Applying the new TrOCA approach to assess the distribution of anthropogenic CO2 in the Atlantic Ocean

Based on the recently proposed composite tracer ‘TrOCA’, a novel and independent approach is developed to estimate the concentration of anthropogenic CO2 in marine systems. It is shown that the precision of this simple model can be as high as 3 to 5.9 μmol kg−1. Several assumptions currently used in some previous back-calculation methods become irrelevant by using the new tracer TrOCA (e.g. constant air–sea disequilibrium, negligible diapycnal mixing, and/or ventilation age of water masses deduced from anthropogenic tracers). The model is then applied throughout the Atlantic Ocean to estimate the concentrations of anthropogenic CO2, using only the four properties TCO2 (dissolved inorganic carbon), TA (total alkalinity), O2 (oxygen), and θ (potential temperature), all available from the CARINA database. The results, which are computed for the 1980s and the 1990s, are in good agreement with the distribution of three independent anthropogenic tracers (Δ14C, 3H, and CFC-11). The anthropogenic CO2 inventory within the Atlantic Ocean (80°N–50°S) reaches 40 gigatons of carbon (Gt C) in the 1980s, and it increases up to 45 Gt C during the 1990s. From one decade to another, the largest increase of inventory occurs in the southern Atlantic Ocean (33.6%), whereas in the northern basin it reaches only 4.3%. The southern Atlantic Ocean is currently playing an important role in sequestrating a large fraction of the excess CO2. The Atlantic Ocean is storing anthropogenic CO2 at a net rate of 0.66 Gt C year−1. Extrapolating this estimate to the world ocean, a total of 1.97 Gt C year−1 could have been effectively stored in the ocean interior over the period 1984–1995.