Implications of nitrate and nitrite isotopic measurements for the mechanisms of nitrogen cycling in the Peru oxygen deficient zone

Oceanic oxygen deficient zones (ODZs) are important regions for oceanic primary productivity, nitrous oxide (N2O) production, and the marine nitrogen (N) budget. These areas are recognized as hotspots for fixed N loss, although the rates and mechanisms of N loss have been difficult to quantify. Stable isotopes of nitrate and nitrite integrate the effects of a complex suite of processes occurring in these regions. Here we examine the distributions of nitrate δ15N and δ18O and nitrite δ15N in the Peruvian ODZ. Our data reveal elevated nitrate δ15N and δ18O values, particularly after correcting for the isotopic contribution of nitrite. Moreover, the isotopic composition of nitrite, a central intermediate in the marine N cycle, provides an additional constraint on the processes occurring in the Peru ODZ. A simple finite difference model is used to interpret the mechanisms and relative rates of N transformation in the waters sampled off the coast of Peru. Nitrite oxidation is found to be an important sink for nitrite, in many cases exceeding the rate of nitrite reduction. In model results, the apparent isotope effect for nitrate reduction, as inferred from a closed system Rayleigh model of nitrate concentration and δ15N, is greater than the prescribed value due to the effects of nitrite oxidation. Accordingly, the isotope effect for nitrate reduction that best fits the data is 12‰, much lower than the commonly inferred 25‰. Furthermore, nitrite oxidation may mediate the δ15N of N2 produced in this water column suboxic zone through its effect on the δ15N values of NO2− and NO3−.