Nuisance to Nutrient: The Journey of Saharan dust from Africa to Amazonia

In the Sahara Desert, dust dominates. From Morocco to Sudan, strong winds continuously pick up dust from the desert floor and loft it into the air, decreasing air quality and exacerbating health problems for millions of people. Coarse particles aggravate respiratory issues such as asthma or pneumonia, while fine particles encourage cardiovascular issues. If the dust contains biomatter such as bacteria or fungi, it can also intensify the symptoms of allergies and lung disease.

There’s no doubt that dust is more than just an inconvenience to the millions living in the Sahara Desert. In fact, it’s a full-blown health hazard, and nowhere in Africa is this more true than near the Bodélé depression.

The Bodélé depression is the site of a once-massive lake called Megachad. At its largest, the lake covered an area of about 400,000 square kilometers, which is larger than all of the great lakes combined. Around 5,000 years ago, increasingly arid conditions in the Sahara region caused the lake to dry up and the organisms living within the lake to perish.

Though Lake Megachad no longer exists, the sediments that rested on the lake’s floor still do. Strong surface winds lift an average of 700,000 tons of these lakebed sediments each day during the winter months. While this can be a major problem for those living near the depression, across the Atlantic, the dust is having an entirely different effect on the South American region of Amazonia. Though it’s detrimental to human health in the Sahara, dust from the Bodélé depression is a crucial component to human and ecosystem health in Amazonia.

Watch this video to see how dust travels from the Sahara desert to the Amazon rainforest.

During the winter, strong surface winds known as the Harmattan winds blow along the southern edge of the Sahara, picking up huge amounts of dust and carrying it nearly 3,000 miles to South America. The dust carried from the Bodélé Depression consists of organic matter from the organisms that used to live in Lake Megachad and includes high levels of key plant nutrients like iron and phosphorus. This makes it very beneficial to plant life in the Amazon. Scientists estimate that each year, billions of pounds of iron and hundreds of millions of pounds of phosphorus are transported to Amazonia from the Bodélé Depression, making this dried-up lakebed a massive nutrient source for one of Earth’s largest and most diverse ecosystems.

Photo by Neil Palmer. 

Photo by Neil Palmer. 

It might be hard to believe that such a desolate place like the Sahara can contribute so much to a region so full of life and so far away. It brings into question why the Amazon is in need of nutrients like iron and phosphorus in the first place. Shouldn’t an ecosystem as biologically productive as the Amazon rainforest have all of its necessary nutrients readily available in the soil?

Surprisingly, no. In fact, Amazonian soil isn’t good for supporting much life at all, let alone the massive rainforest that grows in it. The main reason for this has to do with the amount of rain that the area receives. Similar to washing your car or clearing off your plate after dinner, water running through the soil in the Amazon washes away a lot of nutrients that plants need to survive. As geologist Charlie Bristow explains, “The Amazon is essentially a leached or leaching system, so although it is very productive, it is actually quite nutrient-poor.”

Charlie Bristow is a researcher and professor at Birkbeck, University of London that has done significant research on Saharan dust. In 2005, he collected samples of Bodélé depression dust and analyzed them for their chemical content. From this study, he and the other scientists working on this project proposed that the Bodélé depression contributes up to 14 billion pounds of iron and 265 million pounds of phosphorus to the Amazon region every year.

Thanks to the research findings of Professor Bristow, his team, and the other scientists conducting research on this subject, we now know that Bodélé depression dust is a major nutrient source for the Amazon rainforest. Without it, Amazonia would be what scientists call a “wet desert”, or an environment that receives plentiful rain but is too poor in nutrients to support life. Whether or not the Amazon rainforest exists might not be a concern for many people, but it should be, because its existence has significant global impacts. Not only does the Amazon rainforest provide many resources that we use in our daily lives, such as rubber, medicine, oils, dyes, fruits, and vegetables, it plays a role in Earth’s climate system as well. Since the rainforest consists mostly of plants, and plants consume carbon dioxide, the Amazon rainforest is helpful in removing some (I repeat, some) of the carbon dioxide that humans have been emitting for decades.

All of this to say, this pile of dust plays a major role in all of our lives. It won’t always, though. The depression’s supply of dust is finite and will eventually run out. Scientists have estimated that it will last for another thousand years, at which point it’s unclear what will become of the Amazon rainforest. Until then, scientists will likely continue studying the dust to learn more about its other roles in Earth’s climate, while the rest of us continue to benefit from such a strange connection between two very different and distant environments.

- Haley Royer, Guest Writer