7 November 2010
Sand is a great traveller. Go to the seaside for the day and it’ll ride home on your shoes or sneak into your picnic sandwiches. You may wonder, as you shake sand from your bag on the beach: ‘where did all this sand come from and how long’s it been here?’
Dr Pieter Vermeesch and colleagues had the same question about the sand in the Namib Sand Sea – one of the world’s oldest and largest sand deserts. We know little about where sand in the Namib, or other large deserts, comes from. How long the sand has been blowing around the 34,000 km square desert was also a mystery, although we know southwest Africa has been dry for at least five million years.
The team tracked the sand’s origins back to sediments in South Africa’s Orange River using natural uranium 238 clocks. Uranium 238 – the most common type of natural uranium – turns into lead over billions of years. Older sand contains more lead. In an area containing sands of varying ages, zircon sand grains can be linked to their source by the amount of uranium and lead they contain.
The next question was how long ago the sand left the Orange River bed and blew north into the Namib Sand Sea. The team again used natural clocks to retrace the sand’s path along the Namibian coastline. This time, they looked at radioactive beryllium (Be), aluminium (Al) and neon (Ne) in sands from quartz-containing rocks.
When sand is near the Earth’s surface, it’s bombarded by cosmic radiation – energetic particles from outer space. These particles collide with atoms of Be, Al and other elements in sand grains, causing them to become unstable. When the grains become buried in dunes, they stop being bombarded and the natural clock starts running. The amount of unstable cosmogenic Be and Al declines over time – indicating how long ago the grains were buried.
Sand in sand dunes can be unearthed and buried several times, resetting the natural Be and Al clocks. To work out how many times the sand was exposed, the team compared the Be and Al to Ne. Ne is not destabilized by cosmic rays so more cosmogenic Ne is formed each time the grains are unearthed.
Dr Vermeesch and colleagues found the Be and Al clocks showed almost the same time right along the coast. This, they believe, shows the sand was rapidly transported north and inland by the wind and waves. Once the first sand arrived at its destination, it stayed buried in the Sand Sea for more than one million years.
This is the first evidence the Sand Sea’s sand is older than its inland dunes, which are thought to be a few thousand years old. The team’s next step is studying why the sand and dunes differ in age. The dune sand might have been buried and uncovered hundreds of times. Alternatively, the Sand Sea could have ‘frozen’ for thousands of years when the climate was wet.
Vermeesch, P., Fenton, C., Kober, F., Wiggs, G., Bristow, C., & Xu, S. (2010). Sand residence times of one million years in the Namib Sand Sea from cosmogenic nuclides Nature Geoscience DOI: 10.1038/NGEO985