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Raim for lifelong help; E. Gwinner forinspiration; and G. Swenson, G. Swenson, and J. Brugge-mann for help with light measurements and thank M.Bowlin, N. Sapir, A. Medina, W. Cochran, J. Cochran, J.Mandel, and S. David for their help, support, and under-standing during this intensive project and W. Wiltschko forconstructive comments on the manuscript. Supported bythe National Geographic Society (M.W.), Princeton Univer-sity (M.W.), Volkswagen Stiftung (H.M.), Oldenburg Uni-versity (to H.M.) and NSF (GB 3155 and 6680 to W.W.C.).Supporting Online Materialwww.sciencemag.org/cgi/content/full/304/5669/405/DC1Materials and MethodsFigs. S1 and S2Tables S1 and S220 January 2004; accepted 10 March 2004Southern Ocean Iron EnrichmentExperiment: Carbon Cycling inHigh- and Low-Si WatersKenneth H. Coale,1* Kenneth S. Johnson,2Francisco P. Chavez,2Ken O. Buesseler,3Richard T. Barber,4Mark A. Brzezinski,5William P. Cochlan,6Frank J. Millero,7Paul G. Falkowski,8James E. Bauer,9Rik H. Wanninkhof,10Raphael M. Kudela,11Mark A. Altabet,12Burke E. Hales,13Taro Takahashi,14Michael R. Landry,15Robert R. Bidigare,16Xiujun Wang,1Zanna Chase,2Pete G. Strutton,2Gernot E. Friederich,2Maxim Y. Gorbunov,8Veronica P. Lance,4Anna K. Hilting,4Michael R. Hiscock,4Mark Demarest,5William T. Hiscock,7Kevin F. Sullivan,10Sara J. Tanner,1R. Mike Gordon,1Craig N. Hunter,1Virginia A. Elrod,2Steve E. Fitzwater,2Janice L. Jones,5Sasha Tozzi,8,9Michal Koblizek,8Alice E. Roberts,6Julian Herndon,6Jodi Brewster,1Nicolas Ladizinsky,1,6Geoffrey Smith,1David Cooper,1David Timothy,12Susan L. Brown,16Karen E. Selph,16Cecelia C. Sheridan,16Benjamin S. Twining,17Zackary I. Johnson18The availability of iron is known to exert a controlling influence on biological produc-tivity in surface waters over large areas of the ocean and may have been an importantfactor in the variation of the concentration of atmospheric carbon dioxide over glacialcycles. The effect of iron in the Southern Ocean is particularly important because ofits large area and abundant nitrate, yet iron-enhanced growth of phytoplankton maybe differentially expressed between waters with high silicic acid in the south and lowsilicic acid in the north, where diatom growth may be limited by both silicic acid andiron. Two mesoscale experiments, designed to investigate the effects of iron enrichmentin regions with high and low concentrations of silicic acid, were performed in theSouthern Ocean. These experiments demonstrate iron’s pivotal role in controllingcarbon uptake and regulating atmospheric partial pressure of carbon dioxide.The Southern Ocean exerts a major control on thepartial pressure of carbon dioxide ( pCO2)intheatmosphere. Because rates of photosynthesis andbiological carbon export are low in Antarctic wa-ters, macronutrients are largely unused, and up-welled CO2entering the atmosphere (1, 2) sus-tains the relatively high interglacial atmosphericpCO2of the present day (3).Southern Ocean surface waters contain ex-tremely low iron concentrations (4, 5), and thelow rates of primary production have been at-tributed to iron deficiency. Recent open-oceaniron enrichment experiments demonstrate thevalidity of this hypothesis in the SouthernOcean (6, 7). Martin (8) proposed that naturalvariations in the atmospheric iron flux ultimate-ly regulate primary production in the SouthernOcean and influence the pCO2of the atmo-sphere, thereby potentially affecting the radia-tive balance of the planet. Syntheses of models,field observations, and paleoceanographic data(3, 9, 10, 11, 12) support a role for iron-regulated changes in Southern Ocean macronutri-ent use. Indeed there is a strong inverse correlationbetween iron-rich dust, marine production, andatmospheric pCO2over the past four glacial cy-cles as recorded in Antarctic ice cores (13). Theseobservations support the “iron hypothesis” as pro-posed by Martin (8), yet the magnitude of the ironenrichment effect on marine production and atmo-spheric pCO2remains uncertain.Although all Southern Ocean surface watershave high concentrations of nitrate and phos-phate, silicic acid concentrations differ marked-ly from north to south. Subantarctic watersnorth of the Antarctic Polar Front Zone (APFZ)have low Si concentrations (1 to 5 ␮M), where-as high Si (⬎60 ␮M) is found to the south (fig.S1). Diatoms, which require Si for growth, arebelieved responsible for much of the carbonexport from the surface to the deep sea (14). In1Moss Landing Marine Laboratories, 8272 Moss LandingRoad, Moss Landing, CA 95039 –9647, USA.2MontereyBay Aquarium Research Institute, 7700 Sandholdt Road,Moss Landing, CA 95039, USA.3Department of MarineChemistry and Geochemistry, Woods Hole Oceano-graphic Institution, Woods Hole, MA 02543, USA.4Nich-olas School of the Environment and Earth Sciences, DukeUniversity, 135 Duke Marine Lab Road, Beaufort, NC28516, USA.5Marine Science Institute and the Depart-ment of Ecology, Evolution, and Marine Biology, Univer-sity of California, Santa Barbara, CA 93106, USA.6Rom-berg Tiburon Center for Environmental Studies, SanFrancisco State University, 3152 Paradise Drive, Tiburon,CA 94920 –1205, USA.7Rosenstiel School of Marine andAtmospheric Research, University of Miami, 4600 Rick-enbacker Causeway, Miami, FL 33149–1098, USA.8En-vironmental Biophysics and Molecular Ecology Program,Institute of Marine and Coastal Sciences and Depart-ment of Geology, Rutgers University, 71 Dudley Road,New Brunswick, NJ 08901– 8521, USA.9Virginia Insti-tute of Marine