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


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.

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Figure 1: Location of the study sites, temporal evolution of the bloom and surface water properties.
Figure 2: Iron fertilization above the plateau.
Figure 3: Carbon export at A3 and C11.


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We thank the captain and the crew of the RV Marion Dufresne. This work was supported by the Institut National des Sciences de L’Univers (INSU) and the Centre National de la Recherche Scientifique (CNRS), l’Institut Paul Emile Victor (IPEV), French-Australian Science and Technology (FAST), the Australian Commonwealth Cooperative Research Centre programme through the Antarctic Climate and Ecosystem CRC, and Belgian Science Policy (BELSPO). The project benefited from collaboration with N. Metzl, leader of Ocean Indien Service d’Observation (OISO) supported by INSU, IPEV and Institut Pierre Simon Laplace (IPSL). We acknowledge the contributions of V. Barthaux, P. Catala, J. Caparros and J. Raz (technical assistance), and M.P. Jouandet (computation of the seasonal carbon budget).

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Correspondence to Stéphane Blain.

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Supplementary Information

This file contains Supplementary Discussion which includes description of the ecosystem structure; Supplementary Table 1 illustrating uncertainties of the basics terms of the DFe and Carbon budgets; Supplementary Table 2 with carbon biomass of major components of the plankton community; Supplementary Table 3 showing contribution of major diatom species at A3 and C11; Supplementary Figure 1 illustrating mean vertical profiles of nitrate, silicic acid and phosphate at A3 and C11 and additional references. (PDF 317 kb)

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Blain, S., Quéguiner, B., Armand, L. et al. Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446, 1070–1074 (2007). https://doi.org/10.1038/nature05700

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