More Than Deoxygenation: Linking Iodate Reduction to Nitrogen, Iron, and Sulfur Chemistry in Reducing Regimes

A striking feature of Oxygen Deficient Zones (ODZs) on the eastern boundary of the Pacific Ocean are large subsurface plumes of iodide. Throughout the oceans, iodate is the predominant and thermodynamically favored species of dissolved iodine, but iodate is depleted within these plumes. The origin of iodide plumes and mechanism of reduction of iodate to iodide remains unclear but is thought to arise from a combination of in situ reduction and inputs from reducing shelf sediments. To distinguish between these sources, we investigated iodine redox speciation along the Oregon continental shelf. This upwelling system resembles ODZs but exhibits episodic hypoxia, rather than a persistently denitrifying water column. We observed elevated iodide in the benthic boundary layer overlying shelf sediments, but to a much smaller extent than within ODZs. There was no evidence of offshore plumes of iodide or increases in total dissolved iodine. Results suggest that an anaerobic water column dominated by denitrification, such as in ODZs, is required for iodate reduction. However, re‐analysis of iodine redox data from previous ODZ work suggests that most iodate reduction occurs in sediments, not the water column, and is also decoupled from denitrification. The underlying differences between these regimes have yet to be resolved, but could indicate a role for reduced sulfur in iodate reduction if the sulfate reduction zone is closer to the sediment‐water interface in ODZ shelf sediments than in Oregon sediments. Iodate reduction is not a simple function of oxygen depletion, which has important implications for its application as a paleoredox tracer. Plain Language Summary Inorganic iodine has two stable forms in the ocean, iodate and iodide. In most of the subsurface ocean, iodate is the predominant compound, except in regions of the ocean without oxygen, where iodate is depleted and iodide accumulates. The causes of iodate to iodide conversion remains unclear, but it is often linked to denitrification since both iodate and nitrate can be used by anaerobic bacteria in respiration. We compared iodine behavior in the coastal margins of Oregon and Mexico's Pacific Coast. The former has no denitrification in the water column, whilst the latter has a vast subsurface zone where denitrification occurs. We sampled the low but nonzero oxygenated waters on the Oregon continental shelf. We did not observe iodide accumulation or iodate depletion, in contrast to the Mexican study area.