Star dunes: Ancient find helps scientists unravel secrets of mysterious sand structures

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Throughout sweeping desert vistas, spectacular star dunes stand out. The distinctive mounds are among the many tallest sand dunes on Earth, and their pyramid shapes are sculpted by a crossfire of winds gusting from a number of instructions, creating spiraling sand ridges that pinwheel outward from a central peak.

Whereas star dunes are generally present in sandy deserts worldwide, scientists have lengthy questioned about their formation and their puzzling absence within the geological file.

Now, an investigation of a star dune in Erg Chebbi, a area of the Sahara Desert in Morocco, has revealed surprises about its age and development — and hinted that historical proof of star dunes might have been hiding in plain sight all alongside.

Utilizing radar scans and evaluation of sand grains buried deep contained in the star dune, scientists mapped the mound’s inner construction. The researchers calculated that the oldest a part of the dune’s base fashioned round 13,000 years in the past. However for about 8,000 years, the analysis staff found, the star dune — which covers 0.4 miles (700 meters) and stands 328 ft (100 meters) tall — didn’t develop a lot in any respect. In truth, many of the development to its current dimension happened over the previous 1,000 years, rather more quickly than anticipated, researchers reported March 4 within the journal Scientific Reports.

“I discovered their outcomes very attention-grabbing as a result of, like most individuals, I had not suspected that star dunes might accumulate so rapidly,” Andrew Goudie, a  professor emeritus of geography on the College of Oxford in the UK, advised CNN in an e-mail. Goudie, who was not concerned within the new analysis, coauthored a study printed in March 2021 that analyzed world distribution of star dunes.

The brand new examine’s scans additionally revealed that the dune was on the transfer.

“The entire thing is migrating,” stated examine coauthor Geoff Duller, chair of the division of geography and Earth sciences at Aberystwyth College in Wales. “It’s transferring about half a meter per yr,” demonstrating that star dunes are about as energetic as most different dunes, Duller advised CNN.

“Understanding how briskly these items are transferring is sort of vital for infrastructure in these areas,” as their migration might have an effect on development of roads or pipelines, he added.

The brand new analysis centered across the Erg Chebbi star dune referred to as Lala Lallia, which suggests “highest sacred level” within the native Berber language. Lead examine writer Charles Bristow, a professor emeritus of sedimentology at Birkbeck Faculty within the College of London, mapped the dune with a staff of geology college students. They gathered their knowledge one step at a time, strolling over Lala Lallia and stopping each 1.6 ft (0.5 meter) to ping the dune with ground-penetrating radar, “which is tough work in shifting sands,” Bristow advised CNN in an e-mail.

When these radio waves bounced again to the receiver’s antenna, they produced high-resolution pictures displaying the shapes of various sediment layers beneath the researchers’ ft, Bristow defined.

The following step was to gather sand samples at completely different depths to seek out out when these sands have been deposited. To do this, the scientists extracted tubes of sand cores from Lala Lallia by digging a shallow pit and hammering hole pipes product of metallic or plastic into the dune “so we find yourself with these little tubes of sand inside an opaque container,” Duller stated. Within the lab, the researchers then peered inside particular person sand grain crystals of quartz and feldspar to measure environmental radiation that collected over 1000’s of years within the dune’s darkish depths.

“There’s radioactivity in all places, at very low ranges,” Duller stated. “A few of it will get saved throughout the crystals.”

Publicity to sunlight scrubs radiation from these crystal reservoirs inside 10 to 30 seconds, he added. However as soon as sand grains are buried, radiation from the atmosphere round them begins increase. Within the laboratory at Aberystwyth, the scientists made the collected grains launch their saved vitality as mild, then analyzed mild depth to calculate their age, a method known as optically stimulated luminescence courting. The researchers shined a light-weight on the minerals to free trapped electrons, producing a luminescent sign that the researchers then measured to find out how lengthy the crystals had been in darkness.

“The brighter the sunshine, the older the sediment,” Duller defined. By measuring the brightness of grains from completely different depths within the dune, the analysis staff was  capable of calculate when the construction first fashioned, when it had its largest development spurt, and its fee of motion.

The brand new findings additionally addressed a longstanding thriller for geologists: The place is all the traditional proof of star dunes?

Desert environments are usually well-preserved within the geological file, and dunes depart behind clues about their distant previous in layers of compressed sandstone. However historical proof of star dunes is exceptionally uncommon, save for one identified instance in Scotland courting to the Permian-Triassic (about 251.9 million years in the past).

“Why is that? The place have all of the star dunes gone?” Duller requested. The reply, the scientists wrote, could also be a matter of perspective. Star dunes are so large; maybe eroded components of their preserved constructions have been beforehand recognized as standalone remnants of different sorts of dunes, the examine authors reported.

“If you take a look at every bit individually of a star dune within the geological file, it’s going to seem like one thing else,” Duller stated. “However if you get all of those items collectively — and you may see these giant troughs of cross-bedded sands within the center, you’ll be able to see these arms stretching out in every path — that’s when you’ll be able to confidently say it’s a star dune.”

One doable rationalization for why historical star dunes have been ignored for thus lengthy  is that, for a very long time, it was unknown simply how frequent they have been, Goudie steered.

“The truth that star dunes haven’t been recognized very a lot within the stratigraphic file might partly be as a result of many geologists weren’t very a lot conscious of star dunes and solely knew about longitudinal dunes and barchans (crescent-shaped dunes),” Goudie stated. “Now, with the assistance of Google Earth, we all know simply how widespread these options are.”

Mindy Weisberger is a science author and media producer whose work has appeared in Reside Science, Scientific American and How It Works journal.