The Sahara desert looms large in Africa, stretching across the northern third of the continent. The intensely arid nature of the Sahara means that wind is able to loft sediment—especially dust— into the air and carry it to all corners of the globe.
The largest warm desert in the world, the Sahara is currently hyperarid, although it has experienced periods of wetter weather in the past. The age of the desert is unknown. Researchers have tried to pin it down using various geologic records but have come up with vastly varying ages and a healthy amount of controversy.
But new research published in Palaeogeography, Palaeoclimatology, Palaeoecology has narrowed in on the age of the Sahara. Using a combination of paleosols, well-dated basalts, and geochemistry, researchers concluded that the Sahara has been an arid, dust-producing region for millions of years.
The Saharan Dust Machine
The Sahara is the largest source of dust in the world, said Daniel Muhs, a Quaternary researcher at the U.S. Geological Survey in Denver, Colo., and lead author of the new study. This dust “is a major source of soil parent material for every place that’s downwind” and has “significance in at least three major ways,” he noted.
First, soils on the nearby Canary Islands, and soils across the ocean in the Amazon, get an influx of dust from Sahara. Second, Muhs said, Saharan dust fertilizes the oceans, providing bioavailable iron to phytoplankton. Last, Saharan dust in the atmosphere can affect albedo, creating a warming or cooling effect on the planet.
While Saharan dust has played (and continues to play) a big role around the world, research has placed “quite variable” ages on the desert itself, Muhs said. Prior estimates “range all the way from [the] Holocene, sometime in the last 10,000 years, back to the Miocene, around 7 million years.”
Muhs and his colleagues wanted to narrow in on the age of the Sahara, so they turned to a stack of paleosols and basalts on the nearby Canary Islands. Muhs said that between basalt flows—previously dated to be about at least 4.8 million years and 0.4 million years old—a half dozen buried soils were stacked. This meant that the paleosols were deposited during that 4.4-million-year stretch.
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The Canarian paleosols contained a lot of dust, but where the sediment originated was unknown. The team collected samples for mineralogy and geochemistry testing to see whether the dust reflected a Saharan source.
Dust Signatures
The researchers looked at both the bulk mineralogy and the clay mineralogy of the soil samples and looked for two specific minerals: mica and quartz. “If you find mica or quartz on a basaltic island, it didn’t come from there,” said Muhs. “It came from somewhere else.”
The presence of quartz and mica meant the dust came from a continental source, but that didn’t necessarily mean Africa. However, Muhs said, “the other alternatives are North America, where the wind’s going the wrong way; Europe, where there the wind’s going the wrong way; Asia, where the wind would have to go all the way around the world to get there…. So the logical player here is Africa.”
To back up their theory of an African source for the dust, the researchers completed a geochemical survey of the sediment. They focused on robust, resistant trace elements like scandium, thorium, and lanthanum in the paleosol samples to infer the source of the material.
Previous work on African dusts in the Caribbean gave the team a great picture of what chemical signatures to look for. “We know what African dust looks like,” Muhs said.
Age of Aridity
Muhs likened the dust contributions on the Canary Islands to ingredients in a tossed salad. “We analyzed the tossed salad and figured out the flavors,” he said. “You have both red peppers and green peppers—some of them are from Africa, some of them are from local volcanics.”
This meant that dust from an arid Sahara had been accumulating on the Canary Islands for much of the Pliocene (~4.8 million years ago) and into the Pleistocene (0.4 million years ago).
“The great novelty of Muhs and his collaborators’ study is that they have analyzed a new type of archives—aeolian dusts preserved in paleosols,” said Mathieu Schuster, a researcher at the University of Strasbourg in France who was not involved in the study.
Schuster added that the researchers used the well-constrained ages of the basalts to help refine the geochronology, leading to “an elegant and robust demonstration of the antiquity of the Sahara desert.”
“If you want to tell and understand the story of the Earth, then it is critical to know the age of any rock, fossil, and geodynamical event,” said Schuster. “Here, the question is not simply to know how old…the Sahara desert [is] but, more precisely, to know since when this large part of Africa experience[d] hyperarid conditions.”
Schuster said that understanding when the Sahara first became arid allows researchers to investigate other important questions. “For example, we do not know if the onset of arid conditions was abrupt or progressive, how ecosystems have been impacted by the aridification, and how prehuman and human societies have adapted.”
Understanding past climates is crucial for understanding the future climate, said Muhs, and that’s why every Intergovernmental Panel on Climate Change report has a chapter on paleoclimate.
“Climate is on all of our minds these days,” he stated. “A lot of what we don’t understand about what we might be headed for as a future climate, we can sometimes try to learn from past climates.”
—Sarah Derouin (@Sarah_Derouin), Freelance Journalist