The role of dust deposition in determining surface water distributions of Al and Fe in the South West Atlantic

Underway dissolved Al and Fe determinations were made on surface-water samp les collected between 34 degreesS and 8 degreesN in the southern and tropic al Atlantic Ocean using a towed surface-water sampler. The general distribu tion of dissolved Al, which ranged from similar to8 nM in the sub-tropical gyre to similar to 61 nM under the Intertropical Convergence Zone (ITCZ) no rth of the Equator, corresponds clearly with atmospheric dust deposition pa tterns. A striking correlation between Al and surface-water salinities betw een 28 degreesS and 8 degreesN further indicates that wet deposition is the predominant mechanism of delivery of this atmospheric source. Iron concent rations, in contrast, were more uniform varying between 0.6 and 0.8 nM sout h of the equator. Highest Fe values (up to 1.4 nM) also were found associat ed with the maximum dust inputs at the ITCZ, indicating an atmospheric sour ce for this element whose input appears to be limited by the solubility of Fe in sea water. The lowest Fe values (similar to 0.4 nM) also were observe d in the equatorial region and are assumed to result from the biological re moval of Fe from nutrient-rich water masses originating in the nearby equat orial and coastal upwelling regions. The high-frequency data acquisition of the towed sampler revealed significant variations (up to 50%) in both Al a nd Fe concentrations over short distances, most notably along the interface between the South Equatorial and Canary Currents. The high variability of a biologically important trace element such as Fe in these dynamic regions is indicative of the interplay between chemical and biological patchiness i n surface waters. (C) 2001 Elsevier Science Ltd. All rights reserved.