This conceptual illustration of the ocean conveyor belt circulation illustrates the 1,000-year long cycle. Warm, shallow water is chilled in the far North Atlantic, grows saltier and sinks. The cold, salty current flows south near the bottom, creating a northward surface layer flow of the warm, less salty water

Physicists track great ocean conveyor belt

Argonne physicists can now detect one argon-39 atom per 10 million billion atoms, the equivalent of detecting the atoms from one pint of liquid mixed into Lake Michigan.

The physicists modified the Argonne Tandem-Linac Accelerator System to detect accelerated argon-39 ions with a 1,000-fold increase in sensitivity, a procedure that has never been accomplished until now.

These superior detection methods will allow scientists to track levels of the radioactive argon-39 isotope in the Earth’s ocean circulation. Isotopes – atoms of the same element with different numbers of neutrons – such as argon-39 help scientists track the changes in ocean circulation, which is thought to play an important role in climate moderation.

Argon-39 is naturally made when cosmic rays strike the stable and more abundant argon-40 in the atmosphere. Dissolved argon-39 sinks with cold, salty water in the North Atlantic and follows a "conveyor belt" undercurrent that rises again in the Indian and Pacific Oceans. On top of this cold undercurrent is a warm surface current that flows through the East Indies and through the Indian Ocean, where it goes around Africa and up to Europe.

"If this ‘conveyor belt’ stops, then the warm water that is brought to Europe will stop," said Argonne physicist Ernst Rehm. "We have some idea that this may cause an ice age in Europe."

The phenomenon of global warming could modify the ocean’s circulation if water temperatures rise and glaciers release more of their locked-up water into northern oceans, causing a decrease in water salinity.

Half the atoms in a sample of argon-39 will decay into potassium-39 in 269 years – its "half-life." An argon-39 atom created on the surface water of the North Atlantic will not resurface for another 500 years. By this time, approximately two half-lives have passed and only a quarter of the original argon-39 will remain.

Cosmic rays do not penetrate far into the water to create additional argon-39 and argon does not react with other elements because it is chemically inert. Therefore, determining how much argon-39 is present at various water depths give scientists an accurate measure of the age of the water samples and an understanding of how fast the water is circulating.

For the last seven years, Argonne physicists have been collaborating in this experiment with former Argonne postdoctoral physicist Philippe Collon, who is now at Columbia University’s Lamont-Doherty Earth Observatory and is the study’s primary investigator, along with other researchers at the Vienna Environmental Research Accelerator in Austria, at Hebrew University in Israel and at the University of Bern in Switzerland.

Argonne physicists will begin measuring isotopic levels of the laboratory’s "elemental homonym" in Atlantic Ocean water samples taken near the South American coast next year. This project is funded by the Department of Energy’s Office of Science, Division of Nuclear Physics.

For more information, please contact Dave Jacqué.

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