9 August 2010
Dust from one of the world's most desolate places is providing essential fertilizer for one of the most lush, scientists have discovered. Significant amounts of plant nutrients have been found in atmospheric mineral dust blowing from a vast central African basin to the Amazon, where it could compensate for poor rainforest soils.
The basin, known as the Bodélé depression, is the site of a once-massive lake in Chad. Bodélé is thought to be the dustiest place on Earth.
Scientists went to Bodélé in 2005 and took atmospheric measurements to learn how the dust travels1. They also took dust samples to learn about its composition (see 'Climate science: The dustiest place on Earth').
With chemical analyses now complete, the dust turns out to contain substantial amounts of the key plant nutrients iron and phosphorus. Although the abundances are not unusually high, prior estimates of the dust plume's nutrient content were very low, particularly for phosphorus. The team estimates the dust to be 38 times richer in this mineral than formerly thought.
The dust in any case is unusual in the sandy Sahara, as it is essentially lakebed sediment originating from the shells of freshwater diatoms, and so richer in nutrients, including iron.
The results were reported in Geophysical Research Letters on 30 July2. Charlie Bristow, a sedimentologist at Birkbeck, University of London, and lead author of the study, says, "A hypothesis we are investigating at the moment is that because it's coming from the lakebed, this iron is going to be more bioavailable than if it were coming from the deeply weathered surface of the Sahara."
"The Amazon is essentially a leached or leaching system," says Bristow. Nutrients in the soil are washed away by rains. "So although it is very productive, it is actually quite nutrient-poor."
So nutrient input is vital to the rainforest. Phosphorus, a major element of commercial fertilizer, is essential for plant growth, respiration and energy transfer and storage. Iron is a micronutrient that plants use to make an enzyme critical for photosynthesis.
Not that you could use the dust to make your potted plants more verdant. "It is certainly not rich enough to do that," Bristow says.
But it is more nutrient-rich than simple sand and, crucially, there is a lot of it. It comes from part of the bed of the dried-up lake Megachad, which once measured about 400,000 square kilometres at its largest. Today only a small remnant exists in the 1,350-square-kilometre modern-day Lake Chad.
Megachad "was bigger than all the Great Lakes of North America put together", Bristow says. "Pretty close to the area of California." So the area left bare, which now contributes the dust, is vast.
On the basis of the height of wind-eroded ridges called yardangs, Bristow has calculated that "up to 4 metres of dust have been removed from across the lakebed in the past 1,000 years"3.
That is a massive amount, even if satellite estimates indicate that only 20% gets all the way to the Amazon.
Joseph Prospero, an atmospheric chemist at the University of Miami, Florida, is in Barbados studying the dust blown across the Atlantic. "Looking at satellite images, day after day you see these huge plumes coming out of that region," he says. The dust takes two seasonal routes across the ocean, in the summer passing by the Caribbean and in the winter swinging further south to the Amazon.
At current rates of dispersal, the dust in the basin will last for perhaps another 1,000 years or more, assuring Amazon fecundity if weather patterns do not change. But what about changes in precipitation, which could have an effect on the amount of dust produced?
Unfortunately, even the latest climate models can't agree on whether that part of Africa will get wetter or drier, says Prospero. "If it becomes drier, we can expect more dust."
That might be bad for nearby residents, leading to more dust storms, but it could be a good thing globally. More dust means more nutrients for the Amazon, which means greater plant growth. Plants in the Amazon help to remove excess carbon dioxide from the atmosphere, which could mitigate global warming by slowing its rate.
But if rains in Africa increase, the dust plume might abate in the shorter term. "If it stops arriving in the Amazon, presumably the fertility will decrease," Bristow says. That would harm the ecosystems of the area, as well as damaging a valuable carbon sink.
Dust in the atmosphere, or the lack of it, may have even wider implications. It increases the amount of sunlight reflecting off Earth into space, potentially offsetting global warming. It could also affect cloud formation, and even tropical storms. "It gets pretty complicated," says Prospero. "One of the primary areas of [global climate] research today is understanding the global cycle of dust."
And further insight into the Bodélé region could be elusive, because fieldwork in Chad is complicated. Bristow calls it a hideous place to work. "I've worked in Antarctica and in deserts all over the world," he says. The Bodélé depression "is the worst. When the wind starts to blow, that dust gets into everything. You become totally encrusted. You can't see, you can't eat. The wind flattened our tents."
There are other risks, too. Bristow says, "The troubles from Darfur are spilling over the border, so you're on the edge of a war zone. In Antarctica, if something goes wrong, you can call a helicopter. It might be grounded for a couple of days, but you can be confident that there will be a helicopter. If something goes wrong in Chad, you'd better be able to get yourself out."