Sea surface distribution of nitrous oxide and methane in European shelf seas and the Atlantic Ocean
Abstract Human activities, including intensive agriculture and fertiliser use, are altering the radiative forcing on the planet by increasing the amount of the climatically active gases, nitrous oxide (N2O) and methane (CH4), in the troposphere. There are currently uncertainties in quantifying the natural sources and sinks of N2O and CH4 in the marine environment. This thesis presents high-­‐resolution surface-­‐ocean and atmospheric data for N2O, CH4, CO2 and CO, collected using Integrated Cavity Output Spectrometry (ICOS, Los Gatos) during the D366 (NW European shelf seas, June/ July, 2011) and Atlantic Meridional Transect 22 (Atlantic Ocean, October/November, 2012) cruises. Interpretation of the N2O and CH4 datasets revealed that shelf seas produced a greater atmospheric source of N2O and CH4 than the open ocean. Shelf sea surface waters were slightly supersaturated in N2O at 102 %, but undersaturated in the open ocean at 97 %, acting as slight atmospheric sink. The cause of undersaturation in surface waters is unclear, but may be attributed to seasonal cooling and a potential microbial N2O consumption pathway. The dominant pathway for N2O formation in the open ocean is nitrification. Both nitrification and denitrification may be important in the production of N2O in shelf seas. Vertical mixing processes and upwelling influence the surface concentration of N2O. Surface waters in the shelf seas and the Atlantic Ocean were supersaturated in CH4 at 118 % and 107 %. Methanogenesis, which occurs within anoxic sediments of shelf seas, rivers and estuaries, was the dominant source of CH4 during D366. Freshwater inputs that were supersaturated in CH4, and a fully mixed water column vertically mixing CH4 from marine sediments, influenced the surface concentration of CH4 in shelf seas. In the open ocean, methanogenesis occurs in-­‐situ in the surface waters. Seasons influenced the surface concentration and saturation of N2O and CH4, with sea surface temperatures, seasonal effects, and stratification affecting these gases.
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