Global distribution of N 2 O and the ΔN 2 O‐AOU yield in the subsurface ocean

We present and analyze a data set of subsurface N 2 O from a range of oceanic regions. Observed N 2 O concentrations are highest in the eastern tropical Pacific (ETP), intermediate in the northern Pacific and Indian Oceans, and relatively low in the Southern and Atlantic Oceans. Tongues of high N 2 O, which propagate along sigma surfaces, provide evidence that N 2 O from the ETP is exported widely. Correlation slopes of ΔN 2 O (the level above atmospheric equilibrium) versus apparent oxygen utilization (AOU) are found to be an unreliable gauge of the biological N 2 O yield per mole O 2 consumed because the slopes are strongly influenced by mixing gradients. Most features of the subsurface data set are consistent with an N 2 O source dominated by nitrification, including the widespread, robust ΔN 2 O‐AOU correlation and the lack of a widespread anticorrelation between ΔN 2 O and N*. In addition, ΔN 2 O/NO 3 − ratios tend to increase with decreasing O 2 in a manner consistent with laboratory studies of nitrifying bacteria. The sensitivity of the nitrifier N 2 O/NO 3 − yield to O 2 can explain much of the variability in ΔN 2 O/AOU observed in the ocean. A parameterization is derived for the instantaneous production of N 2 O per mole O 2 consumed as a nonlinear function of O 2 and depth. The parameterization is based on laboratory and oceanic data and is designed for use in ocean biogeochemistry models. It is coupled to a global dissolved O 2 climatology and ocean carbon model output to estimate a total oceanic N 2 O inventory of 610–840 Tg N and a global production rate of ∼5.8 ± 2 Tg N/y.