Oceanographic tracer release experiments using sulphur hexafluoride

We review recent results of experiments using sulfur hexafluoride tracer re leases to investigate ocean mixing, gas exchange, and response to iron fert ilization. A release method has been devised that allows large-scale mixing experiments to be initiated using similar to 100 kg or more of tracer accu rately targeted on a given density surface. Combined with the very low dete ction limit of SF6, this means that these experiments can last up to severa l years and cover thousands of kilometers. The experiments have revealed th at 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 approximate ly 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 ex periments on well-defined volumes of water. These experiments take advantag e of the fact that in surface water, continuous rapid analysis of sulfur he xafluoride enables the tracer to be tracked and mapped by the observing shi p. Recent gas exchange studies using the "dual-tracer" method are summarize d; we suggest that they may be reconciled to values based on global C-14 ex change by appealing to the effect of organic films observed in coastal wate rs by Frew [1997]. The tracer technique makes possible biogeochemical "patc h" studies such as the Ironex I and II experiments in the equatorial Pacifi c. The use of the tracer measurements to guide and normalize measurements o f the effect of added iron is illustrated.