Redox state of iron in the offshore waters of Peru1

The total concentration and redox state of iron were examined along a transect across the continental shelf off the Peruvian coast during January 1984. Total and dissolved iron (0.4‐ µ m filter) were measured by the Co‐APDC coprecipitation method. Fe(II) was measured by a preconcentration step with 8‐hydroxyquinoline bonded to silica as the stationary phase, followed by elution and the ferrozine method. Up to 40 nmol kg −1 of Fe(II) was detected in the bottom water at 5–10 km offshore and decreased markedly upward in the water column and with distance offshore. A good correlation between the distribution of Fe(II) and nitrite in the bottom water indicated a common source from the shelf sediments. Elevated Fe(II) concentrations near the sea surface and a diel change were probably due to photochemical reactions involving iron. Total iron levels were >300–500 nmol kg −1 in the surface and the bottom water at 5–6 km offshore. About 80–90% of the iron was in the particulate form, indicating a substantial input of iron from continental dust and from the sediments on the shelf. The total iron level decreased considerably within 35 km of the coastline and the iron seemed to be trapped on the shelf.