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 δ 13 C DIC (Δδ 13 C DIC ) and dissolved inorganic carbon (DIC) (ΔDIC) between 1978 and 1998, due to the accumulation of anthropogenic CO 2 . The observed decrease in Δ 13 C DIC at the surface, the Suess effect, was −0.015 ± 0.003‰ yr −1 in the Sub‐Antarctic Zone and −0.005 ± 0.003‰ 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 Δδ 13 C DIC with ΔDIC, we used the ratio of these two anomalies (ΔRC=–Δδ 13 C DIC /ΔDIC‰ (μmol; kg −1 ) −1 ). Along the section, ΔRC ranged from 0.015 ± 0.005 at 42°S to 0.007 ± 0.005‰ (μmol; kg −1 ) −1 at 54°S. The spatial variability in ΔRC in the Southern Ocean reflects different timescales for processes controlling the uptake of 13 C from those controlling the uptake of 12 C and indicates that Δδ 13 C DIC decouples from ΔDIC poleward of the Sub‐Antarctic Zone. The variations of ΔRC along the section suggest that the δ 13 C DIC anomaly is not a good predictor of the anthropogenic CO 2 inventory in the Southern Ocean. Some methods for determining anthropogenic CO 2 uptake both on the global and regional scale assume the penetration depths of Δδ 13 C DIC to be the same as ΔDIC, which implies a constant value for ΔRC in the ocean at ∼0.016‰ (μmol; kg −1 ) −1 [ Heimann and Maier‐Reimer, 1996; Ortiz et al., 2000; Bauch et al., 2000]. The use of a constant ΔRC and the observed Δδ 13 C DIC to estimate anthropogenic CO 2 could lead to an underestimate in the inventory of anthropogenic CO 2 for the Southern Ocean by ∼50%.