Response of the Southern Ocean biological carbon pump to climate-related changes in iron supply

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The Southern Ocean (SO) plays a fundamental role in regulating the oceanic carbon uptake and thus the Earth's climate. Higher dust deposition during glacial periods was hypothesised to have alleviated iron limitation in the SO and contributed to the drawdown of atmospheric CO2. Observations of natural iron fertilisation events in the SO and modern artificial ocean iron fertilisation experiments (OIFs) confirmed the positive response of phytoplankton growth to iron addition. It is however debatable if such an increase of primary production leads to an increase of carbon export and storage, and how the response of the marine carbon pump to changes in iron supply is affected by climate change. EIFEX (the European Iron Fertilisation Experiment, carried out in 2004 in the SO) is the only OIF during which a significant increase in carbon export in the fertilised waters was detected. This increased export flux was attributed to the formation of aggregates by diatom species and mass sinking during the bloom termination. The previous modelling study however, failed to reproduce the observed changes in carbon export, indicating that the model parameterisations of iron supply and utilisation, particle dynamics and of processes controlling phytoplankton bloom evolution need to be revised. Further, a significant contribution to carbon export in naturally fertilised regions is made by diatom resting spores, which is so far completely ignored in global modelling of the marine carbon cycle. The proposed project will determine the key drivers of the SO biological carbon pump (e.g. phytoplankton community composition, particle and nutrient dynamics) based on EIFEX observations during EIFEX and of natural iron fertilisation events, improve the model representation of these key drivers, reassess the impact of iron fertilisation in the SO on carbon sequestration and predict responses of marine carbon cycle to future changes in iron supply.