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General Context

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General context: biogeochemistry in the Southern Ocean, iron fertilization of the ocean

The Southern Ocean is a key region in the global carbon cycle because it represents an important CO2 sink. The magnitude of this sink in the present and in the past is still highly debated. In a recent study (Le Quéré et al., 2007) has shown that the Southern Ocean CO2 sink has weakened between 1987 and 2004 by 0.08 pgC yr-1, very likely due to the increase of Southern Ocean wind related to climate change. Theory and model studies also point out the sensitivity of atmospheric CO2 to changes in ocean biology in the Southern Ocean (Marinov et al., in press). This sensitivity is related to nutrient utilization in the surface waters. It is expected that increasing the deep ocean ventilation due to increased vertical mixing or due to an enhancement of Southern Ocean winds results in an increase in the atmospheric CO2. By contrast, stratification of the Southern Ocean decreases the atmospheric CO2 sensitivity due to surface nutrient depletion. In the surface waters of the Southern Ocean, the utilisation of nutrients is first limited by the availability of iron, as demonstrated by artificial and natural iron fertilisation experiments (Blain et al., 2007; Boyd et al., 2007; Coale et al., 2004; Pollard et al., 2007). Therefore, fertilization (naturally or artificially) of this region is expected to have in principle a large impact on atmospheric CO2. However, other properties like elemental ratios (i.e. C:P), gas exchange (Gnanadesikan and Marinov, 2008) are important in this context.

Except for a few locations where natural iron fertilization was clearly demonstrated, the source of iron supporting higher productivity in the Southern Ocean is a matter of debate (see for example the case of an aeolian source in (Boyd and Mackie, 2008; Cassar et al., 2007; Cassar et al., 2008)).

KEOPS and CROZEX have demonstrated that the natural fertilization of the Southern Ocean resulted in dramatic changes in the functioning of the ecosystem with large impacts on the biogeochemical cycles. We also learned from both projects that the responses can largely differ from one site to another. These projects highlighted the interest of natural laboratories in the context of iron fertilization of the ocean, but they could cover only a small part of the potential new findings in these area.

Blain, S., Quéguiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Brussaard, K., Carlotti, F., Christaki, U., Corbière, A., Durand, I., Ebersbach, F., Fuda, J.L., Garcia, N., Gerringa, L.J.A., Griffiths, F.B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, C., Laan, P., Lefèvre, D., Lomonaco, C., Malits, A., Mosseri, J., Obernosterer, I., Park, Y.H., Picheral, M., Pondaven, P., Remenyi, T., Sandroni, V., Sarthou, G., Savoye, N., Scouarnec, L., Souhault, M., Thuillers, D., Timmermans, K.R., Trull, T., Uitz, J., Van-Beek, P., Veldhuis, M.J.W., Vincent, D., Viollier, E., Vong, L. and Wagener, T., 2007. Effect of natural iron fertilisation on carbon sequestration in the Southern Ocean. Nature, 446(7139): 1070-1075.

Boyd, P.W., Jickells, T., Law, C., Blain, S., Boyle, E.A., Buesseler, K.O., Coale, K.H., Cullen, J.J., De Baar, H.J.W., Follows, M., Harvey, M., Lancelot, C., Levasseur, M., Owens, N.J.P., Pollard, D.A., Rivkin, R.B., Sarmiento, J.L., Schoemann, V., Smetacek, V., Takeda, S., Tsuda, A., Turner, D.R. and Watson, A., 2007. Mesoscale iron enrichment experiments 1993-2005: Synthesis and future directions. Science, 315: 612-617.

Boyd, P.W. and Mackie, D., 2008. Comment on “The Southern Ocean Biological Response to Aeolian Iron Deposition”. Science, 359: 159.

Cassar, N., Bender, M.L., Barnett, B.A., Fan, S., Moxim, W.J., Levy II, H. and Tilbrook, B., 2007. The Southern Ocean Biological Response to Aeolian Iron Deposition. Science, 317: 1067-1070.

Cassar, N., Bender, M.L., Barnett, B.A., Fan, S., Moxim, W.J., Levy II, H. and Tilbrook, B., 2008. Response to comment on “The Southern Ocean Biological Response to Aeolian Iron Deposition”. Science.

Coale, K.H., Johnson, K.S., Chavez, F.P., Buesseler, K.O., Barber, R.T., Brzezinski, M., Cochlan, W.P., Millero, F.J., Falkowski, P.G., Bauer, Wanninkhof, R.H., Kudela, R.M., Altabet, M.A., Hales, B.E., Takahashi, T., Landry, M.R., Bidigare, R.C., Wang, X., Chase, Z., Strutton, P.G., Friederich, G.E., Gorbunov, M.Y., Lance, V.P., Hilting, A.K., Hiscock, M.R., Demarest, M., Hiscock, W.T., Sullivan, K.F., Tanner, S.J., Gordon, R.M., Hunter, C.N., Elrod, V.A., Fitzwater, S.E., Jones Janice L., Tozzi Sasha, Koblizek Michal, Roberts Alice E., Herndon, J., Brewster, J., Ladizinsky, N., Smith, G., David, C., Timothy, D., L. Brown, S., Selph, K.E., Sheridan, C.C., Twining, B.S. and Johnson, Z.I., 2004. Southern Ocean Iron Enrichment Experiment: Carbon Cycling in High- and Low-Si Waters. Science, 304: 408-414.

Gnanadesikan, A. and Marinov, I., 2008. Export is not enough: nutrient cycling and carbon sequestration. Marine Ecology Progress Series, 364: 289-294.

Le Quéré, C., Rodenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N. and Heimann, M., 2007. Saturation of the Southern Ocean CO2 sink due to recent climate change. Science: doi:10.1126/science.1136188.

Marinov, I., Follows, M., Gnanadesikan, A., Sarmiento, J. and Slater, R.D., in press. Atmospheric pCO2 sensitivity to surface nutrient depletion in the ocean - theory and models. Journal of Geophysical Research.

Pollard, R., sander, S., Lucas, M. and Statham, P.J., 2007. the Crozet natural iron bloom and export experiment (CROZEX). Deep sea Research II, 54: 1905-1914.


Last Updated on Thursday, 26 January 2012 14:46  


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