Impacts of iron control on phytoplankton production in the modern and glacial Southern Ocean

Paleoceanographic evidence points to the Southern Ocean as a strong sink for atmospheric CO2 during the Last Glacial Maximum (LGM), but no consensus about the responsible mechanism has yet been reached. Martin (Paleoceanography 5 (1990) 1) postulated that greater iron input during the LGM could have stimulated phytoplankton to consume the surface nutrients in the Southern Ocean, increasing carbon export substantially. We use a simple ecological model to elucidate the extent to which iron availability affects export production in the southwest Pacific sector. The model includes the effect of iron in a semi-explicit way. Based on the physiological response of the photosynthetic apparatus, the simulated phytoplankton growth rates are explicitly dependent on iron. The cycling of iron in the food web is not tracked, since uncertainties persist about the dynamics of iron uptake, transformation and release processes within the pelagic community. A simulation of the present ocean that uses the semi-explicit approach to include iron is compared with a simulation that accounts for iron only implicitly by using model parameters that are typical for low iron conditions. The simulations agree within the range of available observations. Glacial scenarios are simulated (assuming an increase in iron supply) and compared to the modern ocean simulation. Primary and export production increase in the glacial simulations, in particular if we assume an adaptation of the Si:N cell quota of diatoms to the higher iron levels. In this case the export doubles north of the Polar Front and in the Seasonal Ice Zone.