Thin skin: desert's fragile crust takes millennia to form but only moments to destroy - Cover Story

Science News, Jan 3, 2004 by Sid Perkins

As they barreled across the desert toward Baghdad during the U.S.-led invasion of Iraq last March, the tanks, trucks, and armored personnel carriers churned up huge clouds of dust. Iraqi vehicles mobilized to defend against the onslaught generated their own plumes of grit. In a literal sense at least, much of the dust in Iraq has begun to settle. If history is a guide, however, the ruts the combat vehicles left behind will spew prodigious amounts of dust for years. That airborne material can cause major environmental and health problems.

As the battalions maneuvered across the arid terrain, their vehicles often broke through a delicate crust known as desert pavement. This type of veneer covers as much as half of the world's arid lands. Despite the connotations of its name, desert pavement isn't robust. It's merely a thin shell of stones that lies atop the dust, soft, or sand. Multi-ton tanks easily breach these fragile mosaics, but so do dirt bikes and all-terrain vehicles. Even trudging hikers, grazing animals, and the scrabbling of rodents and birds can disrupt desert pavements, exposing subsurface material to erosion and disrupting fragile ecosystems of fungi and algae.

Damage can happen in a moment, out me processes that sculpt desert pavement typically act slowly, taking centuries to generate a surface resistant to wind erosion. Therefore, when desert pavements and their biota are wounded by human activity, it will take human action to heal them on a shorter timescale.

Scientists are currently developing large-scale experiments to determine what methods might be most effective for mending a scarred desert. Successful techniques, far from being of interest only in war-torn regions, might also address a plague of problems that could result from an increase in the recreational use of environmentally sensitive public lands in the American Southwest.

BACK TO THE FUTURE In August 1990, Iraqi troops invaded and captured Kuwait. For the next 6 months, they dug pits in the desert to hide equipment, excavated trenches to protect soldiers, and built 2-meter-tall sand berms to slow opposing troops. Military maneuvers during the liberation of Kuwait in February 1991 scarred the desert further. A comparison of satellite images taken just before and after the conflict show that almost 950 square kilometers of desert pavement were destroyed by such activity, says Farouk El-Baz, a geologist at Boston University. The planting or removal of land mines denuded another 3,500 [km.sup.2] or so. In all, the desert pavement across more than 20 percent of Kuwait's land area was disrupted.

As a result, immense quantities of the areas fine-grained soft, previously locked beneath a stony blanket, were lofted by winds. Airborne dust reduced visibility and exacerbated a variety of health problems, such as asthma and other respiratory ailments, says El-Baz. Within months, the fine sands accumulated in dune fields on the northern shore of Kuwait Bay. In western portions of the country, sheets of sand smothered some roads and farms.

All that material had been brought to the area over the cons by the Tigris and Euphrates Rivers, says El-Baz. Water flow in the rivers decreased when the region's climate changed about 5,000 years ago and wind began to winnow the river delta's dried sediments. Dust and sand in the upper layers of sediment were blown elsewhere, but gravel and pebbles were left behind and eventually compacted into a thin armor against further wind erosion.

With today's drier climate in Kuwait and southern Iraq, it would probably take millennia for the wind alone to resculpt a fully mature desert pavement over the regions damaged by this year's conflict in Iraq, says El-Baz. Restoration of the landscape to its original flat contours would probably limit erosion, minimize health problems related to dust, and decrease the amount of time needed to regenerate the war-torn desert pavement, he notes.

However, El-Baz doesn't hold much hope for the war-torn soils of Iraq. "Everybody's thinking about the security of the soldiers there, and rightly so," he notes. "Nobody's even thinking about the security of the soil."

FLOATING ROCKS Although the initial phases of a desert pavement's formation are characterized by the wind-driven removal of surface dust and sand, other factors sculpt the rocky shell.

When the surface of nascent desert pavement becomes rough enough to interfere with airflow, small, windborne particles can drop out of that slow-moving boundary layer and fall between the cracks of the pavement's stone mosaic. The trapped dust can even work its way underneath heavy pebbles, says Peter K. Haff, a geologist at Duke University in Durham, N.C. Any force that jostles surface pebbles--the impact of raindrops or the expansion and contraction of clay particles in the soil as they absorb and release moisture, for instance--can make the stones ride up on smaller particles that have recently fallen.

Even the footfalls of tiny animals can do the trick, says Haff. He once conducted an informal experiment in a terrarium on his desk, in which he placed a mixture of stones, pebbles, sand, and a few desert beetles large enough to budge marble-size stones as they propelled themselves across the ground.