Oceanographic tracer release experiments using sulphur hexafluoride

We review recent results of experiments using sulfur hexafluoride tracer releases to investigate ocean mixing, gas exchange, and response to iron fertilization. A release method has been devised that allows large-scale mixing experiments to be initiated using similar to 100 kg or more of tracer accurately targeted on a given density surface. Combined with the very low detection limit of SF6, this means that these experiments can last up to several years and cover thousands of kilometers. The experiments have revealed that in the open ocean pycnocline, rates of mixing are low (0.1-0.2 cm(2) s(-1) at 300 m in the subtropical North Atlantic, for example). An approximately inverse relationship between diapycnal mixing and buoyancy frequency is suggested by most (but not all) tracer investigations to date, though this may not hold in the abyssal ocean far from any boundary. Much smaller scale releases into surface waters have been used to obtain measurements of gas exchange and to enable biogeochemical studies such as iron fertilization experiments on well-defined volumes of water. These experiments take advantage of the fact that in surface water, continuous rapid analysis of sulfur hexafluoride enables the tracer to be tracked and mapped by the observing ship. Recent gas exchange studies using the dual-tracer method are summarized; we suggest that they may be reconciled to values based on global C-14 exchange by appealing to the effect of organic films observed in coastal waters by Frew [1997]. The tracer technique makes possible biogeochemical patch studies such as the Ironex I and II experiments in the equatorial Pacific. The use of the tracer measurements to guide and normalize measurements of the effect of added iron is illustrated.