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Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron^,. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO₂. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO₂ and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record …