Redox state of iron in the offshore waters of Peru’

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~pm filter) were measured by the Co-APDC coprecipitation method. Fe(II) was measured by a preconcentration step with S-hydroxyquinoline bonded to silica as the stationary phase, followed by elution and the ferrozine method. Up to 40 nmol kg-l 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-’ 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. Iron is found in natural waters in both Fe(II) and Fe(III) oxidation states. The distribution of these two forms of iron is governed by several factors, including redox potential, pH, and the presence of organic material. From thermodynamic considerations, the concentration of reduced forms of iron in oxic natural waters will be much lower than that of the oxidized forms of iron due to the rapid oxidation of Fe(II) by Oz. Nevertheless, it has been suggested that nonequilibrium processes may enable Fe(II) species to persist at appreciable concentrations in natural waters even in the presence of oxygen. McMahon (1969) suggested that annual and diurnal variations of acid-soluble ferrous iron in lake water were a result of photochemical reactions or of metabolic activity of microorganisms. Recently, the photochemistry of iron in natural waters has been emphasized by several studies as reviewed by Zafiriou ( 1983). Miles and Brezonik (198 1) showed that the oxygen consumption in humic-colored freshwaters involved a photochemical ferrous-ferric catalytic cycle. Waite and Morel (1984)