Ancient discovery offers new clues in the desert star dune mystery

By | March 4, 2024

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Spectacular star dunes stand out against the vast desert vistas. The distinctive hills are among the tallest sand dunes on Earth, and their pyramid shapes are formed by a crossfire of gusts from multiple directions, creating spiraling sand ridges that spiral 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 puzzling absence from the geological record.

Now, an examination of a star dune in Erg Chebbi, a region of Morocco’s Sahara Desert, has revealed surprises about its age and growth – and suggested that ancient evidence of star dunes may have been hiding in plain sight all along has kept.

Using radar scans and analysis of grains of sand deep within the star dune, scientists mapped the mound’s internal structure. The researchers calculated that the oldest part of the dune base was formed about 13,000 years ago. But for about 8,000 years, the research team found, the star dune – which spans 700 meters and is 100 meters high – did not grow much at all. In fact, most of the growth to its current size occurred in the past thousand 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 had not suspected that star dunes could accumulate so quickly,” Andrew Goudie, professor emeritus of geography at the University of Oxford in the United Kingdom, told me in an email. CNN. 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 scans from the new research also showed that the dune was moving.

“The whole thing is migrating,” says co-author Geoff Duller, chairman of the department of geography and earth sciences at Aberystwyth University in Wales. “It moves about half a meter per year,” showing that star dunes are about as active as most other dunes, Duller told CNN.

“Knowing how fast these things are moving is very important for infrastructure in these areas,” because their migration could affect road or pipeline construction, he added.

Below the surface

The new research focused around the Erg Chebbi star dune, known as Lala Lallia, which means ‘highest sacred 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 step by step, walking across Lala Lallia and stopping every 0.5 meters to ping the dune with ground-penetrating radar, “which is hard work in shifting sands,” Bristow told CNN in an email.

Geology students from the University of London's Birkbeck College investigate the star dune at Erg Chebbi.  -Charlie Bristow

Geology students from the University of London’s Birkbeck College investigate the star dune at Erg Chebbi. -Charlie Bristow

When these radio waves bounced back to the receiver’s antenna, they produced high-resolution images that showed 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 find out when that sand was deposited. To do that, the scientists extracted tubes of sand cores from Lala Lallia by digging a shallow hole and hammering hollow tubes of metal or plastic into the dune “so that we end up with these little tubes of sand in an opaque container,” Duller said. . In the laboratory, the researchers then looked into individual sand grain crystals of quartz and feldspar to measure the ambient radiation that had accumulated over thousands of years in the dark depths of the dune.

“There is radioactivity everywhere, at very low levels,” Duller said. “Some of it is stored in the crystals.”

Exposure to daylight removes radiation from these crystal reservoirs within 10 to 30 seconds, he added. But once grains of sand are buried, radiation from the environment around them begins to build up. In Aberystwyth’s laboratory, the scientists allowed the collected grains to release their stored energy in the form of light. They then analyzed light intensity to calculate their age, a technique called optically stimulated luminescence dating. The researchers shined a light on the minerals to release trapped electrons, creating 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 grains at 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 how quickly it moved.

A mystery solved

The new findings have also addressed a long-standing mystery for geologists: Where is all the ancient evidence of star dunes?

Desert environments are typically well preserved in the geological record, and dunes leave clues to their ancient past in layers of compressed sandstone. But ancient evidence for star dunes is exceptionally rare, with the exception of one known example in Scotland dating to the Permian-Triassic (about 251.9 million years ago).

“Why is that? Where have all the star dunes gone?” Duller asked. The answer, the scientists wrote, may be a matter of perspective. Star dunes are so big; perhaps eroded parts of their preserved structures were previously identified as self-contained remains of other types of dunes, the study authors reported.

“If you look at each piece of a star dune in the geological record separately, it will look like slightly different,” Duller said. “But when you put all these pieces together — and you see these big troughs with sand crossed in the middle, you see these arms stretching out in every direction — then you can confidently say it’s a star dune.”

One possible explanation why ancient star dunes were overlooked for so long is that it was long unknown how common they were, Goudie suggested.

“The fact that star dunes have not been widely identified in the stratigraphic data may be partly because many geologists were not very aware of star dunes and only knew about longitudinal dunes and barchans (crescent-shaped dunes),” Goudie said. “Now we know, with the help of Google Earth, how widespread these features are.”

Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.

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