Superoxide decay kinetics in the Southern Ocean

Measurements of superoxide (O₂^-) reaction kinetics were made during a transect with the research icebreaker Polarstern (ANT24-3) in the Antarctic through the Drake Passage in austral autumn 2008. Our sampling strategy was designed to investigate the sinks of superoxide in Polar waters; principally through reactions with dissolved organic matter (DOM) or metals (copper and iron). We modified an existing chemiluminescence flow injection system using methyl Cypridina luciferin analog (MCLA) for the detection of O₂^- and added O₂^- using KO ₂ as the source. Our results indicate that O₂^- in ambient seawater had a half-life ranging from 9.3 to 194 s. DTPA additions to seawater, to remove the effects of reactions with metals, revealed O ₂^- decay rates consistent with a second order reaction, indicating that the dismutation reaction dominated and that reactions with DOM were not significant. Titrations of seawater by the addition of nanomolar amounts of iron or copper revealed the importance of organic chelation of Fe and/or Cu in controlling the reactivity with O₂^-. Throughout the water column reactions with Cu appeared to be the major sink for superoxide in the Southern Ocean. This new strategy suggests an alternative approach for speciation measurements of Fe and Cu in seawater.