|Oasis Sediments Archive History|
|By Johann Grolle, SPIEGEL||May 24, 2013, 9:34 AM|
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.
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.
Various traces of settlements show that man took advantage of this opportunity. The Sahara became a center of cultural development, where ceramics were created at a very early stage, nomads domesticated cattle and goats, and humans documented their daily lives in spectacular scenes painted on cliff walls.
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.
The 'Man of the Desert' Kröpelin is a wiry, hands-on man with seemingly inexhaustible energy, whoseems youthful despite his 61 years. In an article titled "Man of the Desert," the journal Nature describes him as "one of the most devoted Sahara explorers of our time." Some see him as part of a tradition established by predecessors like Heinrich Barth and Gustav Nachtigal.
Kröpelin, like these great scholars of the 19th century, is a universalist. He is equally at home with such diverse subjects as stone-age settlements, the ecological importance of solitary wasps and the angles of sickle-shaped migrating dunes.
In Sudan, Kröpelin explored Wadi Howar, a dry valley that was once filled with a large desert tributary of the Nile. He has retraced the trading routes of caravans from the days of the pharaohs, and he has documented climate change on the basis of tentatively sprouting camel grass in the desert.
Most of all, Kröpelin is a gifted storyteller. "I was just in the shower, when…" he begins, and before long he is in the middle of a gripping story. Shortly before his departure, he says, he received a call on his satellite phone from a French colleague. The Frenchman and his camel expedition were lost in the canyons of the Erdi-Ma, a barren region in northeastern Chad into which hardly any humans had ever set foot. To help the lost Frenchman, Kröpelin used old GPS notes to direct him to one of the rare watering holes. "He must be out there somewhere," says Kröpelin, pointing to the rocky landscape north of the Ounianga lakes. "I hope he makes it."
"The desert attracts a special sort of person," says Kröpelin, as he seamlessly launches into his next story. This one is about a pedantic botanist, whose Jeep overturned while he was maneuvering his way around the cracked surface of a dry lakebed. But the botanist did not crawl out of the vehicle until he had painstakingly documented the accident in his field book. And then he talks about the time Sudanese soldiers pursued him through the desert for days, until they caught up with him and demanded some of his diesel fuel.
He launches into a discussion of politics. The rumors about Islamist terrorists, the horrific stories from Darfur and now the war in Mali -- all of this, he says, doesn't make working in the region any easier. But the dangers are exaggerated, says Kröpelin, noting that walking through some neighborhoods of New York in the evening is riskier than spending a week in the desert. Nevertheless, fear has driven away some of his fellow geologists. The French, once so numerous, aren't coming anymore, he says. It's also become difficult to find doctoral students in Germany willing to put up with the hardships of working in the Sahara.
But this time the 21st century has found its way into Kröpelin's realm. Before departing from Marseille, the desert explorer excitedly took a picture of the departure board at the airport. He could hardly believe his eyes: The board listed a 3 a.m. departure for a charter flight bound for Faya-Largeau.
Point-Afrique, an adventure travel company, had managed to make use of an old French military airport in the small oasis city in northern Chad for tourism purposes. Now it offers charter flights in the winter season, taking travelers on board a Boeing 737 to one of the most remote spots on the planet. "Unbelievable," says Kröpelin. Making the round trip from his home to his research site within a week is something he has never experienced throughout his entire academic career, he says.
From Faya, it's only a one-day trip in an off-road vehicle to the Ounianga lakes. The journey passes through a wasteland that stretches to the horizon, with one of the giant sickle dunes occasionally blocking the way. Slowly and inexorably, the dunes move across the plain in a southwesterly direction, traveling about one kilometer a century.
Dust-covered tire tracks are the only evidence of a road here, along the main artery between Libya and Chad, as well as the truck tires, scattered at regular intervals, that former Libyan leader Moammar Gadhafi had his troops leave behind during his expedition into Chad. Like Hansel and Gretel in the forest, Gadhafi wanted to mark the route for the trip home.
