THE PREINDUSTRIAL ATMOSPHERIC (CO2)-C-14 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 Delta(14)C in response to oceanic boundary conditions. Regional atmosphere-ocean (CO2)-C-14 fluxes are controlled by regional wind-dependent gas exchange coefficients (E), air-sea pCO(2) differences (Delta pCO(2)) and oceanic C-14/C-12 deficiencies. We find that various preindustrial oceanic scenarios reconstructed from reasonable sets of such air-sea variables all produce model latitudinal gradients in atmospheric Delta(14)CO(2) (the ''NS Delta(14)C'') significantly greater than the measured preindustrial NS Delta(14)C Of +4.4 +/- 0.5 parts per thousand (45 degrees N to 45 degrees S), as estimated from pre-twentieth century tree rings. The NS Delta(14)C is insensitive to regional Delta pCO(2) but is strongly contingent on the chosen values for surface ocean Delta(14)C and E of southern high latitudes (>50 degrees 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 Delta(14)C. Terrestrial C-14 sinks with longer turnover times (such as peatlands) also potentially may help to reduce the NS Delta(14)C. With our best estimates for preindustrial surface ocean Delta(14)C, Delta pCO(2), and E values for other regions, we find a ''best fit'' ocean scenario in which the preindustrial southern surface ocean Delta(14)C is -95 +/- 10 parts per thousand, its pCO(2) is O +/- 20 ppmv, and its E is 0.088 +/- 0.010 M m(-2) yr(-1) mu atm(-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 C-14 uptake is an important constraint on global mean and Antarctic E.