DUST IS OUR SOURCE OF FERTILITY. The glaciers and the deserts feed the stable platforms, or cratons, like those broad prairies on which I’d found a former sea. From the frozen and the arid wastes come the materials for luxuriant growth.
The surface of the Earth is a thin film that exists between two boiling systems. Deep beneath the ground, the roiling of the hot mantle propels the crust’s tectonic plates. Above the ground, the winds of the world execute the same ballet, girdling the planet from east to west. The winds ascend and descend in the spirals of convection cells, rising when warmed and sinking as they cool. If you could see these currents, it would look as though the Earth were wearing a knitted sweater of horizontal bands.
When the winds are descending, they gather moisture and heat energy. Little rain falls beneath them. When they ascend, the wet air cools, clouds form, and it rains, sometimes copiously. The Sahara and the Gobi Desert are located where the wind is descending; the rainforests of the equatorial zone occur where it is ascending.
Where the ground is dry and the soil is fine, a descending convection cell can start a dust storm, the same thing that happens when you come down fast on dust bunnies with a broom. The cooled air spreads across the ground like a splash, whipping up the silt into turbulent eddies. The dust drifts into the upward-bound part of the cell and quickly finds itself ten thousand feet in the air. Such storms, traveling at speeds of up to 650 feet per second, can drop the dust hundreds or even thousands of miles away.
This is what happened in the periods of glacial retreat that punctuated the Pleistocene Age, the last one beginning some fifteen thousand years ago. When the glaciers drew back and the climate warmed, the former ice sheets released tons of rock flour from their masses, leaving it along the watercourses of the outwash plains. Then, as the climate dried and cooled, the silts rose with the winds and were scattered over the upper Midwest and down the course of the Mississippi River Valley.
The dust storms that raged then would have made the 1930s’ Dust Bowl look like a wheeze. Landscapes that started out relatively naked were covered with porous silty soils more than a hundred feet deep within a few centuries. It is still possible to trace the course of these winds over the land, because as they traveled they dropped the heaviest particles nearest to the source, the finer particles remaining longer in suspension before they finally settled to form the best of the prairie soils. These fine soils were both rich in nutrients and porous in structure, so that water did not pool in them, and so that nitrogen, potassium, and phosphorus quickly became available for plants. Once the prairies were broken for corn, these soils yielded three times the harvest of nearby claypan soils.
The American prairie soils are not unique. The Russian chernozem, the amazingly productive black soils studied by V.V. Dokuchaev, the pioneer of soil science, also have their origin in this windblown dust called “loess.” The deepest soils in the world, the yellow loams of northern China, some of them hundreds of feet in depth, from whose sediments the Yellow River takes its name, were formed by dusts that are still being blown into the region from the Gobi Desert. Some of the people of this region traditionally excavate their houses directly into the material of these dirt cliffs.
So the Dust Bowl of the American Midwest in the 1930s was not an aberration, but a miniaturized replay of the forces that caused the region’s fertility in the first place. Unfortunately, it worked in reverse. The rains failed, and the cultivated soil became once more the mobile dust that it had begun as. A writer for the U.S. Department of Agriculture put the matter movingly in 1938:
Everything goes well as long as the rains continue. But inevitably the drought years come, as they have with dismaying regularity since 1930. The crops fail and the pastures and range grasses stop growing. Stock-water reservoirs dry up, and cattle are rushed to market to prevent them from starving or dying of thirst. The dust begins to blow, and black blizzards lay bare the soil down to the furrow bottoms, pile drifts of dust around the farmstead and in the fencerows, and make life unbearable for all the people within a radius of several hundred miles.
When a dust storm rose, the sun turned red and the day darkened to dusk. As slippery and small as its particles are, the silt got in every-where, even under sills stuffed with towels. People caught in the storms sometimes choked to death on the silt.
In 1937, a dust storm started out in the Texas and Oklahoma panhandle country, crossed five states, and dropped the last of its load in Canada. As the storm churned, it picked up much of the fine soil at the source, leaving only coarse sand behind. As it traveled, it dropped the coarsest materials first, leaving the finest only at its last stop in Canada. When tested five hundred miles from its source, the storm’s dust was found to have ten times as much organic matter, nine times as much nitrogen, and nineteen times as much phosphoric acid as did the dune sands that were left at the storm’s source.
Five hundred miles is piddling. The fact is that the Brazilian rainforest is an artifact of the Sahara, some thousands of miles across the Atlantic. The silt originating near Lake Chad in the southern Sahel is what fertilizes the rainforest. Great plumes of dust rise from the desert and drift west, dropping their coarser particles in North Africa. As they pass out over the Atlantic, they meet the edges of the storms that will soon pelt the Amazon Basin. University of Virginia researchers estimate that twelve million tons of Sahara dust drop on Amazonia every year, bringing to the nutrient-poor soils a shot of fertility. Particularly important is the available phosphate, about a pound per acre, that the storms bring; there is virtually no other available phosphate in the old, deeply leached Amazon soils.
The scientists were not even looking for dust when they learned this startling fact. They were part of a 250-man international team funded by NASA and its Brazilian equivalent to learn more about how the hydrocarbons released in Amazon burning might be affecting global warming. “We were looking for hydrocarbons,” recalls Robert Swap. “When we started finding soil particles in the air, it blew us away.”
At first, they thought that the dust must have come from the arid lands in northeastern Brazil, but tracing the path of the huge storms, they determined that the only possible path was across the ocean from Africa. Checking satellite photos, they were even able to distinguish a steady plume of dust and trace its course across the Atlantic. It was well known that Sahara dust had been able to reach Guyana and the Caribbean, and way back in 1846 Darwin had reported a snow of white dust settling on the Beagle as it rode the swells off South America, but the idea of the Sahara feeding the Amazon was a new one.
Now the team is studying soil cores to determine what possible effect this process might have had on the genesis of the forests, and they are trying to assess its effect on marine organisms, particularly planktons, that thrive in the paths of the storms.
Once, Lake Chad was ten times its current size, and the sediments around it are rich in diatomaceous earths and salts. From its present desolation comes the dense life of the rainforest.