THE RESPONSE OF TRACE-ELEMENT REDOX COUPLES TO SUBOXIC CONDITIONS IN THE WATER COLUMN

Depth profiles for a series of redox-sensitive elements were determine d in the productive eastern boundary of the tropical North Pacific. Su boxic conditions (regions with dissolved oxygen concentrations <5 mu m ol kg(-1) and evidence of nitrate reduction) was observed at depths be tween 140 and 600 m. The behavior of a suite of trace (less than or eq ual to 1 mu M) redox-sensitive species such as oxidized and reduced fo rms of iodine, manganese, chromium, selenium, and iron was examined, i n order to better characterize redox conditions within such suboxic zo nes. Iodate was completely reduced to iodide, and leachable particulat e 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. (C) 1997 Elsev ier Science Ltd.