Transport times and anthropogenic carbon in the subpolar North Atlantic Ocean

Simultaneous measurements of chlorofluorocarbons (CFCs), tritium, and helium are used to estimate the first two moments of the surface-to-interior transit times within the subpolar North Atlantic Ocean. The observed relationships among the different tracers implies that the transit-time distributions are broad, with width Δ approximately equal to the mean age Γ , indicating that mixing plays an important role in transport over decadal timescales. These broad transit-time distributions are further shown to be consistent with the observed time variations of mid-depth tritium in the Newfoundland Basin and Northeastern Atlantic between 1972 and 1997. We use the transit-time distributions inferred from the tracers to estimate the distribution, and change over the last two decades, of anthropogenic carbon in the subpolar North Atlantic. The values obtained are smaller than previous estimates using methods that have assumed weak mixing, with largest differences occurring in the Newfoundland Basin and Northeastern Atlantic. In the case of the uptake over the last two decades our estimate is 20–30% less than estimates that neglect mixing and use pCFC ages as water mass ages.