Sahara Desert Is at Least 4.6 Million Years Old, Ancient Dust From Vast Landscape Reveals

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Scientists have cast new light on the age of the Sahara—the largest hot desert in the world, which spans a vast swathe of the African continent. The desert's age has been a topic of contention for decades, with various groups of researchers proposing differing estimates. "People have been trying to figure it out for several decades," Daniel Muhs, a geologist with the U.S. Geological Survey in Denver, Colorado, said in a statement. "More recent studies said it was the beginning of the Pleistocene [about 2.6 million years ago.]" Others suggest that the desert could have formed even further back—around seven million years. Furthermore, other researchers still say that the desert was wet and green around 5,000 years ago, covered in swamps and lakes Now, research conducted by Muhs and his colleagues indicates that the desert is at least 4.6 million years old, contributing new evidence to the long-running debate over its age. Muhs drew his conclusions after analyzing ancient Saharan dust that had blown over to the Spanish Canary Islands—which lie off the coast of northwestern Africa. The Canaries are affected by a weather phenomenon known locally as the "Calima" which occurs every year and drags vast quantities of dust from the Sahara towards the Atlantic Ocean. On two of the islands—Fuerteventura and Gran Canaria—the scientists investigated sediments to identify and date any of this ancient dust in so-called paleosols—buried, ancient soils. "With some Spanish geologist colleagues at the University of Las Palmas, we looked for buried soils, sandwiched in between dated volcanic rock (basalt) layers," Muhs told Newsweek. "There are places on both the island of Gran Canaria and the island of Fuerteventura where buried soils like this occur, at times in between volcanic eruptions." "One thick buried soil on Gran Canaria is found between lava flows that are both about 3 million years old. On Fuerteventura, there is a series of 6 buried soils between dune sands," he said. "Below the oldest dune sand is a lava dated to about 4.8 million years old and the youngest (uppermost) one is dated to about 2.8 million years. So, all 6 of these buried soils are between 4.8 million years and 2.8 million years old." The team sampled the soils on both islands, studied them in laboratories at the U.S. Geological Survey, and found that they all contain quartz and mica. These are abundant in African dust but rare in the volcanic rocks that dominate the Canaries. "That tells us that these minerals are exotic, and the simplest way to get them to the Canary Islands is to move them by wind, in dust storms, from the nearby Sahara," Muhs said. "Some of the dust keeps moving west, but some dust is trapped by vegetation on Canary Islands and is added to the soils there." The researchers were able to approximately date when this dust may have blown over with the help of the layers of volcanic rock which contain minerals that act like geological clocks, allowing geologists to determine when lava cooled and solidified to form them. The team's findings indicate that even the oldest buried soil, perhaps just a bit younger than 4.8 million years, contains Saharan minerals. "That tells us that the Sahara is several million years old, at least, and that it has likely been fertilizing soils downwind, like the Canary Islands, for much longer than we thought," Muhs said. "This could even include adding nutrients to the nutrient-poor soils of the Amazon Rain Forest, because Saharan dust travels that far almost every year at the present time." "The same process of Saharan dust fertilization of the Amazon Rain Forest may have been going on, therefore, for several million years," he said. These results agree with data collected from deep-sea sediments that indicate increases in Saharan dust being blown over the Atlantic at least 4.6 million years ago. While this suggests that the desert is at least this old, the findings do not rule out the possibility that it formed earlier. If the team were able to find older paleosols containing Saharan dust, it would be possible to conclude that the desert is even older. Mathieu Schuster, a scientist from the University of Strasbourg, who was not involved in the research, said the latest work brings "a new type of evidence for the antiquity of the Sahara Desert." Previously, he took part in research which proposed that the desert was around 7 million years old. "The result of Professor Muhs and his team fits well with existing knowledge based on remote aeolian dust found in the marine record as well as on in situ aeolian dune deposits from the Chad Basin that my colleagues and I published some years ago." Schuster said that in the past decades, the origin of the Sahara was considered to be relatively recent and linked to the Quaternary Ice Age, which began about 2.58 million years ago. "It was then quite controversial for us to propose that the Sahara Desert was some 7 million years old," he said. "I am therefore very excited to read about the work and the results of Pr Muhs This work is significant because we now have a decent set of independent results that strongly suggest the antiquity of the Sahara Desert." This article was updated to include additional comments from Daniel Muhs and Mathieu Schuster.