Sulphur hexafluoride as a tracer of biogeochemical and physical processes in an open-ocean iron fertilisation experiment

The first open-ocean experiment to test the iron hypothesis in the equatorial Pacific was undertaken using the tracer gas sulphur hexafluoride (SF6) to locate and track the fertilised surface water. Continuous surface measurements showed that the SF6 patch spread rapidly in the first 24 h, from an initial release area of ∼64 km2 to a total area of 214 km2, and remained relatively constant in size for the following three-day period. SF6 data was mapped in a Lagrangian frame of reference by the use of a drogued GPS buoy released at the centre of the patch. The SF6 patch remained coherent and exhibited a slow, anti-cyclonic oscillation during the first four days. The buoy was transported downwind of the patch in a northwesterly direction within two days, which has implications for the future use of buoys in surface-water advection studies. Following subduction below a low-salinity front 3–4 days after release, the patch centre was relocated by its SF6 signal at a depth of 25–30 m to the east of the residual surface patch. The latter spread rapidly to the southwest during the remainder of the experiment, whilst the subducted patch remained relatively stationary. Density-corrected SF6 profiles were used to calculate a mean vertical eddy diffusivity of 0.25 cm2/s across the thermocline following the subduction event. A vertical flux of nitrate of 2.5 mmol/m-2 d-1 into the mixed layer was estimated, which implied an f-ratio value of 0.4 on comparison with productivity data. The results demonstrate that SF6 is a successful tracer of water masses, and emphasise the potential of this technique for the in situ measurement and manipulation of open-ocean processes.