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

Author(s): Pollard RT, Salter I, Sanders RJ, Lucas MI, Moore CM, Mills RA, Statham PJ, Allen JT, Baker AR, Bakker DCE, Charette MA, Fielding S, Fones GR, French M, Hickman AE, Holland RJ, Hughes JA, Jickells TD, Lampitt RS, Morris PJ, Nédélec FH, Nielsdóttir M, Planquette H, Popova EE, Poulton AJ, Read JF, Seeyave S, Smith T, Stinchcombe M, Taylor S, Thomalla S, Venables HJ, Williamson R, Zubkov MV

Published: January, 2009

Publisher: Nature

DOI: 10.1038/nature07716

Tags: Ocean Fertilization

URL: http://www.nature.com/nature/journal/v457/n7229/full/nature07716.html

Abstract: The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. 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 quantified. Here we report data from the CROZEX experiment 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 ocean. The CROZEX sequestration efficiency (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 iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.


Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions

Author(s): Boyd PW, Jickells TD, Law CS, Blain S, Boyle EA, Buesseler KO, Coale KH, Cullen JJ, de Baar HJW, Follows M, Harvey M, Lancelot C, Levasseur M, Owens NPJ, Pollard RT, Rivkin RB, Sarmiento JL, Schoemann V, Smetacek V, Takeda S, Tsuda A, Turner SM, Watson AJ

Published: February, 2007

Publisher: Science

DOI: 10.1126/science.1131669

Tags: Ocean Fertilization, Research

URL: http://www.sciencemag.org/content/315/5812/612.abstract

Abstract: Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.


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