As part of the US GEOTRACES Eastern Pacific Zonal Transect cruise, marine particles were collected by in situ filtration along a greater than 7000 km transect of the Pacific Ocean from the Peruvian Coast to the central South Pacific. The sample set includes full water column profiles of two major oceanographic features hosting strong geochemical gradients: (1) the Peruvian Oxygen Minimum Zone (OMZ); and (2) the East Pacific Rise (EPR) mid-ocean ridge hydrothermal plume at 15oS. In addition, samples of seafloor sediments were obtained. Iron (Fe) speciation measurements using X-ray microprobe (10.3.2) and scanning transmission X-ray microscopy (STXM, 5.3.2.2) instruments at the Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, are on-going. Iron (1s and 2p) X-ray absorption near edge structure (XANES) spectra are being used to measure the oxidation-reduction (redox) status and chemical/mineralogical form of particulate Fe. In the OMZ, we hypothesized that the Fe(III) oxyhydroxide concentrations would be highest within strong dissolved oxygen (DO) gradients where dissolved Fe(II) is consumed by oxidation. In contrast, the observations to-date show that the highest Fe(III) oxyhydroxide concentrations were in oxygen deficient waters where dissolved Fe(II) was highest. The Fe(III) oxyhydroxides observed in the high DO gradients of the OMZ were dominated by lepidocrocite (γ-FeOOH); a mineral generally thought to form from dissolved Fe(II). Lepidocrocite has been shown to stimulate Fe(II)-dependent nitrite reduction in soils, and we suggest that it may also be a reactant/product of nitrogen cycling in the Peruvian OMZ. In the EPR 15oS hydrothermal plume, we hypothesized that carbon (C) and sulfur (S) would slow Fe(II) oxidation in plume particles. This hypothesis was based on recent observations at EPR 9oN, as well as historical plume studies indicating abundant S along the EPR 15oS segment of the mid-ocean ridge. In contrast, the 15oS plume and underlying sediments were depleted in both particulate S and reduced Fe. These observations lead us to suggest that S is an essential component in protecting hydrothermal Fe from rapid oxidation processes in the water column and sediments.

Speciation of particulate iron in marine geochemical gradients

249th National Meeting (2015)

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