Anthropogenic carbon accumulation in the subtropical North Atlantic

Recent data suggest the accumulation of anthropogenic carbon dioxide (?Canth) 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 (TCO 2) 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 mmol kg-1 for 1998-2004. In deeper waters, ?Canth 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 ?Canth 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 ?Canth 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 ?Canth 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 C anth storage away from the surface.