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 samples collected between 34°S and 8°N in the southern and tropical Atlantic Ocean using a towed surface-water sampler. The general distribution of dissolved Al, which ranged from ∼8 nM in the sub-tropical gyre to ∼61 nM under the Intertropical Convergence Zone (ITCZ) north of the Equator, corresponds clearly with atmospheric dust deposition patterns. A striking correlation between Al and surface-water salinities between 28°S and 8°N further indicates that wet deposition is the predominant mechanism of delivery of this atmospheric source. Iron concentrations, in contrast, were more uniform varying between 0.6 and 0.8 nM south of the equator. Highest Fe values (up to 1.4 nM) also were found associated with the maximum dust inputs at the ITCZ, indicating an atmospheric source for this element whose input appears to be limited by the solubility of Fe in sea water. The lowest Fe values (∼0.4 nM) also were observed in the equatorial region and are assumed to result from the biological removal of Fe from nutrient-rich water masses originating in the nearby equatorial and coastal upwelling regions. The high-frequency data acquisition of the towed sampler revealed significant variations (up to 50%) in both Al and 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 in surface waters.