How old is the water in your drinking glass? What about the ice cubes
floating in it? Any answer is bound to make reference to the water cycle (evaporate, rain, repeat). Still, for most practical purposes, water is both eternal and constantly replenished.
But when water flows underground or freezes into glacial ice, a clock
starts ticking from the moment it loses contact with the air. In order
to read the time, scientists must trap and count almost infinitesimally
small quantities of the radioactive isotope krypton 81. Physicist Zheng-Tian Lu
and his team at the U.S. Department of Energy’s Argonne National
Laboratory outside of Chicago have honed the technique, called Atom Trap Trace Analysis (ATTA), over more than a decade and successfully used it to map the flow of water in the Nubian Aquifer, two miles beneath the Sahara Desert.
If scientists can determine how long water has been in an underground
aquifer, they’ll know how fast it travels and how fast it can be
replenished in certain areas – critical information in desert
climates where the population depends on groundwater. Krypton 81 can
help tell water’s story. Dating glacial ice, a new frontier for the
technology, can tell scientists about the atmosphere hundreds of
thousands of years ago.
As the technology improves, radio-krypton dating is set to become
part of the scientist’s toolkit, going far beyond the range of carbon 14
dating to pinpoint the age of samples 150,000 – 1.5 million years
old. But the journey to this discovery has been slow.
“In 1999, when we first demonstrated the principle of ATTA, it would
take millions of liters of water for a krypton 81 analysis,” Lu said.
“Basically it was impossible.”
These days, sample sizes are 100 liters but Lu hopes to get it down to 1 liter within his career.
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