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


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.

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Figure 1: Chlorophyll a images of Crozet region.
Figure 2: Time series of chlorophyll a and particulate organic carbon (POC).


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We thank the operators, master and crew of RRS Discovery for their support. CROZEX was a component of Biophysical Interactions and Controls on Export Production, a five-year project at the National Oceanography Centre, Southampton, supported by the Natural Environment Research Council (NERC).

Author Contributions R.T.P. led the project, the first cruise and the physics analysis (J.T.A., J.F.R., H.J.V). R.J.S. led the second cruise and the nutrient chemistry analysis (M.F., R.S.L., P.J.M., I.S., M.S., S.T.), P.J.S. the iron and radium chemistry analysis (A.R.B., M.A.C., G.R.F., T.D.J., F.H.N., H.P.), M.I.L. the biology analysis (C.M.M., S.F., A.E.H., R.J.H., A.J.P., S.S., R.W., M.V.Z.), D.C.E.B. the carbon dioxide chemistry analysis (M.N.), R.A.M. the sediment chemistry analysis (S.T.) and J.A.H. the benthic biology analysis (T.S.). R.T.P. wrote the paper, assisted by R.J.S., C.M.M., I.S. (sediment traps), H.F.P. (iron), P.J.M (234Th), R.A.M. (cores) and M.I.L. (biology), with all authors commenting.

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Correspondence to Richard J. Sanders.

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Data are held at the British Oceanographic Data Centre (

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Pollard, R., Salter, I., Sanders, R. et al. Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature 457, 577–580 (2009).

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