Limited iodate reduction in shipboard seawater incubations from the Eastern Tropical North Pacific oxygen deficient zone

The relative abundance of the inorganic iodine species, iodide and iodate, are applied to characterize both modern and ancient marine oxygen deficient zones (ODZs). However, the rates and mechanisms responsible for in situ iodine redox transformations are poorly characterized, rendering iodine-based redox reconstructions uncertain. Here, we provide constraints on the rates and mechanisms of iodate reduction in the Eastern Tropical North Pacific (ETNP) offshore ODZ using a shipboard tracer–incubation method. Observations of iodate reduction from incubations were limited to the top of the oxycline (σθ∼25.2kgm−3) where native oxygen concentrations were low, but detectable (≈11 μM). Incubations from additional depths below the oxycline—where O₂ was <2 μM—yielded no detectable evidence of iodate reduction despite hosting the lowest iodate concentrations. These experiments place an upper limit of iodate reduction rates of generally <15 nM day⁻¹ but as low as <2.3 nM day⁻¹, which are based on variable precision of individually incubated replicates between experiments. Experimental inferences of limited or slow iodate reduction in the ODZ core relative to that observed in the oxycline are generally consistent with iodate persistence of up to 70 nM and low biological productivity in this zone. We also compare dissolved iodine and oxygen concentrations between variable water masses of the ETNP and globally distributed open ocean ODZs. Consistent with sluggish reduction rates, comparison of iodate concentrations with previously published water mass analyses within the ETNP ODZ (σθ=26-27kgm−3; O<27μM) demonstrate iodate as a semi-conservative tracer at least partially reflecting regional water mass mixing. A compilation of iodate and dissolved oxygen concentrations from global ODZs generally supports that at least some iodate variations in both vertical and lateral transects largely reflect variable combinations of relatively slow reduction and mixing of iodate reduction signals generated in adjacent regions—as opposed to solely rapid in situ processes. In this context, the variations in iodine speciation inferred for the ancient and modern ocean represent a combination of in situ processes and regional mixing between water masses that retain variable spatially and temporally integrated redox histories.