Previous river actions in North Africa present a number of greenings within the Sahara – Watts Up With That?

The analysis of sediment cores from the Mediterranean in combination with earth system models tells the story of the great environmental changes in North Africa over the last 160,000 years

GFZ GEOFORSCHUNGSZENTRUM POTSDAM, HELMHOLTZ CENTER

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PICTURE: ENGRAVING BY GIRAFFES NEAR GOBERO IN NIGER, APPROX. 8,000 years old, witness green times in the desert. Show more CREDIT: MIKE HETTWER, 2006, WWW.HETTWER.COM

Much of today’s Sahara was green thousands of years ago. Prehistoric engravings of giraffes and crocodiles bear witness to this, as well as a Stone Age cave painting in the desert, which even shows people swimming. However, these images only give a rough picture of the living conditions. More recently, more detailed knowledge has been gained from sediment cores obtained from the Mediterranean Sea off the coast of Libya. An international research team examined these cores and discovered that the layers of the ocean floor tell the story of the great environmental changes in North Africa over the past 160,000 years. Cécile Blanchet from the German Research Center for Geosciences GFZ and her colleagues from Germany, South Korea, the Netherlands and the USA report on this in the journal Nature Geoscience.

Together with the GEOMAR Helmholtz Center for Ocean Research 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 the rivers that are currently dry when the Sahara was green would have been active and would have brought particles into the Gulf of Sirte,” says lead author Cécile Blanchet. Such sediments would help to better understand the timing and circumstances for the reactivation of these rivers.

Using a method called “piston drilling”, the scientists were able to extract 10 meter long columns from sea mud. “You can imagine a huge hollow cylinder being pushed into the sea floor,” says co-author Anne Osborne from GEOMAR, who was on board the research vessel. “The sea mud layers contain rock fragments and plant remains that are transported from the nearby African continent. They are also full of shells of microorganisms that grew in seawater. Together, these sediment particles can tell the story of past climatic changes, ”explains Blanchet.

“By combining the sediment analyzes with the results of our computer simulation, we can now understand the climate processes at work in detail to explain the drastic changes in North African environments over the last 160,000 years,” adds co-author Tobias Friedrich of the University of Hawai ‘I.

It was already known from previous work that several rivers flowed episodically through what is now one of the driest areas on earth. The team’s unprecedented reconstruction spans the past 160,000 years. It provides a comprehensive picture of when and why there was enough rainfall in the central Sahara to reactivate these rivers. “We found that it is the small changes in the earth’s orbit and the waxing and waning of the polar ice sheets that accelerate the alternation of humid phases with high precipitation and long periods of almost complete dryness,” explains Blanchet.

The fertile periods generally lasted five thousand years and the humidity spread over North Africa to the Mediterranean coast. For the people of the time, this led to drastic changes in living conditions, which probably led to large migration movements in North Africa. “With our work, we have added some important puzzle pieces to the picture of past changes in the landscape in the Sahara that will help us to better understand human evolution and the history of migration,” says Blanchet. “The combination of sediment data with computer simulation results was critical to understanding what controlled the past succession of wet and dry phases in North Africa. This is particularly important as this region is expected to experience severe droughts as a result of man-made climate change. “

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