We use the adjoint of a global model of coupled oceanic cycles of carbon, phosphorus and iron to comprehensively and efficiently map the sensitivity of global biological productivity and air-sea carbon fluxes to local perturbations of the atmospheric iron source sustained for a decade or more. Modeled productivity and carbon flux are found to be most sensitive to enhanced iron sources in high nitrate low chlorophyll regions. The relative response of productivity to an enhanced iron source is greatest in the Equatorial Pacific. Although surface macro-nutrients are more abundant in the Southern Ocean, nutrient utilization here is critically regulated by light limitation. Our results differ from those of previous studies which imposed depletion of surface nutrients and ignore the availability of light. However, the enhancement of oceanic carbon storage per unit increase in productivity is strongest in the high latitude oceans.