Investigating Methane Across Ocean and Atmosphere from the Atlantic to the Polar Regions

The total amount of methane (CH4) released from the global ocean is uncertain. It is important that this component of the global CH4 budget is better constrained, as a more comprehensive understanding of global CH4 sources and sinks is necessary for controlling future emissions.
Using shipborne eddy-covariance measurements collected in the Arctic and Southern Oceans, the first results chapter of this thesis quantifies sea–air CH4 fluxes and explores their spatial variability. On-shelf regions in both polar oceans are identified as sources of atmospheric CH4, with elevated fluxes near seabed seepage sites. Off-shelf, CH4 uptake is observed in the Southern Ocean, likely driven by upwelling of methane-depleted water, while the Arctic Ocean is found to be a small CH4 source.
The second results chapter focuses on the Scotia and Weddell seas of the Southern Ocean, including the South Georgia shelf, and links seabed activity, water column CH4, and atmospheric CH4. All investigated regions were found to be small sources of CH4, with spatial variability controlled by oceanographic fronts, freshwater influence, and seabed seepage. This chapter adds much needed data from the Southern Ocean, which is an
understudied and poorly quantified component in the global CH4 cycle.
The third results chapter investigates atmospheric CH4 isotope and mole fraction measurements from several Atlantic meridional transect cruises. The first Atlantic shipborne transect of both carbon-13 (δ 13C) and deuterium (δD) isotopes of atmospheric CH4 are presented and reveal that CH4 in the Northern Hemisphere is isotopically depleted in these isotopes. Additionally, a relationship between the hemispheric difference in CH4 and El Niño-Southern Oscillation (ENSO) is observed, with ENSO explaining up to 25% of its variability. El Niño events appear to enhance the hemispheric difference, likely due
to reduced CH4 emissions from tropical wetlands in the Southern Hemisphere.
Together, these chapters emphasise the need for continued monitoring of CH4 in the ocean, above the ocean, and the sea-air flux to understand if this component of the methane budget is changing, and if it will continue to change under future climate warming.