WHAT CONTROLS DISSOLVED IRON CONCENTRATIONS IN THE WORLD OCEAN

Citation
Ks. Johnson et al., WHAT CONTROLS DISSOLVED IRON CONCENTRATIONS IN THE WORLD OCEAN, Marine chemistry, 57(3-4), 1997, pp. 137-161
Citations number
73
Categorie Soggetti
Oceanografhy,Chemistry
Journal title
ISSN journal
03044203
Volume
57
Issue
3-4
Year of publication
1997
Pages
137 - 161
Database
ISI
SICI code
0304-4203(1997)57:3-4<137:WCDICI>2.0.ZU;2-L
Abstract
Dissolved (< 0.4 mu m) iron has been measured in 354 samples at 30 sta tions in the North and South Pacific, Southern Ocean and North Atlanti c by the Trace Metals Laboratory at Moss Landing Marine Laboratories, These stations are all more than 50 km from a continental margin. The global distribution of dissolved iron, which is derived from these pro files, is remarkable for several reasons, The dissolved iron profiles have a uniform shape with a nutrient-like profile at each station. Con centrations at the surface are all <0.2 nmol kg(-1) and average 0.07 n mol kg(-1). Below 500 m, the average concentration is 0.76 nmol kg(-1) . The largest value in the data set is 1.38 nmol kg(-1). There is no i nter-ocean fractionation, which is unique for an element with a nutrie nt-like profile. Published estimates of the iron residence time are on the order of 100 to 200 yr, indicative of rapid removal. Other elemen ts with such short residence times are characterized by vertical profi les that decrease with depth and deep concentrations that decrease wit h age as water passes from the Atlantic to the Pacific. This is not th e case for iron. The largest horizontal changes in dissolved iron are observed in gradients from the continental margin. There is only a fac tor of three difference between the minimum (0.4 nmol kg(-1)) and maxi mum (1.3 nmol kg(-1)) value in the data set at a depth near 750 m, whe re variability is at a maximum. The minimum concentrations are found a t stations in the remote central Pacific and the maximum values occur at stations adjacent to the continental margin. The major source of ir on in the deep sea is generally aeolian deposition. Integrated (surfac e to 500 m) concentrations of iron at each station are only weakly cor related with the aeolian iron deposition flux, however. This contrasts with other elements such as lead that also have strong atmospheric so urces, These observations lead us to conclude that the nutrient-like p rofile is maintained by a mechanism that reduces the scavenging rate o f dissolved iron at concentrations less than 0.6 nmol kg(-1) This mech anism may be complexation by strong iron binding ligands, which have b een found in both the Atlantic and Pacific at concentrations near 0.6 nM. This apparent solubility would act to diminish inter-ocean fractio nation. It would allow a nutrient-like profile to develop before scave nging began to remove iron,In order to test the concept, we developed a numerical model to make quantitative predictions of dissolved iron c oncentrations from place to place. The dissolved iron source in the oc ean interior is remineralization from sinking particulate organic matt er, Scavenging removes dissolved iron only at concentrations greater t han the apparent solubility. The only geographically variable paramete r in the model is the export flux of carbon from the surface layer, wh ich carries iron with it. The model generated dissolved iron profiles, based on measured or estimated values of the carbon export flux, are in remarkable agreement with the observed profiles at all stations fro m the North Atlantic through the Southern Ocean to the North Pacific. (C) 1997 Elsevier Science B.V.