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Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2


Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron1,2. One implication of this limitation is that a larger iron supply to the region in glacial times3 could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean4, which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok3 ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial 40 p.p.m. of glacial–interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known.

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Figure 1: Seawater f CO 2 in surface water measured from the RV Tangaroa as a function of time during the experiment.
Figure 2: Vostok ice-core record3 of dust and atmospheric CO2 compared to the atmospheric CO2 concentration from our carbon-cycle model.


  1. Martin, J. H. Glacial-interglacial CO2 change; the iron hypothesis. Paleoceanography 5, 1–13 (1990).

    Article  ADS  Google Scholar 

  2. Kumar, N. et al. Increased biological productivity and export production in the glacial Southern Ocean. Nature 378, 675– 680 (1995).

    Article  ADS  CAS  Google Scholar 

  3. Petit, J. R. et al. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399, 429–436 (1999).

    Article  ADS  CAS  Google Scholar 

  4. Boyd, P. W. et al. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407, 695–701 (2000).

    Article  ADS  CAS  Google Scholar 

  5. Watson, A. J., Liss, P. S. & Duce, R. A. Design of a small scale iron enrichment experiment. Limnol. Oceanogr. 36, 1960– 1965 (1991).

    Article  ADS  Google Scholar 

  6. Martin, J. H. et al. Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean. Nature 371, 123– 129 (1994).

    Article  ADS  CAS  Google Scholar 

  7. Coale, K. H. et al. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature 383, 495–501 ( 1996).

    Article  ADS  CAS  Google Scholar 

  8. Cooper, D. J., Watson, A. J. & Ling, R. D. Variation of P CO 2 along a North Atlantic shipping route (UK to the Caribbean): A year of automated observations. Mar. Chem. 60, 147–164 (1998).

    Google Scholar 

  9. Abraham, E. R. et al. Importance of stirring in the development of an iron-fertilized phytoplankton bloom. Nature 407, 727– 730 (2000).

    Article  ADS  CAS  Google Scholar 

  10. Takeda, S. Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters. Nature 393, 774– 777 (1998).

    Article  ADS  CAS  Google Scholar 

  11. Hutchins, D. A. & Bruland, K. W. Iron-limited diatom growth and Si:N uptake ratios in a coastal upwelling regime. Nature 393, 561–564 ( 1998).

    Article  ADS  CAS  Google Scholar 

  12. Duce, R. A. & Tindale, N. W. Atmospheric transport of iron and its deposition in the ocean. Limnol. Oceanogr. 36, 1715–1726 (1991).

    Article  ADS  CAS  Google Scholar 

  13. Lefèvre, N. & Watson, A. J. Modelling the geochemical cycle of iron in the oceans and its impact on atmospheric carbon dioxide concentrations. Glob. Biogeochem. Cycles 13 , 727–736 (1999).

    Article  ADS  Google Scholar 

  14. Sohrin, Y. et al. The distribution of Fe in the Australian sector of the Southern Ocean. Deep-Sea Res. 47, 55– 84 (2000).

    Article  CAS  Google Scholar 

  15. Broecker, W. S. & Peng, T.-H. The role of CaCO 3 compensation in the glacial to interglacial atmospheric CO2 change. Glob. Biogeochem. Cycles 1, 15– 29 (1987).

    Article  ADS  CAS  Google Scholar 

  16. Stevens, D. P. & Ivchenko, V. O. The zonal momentum balance in an eddy-resolving general-circulation model of the Southern Ocean. Q. J. R. Meteorol. Soc. 123, 929– 951 (1997).

    Article  ADS  Google Scholar 

  17. Mahowald, N. et al. Dust sources and deposition during the last glacial maximum and current climate: a comparison of model results with paleodata from ice cores and marine sediments. J. Geophys. Res. 104, 15895–15916 (1999).

    Article  ADS  Google Scholar 

  18. Jickells, T. D. & Spokes, L. J. in The Biogeochemistry of Iron in Seawater (eds Turner, D. R. & Hunter, K. A.) (Wiley, New York, in the press).

  19. Sunda, W. G. & Huntsman, S. A. Iron uptake and growth limitation in oceanic and coastal phytoplankton. Mar. Chem. 50 , 189–206 (1995).

    Article  CAS  Google Scholar 

  20. Levitus, S., Burgett, R. & Boyer, T. World Ocean Atlas 1994 Vol. 3, Nutrients (NOAA Atlas NESDIS, US Dept of Commerce, Washington DC, 1994).

    Google Scholar 

  21. Sarmiento, J. L. & Toggweiler, J. R. A new model for the role of the oceans in determining atmospheric P CO 2 . Nature 308, 621– 624 (1984).

    Google Scholar 

  22. Smith, H. J., Fischer, H., Wahlen, M., Mastroianni, D. & Deck, B. Dual modes of the carbon cycle since the Last Glacial Maximum. Nature 400, 248– 250 (1999).

    Article  ADS  CAS  Google Scholar 

  23. Broecker, W. S. & Henderson, G. M. The sequence of events surrounding Termination II and their implications for the cause of glacial-interglacial CO2 changes. Paleoceanography 13, 352–364 ( 1998).

    Article  ADS  Google Scholar 

  24. Francois, R. et al. Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period. Nature 389, 929–935 (1997).

    Article  ADS  CAS  Google Scholar 

  25. Sigman, D. M., Altabet, M. A., Francois, R., McCorkle, D. C. & Gaillard, J. F. The isotopic composition of diatom-bound nitrogen in Southern Ocean sediments. Paleoceanography 14, 118–134 ( 1999).

    Article  ADS  Google Scholar 

  26. Mortlock, R. A. et al. Evidence for lower productivity in the Antarctic Ocean during the last glaciation. Nature 351, 220– 222 (1991).

    Article  ADS  Google Scholar 

  27. Rickaby, R. E. M. & Elderfield, H. Planktonic foraminiferal Cd/Ca: Paleonutrients or paleotemperature? Paleoceanography 14, 293–303 ( 1999).

    Article  ADS  Google Scholar 

  28. Stephens, B. B. & Keeling, R. F. The influence of Antarctic sea ice on glacial–interglacial CO2 variations. Nature 404, 171–174 (2000).

    Article  ADS  CAS  Google Scholar 

  29. Toggweiler, J. R. Variation of atmospheric CO2 by ventilation of the ocean's deepest water. Paleoceanography 14, 571– 588 (1999).

    Article  ADS  Google Scholar 

  30. Peng, T. H. & Broecker, W. S. Dynamic limitations on the Antarctic iron fertilization strategy. Nature 349, 227–229 (1991).

    Article  ADS  CAS  Google Scholar 

  31. Keir, R. S. On the late Pleistocene ocean geochemistry and circulation. Paleoceanography 3, 413–445 ( 1988).

    Article  ADS  Google Scholar 

  32. Archer, D. Modeling the calcite lysocline. J. Geophys. Res. 96 , 17037–17050 (1991).

    Article  ADS  Google Scholar 

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We thank K. Tanneburger, U. Schuster, and the officers and crew of the RV Tangaroa for their support. This work was supported by the United Kingdom Natural Environment Research Council and the New Zealand National Institute for Water and Atmospheric Research. We acknowledge support from the CEFIC iron salts committee.

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Correspondence to A. J. Watson.

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Watson, A., Bakker, D., Ridgwell, A. et al. Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Nature 407, 730–733 (2000).

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