Why the world’s tallest sand dunes aren’t in the geological record

A mighty Saharan sand dune has been discovered to have a 13,000-year-old base, but most of its current height has increased in the past 1,000 years. The information changes the way geologists think about star dunes, the highest category of sand dunes. Because star dunes occur not only in the deserts of Africa, Asia and North America, but also on Mars and Titan, the work could have broad applications. It also explains why geological searches in the past rarely found signs of it.

When people who have never been there picture deserts, they might think of star dunes – named for the way their arms extend outward when you view them from above. Although they cover only a small portion of the world’s deserts, they are photogenic enough to be widely filmed when directors want to convey morale-sapping journeys. Some in China are 300 meters high, possibly the largest sand dunes on earth.

It was thought it would take a long time to form something that large, but ancient records of deserts preserved as rocks reveal no clues to the star dunes’ past, aside from a Triassic example, when dinosaurs were young. Professor Geoff Duller from Aberystwyth University and Professor Charlie Bristow from Birkbeck University went to the Moroccan star dune Lala Lallia to look for answers.

Lala Lallia means ‘highest sacred point’ in Berber and is 100 meters high and about 700 meters wide. It’s located in a part of southeastern Morocco that film crews flock to when they want a menacing-looking desert.

“By using ground-penetrating radar to look inside this star dune, we were able to show how these immense dunes are formed and develop a new model to help geologists better know what to look for in the rock to identify these amazing desert features,” says Bristow. a statement. The team also used luminescence dating, which shows when certain minerals were last exposed to sunlight.

These approaches revealed a dune that formed during the Younger Dryas event, when Earth briefly plunged back into an ice age after digging its way out for thousands of years.

During the Younger Dryas, northerly winds produced the 30-meter-high base of Lala Lallia. The dune then stopped growing for at least 8,000 of the next 12,000 years. During this time, evidence of fulgurites, plants and pottery shards accumulated on the east side of the dune. This indicates that the climate was wet enough for vegetation to stabilize the dunes in the area, filling a gap in our knowledge of the area’s climate at the time.

When growth resumed, it was slow at first, but over the past 900 years Lala Lallia expanded some 70 meters and quickly (by the standards of geology) migrated westward. The arms that are the distinguishing feature of star dunes are usually younger than the main body, in one case only 15 years old.

Using this data, the team was able to map the dominant wind direction in the area at different times since shortly after the last ice age.

The pair were just as interested in what their findings could tell them about other dunes and their apparent absence as this particular example.

“This research is really about the missing sand dune – it was a mystery why we couldn’t see them in the geological record. It is only thanks to new technology that we can now discover their secrets,” said Duller. “These findings will probably surprise many people because we can see how quickly this enormous dune formed and that it is moving across the desert at a height of about 50 cm. [20 inches] a year.”

The authors think that ancient star dunes that have turned to stone have been misclassified as once linear or barchanoid dunes, which look different on the surface but are likely quite similar once buried. They think their work will help differentiate the sedimentary structures of star dunes from the remains of their counterparts, hopefully reconciling their absence from history.

The research is published open access in Scientific Reports.

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