Miracle in the Sahara: Oasis Sediments Archive Dramatic History
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 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.
Doing this kind of research in the middle of the Sahara is an adventure that requires stamina. Kröpelin has experienced it all -- passport theft, a life-threatening schistosomiasis infection, sandstorms lasting for weeks -- and yet the scientist remains undeterred. Even when local inhabitants turned up at his camp and threatened him, because they believed that his drilling activities were disturbing the virgin of the lake, he managed to appease them.
Every day, he and his team took a boat out to the raft they had anchored in the middle of Lac Yoa. Earlier, they had lowered a steel cylinder to the lake floor, at a depth of 25 meters. Now, using nothing but muscle strength, they rammed it deeper into the subsurface, millimeter by millimeter.
There was no canopy to provide protection from the fierce sun. One of the men would give the cylinder 30 to 40 blows with a 30-kilogram (66-pound) hammer before, dripping with sweat, handing it to the next man. Of course, they could only work when the wind, which sweeps across the flat desert from Libya, wasn't constantly blowing fine sand into their eyes.
They drove the pipe 16 meters into the sediment before reaching the ice-age desert floor. The geologists had penetrated all the way to the original bottom of the lake.
After cutting it into one-meter segments and protecting it from impact and drying with a Plexiglas sleeve, the scientists took their prize out of the country. They traveled in a Toyota Land Cruiser across 1,200 kilometers of desert tracks to the capital N'Djamena. Then the drilling cores were sent to Cologne by airfreight.
There the experts were able to examine the clay-like deposits one layer at a time. The layers of mud were deposited on top of each other, not unlike tree rings, at an average rate of about a millimeter a year. Even in the desert, there are sufficient differences between the seasons to be clearly recognizable in the sediment.
History by the Layer
Three employees were entrusted with the exhausting task of counting, eventually arriving at 10,940 layers, each representing one year. Not even radiocarbon dating is this precise. The method was off by about 50 years.
More importantly, the geologists began analyzing the individual layers. Using a mass spectrometer, X-rays, laser beams and a scanning electron microscope, they tried to wrest the secrets from the drilling core. They measured particle sizes, the material's chemical composition and magnetic susceptibility, and placed thin sections of their samples, only 25 micrometers thick (less than one-thousandth of an inch), under a polarizing microscope.
But the most precious source of information is the pollen trapped in the sediment, because it reflects changes in the climate more faithfully than anything else. For instance, when primarily grass pollen was deposited on the lake floor, it means that there must have been steppes extending along the shore. Fern spores indicate that rivers emptied into the lake, presumably from the nearby Tibesti Mountains. Pollen from sagebrush or the toothbrush tree, on the other hand, is a sign that the area was dominated by desert at the time.
Kröpelin can also identify individual events in his stony climate archive. Earthquakes, wildfires and especially violent dust storms leave behind telltale traces in the lake sediments.
Kröpelin is still assembling the final details of his work. When he is finished, he and his colleagues plan to publish the fruits of their backbreaking work in the journal Nature. Climate modelers from around the world are already waiting for the results. "Stefan's drilling core will enable us to precisely tracks how the African monsoon system has shifted," says Martin Claußen of the Max Planck Institute for Meteorology in Hamburg, one of the leading experts on simulation of the Sahara climate.
Kröpelin is already thinking about his next step. For him, the drilling core from Lac Yoa is much more than a chronicle of climate in the region. He is convinced that analyzing the sediments will also offer a glimpse into an entire chapter of human history. In a place now covered by the largest desert on earth, human settlers once ruled the savannah.
Until the end of the ice age, about 11,000 years ago, Kröpelin explains, the Sahara constituted the northern border of the areas settled by Homo sapiens. The wasteland was too inhospitable for humans to traverse it.
But when the glaciers melted in Europe, the monsoon system shifted in North Africa, and rain clouds were driven inland from the Gulf of Guinea. As the East African savannah continued to expand northward, a path was opened up to Homo sapiens to travel to more distant lands.
Only when the life-bringing monsoon slowly diminished about 5,000 years ago did the desert gradually return. The grass withered, the rivers ran dry and the animal herds disappeared, making it difficult for humans to survive. A portion of the Sahara population moved south to the more fertile Sahel zone, while the rest settled along the Nile River. This migration away from the desert, says Kröpelin, probably paved the way for the advanced civilization of ancient Egypt.
- Part 1: Oasis Sediments Archive Dramatic History
- Part 2: The 'Man of the Desert'
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