Ancient african lake fertilises the amazon

Ancient African lake fertilises the Amazon
02 July 2010
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WINDBLOWN dust from a dried-out African lake that was once the size of California is nourishing rainforests in the Amazon and algae in the Atlantic.

Previous modelling studies estimated that the Bodélé depression in Chad, which formed when the largest lake in Africa dried out about 1000 years ago, is responsible for about 56 per cent of the dust from Africa reaching the Amazon, amounting to millions of tonnes per year.

Now, researchers have estimated how much fertiliser, in the form of iron and phosphorus, is in the dust. Although the Bodélé depression is near the war zone in Darfur, Sudan, Charlie Bristow of Birkbeck, University of London, and his colleagues managed to collect and analyse 28 samples during an expedition in 2005.

Their analysis suggests that the depression is the source of 6.5 million tonnes of iron and 120,000 tonnes of phosphorus per year, with about 20 per cent reaching the Amazon, half falling in the Atlantic, and the rest dropping in west Africa (Geophysical Research Letters, in press). "It's a missing part of the jigsaw that they've solved: what's actually in the dust chemically," says Richard Washington of the University of Oxford, who studies dust clouds from the Sahara.

Bristow hopes to mount another expedition to the depression to find out how thick the sediment is and estimate how much natural fertiliser is left.