Ancient African Fish Feed Today’s Amazon
It begins with a darkening sky, then a distant roar. Leaves rustle as monkeys scamper through the trees above. Drops of their pee sprinkle down through the leaves. Then they start to howl. It is more like a deafening yowl. Unseen in the trees overhead, a chorus of howler monkeys scream at the approaching storm.
Such storms are frequent in the Amazon. When Robert Swap was there years ago, he could sometimes see the dark wall of rain sweeping forward over the tops of the Amazon’s trees. He would be soaked the instant the storm arrived. A downpour might last 30 minutes. Then it would give way to a gentle sprinkle and the musty smell of wet leather. But what Swap remembers most of all is the dust.
Once a squall passed, he would find a film of red or brown dust blanketing his car. It was spattered on the windows, hood and roof of his little VW Gol. “Everything was wet,” he says. “It was humid. We had leather rotting. We had mold growing on everything. You couldn’t get dry. And yet you’d have this film of dust on top these cars. Where is this dust coming from?” Swap’s curiosity about that dust would lead to a surprising discovery about the secret life of the Amazon.
Swap was 20 years old during that trip in February and March of 1987. He had taken a semester off from college for what sounded like an exciting job. Michael Garstang was an atmospheric scientist at Swap’s school, the University of Virginia in Charlottesville. He needed helpers for a project in Brazil. Garstang was assembling a team to study late-afternoon storms. The group would drive into the Amazon on dirt roads. There, they would assemble metal towers that rose above the tree tops. From these lofty towers, they would measure the temperature, movement and moisture of air drifting over the trees.
For Swap, it turned out to be more hard work than high science. He hauled heavy loads and hammered stakes. He twisted metal screws two meters (6.5 feet) into the ground to anchor the wobbly towers. Dust had nothing to do with this project. Back then, few scientists cared about dust. But today, those old attitudes are starting to change.
Scientists have documented billions of kilograms of dust traveling the world. They increasingly see dust as a powerful force shaping Earth’s environment. Scientists now believe that without dust, the Amazon and some other big tropical forests might not exist. Instead, they might have died from malnutrition. At the very least, they would not host as much diverse plants and animals as they do today.
If that seems crazy, consider this: The lush, green Amazon actually lives on the edge of starvation.
To grow, plants need nutrients — such as phosphorus, calcium and magnesium. These minerals come from bedrock as it crumbles over thousands of years into soil. But they don’t stay put. Those minerals dissolve in water the same way table salt, also a mineral, will dissolve in a glass of water. Over millions of years, rainwater has leached rocky nutrients out of the Amazon’s soil. It has carried them into streams, rivers — and eventually out to sea. This is why the oceans are so salty. They are full of minerals washed off the land throughout Earth’s long history.
So it might seem strange, but the same heavy rain that quenches the thirst of the Amazon’s forests also steals their food. More rain steals more minerals, more quickly.
Ecologists have estimated that the Amazon needs twice as much phosphorus as comes out of its bedrock. But since the trees aren’t dying off, they have to be getting it from somewhere. The question has been: Where?
Swap stumbled onto the answer when he noticed all of that dust on his car in 1987.
Pyramids of dust
Before leaving Brazil to go home, Swap measured how much dust was collecting on the car. Months later, after getting home, he was also able to estimate how much dust was in the air high above the forest. Airplanes had collected samples of that air when Swap had been in Brazil. Those samples contained some dust that he was able to measure back home.
Over the next couple of years, Swap used this data to calculate how much dust was raining down onto the Amazon. He found that a single storm can drop nearly half a million tons of dust. And the entire Amazon might get 26 times that much each year. Pile it up, and you could build about five Egyptian pyramids!
Even if you look at smaller pieces of forest, that still works out to a lot of dust. A forested area the size of a U.S. football field would receive about 100 kilograms (220 pounds) of dust each year. That works out to between 1 and 2 kilograms (2 and 4 pounds) of phosphorus falling out of the sky each year. To buy that much phosphorus fertilizer for the entire Amazon, each year, would cost many millions of dollars.
So where was the dust coming from?
To answer that, Swap used data from weather satellites. These helped him reconstruct the air currents that delivered the dust. His findings, published almost a quarter-century ago, showed that those currents had to come from somewhere in North Africa. Most likely, the dust had come from the Sahara, a vast desert that covers most of that region. In just 10 or 12 days, he found, the dust travels 5,000 kilometers (more than 3,000 miles) across the Atlantic Ocean.
“It was pretty … cool,” he says, to find that something that is happening so far away could boost the health of the Amazon.
It is amazing enough to think that dust from the Sahara fertilizes the Amazon. But the truth is even more startling. Half of this dust actually comes from a single tiny source. It comes from an ancient, dry African lake bed. It covers an area of 10,000 square kilometers (4,200 square miles), equal to just two-tenths of one percent of the size of the Amazon forest it feeds.
