Patrick J. Monreal, Colette L. Kelly, Nicole M. Travis, Pascale A. Baya, Karen L. Casciotti
Although it is less prevalent in the atmosphere than carbon dioxide, nitrous oxide (N2O) is a powerful greenhouse gas with a potency 300 times as great. Oceanic sources account for up to one third of the total flux of nitrous oxide to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes — nitrification, nitrifier-denitrification, and denitrification — that are impacted by a variety of oceanographic variables and substrates. In this study, isotopomers of N2O from a 2016 cruise to the ETNP aboard R/V Sikuliaq were analyzed to look at spatial variability in the concentration and isotopic composition of N2O between stations. Three distinct regimes of N2O cycling, delineated by its isotopic composition, were observed in the 12 stations that were included in analysis. The high N2O regime was distinguished by high near-surface [N2O], reaching 126.07 ± 12.6 nmol/kg, corresponding to low-oxygen conditions that expand into near-surface isopycnals. Keeling plot analyses point to a different near-surface N2O source in the high N2O regime, with the intercepts for δ15Nα and δ15Nβ 3-4‰ lower at these stations than the rest. The central ODZ regime was characterized by strong consumption and production signals in the anoxic core of the ODZ, indicated by high δ18O (> 90‰) and low δ15Nβ (< -10‰) values. The “background” regime (closest to the Baja peninsula) exhibited less dynamic cycling than the other two — regression between δ18O and δ15Nα or δ15Nbulk did not follow the relationship expected for reduction. A time-dependent model was used to analyze the drivers of this variability, and preliminary findings suggest some of the stations could be approaching steady state while others do not.