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PODCAST: Dating Ancient Water

Image: Böhringer Friedrich
Hey Folks! I'm back from the APS April Meeting that took place in Denver, Colorado this year. While I was there I heard a talk by Zheng-Tian Lu, who is a physicist at Argonne National Lab and a professor of physics at the University of Chicago. Lu is the kind of physicist who spends most of his time inside. In fact, because he often works with lasers, many of the windows in his laboratory are sealed off (for safety—can't have stray laser beams bouncing around). But in the past year Lu has been out in the field visiting with geologists; and those geologists are sending water and ice samples to Lu and his colleagues at Argonne. Why?

Lu and his group have developed a method for dating water samples. It's called the Atom Trap Trace Analysis, or ATTA. These samples come from underground wells, pockets of isolated ocean water, and glaciers, and the ATTA method can determine how long they have been sealed off from the atmosphere. Dating these samples could reveal when glaciers formed, show the path of ocean currents, and help hydrologists figure out which underground water sources will be the best suppliers for humans.

The ATTA method determines the age of water samples using something called radiometric dating. Radiometric dating (which includes radiocarbon dating) takes advantage of a cool physics concept: certain atoms break down over time, and effectively disappear (of course nothing just vanishes; they do leave behind traces of their presence). This is called radioactive decay, and the atoms that do this are called radioactive isotopes. What's great for scientists is that the isotopes do this on a very regular schedule. So if you have a sample and you know how much of a particular radioactive isotope was in the sample when it went into the ground, and you can measure how much is in it today, then you can figure out how old the sample is. Radiometric dating is what tells scientists the age of rocks and fossils and dead bodies and even the Earth itself.

There are a number of radio active isotopes in our atmosphere, and these get into both organic and inorganic material; when those materials become isolated from the atmosphere, they stop absorbing those isotopes, so they contain a (mostly) fixed amount. Carbon is the most commonly used radioactive isotope for radiometric dating, but it isn't the only radioactive isotope in our atmosphere. The ATTA method tests for Argone 39, Krypton 81 and Krypton 85.

You'll have to listen to the podcast to hear more. Find it here or subscribe via iTunes!


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