Recent history of atmospheric trace gas concentrations deduced from measurements in the deep sea: Application to sulphur hexafluoride and carbon tetrachloride

On a time scale of several decades, an increase in the atmospheric burden of certain stable trace gases results in a characteristic oceanic depth profile for the concentration of the dissolved gas. If the atmosphere is the only source of the gas to the sea, the time delay inherent in its downward penetration from the surface results in a profile which decreases with depth. By referencing to compounds such as Freon 11 or Freon 12, the atmospheric histories of which are relatively well known, limits can be placed on the increase of a trace gas whose history is unknown. The method may be particularly valuable in distinguishing the contributions of natural and anthropogenic sources of gases such as CCl₄ and CF₄, which may have both. The method is here applied to estimate the concentration of atmospheric SF₆ since 1970. Both exponential and linear fits are investigated, but the best fit is a linear increase, C = 0.34 + 0.084 (Yr-1970), where Yr is the calendar year and C is the concentration in pptv. A preliminary look at two CCl₄ profiles suggests that at least 50% of the atmospheric burden is of recent anthropogenic origin.