Global Iron Connections Between DesertDust, Ocean Biogeochemistry, and ClimateT. D. Jickells,1Z. S. An,2K. K. Andersen,3A. R. Baker,1G. Bergametti,4N. Brooks,1J. J. Cao,2P. W. Boyd,5R. A. Duce,7K. A. Hunter,6H. Kawahata,8N. Kubilay,9J. laRoche,10P. S. Liss,1N. Mahowald,11J. M. Prospero,12A. J. Ridgwell,13I. Tegen,14*R. Torres15The environmental conditions of Earth, including the climate, are determined byphysical, chemical, biological, and human interactions that transform and transportmaterials and energy. This is the ‘‘Earth system’’: a highly complex entity characterizedby multiple nonlinear responses and thresholds, with linkages between disparatecomponents. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans,affecting ocean biogeochemistry and hence having feedback effects on climate and dustproduction. Here we review the key components of this cycle, identifying criticaluncertainties and priorities for future research.Iron is an essential nutrient for all organisms,used in a variety of enzyme systems,including those for photosynthesis, respira-tion, and nitrogen fixation (1, 2). However, ironis very insoluble under oxidizing conditionsabove pH 4 (3). For marine phytoplankton,separated from the iron-rich sediment of theocean floor by considerable water depths,physiological iron requirements must be metfrom within the water column. Iron supply is alimiting factor on phytoplankton growth overvast areas of the modern ocean, although thismay not have been so in the distant past, whenprokaryotes first evolved in a less oxic ocean (1).Iron supply reaches the oceans mainly fromrivers as suspended sediment in a vast globaltransport system (Table 1). However, fluvial andglacial particulate iron is efficiently trapped innear-coastal areas (4), except where rivers dis-charge directly beyond the shelf. Hydrothermalinputs are rapidly precipitated at depth in theoceans. Hence, the dominant external input ofiron to the surface of the open ocean is aeoliandust transport, mainly from the great deserts ofthe world. Currently hyper-arid areas such as theSahara desert occupy 0.9 billion hectares anddrylands occupy 5.2 billion hectares, which isone-third of global land area. These environ-ments are particularly sensitive to global changepressures (5, 6), and such changes could alterocean productivity and hence climate. There areother possible contributors to atmospheric ironsupply, including volcanic, anthropogenic, andextraterrestrial sources (7, 8), whose iron may bemore soluble than iron in soil aluminosilicates(7), and these merit further study.Dust produced in arid areas has importantand disparate effects throughout the Earthsystem, as illustrated in Fig. 1 and discussedbelow. These need to be incorporated intoclimate models to correctly predict impacts ofglobal change pressures. We first considereach component of the system before attempt-ing a global synthesis.Climate Effects on Dust/Iron FluxesSatellite imagery has greatly increased ourknowledge of large-scale dust source regions,emphasizing the importance of localizedsources, which vary seasonally. There are sim-ilar climatic and geomorphological controls onmany source regions (6), and dried-out lakesystems such as the Bodele Depression inNorth Africa appear to be particularly impor-tant. Dust production depends on the supply ofwind-erodible material, which ironically usu-ally requires fluvial erosion, often fromadjacent highlands, followed by subsequentdrying out and the loss or absence of veg-etative protection (6, 9–11). Dust productionarises from saltation or sandblasting, whenwinds above a threshold velocity transport soilgrains horizontally, producing smaller par-ticles, a small proportion of which get carriedup into the atmosphere for long-range trans-port. These processes depend on rainfall,wind, surface roughness, temperature, topog-raphy, and vegetation cover, which are inter-dependent factors linked to aridity and climatein a highly nonlinear way. Wind tunnel studiesshow dust production to be proportional to thecube of wind speed (5).Desert dust aerosol is dominated by par-ticles of diameter 0.1 to 10 mm, with the meansize being around 2 mm. Such aerosols have alifetime of hours to weeks, allowing long-range transport over scales of thousands ofREVIEW1School of Environmental Sciences, University of EastAnglia, Norwich NR47TJ, UK.2State Key Lab of Loessand Quaternary Geology, Institute of Earth Environ-ment, Chinese Academy of Sciences, AS, 10 FenghuiSouth Road, Post Office Box 17, China.3Niels BohrInstitute, University of Copenhagen, Juliane Maries Vej30, 2100 Copenhagen, Denmark.4Laboratoire Inter-universitaire des Syste`mes Atmospherique, Universite´sParis 7 and Paris 12, UMR CNRS 7583, Paris, France.5National Institute of Water and Atmospheric ResearchCentre for Chemical and Physical Oceanography, De-partment of Chemistry;6Department of Chemistry, PostOffice Box 56; University of Otago, Dunedin, NewZealand.7Departments of Oceanography and Atmo-spheric Sciences, Texas A&M University, TAMU 3146,College Station, TX 77843–3146, USA.8GeologicalSurvey of Japan, National Institute of AdvancedIndustrial Science and Technology (AIST), Tsukuba-higashi 1-1-1, Ibaraki 305–8567, Japan.9Institute ofMarine Sciences, Middle East Technical University,Post Office Box 28, Erdemli-Mersin 33731, Turkey.10Leibniz-Institute fu¨r Meereswissenchaften IFM-GEO-MAR, Marine Biogeochemistry, Geba¨ude Westufer,Du¨sternbrooker Weg 20, 24105 Kiel, Germany.11Na-tional Center for Atmospheric Research. Post OfficeBox 3000, Boulder, CO 80307, USA.12RosenstielSchool of Marine and Atmospheric Sciences, Universityof Miami, 4600 Rickenbacker Causeway, Miami, FL33149–1089, USA.13Department of Earth and OceanSciences, University of British Columbia, 6339 StoresRoad, Vancouver, British Columbia V6T 1Z4 Canada.14Max-Planck-Institute for Biogeochemistry. Post Of-fice Box 10, 01 64 07701 Jena, Germany.15Universi-dad de Conception, Departamento de Oceanografia,Casilla 160C, Chile.*Present address: Institute for Tropospheric Research,Permoserstrasse 15 04318, Leipzig, Germany.Table 1. Global iron fluxes to the ocean (in Tgof Fe year–1). From Poulton and Raiswell (4),with modified atmospheric inputs from Fig.2. ‘‘Authigenic fluxes’’ refer to releases fromdeep-sea sediments during diagenesis. Wedistinguish only separately dissolved