Vast portions of today’s Sahara Desert were green thousands of years ago
Past riverine activity in North Africa reveals several greening areas of the Sahara
Analysis of sediment cores from the Mediterranean Sea combined with Earth system models tells the story of major environmental changes in North Africa over the past 160,000 years.
Large parts of today’s Sahara Desert were green thousands of years ago. Prehistoric engravings of giraffes and crocodiles bear witness to this, as does a rock painting from the Stone Age in the desert that even shows humans swimming. However, these illustrations only give an approximate picture of living conditions. Recently, more detailed information has been obtained from sediment cores extracted from the Mediterranean Sea off the Libyan coast. An international research team examined these cores and found that the seabed layers tell the story of major environmental changes in North Africa over the past 160,000 years. Cécile Blanchet from the German Geoscience Research Center GFZ and her colleagues from Germany, South Korea, the Netherlands and the United States discuss it in the journal Geosciences of nature.
In collaboration with the GEOMAR Helmholtz Center for Ocean Research in Kiel, a team of scientists organized a research cruise on the Dutch ship Pelagia to the Gulf of Sirte in December 2011. “We suspected that when the Sahara desert was green, the rivers which are currently dry would have been active and would have brought particles into the Gulf of Sirte, ”explains lead author Cécile Blanchet. Such sediments would help to better understand the timing and circumstances of the reactivation of these rivers.
Using a method called “piston coring”, scientists were able to recover columns of marine mud 10 meters long. “You can imagine a giant hollow cylinder pushed into the seabed,” says co-author Anne Osborne of GEOMAR, who was on board the research vessel. “The layers of marine mud contain rock fragments and plant remains transported from the neighboring African continent. They are also full of the shells of microorganisms that have grown in seawater. Together, these sediment particles can tell us the story of past climate change, ”explains Blanchet.
“By combining the sediment analyzes with the results of our computer simulation, we can now accurately understand the climatic processes at work to explain the drastic changes in North African environments over the past 160,000 years”, adds the co -author Tobias Friedrich of the University of Hawaii. ‘I.
From previous work, it was already known that several rivers flow episodically through the region, which today is one of the driest areas on Earth. The team’s unprecedented rebuilding spans the past 160,000 years continuously. It offers a full picture of when and why there was enough rainfall in the central Sahara to reactivate these rivers. “We have observed that it is the slight changes in the Earth’s orbit and the growth and decline of the polar ice caps which punctuate the alternation of wet phases with heavy precipitation and long periods of almost complete aridity”, explains Blanchet. .
The fertile periods generally lasted five thousand years and the humidity extended over North Africa to the Mediterranean coast. For the people of the time, this resulted in drastic changes in living conditions, which probably led to large migratory movements in North Africa. “With our work, we have added essential puzzle pieces to the image of past changes in the Saharan landscape that help to better understand human evolution and the history of migration,” explains Blanchet. “The combination of sediment data and computer simulation results was crucial to understanding what controlled the past succession of wet and arid phases in North Africa. This is especially important as this region is expected to experience severe droughts due to human-induced climate change. “
Reference: “Drivers of river reactivation in North Africa during the last glacial cycle” by Cécile L. Blanchet, Anne H. Osborne, Rik Tjallingii, Werner Ehrmann, Tobias Friedrich, Axel Timmermann, Warner Brückmann and Martin Frank, January 28, 2021, Geosciences of nature.
DOI: 10.1038 / s41561-020-00671-3