SULFUR-HEXAFLUORIDE AND HE-3 AS SEA-WATER TRACERS - DEPLOYMENT TECHNIQUES AND CONTINUOUS UNDERWAY ANALYSIS FOR SULFUR-HEXAFLUORIDE

Methods are described for the preparation and release of sulphur hexafluoride (SF6) and He-3 to surface sea water for studies of gas exchange and as oceanographic tracers. Known amounts of the tracers (0.28 mol SF6 and 0.05 mol He-3, measured by gas chromatography with thermal conductivity detection) were dissolved in 1000 l of sea water in a gas-tight steel tank and later released at a depth of 10 m without modification of the initial He-3:SF6 ratio, by excluding all air from the tank. Techniques for deploying the tracers as a discrete patch in shallow, tidally active waters (30 +/- 2 m, S.E. North Sea) and the labelling of the patch with drifter buoys are also given. Concentrations of SF6 in the patch were mapped from a survey vessel using an analysis system that determined concentrations in surface water by purge and trap followed by gas chromatography with electron-capture detection, in a fully automated, continuous sequence of analyses with a 3-min repeat time. Data were computer-matched with the ship's navigational data and used to construct a near real-time plot of the tracer patch in a reference frame which moved with the water mass. The plot was used to aid further navigation of the ship around the patch. The results from a typical mapping exercise are presented.