Ancient rivers reveal more greenery of the Sahara desert

Large parts of the Sahara Desert were green thousands of years ago, as evidenced by prehistoric desert engravings of giraffes, crocodiles, and a Stone Age rock painting showing humans swimming. Recently, more detailed information has been obtained from a combination of sediment cores extracted from the Mediterranean Sea and results of computational climate modeling, that an international research team, including Tobias Friedrich, an oceanographic researcher at the University of Hawaii at Mānoa, reviewed for the first time.

The layers of the seabed tell the story of major environmental changes in North Africa over the past 160,000 years. The study, co-authored by Friedrich and led by Cécile Blanchet of the German Geoscience Research Center GFZ, was published in Geoscience of nature.

Climatic context of past populations

In collaboration with the GEOMAR Helmholtz Center for Ocean Research Kiel, a team of scientists organized a research cruise to the Libyan Gulf of Sirte.

“We suspected that when the Sahara Desert was green, the currently dried up rivers would have been active and would have brought particles into the Gulf of Sirte,” Blanchet said.

The analysis of these sediments would help to better understand the time and circumstances of the reactivation of these rivers and would provide a climatic context for the development of past human populations.

Using a method called piston coring, scientists drove giant cylinders into the seabed and were able to retrieve columns of sea mud nearly 30 feet long.

The mud layers contain particles of sediment and plant remains transported from the neighboring African continent, as well as shells of microorganisms that have grown in seawater, telling the story of past climate change.

“By combining the sediment analyzes with the results of our computer simulation, we can now precisely understand the climatic processes at work to explain the drastic changes in North African environments over the past 160,000 years,” said Friedrich .

Climate change can cause migration

From previous work, it was already known that several rivers flow episodically in the region, which today is one of the driest regions on the planet. The team’s unprecedented rebuilding spans the past 160,000 years continuously. It offers a full picture of when and why there was sufficient rainfall in the central Sahara to reactivate these rivers.

“We have found that it is the slight changes in the Earth’s orbit and the rise and fall of the polar caps that have punctuated 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 spread over North Africa to the Mediterranean coast. For the people of that 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 some 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,” said Blanchet. “Combining the sediment data and computer simulation results was crucial to understanding what controlled the succession of wet and arid phases in North Africa in the past. This is especially important as this region is expected to experience severe droughts due to human-induced climate change. ”