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Pollard 2009: Southern Ocean deep-water carbon export enhanced by natural iron fertilization Print E-mail
Thursday, 29 January 2009 16:38


Nature 457, 577-580 (29 January 2009) | doi:10.1038/nature07716; Received 23 October 2008; Accepted 8 December 2008

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

Raymond T. Pollard1, Ian Salter1,2, Richard J. Sanders1, Mike I. Lucas3, C. Mark Moore1, Rachel A. Mills1, Peter J. Statham1, John T. Allen1, Alex R. Baker4, Dorothee C. E. Bakker4, Matthew A. Charette5, Sophie Fielding6, Gary R. Fones7, Megan French4, Anna E. Hickman8, Ross J. Holland1, J. Alan Hughes1, Timothy D. Jickells4, Richard S. Lampitt1, Paul J. Morris1, Florence H. Nédélec9, Maria Nielsdóttir1, Hélène Planquette10, Ekaterina E. Popova1, Alex J. Poulton1, Jane F. Read1, Sophie Seeyave1, Tania Smith1, Mark Stinchcombe1, Sarah Taylor1, Sandy Thomalla11, Hugh J. Venables6, Robert Williamson11 & Mike V. Zubkov1

  1. National Oceanography Centre Southampton, Natural Environment Research Council and University of Southampton, European Way, Southampton SO14 3ZH, UK
  2. Observatoire Océanologique, Avenue de Fontaulé, BP44, F-66651 Banyuls-sur-Mer, France
  3. Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa
  4. School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
  5. Department of Marine Chemistry and Geochemistry MS25, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
  6. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
  7. School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK
  8. Department of Earth and Ocean Sciences, University of Liverpool, L69 3GP, UK
  9. Laboratoire Environnement et Resources de Normandie, IFREMER, Avenue du Général de Gaulle - B.P.32, 14 520 Port-en-Bessin, France
  10. Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA
  11. Department of Oceanography, University of Cape Town, Rondebosch 7701, South Africa

Correspondence to: Richard J. Sanders1 Correspondence and requests for materials should be addressed to R.J.S. (Email:  This e-mail address is being protected from spambots. You need JavaScript enabled to view it ).

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon1, 2, 3. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified3. Here we report data from the CROZEX experiment4 in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean5. The CROZEX sequestration efficiency6 (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron7, but 77 times smaller than that of another bloom8 initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.