Marine pathways to nitrous oxide

There is no doubt that oceanic N2O emissions play a major role in the atmospheric N2O budget. The quantification of the oceanic N2O emissions and the identification of the marine pathways of N2O formation and consumption have received increasing attention during the last few decades. The very first study of oceanic N2O (in the South Pacific Ocean) was published by Craig and Gordon (1963), followed by studies in the North Atlantic Ocean by Junge and Hahn during the late 1960s and early 1970s (Junge and Hahn, 1971; Hahn, 1974). Junge and Hahn were the first to quantify the oceanic source of atmospheric N2O. In 1976 Yoshinari published his now ‘classical’ study of N2O profiles in the Sargasso and Caribbean Seas, which turned out to be groundbreaking because it was the first study to report the inverse correlation between N2O and O2 concentrations in the water column (Yoshinari, 1976). He also introduced the term N2O (for a definition see below) as a measure of the ‘apparent N2O production’ and found a linear correlation between N2O and AOU (apparent oxygen utilization) (Yoshinari, 1976). Based on this, he suggested that ‘N2O production in the sea is related in some way to the oxidation sequence of organic matter’, which was an early hint of nitrification (i.e. microbial oxidation of NH4 to NO3) as a major N2O formation process in the ocean. Cohen and Gordon (1978), Cohen (1978) and Elkins et al (1978) were the first to report a significant N2O consumption in the oxygen minimum zone in the subsurface waters of the eastern tropical Pacific Ocean and the anoxic waters of the Saanich Inlet basin (off Vancouver Island). They attributed the N2O loss to microbial reduction of N2O to N2 (i.e. denitrification). In order to verify the marine pathways to N2O, isotope studies have been introduced in recent years: first, measurements of the 15N value of dissolved N2O were presented by Yoshida et al (1984) and nine years later Kimand Craig (1993) published the first measurement of the dual isotope signature (15N and 18O) of oceanic N2O. This was followed by the publication of the isotopomeric signature of N2O (which makes it possible to distinguish the position of 15N within the asymmetric N2O molecule: NNO) by Popp et al (2002) and Toyoda et al (2002).