Iron speciation in the eastern tropical South Pacific oxygen minimum zone off Peru

Vertical profiles of total dissolved iron (DFe), Fe(II), and hydrographic parameters were obtained along three transects across the continental shelf off Peru in October 2005. Fe(II) and DFe concentrations were determined using luminol chemiluminescence and isotope dilution inductively coupled plasma mass spectrometry (ICP-MS), respectively. The objective was to study the relationships among redox chemistry, nitrogen-cycle processes, bottom topography, and the distribution and chemistry of iron in the region. The results are the most thorough survey to date of Fe geochemistry in this region. Exceedingly high DFe (similar to 50-75 nmol L-1) was measured over the wide northern continental shelf, with most of the dissolved Fe present as Fe(II) below the oxycline. In southern Peru, the shelf is narrower, and dissolved Fe concentrations were 10-fold lower. Moreover, a smaller fraction of the dissolved Fe was present as Fe(II) in the south, even below the oxycline. At the western ends of the transects, Fe(II) maxima were coincident with deep (i.e., secondary) nitrite maxima. This suggests a relationship between nitrate reduction and Fe(II) accumulation in the water column. However, over the shelf, Fe(II) was also influenced by benthic processes. Large lateral gradients in dissolved Fe across the shelf-slope break reflect Fe removal by oxidative scavenging, and it seems plausible that much of the Fe is trapped by redox cycling on the shelf. Nevertheless, the maintenance of dissolved Fe(II) filaments within the secondary nitrite maxima constitutes an important mechanism for Fe transport offshore.