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|Revolutionary Namib Desert sand dune study reveals new information on dune age, structure and sheds new light on past climate change|
Article in GSA's online journal GEOLOGY this month
FOR IMMEDIATE RELEASE: May 25, 2007
Reno, Nev.—Linear sand dunes, like those found in the Namib Desert in southwestern Africa, are the most widespread type of desert sand dune around the world, and move over time as if alive—stretching farther in long lines as winds blow across the desert, piling more sand on them. Until recently, insufficient data led to challenges with determining the age of these sand dunes, as well as how they were formed. DRI's Dr. Nick Lancaster, with colleagues Dr. Geoff Duller of the University of Aberystwyth and Dr. Charlie Bristow of the University of London, used ground penetrating radar and optically stimulated luminescence dating to dig deep inside the dunes to reconstruct the sedimentary layers and to solve the mystery of their age. The results, which appear in an article in the new issue of the journal "Geology," are monumental: the dunes were younger than expected; there was firm evidence of lateral migration—or shifting sideways—of linear dunes; and they were constructed by winds from different directions than previously thought. The results have important implications for understanding how all these ancient sand dunes formed, whether they were formed long ago during the Jurassic Period (206 to 144 million years ago) such as the Navajo Sandstone, or in the last 5,000 years like those in the Namib Desert.
WHAT IS GROUND PENETRATING RADAR (GPR)? A GPR system, as shown above in the first picture, sends electromagnetic signals down below the ground that are then reflected back through the sediment. How the signals respond depends on the size of the sand and its moisture content, and sends back signals to a computer allowing researchers to create an image of the sedimentary structure in the sand dune.
WHAT IS OPTICALLY STIMULATED LUMINESCENCE (OSL) DATING? OSL dating is a method that measures the amount of light released when a grain of sand is exposed to a controlled source of light in a laboratory. Using it, researchers can determine the ages of ancient earthquakes and volcanic eruptions, or discover when climate shifts changed a forest or grassland into a sea of sand. With luminescence dating, researchers can determine the ages of many materials that are too old for the radiocarbon dating technique (older than 55,000 years) or do not contain organic material (like sand dunes). It measures the time elapsed since sand grains were last exposed to sunlight.
"Using GPR allowed us to pick out sedimentary units, or layers formed over time with different wind direction," Lancaster said. "The resulting images showed us that the winds during the Jurassic Period were different than we thought. The unique combination of the GPR with the luminescence dating study provided a clear picture of how the dunes developed over thousands of years, and even revealed a break in the migration of the dune, leading us to ask what happened with the wind regime and whether there was some significant climate change, such as increased rainfall."
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