Use of SF6 to estimate anthropogenic CO2 in the upper ocean

Abstract

The highest concentrations of anthropogenic carbon (Cant) are found in the upper layers of the world ocean. However, this is where seasonal variability of inorganic carbon and related parameters due to thermal and biological effects complicates use of back‐calculation approaches for Cant. Tracer based approaches to Cant estimation are unaffected by biological variability and have found wide application. However, slow‐down, even reversal, of the atmospheric growth of chlorofluorocarbons (CFCs) restricts use of these tracers for Cant estimation for waters ventilated since the mid 1990s. Here we apply SF6, a tracer that continues to increase in the atmosphere, as a basis for the Cant estimation, using samples collected in the midlatitude North Atlantic in 2004. Cant estimates derived from water mass transit time distributions (TTDs) calculated with SF6 are compared to those based on CFC‐12. For recently ventilated waters (pCFC‐12 > ∼450 ppt), the uncertainty of SF6 based estimates of Cant is ∼6 μmol kg−1 less than that of CFC‐12 based estimates. CFC‐12 based estimates remain more reliable for older (deeper) water masses, as a result of the longer input history and more readily detectable concentrations of CFC‐12. Historical data suggest that the near‐surface saturation of CFC‐12 has increased over time, in inverse proportion to its atmospheric growth rate. Use of a time‐dependent saturation of CFC‐12 in TTD calculations appears to provide more reliable estimation of Cant.