Ma, Xiao (2020) From coastal waters to the open ocean: the variability and emissions of methane and nitrous oxide. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 100 pp.

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Abstract

Methane (CH 4 ) and nitrous oxide (N2 O) are potent greenhouse gases which are involved in atmospheric chemistry and global warming. Oceans, especially coastal regions, are important sources of these gases. Long-term observations and field investigations on a global scale are effective tools to constrain the uncertainty of oceanic CH4 and N2O emissions, to assess trends and to predict potential changes in the future. In this study, the variability and emissions of CH4 and N2 O were investigated at the Boknis Eck (BE) time-series station located in the coastal SW Baltic Sea during 2005–2017, and the results of continuous underway measurement and depth profiles of N2 O were reported from a cruise to the SW Indian Ocean. Hydroxylamine (NH 2 OH), a short-lived intermediate in the nitrogen cycle and precursor of N2 O, was also measured at several locations during the same cruise, in order to decipher the N2 O production pathway in the ocean. The major findings are:
1) Dissolved CH 4 concentrations fluctuated between 2.9 and 695.6 nM, with an average of 51.2 ± 84.2 nM during 2006–2017 at the BE time-series station. In general, CH4 concentrations were higher in the bottom water than in the surface layer, indicating the release of CH4 from the sediments. CH 4 maxima were usually observed in the bottom water in February, June and October. Enhanced CH 4 concentrations in the upper water layer as, for example, observed in December 2014, could be explained by a major Baltic inflow of North Sea water. This event significantly promoted CH4 emissions to the atmosphere. During 2006–2017, sea-to-air CH4 flux densities ranged between 0.3 and 746.3 μmol m-2 d-1 , with an average of 43.8 ± 88.7 μmol m-2 d-1, while in December 2014, the value was 3104.5 μmol m-2 d-1. Throughout the sampling period, surface BE water was supersaturated with CH 4 and thus emitting CH 4 to the atmosphere.
2) During 2005–2017, N2 O concentrations at the BE time-series station varied between 1.2 and 37.8 nM, with an overall average of 13.9 ± 4.2 nM. N2O concentrations showed a significant seasonal cycle with low concentrations (undersaturation) in autumn, coinciding with hypoxic/anoxic conditions in the bottom water. Relatively low N2O concentrations were observed during October 2016–April 2017, and relatively high N2 O concentrations were observed in November 2017, but no significant N2O trend was detected during 2005–2017. N2O flux densities were estimated to range between -19.0 and 105.7 μmol m-2 d-1, with an average of 3.5 ± 12.4 μmol m-2 d-1 , indicating that BE is a moderate source of atmospheric N2O.
3) Surface N2O concentrations in the southwestern Indian Ocean ranged between 5.8 and 8.0 nM (mean ± SD: 6.9 ± 0.6 nM). Surface waters were mostly supersaturated with respect to atmospheric equilibrium (mean ± SD: 104.1 ± 2.8%), and most of the outgassing occurred along the zonal band between 5°S and 10°S. Depths profiles showed a generally inverse relationship between N2O and oxygen with N2O maxima at about 1000 m. For waters above and below 1000 m, different slopes were found in the correlations between ΔN2 O (excess N2)/AOU (apparent oxygen utilization) and ΔN2 O/NO 3-. NH2OH concentrations in the sampling area varied between below detection limit and 6.76 nM. Although NH2OH concentrations were variable in the Indian Ocean, we suggest that nitrification might be the dominant pathway of N2O formation. In summary, the distributions and variations of CH4 and N2O in coastal and open ocean environments are driven by a complex interplay of various biological and physical processes, and the situation might be even more complicate in coastal waters because of the strong anthropogenic influences. Coastal waters and the open ocean are important sources of atmospheric CH4 and N2O, but the answer to the emerging question of how these emissions will change with the further development of global warming and ocean deoxygenation remains ambiguous.

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