Nitrous oxide cycling in the Eastern Tropical South Pacific as inferred from isotopic and isotopomeric data

The ocean accounts for up to 25% of global emissions of nitrous oxide (N2O), a potent greenhouse gas. Much of this N2O flux occurs in upwelling regions near the ocean's oxygen deficient zones (ODZs), areas known for intense N2O cycling. The Eastern Tropical South Pacific (ETSP) ODZ is one such area, and large uncertainties surround the balance of processes regulating N2O production and emission in this region. Here we examined the distributions of dissolved N2O concentration and stable isotopic composition, in concert with nitrate (NO3-) and nitrite (NO2-) isotopic ratios, to understand the mechanisms that drive N2O production, consumption, and emission from the ETSP ODZ. Keeling plot analysis identified N2O production from both nitrification and denitrification (or nitrifier-denitrification) in the near surface and in the oxycline, where the largest accumulations of N2O were found. In the N2O concentration maximum that occurs below the ODZ, a higher N-15 site preference (SP) indicated nitrification was more prominent. Within the ODZ, significant enrichments were apparent in delta(15)Nbulk (14-22 parts per thousand), delta O-18(N2O) (68-100 parts per thousand) and SP (39-60 parts per thousand), implying active N2O consumption. Further scrutiny of N2O isotope data in the ODZ highlights a deviation from the relative increases in delta O-18(N2O) and SP expected for bacterial denitrification. At high levels of N2O consumption, SP increased more than expected for the increase in delta O-18(N2O). This appeared to be due, at least in part, to a decrease in delta N-15(beta) driven by N2O production in the ODZ, rather than further increases in delta N-15(alpha). Isotopic analysis of co-occurring NO3- and NO2- suggests that NO3- may be the dominant source of N2O in the offshore ETSP ODZ.