The age of water

Radioactive dating has long been used to get information on the age of rocks, water and fossils. Each transient tracer can help to tackle a specific age range. ³⁹Ar is useful for the 50–1000 year range and is thus ideal to investigate the age of deep ocean circulations. However, extremely low concentrations of ³⁹Ar in the ocean make the dating process costly, as it requires long measurements and hundreds of litres of sample. Now, writing in Nature Communications, a group at Heidelberg University led by Werner Aeschbach and Markus Oberthaler in collaboration with the GEOMAR Helmholtz Centre for Ocean Research Kiel have reported a new method for the quantification of ³⁹Ar that, by taking advantage of applied quantum technologies, drastically reduces the sample size and enabled the team to unveil some of the history of our oceans.

Credit: Adapted from Ebser, S. et al. (2018), CC-BY-4.0

Ocean ventilation is the process by which gas molecules from the atmosphere, after exchanging with those in surface water, are transported into the deep ocean. Deep ocean water thus retains information about the atmospheric conditions from when it was at the surface. “Knowing the ventilation age allows us to calculate, for instance, the storage of anthropogenic CO₂ or the supply of oxygen in the deep water,” explains Sven Ebser, lead author of the article. Several tracers are currently used for the measurement of the ventilation age; for example, chlorofluorocarbons and sulfur hexafluoride enable studies of recent history (up to 70 years), while the age of ancient deep waters (~1000 years) can be estimated from ¹⁴C measurements. ³⁹Ar has a half-life of 269 years, making it particularly well suited for the study of the dynamics involved in ocean ventilation.

The first attempts to use ³⁹Ar to study ocean ventilation date back to the early 1980s and were complicated by the very low concentration of ³⁹Ar in deep water. “In modern air or ocean surface water there is only one atom of ³⁹Ar in about one quadrillion (³⁹Ar/Ar = 10⁻¹⁵) stable Ar atoms. In the older deep water, there is even less ³⁹Ar,” remarks Ebser. Because of the low concentration and long half-life of ³⁹Ar, samples of ~1000 litres and several weeks of measurement were needed. The team proposes a new method — argon trap trace analysis (ArTTA) — that reduces the sample size to 5 litres and measurement time to 24 hours, and enables the detection of rare isotopes down to 10⁻¹⁶. The method uses a laser to induce fluorescence in a particular radioisotope. The difference in mass number and nuclear spin of the different isotopes result in a shift of the optical resonance frequency that enables the detection of different nuclides. “In our approach, the atoms are slowed down, trapped inside a magneto-optical trap and single ³⁹Ar atoms are counted selectively,” explains Ebser. “The millions of resonant interactions between the atoms and the laser light required for trapping and detection increase substantially the signal-to-noise ratio, making the method extremely selective for the targeted ³⁹Ar atoms.”

The new measurements revealed that deep waters in the eastern tropical North Atlantic are less strongly mixed than previously reported. This then enabled a calculation of CO₂ uptake in these deep waters, which was found to be higher than expected. “This is a perfect example of how fundamental research in the field of atomic physics can lead to unexpected new insights in other fields, which might not seem to be related,” points out Oberthaler.

This is a perfect example of how fundamental research in the field of atomic physics can lead to unexpected new insights in other fields, which might not seem to be related

The new method proposed by Aeschbach, Oberthaler and co-workers can also be applied to the study of lakes, groundwater, freshwater reservoirs and glacier ice as a climate archive. “With these new capabilities to measure ³⁹Ar from small volume samples, we will ultimately be able to generate a global ocean ³⁹Ar data set, which will enable a much more accurate study of the ventilation of the oceans than was previously possible. Ventilation is important, because it also transports dissolved substances, such as oxygen or anthropogenic CO₂, into the deep ocean,” says Aeschbach. Toste Tanhua, a leading researcher from GEOMAR and co-author, concludes “I am sure that a global oceanic ³⁹Ar data set will lead to completely new insights into ocean circulation and the transport of the greenhouse gas CO₂ into the deep ocean.”