Methane release from the East Siberian Arctic Shelf: The role of subsea permafrost and other controlling factors as inferred from decadal observational and modeling efforts

Abstract:

Sustained methane (CH₄) release from thawing Arctic permafrost to atmosphere may be a positive, major feedback to climate warming. East Siberian Arctic Shelf (ESAS) atmospheric CH₄ venting was reported as on par with flux from Arctic tundra. Unlike release when ancient carbon in thawed on-land permafrost is mobilized, ESAS CH₄ release is not determined by modern methanogenesis. Pre-formed CH₄ largely stems from seabed deposits. Our investigation, including observational studies using hydrological, biogeochemical, geophysical, geo-electrical, microbiological, and isotopic methods, and modeling efforts to assess current subsea permafrost state and the ESAS’ contribution to the regional CH₄ budget, have clarified processes driving ESAS CH₄ emissions. Subsea permafrost state is a major emission determinant; rates vary by 3-5 orders of magnitude. Outer ESAS CH₄ emission rates, where subsea permafrost is predicted to be degraded due to long submergence by seawater, in places are similar to near-shore rates, where deep/open taliks can form due to combined heating effects of seawater, river runoff, geothermal flux, and pre-existing thermokarst. Progressive subsea permafrost thawing and decreasing ice extent could significantly increase ESAS CH₄ emissions. Subsea permafrost drilling results reveal modern recently submerged subsea permafrost degradation rates, contradicting previous hypotheses that thousands of years required to form escape paths for permafrost-preserved gas. We used a decadal observational ESAS water column and atmospheric boundary layer (ABL) data set to define the minimum source strength required to explain observed seasonally-increased ABL CH₄ concentration. Modeling results agree with estimates from in-situ sonar data. In <10 m shallow water ≤72% of CH₄ remains in surfacing bubbles. Dissolved CH₄ fate largely depends on 3 factors: dissolved CH₄ water column turnover time, water column stability against vertical mixing, and turbulent diffusion and lateral advection rates. Dissolved outer ESAS CH₄ takes ≤1000 days to be oxidized because oxidation rates are low. Storms could release some aqueous CH₄ to atmosphere; dissolved CH₄, captured beneath ice in winter, can spread via currents and escape to atmosphere through breaks in the ice.