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Magnificent star dunes stand out among the breathtaking desert landscapes. The distinctive dunes are among the tallest dunes on Earth, and their pyramid shapes are shaped by the crossfire of winds blowing from multiple directions, creating spiral sand ridges that pinwheel outward from a central peak.
Although star dunes are commonly found in sandy deserts around the world, scientists have long wondered about their formation and their surprising absence from the geological record.
Now, an investigation of a star dune in Erg Chebbi, a region of the Sahara Desert in Morocco, has revealed surprises about its age and growth, and hints that ancient evidence of star dunes may have been hiding in plain sight all along.
Using radar scans and analysis of sand grains buried deep within the star dune, scientists mapped the internal structure of the mound. The researchers calculated that the oldest part of the dune’s base formed about 13,000 years ago. But the research team found that for about 8,000 years, the star dune, which covers 0.4 miles (700 meters) and has a height of 328 feet (100 meters), did not grow much. In fact, most of the growth to its current size occurred in the last 1,000 years, much faster than expected, researchers reported March 4 in the journal Scientific Reports.
“I found their results 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 revealed that the dune was in motion.
“Everything migrates,” said study co-author Geoff Duller, head of the department of geography and Earth sciences at Aberystwyth University in Wales. “It moves about half a meter per year,” Duller told CNN, indicating that star dunes are as active as most other dunes.
“Knowing how fast these things move is pretty important for infrastructure in these areas,” he added, because their migration could affect the construction of roads or pipelines.
beneath the surface
The new research centered around the Erg Chebbi star dune, known as Lala Lallia, meaning “highest sacred spot” in the local Berber language. The study’s lead author, Charles Bristow, professor of sedimentology at Birkbeck College, University of London, mapped the dune with a team of geology students. They collected their data step by step, walking over Lala Lallia and stopping every 1.6 feet (0.5 meters) to ping the dune with ground-penetrating radar, Bristow told CNN in an email. “This is a difficult task to shift sand,” he said.
Bristow explained that when these radio waves are reflected back to the receiver’s antenna, they produce high-resolution images that show the shapes of the different layers of sediment beneath the researchers’ feet.
The next step was to collect sand samples from different depths to find out when these sands were deposited. To do this, scientists extracted sand core tubes from Lala Lallia by digging a shallow hole and driving hollow tubes made of metal or plastic into the dune, “so we found these little sand tubes in an opaque container,” Duller said. . In the laboratory, researchers then looked inside sand grain crystals made of quartz and feldspar to measure environmental radiation accumulated over thousands of years in the dune’s dark depths.
“There are very low levels of radioactivity everywhere,” Duller said. “Some are hiding in crystals.”
He added that exposure to sunlight clears radiation from these crystalline reservoirs within 10 to 30 seconds. However, once the sand grains are buried, radiation from the surrounding environment begins to accumulate. In the laboratory in Aberystwyth, scientists caused the collected grains to release their stored energy as light, then analyzed the light intensity to calculate their age using a technique called optically stimulated luminescence dating. The researchers shined light on the minerals to release trapped electrons, producing a bright signal that the researchers then measured to determine how long the crystals remained in the dark.
“The brighter the light, the older the sediment,” Duller explained. By measuring the brightness of the grains at different depths of the dune, the research team was able to calculate when the structure first formed, when it experienced its biggest growth spurt, and the speed 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 are generally well preserved in the geological record, and dunes leave behind clues to their distant past in the form of layers of compressed sandstone. But ancient evidence of star dunes is extremely rare; Except for one known specimen in Scotland dated to the Permian-Triassic period (about 251.9 million years ago).
“Why? Where did all the star dunes go?” Duller asked. The answer, the scientists wrote, may be a matter of perspective. Star dunes are so large that eroded portions of perhaps preserved structures have previously been identified as independent remnants of other dune types, the study authors reported.
“When you look at each piece of a star dune in the geological record individually, it will look like something else,” Duller said. “But when you put all these pieces together – and you can see these big pits of cross-layered sand in the middle, branches extending in all directions – then you can say with confidence that this is a star dune.”
Goudie suggested that one possible explanation for why ancient star dunes were overlooked for so long was that it was not known for a long time how widespread they were.
“The fact that star dunes are not very well described in the stratigraphic record may be due in part to the fact that many geologists do not know much about star dunes and only know about longitudinal dunes and barchans (crescent-shaped dunes),” Goudie said. “We now know, with the help of Google Earth, how common 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 magazines.
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