Miracle in the Sahara: Oasis Sediments Archive Dramatic History

PHOTO: Tchad, geological formation on Ounianga Serirs lake in Sahara.

A marvel of nature, the lakes of Ounianga in the Sahara Desert have lasted thousands of years and withstood dramatic climate change. Now, a German geologist has analyzed lakebed sediments to shed light on a spectacular chapter in human history.

"Water," says Stefan Kröpelin, "water as far as the eye can see." He is pointing to the south, where there is only one thing stretching to the horizon: sand, sand and more sand.

Kröpelin describes the reeds billowing along the shore, the gazelles and giraffes drinking from the lake and the hippos and crocodiles lounging in its waters. But the desert before him is so inhospitable that it could hardly be home to more than a few darkling beetles.

Kröpelin is no fabulist. In fact, he knows what he's talking about. A fertile, wet savannah once covered this region, where not a single blade of grass grows today.

The evidence lies at Kröpelin's feet. He has just dislodged a few white chunks from the underlying bedrock with his geologist's hammer. Using his hand, he picks dozens of small shells from the limestone. "Freshwater snails," he says with satisfaction.

The geologist records the GPS coordinates in his blue field notebook. Then he places the three pieces of rock into plastic bags and labels them with a site number: "W 76." Back home, at the Africa Research Center at the University of Cologne, he will determine the age of the rocks. "About 10,000 years old," he estimates. At least that was the age of the samples he took home after his last visit to this region of northern Chad.

The view to the north offers an idea of the lost paradise Kröpelin is talking about. There, in a basin about 40 meters (131 feet) lower than the surrounding area, is a lake lined with green vegetation. The massive sand dunes that reach into the water like giant fingers will eventually bury the entire oasis, but now there are still date palms growing there.

The lakes of Ounianga are a miracle of nature. These unusual green islands in a sea of sand have lasted thousands of years. There are no other comparable stretches of open water within a radius of more than 800 kilometers (500 miles).

And why should there be? The scorching sun over the Sahara evaporates a water column of more than six meters a year, while the sky yields less than five millimeters of annual precipitation. Under these conditions, even an ocean would soon disappear. But in Ounianga a vast reservoir of fossil ground water beneath the surface constantly replenishes the water lost to evaporation.

The Earth's Archive

Kröpelin first set up camp there more than 14 years ago. His goal was to recover sediments from the floor of the largest of the lakes, Lac Yoa, deposits that have formed in the lake's roughly 11,000-year history.

These sediments are a unique archive of the history of the earth. They contain evidence of what is probably the most impressive and dramatic change in the climate occurring on the planet since the end of the last ice age. The mud on the lake floor tells the story of the greening of the biggest desert on earth, which then dried up a few millennia later.

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