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 the entire eastern Pacific Ocean.

Oxygen deficient zones (ODZs) are persistent regions of the ocean in which rapid heterotrophic respiration and poor ventilation result in oxygen (O 2 ) concentrations below detection by conventional means (Margolskee et al., 2019). Two of the largest ODZs occur at pelagic depths within the Eastern Tropical North and South Pacific (ETNP and ETSP), where dissolved O 2 concentrations <10 nmol kg −1 have been observed (Thamdrup et al., 2012). Within these regions, anaerobic microbial metabolisms, including denitrification and anammox, are possible, causing ODZs to have a significant impact on global biogeochemical cycles of nitrogen, and by consequence, carbon. ODZs generate roughly half of marine nitrous oxide emissions despite occupying only 0.35% of the ocean's volume (Codispoti, 2010;Karstensen et al., 2008) and are responsible for 20%-40% of marine bio-available nitrogen loss (Brandes & Devol, 2002;DeVries et al., 2013). In addition, significant oxygen loss from the oceans has been observed in the past 50 years (Schmidtko et al., 2017;Stramma et al., 2008), and climate models generally predict further deoxygenation due to thermal effects on solubility (Cabré et al., 2015). Predicted trends for ODZs are more complex, however, as increased stratification in the tropics can curtail localized

The Eastern Tropical North Pacific (ETNP), like the other marine oxygen deficient zones (ODZs), is characterized by an anoxic water column, nitrite accumulation at the anoxic core, and fixed nitrogen loss via nitrite reduction to N2O and N2 gases. Here, we constrain the relative contribution of biogeochemical processes to observable features such as the secondary nitrite maximum (SNM) and local pH maximum by simultaneous measurement of inorganic nitrogen and carbon species. High‐resolution sampling within the top 1 km of the water column reveals consistent chemical features previously unobserved in the region, including a tertiary nitrite maximum. Dissolved inorganic carbon measurements show that pH increases with depth at the top of the ODZ, peaking at the potential density of the SNM at σθ = 26.15 ± 0.06 (1 s.d.). We developed a novel method to determine the relative contributions of anaerobic ammonium oxidation (anammox), denitrification, nitrite oxidation, dissimilatory nitrate reduction to nitrite, and calcium carbonate dissolution to the nitrite cycling in the anoxic ODZ core. The calculated relative contributions of each reaction are slightly sensitive to the assumed C:N:P ratio and the carbon oxidation state of the organic matter sinking through the ODZ. Furthermore, we identify the source of the pH increase at the top of ODZ as the net consumption of protons via nitrite reduction to N2 by the denitrification process. The increase in pH due to denitrification impacts the buffering effect of calcite and aragonite dissolving in the ETNP.