New images from NASA indicate that the Amazon rainforest is not as self-sufficient as many believe.
It turns out that the renowned biodiversity hotspot is actually reliant on dust blown overseas from another continent.
According to a recent NASA study, an average of 182 million tons of dust is annually blown west from the Sahara Desert in Northern Africa.
Approximately 27.7 million tons of that completes the 1,600-mile journey across the Atlantic Ocean to the Amazon rainforest.
The phenomenon, dubbed the dust migration, is critical to life in the rainforest.
Although the Amazon has a steady supply of nutrients from decaying plant matter in its immediate vicinity, one easily eroded nutrient is phosphorus.
Enter the Sahara Desert. The Bodele Depression, an ancient dried lake bed in Chad, is rich in phosphorus thanks to an abundance of dead microorganisms in the rocks there, according to CNET’s Michael Franco.
“Nutrients…are in short supply in Amazonian soils. Instead, they are locked up in the plants themselves,” said Ellen Gray of NASA’s Earth Science News Team:
“Decomposing leaves and organic matter provide the majority of nutrients, which are rapidly absorbed by plants and trees after entering the soil. But some nutrients, including phosphorus, are washed away by rainfall into streams and rivers, draining from the Amazon basin like a slowly leaking bathtub.”
According to Hongbin Yu, an atmospheric scientist at the University of Maryland and the report’s lead author, roughly 22,000 tons of phosphorus is deposited in the Amazon rainforest from the dust migration every year.
This is approximately the amount that is annually eroded from the native soils.
But the equivalency is not quite perfect, the study cautions; the amount of dust making the journey each year is highly variable—dependent as it is on weather conditions spanning two continents and the ocean separating them.
According to Yu, the volume of migrating dust shrank by 86 percent between 2007 (the highest recorded year) and 2011 (the lowest recorded year).
The prime suspect for this variability is the Sahel: a ribbon of semi-arid land bordering the Sahara Desert along its southern edge.
The moisture content of Sahel’s soils is linked to the following year’s dust migration. The greater the water content is, the lower the next year’s dust volume is.
Researchers have not determined the reason for this relationship. The study suggests that rainfall is directly tied to the conditions that also govern wind circulation, which is the primary driver of the dust migration.
Alternatively, the researchers propose that higher rainfall produces more vegetation in the Sahel, which then anchors sediment to prevent its erosion and escape across the sea.
The migration is Earth’s largest dust transportation mechanism, according to Gray. Yu’s study compiled observations taken from 2007 to 2013 by NASA’s “Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation,” or CALIPSO.
A lidar instrument aboard the satellite detects atmospheric particles by bouncing light off them. To the optical equipment, dust is discernable from other particles in Earth’s atmosphere.
CALIPSO’s observations were particularly relevant to the study because they recorded the altitudes at which the dust flowed from the Sahara to the Amazon.
These data bring researchers a step closer to designing computer models to predict the migration patterns for future years.
Yu and his fellow researchers intend to expand their study to include other atmospheric aerosols in the Amazon, such as smoke and fungal pores, and how they interact with sediment originating in the Sahara.
“It’s a small world,” he said. “And we’re all connected.”
