A first study of SF6 as a transient tracer in the Southern Ocean

Halogenated transient tracers such as the chlorofluorocarbons CFC-11, CFC-12 and CFC-113 are commonly used in oceanographic studies. These compounds enter the ocean via the atmosphere and their transient atmospheric concentrations make them valuable as oceanic tracers. The trends of rapidly rising atmospheric concentrations of these tracers are however broken, and the oceanic signal becomes increasingly more difficult to decipher. There is a need for a new transient tracer to complement the existing suit of tracers, especially for recently ventilated water masses. One compound that looks promising in this respect is sulphur hexafluoride (SF6), an inert gas whose atmospheric concentration is rising rapidly. In this paper, the use of SF6 as a transient tracer in recently ventilated waters is discussed and the method for determination of SF6 in seawater is described. Tracer data from a section in the Atlantic sector of the Southern Ocean along 6°E, from 60°S to 40°S, occupied during January 1998, are presented. The Antarctic Polar Front, found close to 50°S, was studied with a densely sampled section down to 400 m depth and SF6/CFC-12 ratios are used to deduce ventilation ages and dilution factors. A comparison of apparent ages derived from a variety of tracers is presented together with the uncertainties in these estimates. This work demonstrates that SF6 is a useful and valuable transient tracer for waters ventilated during the last 20 years.