Tracer release experiments using sulfur hexafluoride (SF6) in the ocean have proven to be a powerful tool in the study of ocean mixing. An alternative to SF6 is desired because releases of SF6 can interfere with its very promising use as a transient tracer invading the ocean from the atmosphere. Only a small fraction of the ocean has been tainted with SF6 thus far, but in prospect is a tracer release experiment in the Southern Ocean, a region for which SF6 would be very useful as a transient tracer.
In this study, researchers at the Woods Hole Oceanographic Institution and Columbia University will evaluate a promising new tracer, trifluoromethyl sulfur pentafluoride (SF5CF3). An important advantage of SF5CF3 is that its background concentration in the ocean is virtually undetectable, while that of SF6 limits the useful duration of prospective tracer release experiments. The researchers have already found that the minimum detectable level of SF5CF3 is as low as that of SF6. A concern about the new tracer is that it may interact with particles in the water strongly enough to not follow the water as well as SF6. Preliminary laboratory measurements of the affinity of SF6 and of SF5CF3 to 1-octanol, to calcium carbonate particles, and to silica particles have shown that transport on particles in the ocean should not be detectable for either compound. On the other hand, SF6 itself seems to have fallen 3 to 10 meters per year in the ocean experiments. Hence they plan to refine and to extend their laboratory measurements. They will also conduct a field test by releasing both tracers together in Santa Monica Basin and following the distributions of the tracers for two years to determine whether SF5CF3 is a viable substitute for SF6 for tracer release experiments.
Broader Impacts: Both the study of mixing in the ocean with tracer release experiments and the study of water mass ages using transient tracers are important aspects of the overall study of the ocean circulation and of the ability of the ocean to take up the large amounts of carbon dioxide that are being released to the environment by the burning of fossil fuels. Adding a new tracer to the oceanographic toolkit would contribute significantly to technology, as was demonstrated to be the case when bomb tracers were added in the 1960s, when helium-3 and CFCs were added in the 1970s, and when SF6 was added in the 1980s. SF6 has also been used successfully in transport and dispersion studies in estuaries. The availability of a second tracer would add versatility and power to such experiments. Finally, as an educational component, the researchers will hire an undergraduate student to work each summer to participate in the cruises in Santa Monica Basin and in the laboratory work.
