The lesson from the past

The atmospheric history of N2O is illustrated by the ice core records which now reach back to 650,000 years before the present (yr BP) (Spahni et al, 2005). It seems that atmospheric N2O concentrations followed glacial climate changes but in a complex way (Spahni et al, 2005). The significant variability of the atmospheric N2O concentrations have been attributed to concurrent changes in both the terrestrial source and in the oceanic source (Sowers et al, 2003; Fluckiger et al, 2004). However, the ice core data do not allow for identification of the key parameters responsible for the abrupt changes of the N2O sources. More recently, coupled climate/biogeochemistry models were applied to investigate the role of the oceanic N2O production during fast climate changes such as the Younger Dryas cold period (~12,000 yr BP) (Goldstein et al, 2003) and the Heinrich event H5 (~48,000 yr BP) (Schmittner and Galbraith, 2008). In both models the oceanic N2O production was parameterized as a function of AOU. The model results of Goldstein et al (2003) suggested that the variability of the oceanic N2O source was the main but not the sole contributor to the observed changes of atmospheric N2O. The model results of Schmittner and Galbraith (2008) showed that the abrupt changes of atmospheric N2O during the Heinrich event H5 could have been caused by variabilities of the oceanic sources alone. They proposed that changes of the ocean circulation results in fast adjustments of the oxygen concentrations in the thermocline, which in turn drives the oceanic N2O production via nitrification (Schmittner and Galbraith, 2008).

Another line of argument is derived from 815N records from sediments underlying sub-oxic denitrification zones in the open ocean: several studies showed that the temporal variations of the denitrification signal in both the Arabian Sea and the eastern tropical Pacific Ocean during the last 23,000 years is paralleled by the reconstructed atmospheric N2O concentration from ice core records (Figure 3.7) (Suthhof et al, 2001; Thunell and Kepple, 2004; Agnihotri et al, 2006; Pichevin et al, 2007). These results imply that variations in the amount of the water column denitrification might have led to changes in the magnitude of N2O formation and its subsequent release to the atmosphere.

Figure 3.7 815N profiles from sediment cores in the Gulf of California/eastern tropical North Pacific, ETNP (core no. JPC56) and Arabian Sea (core no. S090-111KL) compared to reconstructed atmospheric N2O data from the GRIP (Greenland Ice Core Project) ice core

Note: YD = Younger Dryas; IS1 = Interstadial 1; H1 = Heinrich event 1; B/A = B0lling/Aller0d event. Source: Suthhof et al (2001)

On the basis of the model results and sedimentary 815N records introduced above we can conclude that the rapid changes observed in the palaeorecord of N2O concentration might be dominated by changes in the oceanic N2O production (nitrification and/or denitrification) via pronounced changes of the dissolved oceanic oxygen concentrations.

The ongoing rapid increase in atmospheric N2O, which started during the 19th century, is mainly attributed to the increase of agricultural activities (Kroeze et al, 1999; Ishijima et al, 2007), which in turn was caused by the expansion of agricultural land and industrialization that came along with the increasing availability of agricultural fertilizers triggered by the development of the Haber-Bosch process (see Chapters 4 and 5). A potential indirect contribution by oceanic sources (for example increased N2O emissions as a result of eutrophication of coastal areas) has not been quantified yet.

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