Iron Oxidation in the Nitrogenous Oxygen Deficient Zone of the Eastern Tropical North Pacific Ocean

Oxygen deficient zones (ODZs) contain elevated concentrations of dissolved, reduced iron (Fe(II)), presumably sourced from shelf sediments. As that Fe(II) is transported offshore, it is oxidized and scavenged to the continental slope. The redox reactions involved and their influence on transport are poorly constrained. Here, the in situ oxidation of Fe(II) by nitrate and or nitrite and incorporation into iron oxyhydroxides was studied in a free floating sediment trap array in the Eastern Tropical North Pacific ODZ. Particle-dependent Fe(II) half-lives ranged from 43 to 132 days, with the slowest rates at each station within the core of the ODZ. The very slowest rates were at an offshore station with the lowest Fe(II) concentrations. We conclude that iron oxidation in this region is likely a microbially driven process. An inverse model described the characteristic distribution of Fe(II) within ODZs by coupling a benthic source with our oxidation rate data. While oxidation was assumed to be first order with respect to Fe(II), apparent second order kinetics yielded the best fit, presumably because microbial Fe(II) oxidizer abundance is proportional to Fe(II) concentration. The fit was also improved by incorporating an Fe(II) source within the ODZ from remineralization of sinking particles. While this source was at odds with thermodynamics in a nitrate-dominated regime, we showed that Fe(II) production occurs in anaerobic, nitrate-replete mesocosm, provided that large particles are present. Such particles may harbor nitrate-depleted microenvironments that create conditions thermodynamically favorable for iron reduction. These experiments provided a justification for incorporating a remineralization term into the model.