Cracking the lid - Methane cycling beneath thinning sea ice

• Main Authors: Uhlig, Christiane, Kirkpatrick, John B., Loose, Brice
• Format: Conference Object
• Language: unknown
• Published: 2017
• Subjects: Arctic; Barrow; Point Barrow; Sea ice; Alaska
• Online Access: https://epic.awi.de/id/eprint/45457/; https://hdl.handle.net/10013/epic.51639

In the shallow marginal seas of the Arctic seawater methane concentrations are frequently in excess of atmospheric saturation, partly caused by reduced sea-air-exchange around sea ice cover. These same regions can host rich ice-attached and water column microbial communities, including methane-oxidizing bacteria. How much methane accumulating beneath seasonal sea ice is oxidized by bacteria and how much escapes to the atmosphere? To determine methane oxidation rates, we incubated seawater from beneath sea ice, collected close to Point Barrow, Alaska under a variety of methane concentrations. Methane concentrations and stable isotope ratios were monitored using a Picarro trace gas analyzer, equipped with a Small Sample Isotope Module (SSIM). High oxidation rates were observed in the presence of high methane concentrations, while at lower concentrations the methane budget was zero or slightly positive. Trends in the mass balance were accompanied by a respective trend in stable isotope ratios, indicating that biological processes are responsible for these changes. Increasing cell densities hint to microbial uptake of the methane and a shift towards methanotrophs in the community. While the low concentration experiments elucidate methane cycling at in situ concentrations, the high amended experiments show the potential of the communities to mitigate events of high methane release from the seabed or while the methane is caught under the sea ice cover. With a decreased sea ice cover or earlier ice breakup it is possible that more methane will be vented to the atmosphere due to decreased residence time in the water column and thus accessibility to the methanotrophic community.