A NOVEL TRACER METHOD TO PROVIDE PRINCIPAL CONSTRAINTS ON SEAWATER IODINE REDOX CHEMISTRY

Ratios of iodine-to-calcium magnesium (I/(Ca+Mg)) in marine carbonate have been applied extensively as a tool to specifically track the oxidized iodine species, iodate, in ancient oceans. Observations of dynamic I/(Ca+Mg) ratios in Paleoproterozoic, Neoproterozoic, and Mesozoic carbonate have been directly tied to shallow oxygen oxygenation as well as widespread marine anoxia. This application is predicated on modern observations of iodate reduction to the reduced iodide in low oxygen seawater. However, the conditions necessary for and catalysing the formation of iodate, the most abundant marine iodine species, are nearly completely unresolved, limiting quantitative applications. To address this issue, we have developed a novel method for tracing iodine redox reactions in seawater incubations. The method uses addition of the radioactive isotope I-129 (t_1/2 ~ 15.7 Myrs) to natural samples (seawater incubations) and liquid ion chromatographic separation of the two main iodine species (iodate-iodide) to trace the production of iodate. The iodine isotope ratios are measured using multicollector-inductively coupled plasma-mass spectrometry coupled to a new iodine sparge introduction technique calibrated for detecting trace iodine contents. We have applied the technique to a series of seawater incubations to track iodide oxidation from variable depths and localities along the coastal Northeastern US and mid-Atlantic. Our tracer results quantify marine iodide oxidation rates under normal marine conditions, which differ between localities and at variable depths. Partial oxidation products form most commonly and preferentially react with dissolved organic carbon, opposed to continued oxidation to iodate. In addition, controls provide evidence for a likely biotic mechanism and support that oxidants stronger than oxygen, such as reactive oxygen species, ultimately catalyze marine iodide oxidation.