Constraints on marine ODZ iodine cycling and implications for paleoredox proxy applications

The relative abundances of the iodine species iodide and iodate have been applied extensively to characterize both modern and ancient marine oxygen deficient zones (ODZs). Here, we directly evaluate iodate reduction rates, oxygen concentrations, and water mass as controls on ODZ iodate variability in an offshore transect within the Eastern Tropical North Pacific (ETNP). We also compare iodine and dissolved oxygen concentrations between globally distributed open ocean ODZs. For one offshore station of the ETNP we performed shipboard experiments tracking iodate reduction through a vertical ODZ profile. Iodate reduction was limited to the suboxic zone of the oxycline, while anoxic depths below the oxycline did not show trends consistent with iodate reduction. This vertical contrast is interpreted to support an important productivity control on iodate reduction even in low oxygen waters. Limited iodate reduction in the ODZ core (26-27 kg m^-3 potential density anomaly) is generally consistent with the ubiquitous presence of iodate in this zone at ours and other localities of the ETNP. Our incubation results within the ODZ core are also consistent with water mass analyses and detailed iodine speciation along a longitudinal, offshore ETNP transect that demonstrate local iodate abundance as a semi-conservative tracer governed in part by advection of regional water masses. A compilation of iodate and dissolved oxygen concentrations from global ODZs supports the view that iodate reduction is a clear feature of ODZs, but that at least some variations in vertical and lateral transects may largely reflect the preservation and exchange of iodine concentrations and speciation generated in adjacent water masses or regions—as opposed to in situ processes. In this context, iodine speciation from the ancient and modern ocean may best indicate mixing between water masses that retain variable spatially and temporally integrated redox histories.