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Across the vast desert vistas, striking star dunes stand out. The distinctive mounds are among the tallest dunes in the world, and their pyramidal shapes are sculpted by a crossfire of wind blowing from different directions, creating spiral ridges of sand that radiate from a central peak.
Although star dunes are commonly found in sandy deserts around the world, scientists have long puzzled over their formation and their puzzling absence in the geologic record.
Now, an investigation of star dunes in Erg Chebbi, a region of the Sahara Desert in Morocco, has revealed surprising facts about their age and growth – and revealed that the star dunes’ ancient evidence may be hidden in plain sight.
Using radar scanning and analysis of sand grains buried deep within the star dune, the scientists mapped the internal structure of the dune. The researchers calculated that the oldest part of the base of the dunes was formed around 13,000 years ago. But for about 8,000 years, the research team found, the star dune — which covers 0.4 miles (700 meters) and stands 328 feet (100 meters) tall — didn’t grow much at all. In fact, most of the growth to its present size occurred in the past 1,000 years, much faster than expected, researchers reported March 4 in the journal Scientific Reports.
“Their results were very interesting because, like most people, I did not suspect that star dunes could accumulate so quickly,” Andrew Goudie, professor emeritus of geography at the University of Oxford in the United Kingdom, told CNN in an email. Goudie, who was not involved in the new research, co-authored a study published in March 2021 that analyzed the global distribution of star dunes.
The new study’s scans also showed that the dune was moving.
“The whole thing is migrating,” said study author Geoff Duller, chair of the Department of Geography and Earth Sciences at Aberystwyth University in Wales. “It’s moving about half a meter a year,” indicating that the star dunes are as active as most of the other dunes, Duller told CNN.
“It’s really important for the infrastructure in these areas to know how fast these things are moving,” because their migration could affect the construction of roads or pipelines, he said.
Beneath the surface
The new research focused on the Erg Chebbi star dunes known as Lala Lallia, which means the “highest holy point” in the local Berber language. Lead study author Charles Bristow, emeritus professor of sedimentology at Birkbeck College, University of London, mapped the dune with a team of geology students. They collected their data one step at a time, walking over Lala Lallia and stopping every 1.6 feet (0.5 meters) to ping the dune with ground-penetrating radar, “moving sand is hard work,” Bristow told CNN in an email.
When these radio waves bounced back to the receiver’s antenna, they produced high-resolution images showing the shapes of different layers of sediment beneath the researchers’ feet, Bristow explained.
The next step was to collect sand samples at different depths to determine when those sands were deposited. To do so, the scientists extracted tubes of sand cores from Lala Lallia by digging a shallow pit and twisting hollow pipes made of metal or plastic into the dune “so these little tubes end up inside an opaque container,” Duller said. . In the laboratory, the researchers then looked inside individual crystals of quartz and feldspar sand grains to measure environmental radiation that had accumulated over thousands of years in the dark depths of the dunes.
“There is radioactivity everywhere, at very low levels,” Duller said. “Some of it is stored within the crystals.”
Exposure to daylight scrubbed radiation from these crystal reservoirs within 10 to 30 seconds, he said. But when grains of sand are buried, the radiation from the environment around them starts to increase. In the laboratory at Aberystwyth, the scientists made the collected grains release their stored energy as light, then analyzed the intensity of the light to calculate their age, a technique known as optically induced luminescence dating. The researchers shone light on the minerals to free trapped electrons, producing a luminescent signal that the researchers then measured to determine how long the crystals had been in the dark.
“The brighter the light, the older the sediment,” Duller explained. By measuring the brightness of the grains from different depths in the dune, the research team was able to calculate when the structure first formed, when it had its biggest growth spurt, and its rate of movement.
A mystery solved
The new findings also addressed a long-standing mystery for geologists: Where is all the ancient evidence of star dunes?
Desert environments tend to be well-preserved in the geological record, and dunes leave clues to their distant past in layers of compressed sandstone. But ancient evidence of star dunes is extremely rare, except for one known example in Scotland that dates back to the Permian-Triassic (about 251.9 million years ago).
“Why is that? Where did all the star dunes go?” Duller asked. The answer, the scientists wrote, could be a matter of perspective. Star dunes are so big; parts of their previously eroded preserved structures may have been identified as independent remnants of other dune types, the study authors reported.
“When you look at each individual piece of star dunes in the geologic record, it’s going to look like something else,” Duller said. “But when you get all these pieces together – and see these big cross-bed sand troughs in the middle, you can see these arms reaching out in all directions – that’s when you can tell with confidence that it is a star dune.”
One possible explanation for why ancient star dunes have been overlooked for so long is that it wasn’t known, for a long time, how common they were, Goudie suggested.
“It could be that the reason that few geologists were recognized in the stratigraphic record was only to an extent that few geologists knew about star dunes and only knew about longitudinal dunes and barchan (crescent dunes),” Goudie said. “Now, with the help of Google Earth, we know how widespread these features are.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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