Anthropogenic carbon accumulation in the subtropical North Atlantic

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Recent data suggest the accumulation of anthropogenic carbon dioxide (ΔCanth) in the subtropical North Atlantic is not occurring at a steady rate throughout the water column. Carbon measurements from three transatlantic cruises along 24.5°N in 1992, 1998, and 2004 were investigated for changes in Canth using both a back‐calculation shortcut technique and extended multiple linear regression. For three time periods (1992–1998, 1998–2004, and 1992–2004) we observed spatial and vertical changes in Canth storage, along with a general increase in total concentration. In the surface layers, total dissolved inorganic carbon (TCO2) and Canth concentrations increased in line with atmospheric CO2 levels: TCO2 +8.8 ± 0.5 μmol kg−1 for 1992–1998 and +8.6 ± 0.5 μmol kg−1 for 1998–2004 and Canth +8.0 ± 0.2 μmol kg−1 for 1992–1998 and +6.8 ± 0.3 μmol kg−1 for 1998–2004. In deeper waters, ΔCanth was significantly different than zero for all depths above 5000 dbar between 1992 and 2004, while on a subdecadal timescale, significant variability was observed for ΔCanth at a depth range of 800–1000 dbar. Evidence is presented for the arrival at 24.5°N at depth of freshly ventilated Labrador Sea Water from the subpolar North Atlantic between 1992 and 1998, as well as consistent smaller ΔCanth signals alongside the Mid‐Atlantic Ridge. This is in addition to low‐level, stable increases identified in the deep eastern basin between 1992 and 2004, the first time that ΔCanth has been detected and confirmed by new measurements of carbon tetrachloride and CFC‐11 from 2004. These results highlight the importance of the subtropics as a site for long‐term Canth storage away from the surface.