Beneath the Saharan sands, a river valley

(credit: Evgeni Zotov)

If someone traveling the Sahara Desert with you said, “I bet there used to be a huge river right about… here,” you would probably think it’s time to stop for food and water. But that’s exactly what some scientists have said, and it turns out they were right.

Obviously, that implies the arid Sahara hasn’t always been quite so arid. We don’t even have to reach back far into the murky depths of geologic time in this case. The cycles in Earth’s orbit include a wobbling top motion called precession that alters the extremity of summer and winter over a period of about 23,000 years. When the summer sun is stronger over northern Africa, the equatorial boundary in Earth’s atmospheric circulation is pushed to the north—and life-giving rain comes with it.

It was actually seafloor sediments that proved this to us. Beyond the mouth of the Nile River, layers of sediment rich in organic material testify to periods of bigger flows. And off the coast of Mauritania to the west, there are also layers of river sediment—and no river. What’s more, surveys of the seafloor along Mauritania’s coastline discovered a large submarine canyon similar to those connected with major rivers elsewhere.

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