Fig 1.
Disequilibrium schematic in Southern Ocean.
In an ocean without biology (A) upwelling of North Atlantic Deep Water (NADW) occurs in the Antarctic divergence. Subsequent flow to the south leads to heat loss to the atmosphere (red wiggly line). This increases its solubility, which causes uptake of carbon (green wiggly line), and a surface flux that tends to increase δ13C (orange wiggly line). However, due to incomplete air-sea gas exchange before waters sink to depths as Antarctic Bottom Water (AABW) negative disequilibria remain for both carbon and δ13C. Soft-tissue biology (B), conversely, causes upwelling of respired carbon that is depleted in δ13C, which leads to outgassing of carbon, but a surface flux that tends to increase δ13C. This results in a positive disequilibrium for carbon, but a negative disequilibrium for δ13C.
Table 1.
Global carbon budget and temperatures.
CA, CO = DIC + DOC, and CL are the atmospheric, ocean, and land carbon contents, respectively, and C = CA + CO + CL is their sum. SAT is surface air temperature and TO is whole ocean temperature.
Fig 2.
Components of global ocean C (panel A, C) in units of Exagram (1Eg = 1018g = 1,000 Pg) and δ13C (panels B, D) in units of permille. Panels A & B show absolute values from the PI (red colums) and LGM (blue columns). Panels C & D show LGM-PI differences. In panel A, the left axis applies to DIC and Csat,phys, whereas the right axis applies to the other components. X-symbols in the leftmost columns indicate the sum of the individual components; compared to the total, they indicate the accuracy of the method. Plus-symbols represent AOU approximations. Other symbols indicate observation-based estimates [1,52–54]. We have corrected the LGM-PI change in δ13CDIC from [52] (-0.32 ‰) by subtracting 0.05 ‰ due to a decrease in global mean [CO3] by 20.5 μmol/kg in the model using the regression slope of -0.0026 ‰/(μmol/kg) estimated by [34].
Fig 3.
Zonally averaged surface values of DIC concentrations (left) and δ13CDIC (right) and their decompositions for the PI (red) and the LGM (blue). Observations (square symbols) of PI DIC and δ13CDIC are from (A) GLODAP2.2021 [55,56] and (B) Kwon et al. [54], respectively, corrected for anthropogenic carbon. Legend in panels (A) and (C) also apply to panels (B) and (D), respectively.
Fig 4.
Horizontally averaged profiles of DIC (left) and δ13CDIC (right) and their decomposition in the PI (red) and LGM (blue) runs. Top panels show DIC (thick solid) and Cpre (dashed). The difference is Creg = DIC–Cpre. Thin lines show the sum of the components for comparison with DIC. Observations (square symbols) are from GLODAP2.2021 [55,56] and Kwon et al. [54] corrected for anthropogenic carbon. Panels C & D show physical components Cphys and Δ13Cphys (solid), and Csat,phys and Δ13Csat,phys (dashed), respectively. The difference is the physical disequilibrium e.g. Cdis,phys = Csat,phys−Cphys. Panels E & F show the biological components.
Table 2.
Global Components for C (Pg) and Δ13C (‰).
Fig 5.
Horizontally-averaged profiles of δ13CDIC and its decomposition in the Atlantic (A,B,C), Southern Ocean (south of 40°S; D,E,F), and Pacific and Indian (G,H,I). Panels A,D&G show absolute values for the PI (red) and LGM (blue). Thick solid lines are model results. Observation-based estimates are shown as thick red dotted [54], tick red dashed [22], thick blue dashed [52] corrected according to calibration LA1 of Schmittner et al. [34], and thin blue dashed line are the uncorrected data from [52]. Panels B,E&H show LGM-PI differences. Panels C,F&I show the three dominant terms in the δ13CDIC decomposition for the LGM-PI. The thick solid purple line is the same as in panels B,E&H. The thin purple line is the sum of the three components (Δ13Csat,phys, Δ13Csoft, Δ13Cdis,bio).
Fig 6.
Modelled distribution of Δ13Cdis,bio (‰) in the PI (A,D,G), LGM (B,E,H) and the difference (LGM-PI; C,F,I) at the surface (A,B,C) and zonally-averaged in the Atlantic (D,E,F) and Pacific and Indian (G,H,I).