Six thousand years ago, what is now Lake Chad in the country of the same name was the largest freshwater lake on Earth. What remains covers less than a hundredth of the same area. The paleolake Mega-Chad, as researchers have dubbed it, at one time more than filled the Bodélé depression, an area which now, six thousand years on, is the largest single source of airborne dust particles in the world.
We now know, thanks to examination of satellite imagery which has identified former shorelines and allowed researchers to date when the lakes had each of them, exactly when the paleolake began to decline, and we have been able to measure the rate. The process is fascinating – but the results are nothing short of amazing.
What’s amazing is that Lake Mega-Chad, having mostly dried up in a short burst (around six hundred years at the outside), was still enough to keep the depression dry a thousand years ago – but since then, the potential of the dust has risen greatly.
The path of the dust takes it from Chad, across the Atlantic, to South Africa, blown by powerful winds, where it settles in the rainforests. Loaded with minerals and other nutrients, it settles in the Amazon basin, which is always in need of soluble nutrients – the growth and activity of a rainforest being enough to burn through such resources swiftly.
It has long been held that the dust from the Bodélé depression is, in this way, essential to the rainforest’s existence, and it certainly seems to be essential to sustaining it today. However, the research into Lake Mega-Chad, performed by teams from Birkbeck College, the University of Royal Holloway, and Kings College London let by Dr Simon Armitage has shown that this can only have been true for the most recent thousand years of the rainforests, and that beforehand they must have obtained their soluble nutrients elsewhere.