Shifting Sands

This is an amazing story:

New Scientist: Amazon forest relies on dust from one Saharan valley

The trees and plants in the Amazon rainforest rely on nutrient-rich dust from a single valley in the Sahara desert for sustenance, researchers have discovered.

Scientists know that millions of tonnes of mineral dust are blown from the Sahara desert to the Amazon basin each year. The dust helps keep the Brazilian rainforest soils fertile.

Now, researchers have found that 56% of this dust comes from one place: the Bodélé depression in Chad, Africa. They also showed that three times more dust than previously thought is transported each year from the Sahara to the Amazon—over 40 million tonnes.

Charles Darwin encountered African dust off the Cape Verde Islands early in the Beagle Voyage. He collected samples and later had them and similar samples obtained from another ship analysed. He wrote about the dust in his account of the Beagle voyage and, in 1846, he produced a short paper for the Journal of the Geological Society of London entitled An account of the Fine Dust which often falls on Vessels in the Atlantic Ocean, in which he wrote:

On the 16th of January (1833) [it was actually 1832], when the Beagle was ten miles off the N.W. end of St. Jago, some very fine dust was found adhering to the under side of the horizontal wind-vane at the mast-head; it appeared to have been filtered by the gauze from the air, as the ship lay inclined to the wind. The wind had been for twenty-four hours previously E.N.E., and hence, from the position of the ship, the dust probably came from the coast of Africa. The atmosphere was so hazy that the visible horizon was only one mile distant. During our stay of three weeks at St. Jago (to February 8th) the wind was N.E., as is always the case during this time of the year; the atmosphere was often hazy, and very fine dust was almost constantly falling, so that the astronomical instruments were roughened and a little injured. The dust collected on the Beagle was excessively fine-grained, and of a reddish brown colour; it does not effervesce with acids; it easily fuses under the blowpipe into a black or gray bead.

Darwin went on to describe how the dust contained considerable quantities of minute aquatic organisms known as Infusoria. He concluded that, while the types of Infusoria found did not suggest an African origin for the dust, the wind direction at the time, which he associated with the seasonal Harmattan wind, clearly did. He confessed that it was an enigma, but concluded:

the circumstance of such quantities of dust being periodically blown, year after year, over so immense an area in the Atlantic Ocean, is interesting, as showing by how apparently inefficient a cause a widely extended deposit may be in process of formation;

Nevertheless, I'm sure Darwin would have been astonished to learn that over 40 millions of tonnes of such dust are transported across the Atlantic to the Amazon each year.

I have also experienced two sandfalls. Last March, while I was on holiday in Sicily, my hotel balcony was one morning covered in gritty dust. The Italian cleaning lady, who spoke as much English as I spoke Italian, pointed at it and said one word: "Sahara!" But far more memorable was one late afternoon in the late 1970s or early 1980s. I was on my way home from school and was walking through the centre of Birkenhead, when the sky suddenly went yellow. It was one of the stranges sights I have ever seen. It was if the sun had suddenly been replaced by a giant sodium lamp. A short while later, it was gone and the sky was blue once again. That evening, a large deposit from the Sahara landed on Manchester.

This planet is a lot more active than we realise.

Writer and photographer Richard Carter, FCD is the founder of the Friends of Charles Darwin. He lives in Hebden Bridge, West Yorkshire.WebsiteFacebookTwitterNewsletterBooks
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