The preindustrial atmospheric 14 CO 2 latitudinal gradient as related to exchanges among atmospheric, oceanic, and terrestrial reservoirs

A three‐dimensional global tracer transport model (derived from a general circulation model) simulates geographic variations in atmospheric Δ 14 C in response to oceanic boundary conditions. Regional atmosphere‐ocean 14 CO 2 fluxes are controlled by regional wind‐dependent gas exchange coefficients ( E ), air‐sea pCO 2 differences (ΔpCO 2 ) and oceanic 14 C/ 12 C deficiencies. We find that various preindustrial oceanic scenarios reconstructed from reasonable sets of such air‐sea variables all produce model latitudinal gradients in atmospheric Δ 14 CO 2 (the “NS Δ 14 C”) significantly greater than the measured preindustrial NS Δ 14 C of +4.4 ± 0.5‰ (45°N to 45°S), as estimated from pre‐twentieth century tree rings. The NS Δ 14 C is insensitive to regional ΔpCO 2 but is strongly contingent on the chosen values for surface ocean Δ 14 C and E of southern high latitudes (>50°S). The simultaneous seasonality in sea ice extent and high‐latitude wind speeds may in part explain the discrepancy between modeled and measured preindustrial NS Δ 14 C. Terrestrial 14 C sinks with longer turnover times (such as peatlands) also potentially may help to reduce the NS Δ 14 C. With our best estimates for preindustrial surface ocean Δ 14 C, ΔpCO 2 , and E values for other regions, we find a “best fit” ocean scenario in which the preindustrial southern surface ocean Δ 14 C is −95 ± 10 ‰, its ΔpCO 2 is 0 ± 20 ppmv, and its E is 0.088 ± 0.010 M m −2 yr −1 μatm −1 (about 20% reduced from the widely accepted value of 0.110). An independent estimate of +6.5 ± 0.5 kg yr −1 for the net preindustrial oceanic 14 C uptake is an important constraint on global mean and Antarctic E .