Source analysis of dissolved methane in Chukchi Sea and Bering Strait during summer–autumn of 2012 and 2013

Abstract

Rapid sea-ice retreat in the Arctic Ocean can promote the release of methane (CH4) from microbe and from sediment on the continental shelf. The released methane can be the cause or effect of strong climate feedback. Nevertheless, few observations have been reported of temporal changes of dissolved CH4 in the Arctic Ocean, especially in western coastal shelf areas (Chukchi Sea and Bering Strait). This study therefore describes the spatial and temporal distributions of the concentrations and stable carbon isotope ratios (δ13C value) of CH4 in the western Arctic Ocean during August–October in 2012 and 2013. In both years, surface water was supersaturated with CH4 with respect to the atmosphere, but concentrations in 2013 were 2–3 times higher than in 2012. In 2012, the concentrations in the bottom layer were higher (up to 55.9 nmol kg−1), whereas δ13C values were lower (down to −63.8‰) than in the surface layer, indicating that CH4 was produced mainly by organic matter degradation in seafloor sediment via methanogens. In 2013, however, because of stronger vertical mixing than in 2012, vertical gradients of CH4 concentration and δ13C value were not prominent. Short-term (19 days) time series observations were also conducted at a fixed station in 2013. An episodic high CH4 concentration in the surface layer (17.2 nmol kg−1 with δ13C = −52.9‰) observed after a strong wind event suggests that CH4 production is accelerated because nutrients are supplied from the bottom of the mixed layer increase plankton activity. These findings suggest that the dynamic variation of dissolved CH4 in the Chukchi Sea in autumn 2012 and 2013 depended mainly on the strength of stratification by sea-ice melt water. Strong winds increase biological activity in the surface layer, consequently accelerating CH4 production.