We examine the relationship between aeolian iron deposition, ocean circulation, and atmospheric CO₂ in the context of a global ocean circulation and biogeochemistry model with a coupled atmospheric reservoir of CO₂. In common with previous models we find only a small reduction of atmospheric pCO₂ in response to an enhanced aeolian iron source consistent with Last Glacial Maximum conditions. We show this to be due to a combination of limiting factors including control of deep ocean iron concentrations by complexation to an organic ligand, regional compensation in changes to export production, and the maintenance of high preformed nutrient concentrations in deep water formation regions. We also demonstrate a significant sensitivity of atmospheric pCO₂ to changes in the residual mean overturning circulation of the Southern Ocean dominated by its regulation of the accumulation of biogenic carbon in the deep ocean. Although it is not enough to explain the full drawdown of pCO₂ to glacial levels, a reduction in overturning can lead to significant reduction in atmospheric pCO₂, providing mechanistic basis for the control by “vertical mixing” inferred from box models.

Copyright 2006 by the American Geophysical Union