This dry lake bed, called the Bodélé (Buh-DAY-lay) Depression, sits in a remote part of Chad. That nation lies on the southern edge of the Sahara. Satellite measurements show that the Bodélé is the dustiest place on Earth. It alone supplies one-sixth of all the dust that spends some time in the planet’s atmosphere. (The dust in your house, though, comes primarily from local sources — like laundry lint.)
Most places in the Sahara release the majority of their dust in summer. That dust rides winds high in the sky to Florida and the Caribbean Sea, to its south. But the Bodélé is different. It releases its dust in winter. Air currents shift at that time of year. And instead of carrying dust to the Caribbean, the winds ferry it southward — all the way to the Brazilian Amazon.
A scientist named Richard Washington led an expedition to the Bodélé Depression in 2005. He is an atmospheric scientist from the University of Oxford. Based on data collected on that trip, Washington estimated that the Bodélé sends 23 billion kilograms (25 million tons) of dust to the Amazon each year. A newer estimate, published in 2015, suggests the true amount could be twice that high. This shows that the estimates Swap made in 1992, using far less information, were in the right ballpark. That study was “way, way ahead of its time,” says Washington. “Ridiculously ahead of its time.”
What’s more, Bodélé dust is full of what the Amazon needs most: phosphorus. Back in 2005, Washington’s team dug many holes in the Bodélé lake bed. The rock they excavated was sprinkled with the bones, scales and teeth of fish. Once they even spotted the meter- (yard-) long skeleton of a fish — a kind called Nile perch — embedded in the hard ground. These animals had died and sunk to the bottom of the lake some 6,000 years ago, when it still held water.
Fish teeth, bones and scales are known to hold lots of phosphorus. That phosphorus is part of a mineral called apatite (AA-pah-tyte). It dissolves exquisitely well in water — far better than other phosphorus minerals. And one benefit of that: Plants easily absorb it.
Up to half of the phosphorus in Bodélé dust actually comes from fish. Charlie Bristow, a scientist who studies lake sediments, reported this in 2014. He works at Birkbeck College at England’s University of London. He was also part of Washington’s 2005 expedition to Bodélé.
Stuff of life
Phosphorus has always been a limiting nutrient. It makes up, in part, a membrane that encloses every cell on Earth. Phosphorus atoms also form part of the backbone of the DNA found in every cell. This DNA provides instructions for the cell.
But phosphorus has always been hard to get. Much is locked away in rocks, deep underground. So all life struggles to acquire whatever phosphorus it can.
Thousands of years ago in Africa, millions of fish inhabited a lake the size of modern-day Pennsylvania. Bit by bit, its fish gathered phosphorus from the insects and algae they ate. In the very act of growing, they locked this precious substance into their bones, scales and teeth. When they died, their remains sank to the lake floor. There, they deposited their lifetime savings of phosphorus. As countless fish left their carcasses entombed in layers of sediment and stone, those stores of phosphorus grew.
Before the lake eventually dried up, billions of kilograms (millions of tons) of phosphorus may have built up in what would become known as the Bodélé Depression. The life’s work of fish that lived 6,000 years ago is now fueling the largest forest on Earth.
The Bodélé may be the most striking example of dust from one part of the world supporting far-away life, but it is not the only one. The more people look, the more common this seems to be.
Tropical forests in parts of the Hawaiian Islands may survive on mineral dust picked up by winds from deserts 8,000 kilometers (5,000 miles) away in Asia. “Hawaii actually receives very, very little dust by comparison to other parts of the Earth,” says Oliver Chadwick. But despite that, “we can very distinctly pick the signal of the dust out.” Chadwick is a soil scientist at the University of California, Santa Barbara. In the 1990s, he examined soils from across the Hawaiian Islands. In it, he found small amounts of several rare elements: thorium, europium (Yu-ROPE-ee-um) and neodymium (NEE-oh-DIM-ee-um). These elements are not normally found in the volcanic rocks that built these islands. That suggests they blew in from elsewhere.
Slimy dust eaters
Something similar happens in the continental United States. Nutrient-rich dust from the California’s Mojave (Mo-HAAV-ee) Desert blows hundreds of kilometers west into a region called the Colorado Plateau. This region spans much of Arizona, Utah, New Mexico and Colorado. A lot of this area sits atop sandstone bedrock that is poor in many of the nutrients that plants need. So the dust blowing in helps sustain forests of juniper and pinyon trees that might otherwise struggle.
Life on the Colorado Plateau has evolved to catch this dust and eat its nutrients. Walk through one of its open woodlands and the crusty soil breaks under your feet. But this “desert crust” is alive!
