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Deep carbon export from a Southern Ocean iron-fertilized diatom bloom


Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

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Figure 1: Experimental eddy and the fertilized patch.
Figure 2: Temporal evolution of chlorophyll and silicate concentrations.
Figure 3: Temporal evolution of dissolved and particulate elements.
Figure 4: Temporal evolution of particulate organic carbon stocks in successive depth layers.
Figure 5: Depth-integrated particulate organic carbon stocks.

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We thank C. Balt, K. Loquay, S. Mkatshwa, H. Prandke, H. Rohr, M. Thomas and I. Vöge for help on board. We are also grateful to U. Struck for POC and PON analyses. The altimeter products were produced by Ssalto/Duacs and distributed by Aviso with support from Cnes. We thank the captain and crew of RV Polarstern (cruise ANT XXI/3) for support throughout the cruise.

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Authors and Affiliations



V.S. and C.K. wrote the manuscript. V.S. directed the experiment and C.K. carried out the budget calculations. V.H.S., P.A., M.M. and D.W.-G. contributed to the preparation of the manuscript. V.H.S., B.C., H.L. and M.L. contributed physical data on mixed-layer depth dynamics, eddy coherence, patch movement and transmissometer data. N.S. provided thorium data. A.W. provided nutrient data. P.A. and J.H. provided phytoplankton and BSi data. F.D. carried out the Lagrangian analysis based on delayed-time altimetry. J.M.A. and G.J.H. provided bacterial data. C.N. and R.B. provided inorganic carbon data. G.M.B., C.K. and M.M.M. provided POC and PON data. P.C. provided the iron data. S.G. and A.T. provided DOM data. I.P. and L.J.H. performed the 14C primary production measurements and provided high-pressure liquid chromatography data. R.R. provided data on photochemical efficiency (Fv/Fm). C.K., M.M.S. and A.T. provided Chl data. U.B., E.S., O.S. and J.S. provided data on the eddy core from a subsequent cruise and satellite Chl images.

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Correspondence to Victor Smetacek or Christine Klaas.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information 1

This file contains Supplementary Text and Data 1-5 (see contents). Each section also includes Supplementary Figures, Supplementary Tables and additional references. (PDF 14229 kb)

Supplementary Information 2

This file contains Supplementary Methods, additional references, Supplementary Figures 1-7 and Supplementary Tables 1-3. This file was replaced on 20 July 2012, as the figures that appeared in the original file were incorrect. (PDF 6258 kb)

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Smetacek, V., Klaas, C., Strass, V. et al. Deep carbon export from a Southern Ocean iron-fertilized diatom bloom. Nature 487, 313–319 (2012).

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