A review of the chemistries of redox sensitive elements within suboxic zones of oxygen minimun regions
This presentation is a review of the chemistries of redox sensitive elements within suboxic regions of extreme oxygen minimum zones. Suboxic regions are defined as extreme oxygen minimum zones with dissolved oxygen < 10 µmol kg-1 (and likely <5 µmol kg-1), and with clear evidence of nitrate reduction, but not sulfate reduction. These are regions with nitrate deficits > 10 µmol kg-1 or N* values of _10 µmol kg-1. Suboxic regions predictably occur in the tropical and subtropical eastern Pacific and the Arabian Sea. These suboxic zones are important in the global biogeochemistry of nitrogen and act as an important sink for fixed nitrogen. Minor and trace element redox couples such as iodate/iodide and leachable particulate Mn/dissolved Mn undergo complete reduction within suboxic regions and clearly define suboxic zones in the water column. Other trace element redox couples such as Cr(VI)/Cr(III) and Se(VI)/Se(IV)/Se(-II) undergo partial reduction and are more complicated due to the chemistries of the different redox species. Iron does not appear to undergo in-situ reduction within suboxic waters, but when the suboxic waters overly shelf sediments dissolved Fe(II) can be supplied to the suboxic water column from the anoxic sediments and be partially stabilized with respect to oxidation to Fe(III) in these suboxic waters.