The crust is a patchwork of mosses and lichens. These are glued together into a mat of tiny organisms called cyanobacteria. Jayne Belnap works at the U.S. Geological Survey in Moab, Utah. There, she has spent years studying these living crusts. “Unlike plants, they don’t have roots,” she says. “They can’t run around and find nutrients” in the dirt. Instead, they catch them from the sky. The cyanobacteria ooze a sticky goo made of polysaccharide (PAH-lee-SAK-uh-ryde). Any fleck of dust that touches it gets caught in its grip so strongly that the wind cannot steal it away again.
“Dust is really a great thing,” says Belnap. “There’s this phenomenal role that dust has played in the evolution of all of these systems.”
Dust may even feed the oceans, notes Joseph Prospero. He’s an atmospheric chemist at the University of Miami in Florida. His calculations indicate that dust from Africa and Asia injects large amounts of iron into the world’s oceans. That iron helps drive the growth of itty-bitty organisms called phytoplankton (FY-toh-plank-tun). They, in turn, feed tiny crustaceans called krill, which are eaten by whales, penguins and some fish. That dust, in other words, helps sustain the entire ocean food web.
Dust “seems to have tentacles into all sorts of arenas,” says Washington. “It’s a connector across all sorts of ecosystems.” These long-distance dust connections have not been constant over time, and neither have the ecosystems that they feed.
Washington’s team made a poignant discovery while they were exploring the Bodélé in 2005. One day they found a human skull and a jawbone jutting from the ground. The bones were still locked in hard stone. These appeared to be the remains of someone who died and sank to the bottom of the lake many thousands of years ago.
Those bones became buried on the lake bottom. And they stayed suspended in those layers as the world above changed and the lake dried. Winds began to chew on the exposed lake bed. Every year, those winds skimmed a little bit more of the sedimentary stone off the ground’s surface. Eventually, this erosion brought the bones back into daylight, for the first time in thousands of years.
Washington estimates that wind continues to blast one to three penny thicknesses of stone off the top of the lake bed each year. On the north edge of the Bodélé, these winds have removed 4 meters (13 feet) of solid stone from the lake bed. Only a few wind-carved ridges of that stone still remain. Washington figures that this same thickness of stone has been blasted off most of the Bodélé lake bed over the past several thousand years.
As such, this source of dust and plant nutrients won’t last forever, says Washington. This became clear one day as he and his companions explored the lake bed. They came to a spot where two nomads lived with a few goats in a tiny shelter of sticks. The goats fed on scraggly tufts of grass growing on the wind-sheltered sides of the dunes. The men probably lived on a mix of goat milk and goat blood. And there, bored into the ground was a hole. It was a well with a bucket for fetching water.
Washington lay on the ground. He tilted his head into the hole. He waited for his eyes to adjust to the dark. Soon he could see the walls of the well. Three or four meters (10 to 14 feet) down, the walls seemed to change color, from white to some darker kind of stone. Based on that, Washington thinks that the Bodélé only has three or four meters (10 or 14 feet) of phosphorus-rich stone left. Half of what was once there has probably already been broken up and blown away, he says.
Bodélé’s tipping point
Washington believes the Bodélé has seen many wet and dry spells in the past. Over and over, it filled with layers of phosphorus-rich sediments, then surrendered them to the winds. Each time, the wind blasted the lake bed into an empty bowl. This left it ready to fill with water during the next wet spell.
At the same time, the world’s dust production, as a whole, also has risen and fallen. Scientists drilling ice cores in Greenland have seen evidence of this. They have found alternating layers of dusty and dust-free ice. These layers go back hundreds of thousands of years.
Swap believes that the Amazon’s forests also have expanded and shrunken many times. Changes in rainfall and temperature likely drove these cycles. But he thinks dust also played a role. An important one.
When dust was plentiful, the Amazon thrived. When dust was scarce, forests could no longer support many of their plant and animal species. Losing those species made ecosystems less diverse. Forests may have even given way to grasslands and shrubs. In other words, Swap says, the Amazon and the Bodélé may have blinked on and off many times in the past, always together, always to the same climate-driven drumbeat.
This raises questions about how the Bodélé will change as the world warms, and how the Amazon will change along with it.
A warmer Earth will have different patterns of wind and rain. If winds over the Bodélé double in speed, the amount of dust they scour out of it could be eight times as high. And if winds over the Bodélé slow down by 50 percent, its rate of dust production could fall to one eighth of what it is now.
Increased rainfall also could snuff out the Bodélé’s dust, by wetting it down.
Washington would very much like to know how the Bodélé will change as the world’s climate fever builds. Changes in the amount of dust, or where the wind carries it, will affect which ecosystems are fertilized and which aren’t. It also will influence how much sunlight — and heat — is absorbed by the atmosphere or reflected out to space. As Earth warms, the Bodélé might make more dust — or it could just as easily make less dust. It could go either way, says Washington: the Bodélé “is very sensitive.”