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Breakups maintain barchan dune fields, somehow

Two new theories try to explain how the crescent-shaped sand mountains persist

DUELING DUNE IDEAS  Barchan dunes like the ones shown in Skeleton Coast, Namibia, should grow out of control. Either calving or collisions could limit dune size, two new studies show. 

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No sand dune is an island.

Interactions between crescent-shaped, or barchan, dunes stabilize the mounds and explain how vast swarms of them can persist over time, two new studies find. But the studies don’t agree on what kind of interactions matter: One team argues that dunes colliding and breaking apart keeps barchan fields from growing into giant sand piles, while the other claims that dunes calving off each other are the key.

Even if they disagree, “both papers are providing convincing evidence that the generation of small dunes in a field is a very important ingredient to explain the structure of barchan corridors,” says Eric Parteli of the University of Erlangen-Nuremberg in Germany.  

Barchans arise in deserts where the ground is hard and flat and strong winds blow sand in one direction. Thousands of the dunes can occupy a narrow strip. Fueled by wind, the dunes travel at different paces and can traverse up to 100 meters in a year.

Scientists have struggled to explain what maintains these assemblages. “It’s unclear why barchan dunes exist at all,” says Pieter Vermeesch, a geologist at the University College London. “Physical models and field measurements unequivocally show that barchan dunes are inherently unstable.” Tiny dunes should shrink and disappear while big ones should grow infinitely large.

Mathieu Génois of Paris Diderot University and colleagues say collisions prevent dunes from growing out of control. If two barchan dunes collide they can merge into one crescent or they can split up into multiple smaller barchans. In computer simulations of crashing dunes, barchans arranged themselves into patterns found in nature — evidence that collisions control dune field behavior, the team concludes August 7 in Geophysical Research Letters.

Stacey Worman of Duke University and colleagues offer an alternative explanation August 12 in Geology. When a dune grows to about 100 meters long, it amasses enough sand on its upwind side for an incipient dune to develop. Since small dunes migrate faster than large ones, the baby dune travels down one of its parent’s horns and buds off. The team simulated calving dunes and also recreated real-world barchan patterns. The simulations included some types of collisions, but they didn’t help stabilize dune size, says coauthor Brad Murray, a geomorphologist at Duke.

Scientists have observed both collisions and calving in barchan fields. It’s possible that one process plays a more important role than the other in shaping dune swarms, says geomorphologist Christopher Hugenholtz of the University of Calgary. The only way to confirm that is with satellite images collected repeatedly over several years to see how dunes evolve. Most likely, says Serina Diniega of the Jet Propulsion Laboratory in Pasadena, Calif., the answer will lie somewhere in between these two studies.

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