Does carbon 13 track anthropogenic CO2 in the Southern Ocean?

A multiparametric linear regression technique was used for waters in the So uthern Ocean to estimate the change in both delta (13) C-DIC (Delta delta C -13(DIC)) dissolved inorganic carbon (DIC) (Delta DIC) between 1978 and 199 8, due to the accumulation of anthropogenic CO2. The observed decrease in d elta C-13(DIC) at the surface, the Suess effect, was -0.015 +/- 0.003 parts per thousand yr(-1) in the Sub-Antarctic Zone and -0.005 +/- 0.003 parts p er thousand yr(-1) in the Antarctic Zone, similar to values reported for th e southern Indian Ocean [Gruber et al, 1999; Sonnerup et al., 2000]. To com pare the Delta delta C-13(DIC) with Delta DIC, we used the ratio of these t wo anomalies (Delta RC = -Delta delta C-13(DIC)/Delta DIC parts per thousan d (mu mol kg(-1))(-1)). Along the section, Delta RC ranged from 0.015 +/- 0 .005 at 42 degreesS to 0.007 +/- 0.005 parts per thousand (mu mol kg(-1))(- l) at 54 degreesS. The spatial variability in Delta RC in the Southern Ocea n reflects different timescales for processes controlling the uptake of C-1 3 from those controlling the uptake of C-12 and indicates that Delta delta C-13(DIC) decouples from Delta DIC poleward of the Sub-Antarctic Zone. The variations of Delta RC along the section suggest that the delta C-13(DIC) a nomaly is not a good predictor of the anthropogenic CO(2)inventory in the S outhern Ocean. Some methods for determining anthropogenic. CO2 uptake both on the global and regional scale assume the penetration depths of Delta del ta C-13(DIC) to be the same as Delta DIC, which implies a constant value fo r Delta RC in the ocean at similar to0.016 parts per thousand (mu mol kg(-1 ))(-1) [Heimann and Maier-Reimer, 1996; Ortiz et al, 2000; Bauch et al, 200 0]. The use of a constant Delta RC and the observed Delta delta C-13(DIC) t o estimate anthropogenic CO2 could lead to an underestimate in the inventor y of anthropogenic CO2 for the Southern Ocean by similar to 50%.