Investigating oceanic sources of methane and their influence on ambient concentrations in polar regions.

Atmospheric methane (CH₄) is a potent greenhouse gas with natural and anthropogenic sources. Concentrations have been significantly increasing over the past few decades, which poses a problem for future climate change goals. The contribution of oceans to the global atmospheric CH₄ cycle is largely uncertain. It is accepted that oceans act as a small net source of atmospheric CH₄. As the polar regions are warming faster than the global average, it is important that we can better quantify CH₄ emissions from the polar oceans. 

In this study, we combine various forms of shipborne data (ambient atmospheric methane concentrations, sea-air CH₄ fluxes and isotopic composition of atmospheric CH₄) taken during cruises in the Arctic and Southern Oceans to present a more complete picture of atmospheric CH₄ above polar oceans, including addressing the question of how much the oceanic component is contributing towards the atmospheric budget in these regions. Measurements  were made around the Barents Sea and Greenland Sea in the Arctic, and in the Atlantic sector of the Southern Ocean, including the Scotia Sea.

Sea-air CH₄ fluxes are measured using the eddy covariance method; indeed, this the first study to use this technique to directly measure how much CH₄ is released from the ocean into the atmosphere in both the Southern and Arctic Oceans. Atmospheric CH₄ measurements are then investigated in order to understand the impact that CH₄ released from the ocean has on the atmospheric burden. We also measure the isotopic composition of CH₄ (δ²H and δ¹³C) in air samples taken onboard polar cruises, to understand the sources of atmospheric CH₄ above these oceans. The isotope measurements can indicate if the CH₄ comes from a biogenic or thermogenic source, which can help determine if anthropogenic or natural processes are behind the production.

We investigate the potential sources of CH₄ released by the polar ocean by looking at areas of known seabed CH₄ seepages, investigating phytoplankton abundance, and investigating the isotopic composition of atmospheric CH₄ in areas of elevated CH₄. 

We find that the region of the Arctic Ocean investigated in this study is a slight atmospheric CH₄ source in boreal summer, while the region of the Southern Ocean investigated is a CH₄ source in areas of shallower water/continental shelves and a CH₄ sink in region of open ocean, in austral summer. This finding is consistent with previous studies that have detected seabed CH₄ emission. Seabed CH₄ seepage at shallower depths is more likely to penetrate the sea-air interface, while CH₄ produced at the seabed at deeper depths gets oxidised as it travels through the water column, making it less likely to reach the surface . We also find evidence of localised “hot spots” of methane emission which will be described.