Iron(II) in the world's oxygen deficient zones

Abstract

Oxygen deficient zones (ODZs) are regions in the marine water column where the concentration of oxygen is very low or zero and respiration occurs using nitrate as the terminal electron acceptor. Under such conditions, iron (Fe) would be predicted to be in the +3 valence state, and its geochemistry would be largely unaffected by the absence of oxygen. In contrast, observations show that Fe(II) is a major and sometimes dominant oxidation state in ODZs, even in the presence of a substantial amount of excess nitrate. Fe(II) plays an important role in Fe transport in ODZs. They are generally enriched in Fe throughout the regions where nitrate reduction is occurring. Here, it is argued that Fe(II) maxima arise as a result of processes common to the Eastern Tropical North and South Pacific as well as the Arabian Sea. Recent work has revealed that lateral advection of Fe(II) from reducing sediments is perhaps the major source and it appears that physical processes that are still poorly understood act over a surprisingly narrow density range. Tracers including iodine and 228Ra have been extremely useful in constraining these processes. The findings reported here have important implications for Fe transport from the continental margin to the ocean interior, and for microbes involved in the nitrogen cycle that have high Fe requirements.