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

Author(s): Smetacek V, Klaas C, Strass VH, Assmy P, Montresor M, Cisewski B, Savoye N, Webb A, d’Ovidio F, Arrieta JM, Bathmann U, Bellerby R, Mine Berg G, Croot P, Gonzalez S, Henjes J, Herndl GJ, Hoffmann LJ, Leach H, Losch M, Mills MM, Neill C, Peeken I, Röttgers R, Sachs O, Sauter E, Schmidt MM, Schwarz J, Terbrüggen A, Wolf-Gladrow D

Published: July, 2012

Publisher: Nature

DOI: 10.1038/nature11229

Tags: Ocean Fertilization

URL: http://www.nature.com/nature/journal/v487/n7407/full/nature11229.html?WT.ec_id=NATURE-20120719

Abstract: 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.


Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

Author(s): Blain S, Quéguiner B, Armand L, Belviso S, Bombled B, Bopp L, Bowie AR, Brunet C, Brussaard C, Carlotti F, Christaki U, Corbière A, Durand I, Ebersbach F, Fuda JL, Garcia N, Gerringa LJA, Griffiths B, Guigue C, Guillerm C, Jacquet S, Jeandel C, Laan P, Lefèvre D, Monaco CL, Malits A, Mosseri J, Obernosterer I, Park YH, Picheral M, Pondeven P, Remenyi T, Sandroni V, Sarthou G, Savoye N, Scouarnec L, Souhaut M, Thuiller D, Timmermans KR, Trull TW, Uitz J, van Beek P, Veldhuis M, Vincent D, Viollier E, Vong L, Wagener T

Published: April, 2007

Publisher: Nature

DOI: 10.1038/nature05700

Tags: Ocean Fertilization

URL: http://www.nature.com/nature/journal/v446/n7139/full/nature05700.html

Abstract: The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales8. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.


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