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 gene ral circulation model) simulates geographic variations in atmospheric Delta(14)C in response to oceanic boundary conditions. Regional atmosp here-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 preindust rial oceanic scenarios reconstructed from reasonable sets of such air- sea variables all produce model latitudinal gradients in atmospheric D elta(14)CO(2) (the ''NS Delta(14)C'') significantly greater than the m easured 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 centu ry 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 D elta(14)C and E of southern high latitudes (>50 degrees S). The simult aneous seasonality in sea ice extent and high-latitude wind speeds may in part explain the discrepancy between modeled and measured preindus trial 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 De lta(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% re duced from the widely accepted value of 0.110). An independent estimat e of +6.5 +/- 0.5 kg yr(-1) for the net preindustrial oceanic C-14 upt ake is an important constraint on global mean and Antarctic E.