1. Laboratoire d'Océanographie et de Biogéochimie, Centre Océanologique de Marseille, CNRS, Université de la Méditerranée, campus de Luminy, case 901, 13288 Marseille Cedex 09, France
2. IPSL/Laboratoire des Sciences du Climat et de l'Environnement, CEN de Saclay, Bât. 701 l'Orme des Merisiers, 91191 Gif-sur-Yvette, France
3. Antarctic Climate and Ecosystems CRC, Hobart, Tasmania 7001, Australia
4. ACROSS, School of Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia
5. LOCEAN-IPSL, UMR 7159, CNRS, Université P. et M. Curie, Case 100, 4 place Jussieu, 75252 Paris Cedex 5, France
6. Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, The Netherlands
7. FRE ELICO, Université du Littoral Côte d'Opale, Maison de la Recherche en Environnement Naturel (MREN), 32 avenue Foch, 62930 Wimereux, France
8. USM402/LOCEAN, Département des Milieux et Peuplements Marins, Muséum National d'Histoire Naturelle, 43 rue Cuvier, F-75231 Paris Cedex 05, France
9. Centre Océanologique de Marseille, Campus de Luminy, 13288 Marseille Cedex 09, France
10. CSIRO Division of Marine and Atmospheric Research, GPO Box 1538, Hobart, Tasmania 7001, Australia
11. LMGEM UMR CNRS 617, Campus de Luminy, case 901, 13288 Marseille Cedex 09, France
12. DT INSU CNRS, Bât. IPEV BP 74, Technopole Brest Iroise, 29280 Plouzané, France
13. Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
14. LEGOS (CNRS/CNES/IRD/UPS), Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France
15. Laboratoire d'Océanographie de Villefranche, Quai de La Darse, BP 8, 06238 Villefranche-sur-Mer, France
16. Université Pierre et Marie Curie-Paris 6, UMR7621, CNRS, F66650 Banyuls-sur-Mer, France
17. UMR 6539/LEMAR/IUEM, Technopole Brest Iroise, Place Nicolas Copernic, 29280 Plouzané, France
18. Observatoire Aquitain des Sciences de l'Univers, UMR CNRS 5805 EPOC, Station Marine d'Arcachon, 2 rue du Pr. Jolyet, 33120 Arcachon, France
19. Laboratoire de Géochimie des Eaux, UMP IPGP 7154, Université Denis Diderot Paris 7, 2 place Jussieu, 75251 Paris Cedex 05, France

Correspondence to: Stéphane Blain¹ Correspondence and requests for materials should be addressed to S.B. (Email: stephane.blain@univmed.fr).

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 cycles^{1, 2, 3, 4, 5}. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments^{6, 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 palaeoclimatic^{9, 10} and future climate change scenarios¹¹—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

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