The growth of microalgae in seawater is an important factor in determining climate as it converts atmospheric carbon dioxide into organic material that can be transported to and stored in the deep sea. This process is inhibited in the Southern Ocean by a lack of iron; this ocean is virtually a biological desert. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. Now analysis of data from the 'KEOPS I' cruise of the research vessel Marion Dufresne has provided insights into carbon cycling from an 'oasis' in that desert — a phytoplankton bloom induced by iron supplied from the waters below. The efficiency of this natural fertilization is at least ten times higher than estimates from short-term experiments. This suggests that natural changes in iron supply to the ocean surface due to changes in upwelling may have a greater effect on atmospheric carbon dioxide than was thought. A study of a phytoplankton bloom in the Southern Ocean induced by the supply of iron from deep waters below finds that the efficiency of fertilization (the ratio of carbon exported to the ocean interior to the amount of iron supplied) is at least ten times higher than estimates from short-term experiments. This suggests that changes in the supply of iron from deep water to the surface ocean may have a greater effect on atmospheric carbon dioxide concentrations than previously thought. The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles1,2,3,4,5. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments6,7. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales8. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments7. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic9,10 and future climate change scenarios11—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean