Distribution of Anaerobic Iron Oxidation in Oxygen Deficient Zones Measured Using Stable Isotope Incubations

Oxygen deficient zones (ODZs) are oceanic regions in which the water column is anoxic. Dissolved iron(II) features have been observed in all three modern ODZs, and it has been hypothesized that these regions are important for the shelf to basin shuttle transporting iron from continental margins to open ocean environments. However, the presence of iron(II) is noteworthy, because ODZs contain high levels of both nitrate and nitrite, which are strong enough oxidants at their environmental concentrations to make iron oxidation thermodynamically favorable. Several observational studies have suggested that iron(II) oxidation is occurring in these regions (Scholz et al. 2016 EPSL; Heller et al. 2017 GCA). Measuring the rate of iron oxidation is challenging because it requires incubations that are both anaerobic and trace metal clean, and most commonly used anaerobic techniques have not been optimized to reduce trace metal contamination. We have developed two incubation systems that are capable of dealing with this problem, one shipboard and one in-situ, and deployed those systems on two cruises to measure the distribution of anaerobic iron oxidation in the Eastern Tropical North Pacific ODZ, allowing us to estimate the rate of iron(II) loss as oxyhydroxides and the implications for iron transport in ODZs.