Dissolved iron in the tropical North Atlantic Ocean
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Seawater from 27 stations was sampled in a zig-zag transect across the tropical North Atlantic from Barbados to the Cape Verde Islands, 7-20N and 21-58W. Under the Northwest African dust plume, surface dissolved Fe (dFe) was elevated, averaging 0.55. . 0.29. nmol/kg (1SD), but showed a wide range from 0.12 to 1.31. nmol/kg with no clear longitudinal or latitudinal gradients. A subsurface dFe maximum in excess of 1. nmol/kg was evident coincident with the oxygen minimum zone. Our goal was to determine whether this maximum occurred as a result of remineralization of high Fe:C organic material formed in the dust-laden (Fe-rich) surface ocean or as a result of lateral mixing of a coastal Fe signal from the African margin. We found that dFe is directly related to apparent oxygen utilization (AOU) with the same slope across all depths sampled, which implies a remineralization source throughout the water column. A continental margin dFe supply was discounted because a modeled margin Fe source should cause curvature in the Fe-AOU relationship that was not observed. We also did not see a significant dissolved manganese (dMn) plume emanating from the margin. Regenerated Fe:C ratios calculated from the dFe:AOU relationships ranged from 9.6 to 12.4. mol Fe/mol C, which is much higher than ratios found in HNLC and dust-poor regions. Deep waters measured at one station in the eastern tropical North Atlantic reflected typical NADW concentrations until 4000. m, where the dFe increased to 0.82. nmol/kg in four discrete samples down to 5000. m. We propose that this increase in dFe may reflect dFe inputs to deep waters as they traverse the Mid-Atlantic Ridge and enter the eastern basin. 2013 Elsevier B.V.
