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The distributions of iodate (IO₃⁻), iodide (I⁻), nitrite (NO₂⁻), and oxygen (O₂) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centeredmore »within a potential density of 26.2–26.6 kg/m³, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP.

Oceanic oxygen deficient zones (ODZs) influence global biogeochemical cycles in a variety of ways, most notably by acting as a sink for fixed nitrogen (Codispoti et al. 2001). Optimum multiparameter analysis of data from two cruises in the Eastern Tropical North Pacific (ETNP) was implemented to develop a water mass analysis for the large ODZ in this region. This analysis reveals that the most pronounced oxygen deficient conditions are within the 13°C water (13CW) mass, which is distributed via subsurface mesoscale features such as eddies branching from the California Undercurrent. Nitrite accumulates within these eddies and slightly below the core of the 13CW. This water mass analysis also reveals that the 13CW and deeper Northern Equatorial Pacific Intermediate Water (NEPIW) act as the two Pacific Equatorial source waters to the California Current System. The Equatorial Subsurface Water and Subtropical Subsurface Water are synonymous with the 13CW and this study refers to this water mass as the 13CW based on its history. Since the 13CW has been found to dominate the most pronounced oxygen deficient conditions within the Eastern Tropical South Pacific ODZ and the Peru‐Chile Undercurrent, the 13CW and the NEPIW define boundaries for oxygen minimum conditions across themore »entire eastern Pacific Ocean.

Iron(II) can catalyze the oxidation of luminol in seawater and this chemiluminescent reaction has been widely used for iron(II) determination. The method is vulnerable to interferences from other analytes that catalyze luminol oxidation. We have shown that addition of diethylenetriamine pentaacetic acid (DTPA) to a sample inhibits the reaction of iron(II) with luminol, while not affecting other substances that also catalyze luminol oxidation under our experimental conditions. DTPA‐treated samples can therefore be used as sample blanks, with the difference between an untreated seawater sample and a DTPA‐treated seawater sample related to the concentration of dissolved iron(II). The DTPA correction has been applied to measure diel variability of iron(II) due to photoreduction in a coastal environment, and to measure vertical distributions of iron(II) in the eastern tropical north Pacific oxygen deficient zone.