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

Abstract

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.