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(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

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The annual phytoplankton bloom occurring north of the Crozet Plateau provides a rare opportunity to examine the hypothesis that natural iron fertilisation can alleviate HNLC conditions normally associated with the Southern Ocean. Therefore, during CROZEX, a large multidisciplinary study performed between November 2004 and January 2005, measurements of total dissolved iron (DFe, 0.2 m) were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples. waters. Enrichment of dissolved iron (>1 nM) at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of ~ 0.07 nM.km -1 at the time of sampling. Using lateral and vertical diffusion coefficients derived from Ra isotope profiles and also estimates of atmospheric inputs, it was then possible to estimate a DFe concentration of ~ 0.55 nM to the north of the islands prior to the bloom event, which is sufficient to initiate the bloom, the lateral island source being the largest component. A similar situation is observed for other Sub-Antarctic Islands such as Kerguelen, South Georgia, that supply dissolved iron to their surrounding waters, thus, enhancing chlorophyll concentrations.Keywords: Dissolved Iron, Crozet Islands, Southern Ocean, HNLC. Planquette et al., 26/04/07 3 The hypothesis that iron can act as a limiting micro nutrient in High Nutrient Low Chlorophyll (HNLC hereafter) regions is now generally accepted and has been investigated on a number of occasions (Boyd et al., 2000;de Baar et al., 2005). IntroductionThe iron hypothesis originally proposed by Martin (1990) has led to numerous studies which all demonstrate that the addition of iron to HNLC waters causes an increase in phytoplankton productivity. Subsequent investigations into iron's role in phytoplankton physiology have also revealed important findings. Among them, one can cite its role in photosynthetic and respiratory electron transport, nitrate reduction, and chlorophyll synthesis (Sunda and Huntsman, 1995; Sunda and Huntsman, 1997). The broader implication is that in HNLC waters, the presence of iron can increase the efficiency of the biological pump and promote drawdown of atmospheric carbon dioxide (Bakker et al., 2001;Bakker et al., 2005; Boyd et al., 2004; Law et al., 2006;Martin et al., 1990).The Southern Ocean is subjected to these HNLC conditions and is depicted as the largest potential sink of anthropogenic CO 2 in the global ocean (Martin, 1990; Tréguer and Pondaven, 2001) and as a key system in the context of climate change (Sarmiento et al., 1998). However, due to the existence of distinct regional sub systems differing in their physical and biological properties (Arrigo et al., 1998; Tréguer and Jacques, 1992), this ocean should not be viewed as one entity.Ther...