The Sahara Desert is the largest warm desert in the world, but its age has been controversial, with estimates ranging from the Miocene epoch (23-5.3 million years ago) to the Holocene epoch (11,650 years ago – present). New research looking into dust that the Sahara blew over to the Canary Islands is providing the first direct evidence that the age of the desert matches that found in deep-sea sediments: at least 4.6 million years old.
The Sahara Desert is the world’s largest hot, non-polar desert.
Dr. Daniel Muhs, a geologist with the U.S. Geological Survey, and colleagues focused on thick layers of fine reddish-brown soil found among layers of volcanic rocks and dune sands on Fuerteventura and Gran Canaria islands.
The islands are off the west coast of North Africa, at the mouth of a spigot that seasonally pours windblown dust off of the Sahara and across the Atlantic Ocean.
The team’s goal was to find, identify, and date any layers of ancient African dust in paleosols (ancient soils).
In one coastal location studied, the authors found layers of dunes made from local shells of sea animals. In another, there were layers of lava from the volcanoes that built the islands.
Both of these geologic archives contained paleosols made of very fine-grained minerals rich in quartz and mica — minerals that do not reflect the local geology of the islands. They do, however, reflect the minerals found on the nearby African mainland.
The lava flows that sandwich the windblown fine-grained quartz and mica layers made it possible to nail down approximate ages of the Saharan dust. This is because volcanic rocks contain minerals with what are essentially isotopic clocks that start ticking when the minerals in the lava cool and solidify.
And since the layers of lava, paleosols, and other local soils are stacked chronologically with the youngest on top, the lava flows provide some boundaries of when the Sahara was dry enough to launch massive dusty storms out over the Atlantic.
In all, the geologists report eight paleosols that record African dust piling up in the Canaries between about 4.8 and 2.8 million years ago, 3.0 to 2.9 million years ago, and at about 400,000 years ago.
The oldest paleosols agree with the deep-sea cores, which put the earliest Sahara dust to the Atlantic at about 4.6 million years ago (Pliocene epoch).
“That’s not to say the Sahara is 4.6-million-years-old. That’s only as old as we could determine based on the paleosols and lavas we found,” the scientists said.
“We could take it further back in time if we can find the paleosols.”
Dr. Muhs and co-authors presented their results September 23, 2019 at the Annual Meeting of the Geological Society of America in Phoenix, Arizona.
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Daniel R. Muhs et al. 2019. The Antiquity of the Sahara Desert: New Evidence from the Mineralogy and Geochemistry of Pliocene Paleosols on the Canary Islands, Spain. GSA Annual Meeting, paper # 76-1
