Project: The East Siberian Arctic Shelf as a Source of Atmospheric Methane: First Approach to Quantitative Assessment

We propose to study methane (CH4) release over the East Siberian Arctic shelf (ESAS), the largest (~10% of the world ocean shelf area) and the shallowest shelf (mean depth <50 m) of the world ocean. Until recently, the ESAS was not considered a CH4 source due to subsea permafrost’s impermeability, which completely isolated it from modern biogeochemical cycles. The ESAS stores the world’s largest hydrocarbon stocks, mostly as shallow Arctic hydrates, and thus represents an enormous potential CH4 atmospheric source that could result from global warming-triggered permafrost degradation. Increased CH4 fluxes could occur as numerous weak seeps or strong bubble plumes over large areas. Due to the shallow nature of the ESAS, the majority of ESAS CH4 likely avoids oxidation and escapes to the atmosphere. To assess whether sudden, large-scale CH4 release occurs or is likely to occur in the future, we will investigate the migration pathway characteristics and identify the controlling factors of CH4 flux from the seabed, in the water column, and to the atmosphere.

Our central hypothesis is that seabed CH4 fluxes are significant year-round sources while atmospheric fluxes are only significant during ice-free periods. To understand both the temporal and spatial variability of CH4 fluxes, we will test the sub-hypotheses that ESAS emissions are strongly modulated by seasonal factors including water-column mixing and ice cover, while seabed CH4 emissions are much more weakly modulated (or not at all), and that the spatial distribution of emission correlates with the location of shallow and deep-water fault zones, and submerged thermokarst areas – primary geologic rather than biologic control. We will: 1) distinguish different CH4 sources 2) quantify the importance of microbial production and consumption of CH4 including seasonal variations vs. release of previously originated CH4 from seabed reservoirs; 3) evaluate contributions of different components (spatial, seasonal and transport components) to the annual CH4 flux; 4) investigate controlling factors and model CH4 release during critical periods such as storms, fall convection and ice-break up period and over critical area such as fault zones and thermokarst-affected areas; 5) use modeling to extend results to the entire ESAS and for climate change scenarios.

Water column hydrography and geochemical measurements will be conducted during a summer cruise and a winter expedition. To distinguish between individual CH4 sources we will measure stable carbon isotope ratios (δ13CCH4, δ13CDIC, δ13C-POC), D/H ratio of CH4 and water (δDCH4, δDH2O), CH4 radiocarbon age (Δ14C), and the abundance of non-CH4 hydrocarbons in water, air and sediment samples. To elucidate importance of microbial production (methanogenesis) we will estimate in situ production in particles; CH4 consumption (oxidation) will be measured in the water column and at the ice-water interface. To evaluate total flux, we will perform direct flux measurements (4 helicopter surveys) using eddy-correlation techniques to obtain integrative rates of CH4 flux and assess strength of the current CH4 source over two area. To assess contribution of ebullition, we will measure seafloor direct bubble flux over 6 control and 2 test sites on a nested range of sonar scales (3.5 kHz, largest), to multibeam sonar scanner (decameter), to turbine tents (meter), and a combination of bubble video imaging and bubble modeling (vent scale). To evaluate factors controlling CH4 fluxes, we will perform investigations of seabed permeability for gases using geophysics and 3He/4He ratio. Modeling will aim to develop a regional flux model.