Westward Penetration of Particulate and Dissolved Iron Redox Species from the Peruvian Margin

The Peruvian margin is known for its intensive oxygen deficient zone (ODZ), which extends westward from the shelf from near surface (~40m) to a depth of ~800m. Observations of dissolved Fe (dFe) near the Peruvian margin obtained during the 2013 GEOTRACES EPZT cruise show that the westward penetration of elevated dFe concentrations is at least as great, if not greater, in the oxic waters below the ODZ compared to the ODZ.

We present here particulate iron (pFe) speciation from in-situ pump samples taken concurrently during the 2013 Geotraces EPZT cruise. Most of the dFe in the ODZ was found in the Fe(II) redox state, whereas the speciation of pFe, which was examined by the use of synchrotron chemical species mapping, XANES, and EXAFS, showed that pFe was dominated by the Fe(III) redox state and was present in the form of Fe oxyhydroxides. Because the usual oxidants for dFe(II), oxygen and hydrogen peroxide, are absent in ODZs, alternative oxidants must be considered. We found that the highest concentrations of pFe and nitrite (NO₂^-) are co-located, suggesting that nitrogen species may play a key role in the oxidation of dFe(II) in the ODZ.

In more oxygenated near-bottom waters on the slope below the ODZ, we found a much higher abundance of pFe(II). XANES spectroscopy identified these pFe(II) phases to be crystalline Fe(II) silicates such as biotite, a common mineral found in a wide variety of igneous and metamorphic rocks, likely derived from the underlying lithogenic sediment in the region. Our data suggests active redox cycling within the ODZ that partially traps shelf-derived dFe from penetrating into the interior. We postulate that elevated dFe levels in the oxic layer below the ODZ may derive from the oxic dissolution of species like those unweathered Fe(II) silicates in the slope sediments. However, it is unclear what processes convert the pFe(II) into dFe and also how dFe is stabilized so that it can be transported over a long distance.