Does carbon 13 track anthropogenic CO2 in the Southern Ocean?

A multiparametric linear regression technique was used for waters in the Southern 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 1998, due to the accumulation of anthropogenic CO2. The observed decrease in delta 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 per thousand yr(-1) in the Antarctic Zone, similar to values reported for the southern Indian Ocean [Gruber et al, 1999; Sonnerup et al., 2000]. To compare the Delta delta C-13(DIC) with Delta DIC, we used the ratio of these two anomalies (Delta RC = -Delta delta C-13(DIC)/Delta DIC parts per thousand (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 Ocean reflects different timescales for processes controlling the uptake of C-13 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) anomaly is not a good predictor of the anthropogenic CO(2)inventory in the Southern Ocean. Some methods for determining anthropogenic. CO2 uptake both on the global and regional scale assume the penetration depths of Delta delta C-13(DIC) to be the same as Delta DIC, which implies a constant value for 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, 2000]. The use of a constant Delta RC and the observed Delta delta C-13(DIC) to estimate anthropogenic CO2 could lead to an underestimate in the inventory of anthropogenic CO2 for the Southern Ocean by similar to 50%.