Oxidation or Escape: What Happens to Methane Trapped at the Ice-Water Interface?
Abstract
Sea water concentrations of the potent greenhouse gas methane and thus its release to the atmosphere is largely dependent on the interplay between its microbial sources and sinks. We are only beginning to explore methane cycling in the Arctic, where numerous sources and massive reserves are found in a complex biogeochemical environment. 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? This is the question that motivates this study. To determine methane oxidation rates under a variety of methane concentrations, we collected seawater from beneath sea ice at several sites in close proximity to Point Barrow, Alaska. The seawater was amended with methane and incubated at 0 °C for up two six weeks from April through May, 2016. Methane concentrations and stable isotope ratios were monitored using a Picarro trace gas analyzer, equipped with a Small Sample Isotope Module (SSIM). In all experiments methane concentrations decreased while the biologically less favored 13C-CH4 accumulated in the residual methane. The rates of methane decrease correlated positively with increasing methane concentrations. Furthermore, increasing microbial cell densities indicated microbial uptake of the methane as substrate and possibly a shift towards methanotrophic species in the community. While the low concentration experiments show methane degradation at in situ concentrations, the high amended experiments indicate 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.B41C0435U
- Keywords:
-
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCESDE: 0475 Permafrost;
- cryosphere;
- and high-latitude processes;
- BIOGEOSCIENCES