The speciation of particulate iron in the Peruvian Oxygen Minimum Zone

Suspended particulate matter (SPM) was collected by in-situ filtration from the Peruvian Oxygen Minimum Zone (OMZ) during the US GEOTRACES Eastern Pacific Zonal Transect cruise. Here, we use X-ray Absorption Near Edge Spectroscopy to examine the redox speciation of marine particulate iron from the Peruvian OMZ to understand the redox cycling processes that generate and maintain elevated Fe concentrations. We hypothesized that Fe(III) oxyhydroxides would be highest at the regions of highest horizontal and vertical oxygen gradients, where dissolved Fe(II) would be lost to oxidization, but found instead that the highest oxyhydroxide concentrations were in oxygen deficient waters on the Peruvian shelf where dissolved Fe(II) was highest. The oxyhydroxides were dominated by the lepidocrocite (gamma-FeOOH) form, which is generally thought to require dissolved Fe(II) for its formation. Although lepidocrocite is relatively common in soils and freshwater systems, there are few reports of its presence in marine systems. Lepidocrocite has been shown to stimulate Fe(II)-dependent nitrite reduction in soils, and we suggest that it may also play a role here in nitrogen cycling.