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.