Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

Nitrous oxide (N 2 O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N 2 O cycling in an ODZ, we analyzed N 2 O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N 2 O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above the σ θ  = 25.0 kg/m 3 isopycnal, Keeling plot analysis revealed two sources to the near‐surface N 2 O maximum, with different δ 15 N 2 O α and δ 15 N 2 O β values, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below the σ θ  = 25.0 kg/m 3 isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N 2 O production by denitrification, and may indicate instead a transient net consumption of N 2 O. Furthermore, time‐dependent model results indicated that while δ 15 N 2 O α and δ 18 O‐N 2 O reflect both N 2 O production and consumption in the anoxic core of the ODZ, δ 15 N 2 O β predominantly reflects N 2 O sources. Finally, we infer that the high (N 2 O) observed at some stations derive from a set of conditions supporting high rates of N 2 O production that have not been previously encountered in this region.