An evaluation of tracer fields and anthropogenic carbon in the equatorial and the tropical North Atlantic

The transit time distribution method was applied to dichlorodifluoromethane and sulfur hexafluoride measurements from four cruises to the tropical North Atlantic between 2006 and 2009 in order to estimate anthropogenic carbon (Cant) concentrations. By assuming an Inverse Gaussian distribution of the transit time distribution the best fit to the data was achieved with the ratio of mean age to width equals 1. Significant differences in the mean age and Cant concentrations between the equatorial belt (5°S–5°N) and the Guinea dome area (5°–15°N) was found. Mean ages are higher and Cant concentrations are lower in the Guinea dome area than at same depths, or densities, in the equatorial belt. The mean column inventories in the upper 1200m are higher by about 3molm−2 in the equatorial belt compared to the Guinea dome area. The mean column inventory of Cant, for the whole water column, in the tropical Atlantic is 32.2molm−2 (error range: 30.6–45.2molm−2), which is significantly lower than the previous estimates. The total Cant inventory in the eastern tropical Atlantic is 2.5Pg (error range: 2.3–3.5Pg) for an area of 6×106km2, comprising the Guinea dome region and the equatorial belt. The equatorial belt has 40% higher storage of Cant compared to the Guinea dome area which reflects the occurrence of relatively young deep waters at the equator, being high in anthropogenic carbon. Our tracer based Cant estimates were compared to Cant concentrations calculated with the TrOCA method applied to measurements conducted in 1999. The TrOCA based estimates are significantly higher than our tracer based Cant estimates. Comparison between tracer measurements in 1999 and the 2006–2009 time-frame revealed possible speed-up of ventilation in the upper water column, increasing the Cant concentration in this depth range at a faster rate and a Cant increase of 12.1μmolkg−1 in the tropical surface water was found. ► Tracers (SF6 & CFC-12) are used to determine mean ages and Cant in the tropical North Atlantic. ► For the TTD method a Δ/Γ ratio of 1 is applicable in the tropical Atlantic. ► Differences in mean age and Cant distribution are found between the equator and the Guinea dome area. ► Cant inventories at the equator are high compared to the area north of it. ► A surface water Cant increase of ∼12μmolkg−1 in 8.5 years and changes in ventilation are found.