A dried-up lake in Chad is quietly fertilizing the Amazon
Fossilized algae from an ancient African lake blow across an entire ocean and feed a rainforest 5,000 miles away.
Thousands of years ago, the Sahara held a lake so large it's now called Mega-Lake Chad — bigger than any lake on Earth today. As the region dried out, the water retreated, and the fossilized remains of the diatoms that once lived in it were left baked into the lakebed: the Bodélé Depression, a basin in northern Chad about 500 kilometers long.
Wind does the rest. Funneled by two mountain ranges on either side, gusts race across the depression on roughly 100 days a year, some of them strong enough to be visible from orbit, lifting an estimated 700,000 tonnes of fine dust into the air on a single winter day. That dust doesn't just drift locally. Carried on trade winds, it crosses the entire Atlantic Ocean.
When it lands, it lands mostly in the Amazon. The Bodélé Depression's dust is rich in phosphorus, a nutrient the Amazon's soil is chronically short of because the rainforest's own relentless rain washes minerals out of the ground faster than local decay can replace them. Research using NASA satellite data has estimated that this single dry lakebed in Chad supplies more than half of the mineral dust that fertilizes the entire Amazon rainforest.
One of the driest, most lifeless-looking places on the planet is functionally feeding one of the wettest, most crowded ecosystems on it — an entire biome kept fertile by a desert basin most people have never heard of, four thousand miles and an ocean away.