The response of trace element redox couples to suboxic conditions in the water column

Depth profiles for a series of redox-sensitive elements were determined in the productive eastern boundary of the tropical North Pacific. Suboxic conditions (regions with dissolved oxygen concentrations < 5 pmol kg−1 and evidence of nitrate reduction) was observed at depths between 140 and 600 m. The behavior of a suite of trace (≤1 μM) redox-sensitive species such as oxidized and reduced forms of iodine, manganese, chromium, selenium, and iron was examined, in order to better characterize redox conditions within such suboxic zones. Iodate was completely reduced to iodide, and leachable particulate manganese was completely reduced to dissolved manganese between the depths of 140 and 600 m. In contrast, no more than 30% of the chromate, selenate, and selenite were reduced, while there was no evidence of leachable particulate iron being reduced. The resulting distributions of trace redox elements in these suboxic waters are influenced both by the extent that they are reduced and by the particular biogeochemical characteristics of their oxidized and reduced species.