Nitrous oxide distribution in the water column

The shapes of N2O profiles generally fall into three categories:

• Cat. I profiles from oceanic regions with dissolved oxygen concentrations [O2] >10pmol litre-1 throughout the water column (for example in the Atlantic Ocean, the South Indian Ocean and the central North Pacific and central South Pacific Oceans);

• Cat. II profiles from regions with sub-oxic environments (0 <[O2] <2-10pmol litre-1, Codispoti et al, 2005) such as found in intermediate water depths from about 200m to about 800m in the Arabian Sea and the eastern North/South Pacific Ocean;

• Cat. III profiles from regions with anoxic deep water masses with [O2] = 0pmol litre-1 and hydrogen sulphide present. Anoxic water masses are found only in a few regions of the world's oceans. Perennial anoxic environments occur in the Black Sea and the Cariaco Basin off Venezuela. Temporarily occurring anoxic conditions have been reported from the deep basins of the central Baltic Sea.

Typical N2O profiles illustrating Cat. I-III profiles are shown in Figure 3.1 (additional examples and references can be found in Bange, 2008). It is obvious that the shapes of the N2O profiles undergo a significant change when [O2] falls below the threshold for sub-oxic conditions. For instance, the one-peaked profiles (Cat. I) observed in the southern Arabian Sea turn into two-peaked profiles in the central Arabian Sea where sub-oxic conditions are found in the intermediate layers (Bange et al, 2001). Cat. III show no pronounced N2O peak at the boundary of the oxic and anoxic water masses (see for example Hashimoto et al, 1983; Walter et al, 2006b; Westley et al, 2006).

The characteristics of the profiles described above are valid for 'static' oceanic systems under steady-state conditions with turnover times much longer than one year. Some coastal areas, however, show a dynamic behaviour, with a rapid seasonal overturning from oxic via sub-oxic to anoxic conditions and vice versa (for example the shelf off West India, the western Baltic Sea, the shelf off Chile, an upwelling area off southwest Africa and the Gulf of Mexico). In these kinds of transient systems, significant amounts of N2O can accumulate temporarily during the short transition time when the system is about to change its oxygen regime. Interestingly, the timing of the N2O accumulation occurs at different transition stages and seems to be characteristic for different coastal systems: in the southwestern Baltic Sea, N2O only accumulates when the system is shifting from anoxic to oxic conditions (Figure 3.2) (Schweiger, 2006), whereas N2O accumulates when the systems are shifting from oxic to sub-oxic (off central Chile) or to anoxic (off West India) conditions (Naqvi et al, 2006; Cornejo et al, 2007). During the transition stages, the accumulation of N2O does not occur in the anoxic zone itself but at the oxic/anoxic boundaries. In anoxic zones, N2O is usually found at very low or even undetectable concentrations.


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100 200 300 400 Dissolved Oxygen [nmoí L"1]

100 200 300 400 Dissolved Oxygen [nmoí L"1]

Figure 3.1 Typical N2O profiles (right column) and dissolved O2 (left column): (panel A) Cat. I profiles from the tropical North Atlantic Ocean; (panel B) Cat. II profiles from the Guinea Dome in eastern Tropical North Pacific Ocean; (panel C) Cat. II profile from the Landsort Deep in the western Gotland Basin (central Baltic Sea)

Note: Sub-oxic (panel B) and anoxic (panel C) layers are indicated by the shading.

Source: Panel A from Walter et al (2006a); panel B from Bange and Walter (2007); panel C from Walter et al


Figure 3.2 (a) N2O saturations and (b) O2 concentrations at the time series station Boknis Eck (southwestern Baltic Sea, 54°31'N, 10°02'E, max. depth 28m) measured on a monthly basis from July 2005 to May 2006

Note:The anoxic event is marked by the vertical dashed lines. The horizontal bold line in (A) depicts the theoretical N2O saturation at equilibrium with the atmosphere. Source: Based on unpublished data from Schweiger (2006)

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