Nitrous oxide (N₂O) is an important climate-relevant trace gas in the Earth’s atmosphere. In the troposphere it acts as a strong greenhouse gas and in the stratosphere it acts as an ozone depleting substance because it is the precursor of ozone depleting nitric oxide radicals. Because of the on-going decline of chlorofluorocarbons and the continuous increase of N₂O in the atmosphere, the contributions of N₂O to both the greenhouse effect and ozone depletion will be even more pronounced in the 21st century.

The ocean - including its coastal areas such as continental shelves, estuaries and upwelling areas - is a major source of N₂O and contributes about 30% to the atmospheric N₂O budget. Oceanic N₂O is mainly produced as a by-product during archaeal nitrification (i.e. ammonium oxidation to nitrate) whereas bacterial nitrification seems to be of minor importance as source of oceanic N₂O. N₂O occurs also as an intermediate during microbial denitrification (nitrate reduction via N₂O to dinitrogen, N₂). Nitrification is the dominating N₂O production process, whereas denitrification contributes only 7-35% to the overall N₂O water column budget in the ocean. The amount of N₂O produced during both nitrification and denitrification strongly depends on the prevailing dissolved oxygen (O₂) concentrations and is significantly enhanced under low (i.e. suboxic) O₂ conditions. N₂O is usually not detectable in anoxic waters because of its reduction to N₂ during denitrification. Thus, significantly enhanced N₂O concentrations are generally found at oxic/suboxic or oxic/anoxic boundaries.

The strong O₂ sensitivity of N₂O production is also observed in coastal characterised by seasonal shifts in the O₂ regime. A biological source of N₂O in the well-oxygenated mixed layer/euphotic zone seems to be unlikely. Global maps of N₂O in the surface ocean show enhanced N₂O anomalies (i.e. supersaturation of N₂O) in equatorial upwelling regions as well as N₂O anomalies close to zero (i.e. near equilibrium) in large parts of the open ocean.

The MEMENTO (The MarinE MethanE and NiTrous Oxide database: https://memento.geomar.de) project has been launched with the aim to collect and archive N₂O data sets and to provide actual fields of surface N₂O for emission estimates.