Until recently, hopelessly overloaded trucks crawled along this path too, bringing goods from Libya to Chad and Sudan. But trade has come to a standstill since Gadhafi was overthrown, and now weeks go by without a single vehicle passing through the region.
Kröpelin's expedition, traveling in three off-road vehicles, also doesn't encounter a single person along the way. A few runaway camels turn up shortly before their destination, where scattered acacia trees indicate that there must be water deep underground there. Then, quite suddenly, the spectacular view of the Ounianga basin appears.
On his first visit to this idyllic green spot in one of the driest places on earth, Kröpelin was so fascinated that he has since fought to place this natural treasure under protection. He achieved his goal last year, when UNESCO declared the lakes a World Heritage site. Kröpelin proudly pulls out a UNESCO map. While there are many sites in Europe, there is only one dot in the vast open spaces of Chad: Ounianga.
But Kröpelin is too restless and enterprising to be satisfied. He has already set his sights on his next goal: to convince UNESCO to add the Ennedi Plateau, more than 200 kilometers farther to the south, to its list. For Kröpelin, the plateau's uniqueness is beyond question. "Monument Valley is nothing by comparison," he says. The region is also culturally significant, he adds. "You won't find stone-age cliff drawings like the ones in the Ennedi anywhere else in the world."
Dreams of a Green Sahara
Kröpelin is fascinated by the relationships among the histories of the climate, the earth and mankind. He is interested in how people responded to change in the Sahara. Here in the inhospitable dryness of the desert, blades and arrowheads made of quartzite or ring-shaped traces of settlements are evidence that Homo sapiens were once omnipresent in the Sahara.
"A Stone-Age burial mound," Kröpelin says, pointing to one of the piles of stones rising from the plain. "What's so fascinating about it is that everything is preserved in just the way it was left thousands of years ago."
During an expedition into the no-man's land east of the Ounianga lakes, Kröpelin even believes he found traces of an ancient Egyptian caravan. He discovered a stone statue of a man, visible from far away on a high plateau, similar to the statues uses on mountains today as guideposts for hikers. Kröpelin suspects that what he had found was a landmark for desert travelers from the days of the pharaohs.
There is evidence that the expeditions from ancient Egypt extended to at least the current Egyptian-Libyan border, says the geologist. A few years ago, hieroglyphics were found there, at Uwaynat Mountain. Kröpelin thinks it is conceivable that traders stopped there to replenish their water supplies before continuing their travels toward Ounianga.
To reinforce his theory, he points to the eroded cliffs that shape the landscape along the shores of the Ounianga lakes. Over the millennia, the constant wind has carved them into step pyramids.
Kröpelin believes that the similarity between this shape and that of structures along the Nile is more than coincidental. He theorizes that gradual desertification drove the Egyptian people out of their original habitat, which is now the Sahara Desert. He points out that silhouettes of the tombs of the pharaohs, visible from a great distance, are characteristic of precisely the region that was once home to the Egyptians.
Will a return ever be possible? Will the Sahara turn green again one day?
Even Kröpelin knows that by answering these questions he is delving into the realm of speculation. Nevertheless, he is gathering evidence.
A rare rainfall over the otherwise dry Sudan in 1988 awakened his suspicions for the first time. If everyone was talking about climate change, why shouldn't the monsoon in Africa be changing, too? Perhaps global warming could drive it back to the state it was in once before, after the ice age.
Since that rainstorm in Sudan, Kröpelin has been recording all signs of climate change during his trips, looking for answers to questions like: Where is camel grass growing more abundantly than in previous years? How productive are the few watering holes? And what are the camel herders and date farmers saying?
Of course, all of this is merely anecdotal evidence that doesn't stand up to scientific scrutiny. Nevertheless, Kröpelin is convinced that the evidence is growing. In fact, he says, he even believes that there is now real evidence of change, and that the desert is getting greener.
The geologist feels validated by recent news from the Faya oasis. Last summer, residents told him, they were surprised by a sudden downpour. Huts were washed away and people drowned. This had never happened before, they said.
Translated from the German by Christopher Sultan