Hemisphere winter, rather than the summer, when most dust emissions from the Sahara occurred, enabling the dust to be blown to South America on the winter winds, rather than towards North America and the Caribbean on the summer winds.
The paper attracted the attention of scientists from the UK who were also studying dust emissions from the Bodele Valley, including Richard Washington from the Climate Research Lab at the Oxford University Center for the Environment, and Martin Todd of the Department of Geography, University College London. The scientists began collaborating and a UK team traveled to Chad in 2005 to analyze dust quantities near the Bodele Valley, on the shore of the Atlantic and at an additional spot above the ocean.
The research team was also joined by 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.
Using information in the field, combined with satellite data, the team came to the conclusion that the Bodele depression, which 10,000 years ago was part of Lake Chad, is an important source of dust because of its shape and geographic features. The valley is flanked on both sides by enormous basalt mountain ridges, which create a cone shaped crater with a narrow opening in the northeast.
Koren says winds that drain into the valley focus on this funnel-like opening, creating a large natural wind tunnel. As a result, gusts of surface wind are accelerated and focused in the tunnel lifting the dust from the ground and blowing it westward toward the Atlantic and on to the Amazon. Koren estimates that on four out of 10 days during the winter, dust packages averaging about 700,000 tons per day, are blown from the Bodele Valley towards the Atlantic. Much of this dust is deposited in the ocean.
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 Bodele 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.
Given the significance of the Bodele Valley to the continuing survival of the Amazon rainforest, Koren says that more study is necessary to understand the delicate natural process. “Many questions remain unanswered,” says the scientist, who studies the Bodele Valley as a part-time project alongside his main study of clouds. “We do not know for how long the Bodele Valley will continue emitting this amount of dust, it is unclear what is the mineralogy of the dust emitted from the valley, and we have no idea what would happen to the Amazon if wind or weather patterns are about to change.”
There is already some indication that wind patterns and dust emissions in the Sahara are changing as a result of global warming. Koren says that over the last four to five years, there is growing evidence that a worrying change is occurring, though he adds that it is far too early to be able to say if this is a long-term trend. In the meantime, Koren plans to continue studying the Bodele Valley dust emissions alongside a growing number of international scientists