Global distribution of N_2O and the ΔN_2O-AOU yield in the subsurface ocean

摘要

[1] We present and analyze a data set of subsurface N_2O from a range of oceanic regions. Observed N_2O 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_2O, which propagate along sigma surfaces, provide evidence that N_2O from the ETP is exported widely. Correlation slopes of ΔN_2O (the level above atmospheric equilibrium) versus apparent oxygen utilization (AOU) are found to be an unreliable gauge of the biological N_2O 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_2O source dominated by nitrification, including the widespread, robust ΔN_2O-AOU correlation and the lack of a widespread anticorrelation between ΔN_2O and N~*. In addition, ΔN2O/NO3? ratios tend to increase with decreasing O_2 in a manner consistent with laboratory studies of nitrifying bacteria. The sensitivity of the nitrifier N_2O/NO_3~- yield to O_2 can explain much of the variability in ΔN_2O/AOU observed in the ocean. A parameterization is derived for the instantaneous production of N_2O 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_2O inventory of 610–840 Tg N and a global production rate of ～5.8 ± 2 Tg N/y.