Abstract
Dissolved iron (DFe) concentrations in oxygen minimum zones (OMZs) of Eastern Boundary Upwelling Systems are enhanced as a result of high supply rates from anoxic sediments. However, pronounced variations in DFe concentrations in anoxic coastal waters of the Peruvian OMZ indicate that there are factors in addition to dissolved oxygen concentrations (O2) that control Fe cycling. Our study demonstrates that sediment-derived reduced Fe (Fe(II)) forms the main DFe fraction in the anoxic/euxinic water column off Peru, which is responsible for DFe accumulations of up to 200 nmol L−1. Lowest DFe values were observed in anoxic shelf waters in the presence of nitrate and nitrite. This reflects oxidation of sediment-sourced Fe(II) associated with nitrate/nitrite reduction and subsequent removal as particulate Fe(III) oxyhydroxides. Unexpectedly, the highest DFe levels were observed in waters with elevated concentrations of hydrogen sulfide (up to 4 µmol L−1) and correspondingly depleted nitrate/nitrite concentrations (<0.18 µmol L−1). Under these conditions, Fe removal was reduced through stabilization of Fe(II) as aqueous iron sulfide (FeSaqu) which comprises complexes (e.g., FeSH+) and clusters (e.g., Fe2S2|4H2O). Sulfidic events on the Peruvian shelf consequently enhance Fe availability, and may increase in frequency in future due to projected expansion and intensification of OMZs.
Highlights
Iron (Fe) forms an important micronutrient, controlling marine phytoplankton growth and nitrogen fixation in vast regions of the global ocean[1,2,3]
The steep dissolved Fe (DFe) concentration gradients and anoxic conditions of the bottom waters facilitate enhanced Fe fluxes out of the sediments reaching 10 to 866 μmol DFe m−2 d−1 8, which result in bottom water DFe concentrations of 20–300 nmol L−1 7,17, with most of the DFe being present in the reduced form Fe(II)[18,19,20,21]
The formation of Fe(II)S) (e.g. mackinawite (FeSm) is reported to occur in anoxic coastal waters and has been observed in permanently euxinic bottom waters of the Framvaren fjord (Norway), where Fe(II) and H2S levels are in the millimolar range[32]
Summary
Iron (Fe) forms an important micronutrient, controlling marine phytoplankton growth and nitrogen fixation in vast regions of the global ocean[1,2,3]. The Peruvian OMZ features extremely low O2 concentrations (
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