Spatial distribution of dissolved methane and its source in the western Arctic Ocean
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Recent Arctic warming and decreasing sea-ice can promote the release of methane (CH4), a greenhouse gas, from the Arctic Ocean, thereby providing a strong climate feedback. However, the dynamics of dissolved CH4 in the Arctic Ocean remain uncertain, especially in western areas. This report describes the horizontal and vertical distributions of concentration and stable carbon isotope ratio (δ13C value) of CH4 in the western Arctic Ocean. Surface layer samples used for this study were supersaturated with CH4 in comparison to the atmosphere. Especially high CH4 concentrations (up to 10.3 nmol kg−1) were observed at stations in the continental shelf area. At the bottom layer of the shelf stations, the CH4 concentration was higher (up to 55.9 nmol kg−1). Its δ13C value was lower (down to − 63.8‰) than in the surface layer, which suggests that CH4 in the shelf water is produced mainly by methanogens in sediment. At deeper stations in the Canada Basin (seafloor > 300 m depth), the maxima of CH4 concentration were detected at depths of 10–50 m and 100–200 m, although δ13C values were lowest at 50 m depth. The shallower CH4 maximum coincided with the DO maximum, suggesting CH4 production by plankton activity or sinking particles. The deeper CH4 maximum corresponded to the nutrient maximum, suggesting horizontal advection of shelf water from the coastal shelf area. From the results, we were able to confirm that the dynamics of dissolved CH4 in the western Arctic Ocean in summer 2012 varied with area and depth.
KeywordsWestern Arctic Ocean Dissolved CH4 concentration Stable carbon isotope ratio Depth profile Chukchi Sea Canada Basin Bering Strait Organic matter degradation from sediment Methanogen Plankton activity
We acknowledge the scientists and crews of the MR12-E03 cruise on R/V Mirai, JAMSTEC, for sampling and providing the hydrographic and nutrient data. This study was conducted under the Green Network of Excellence (GRENE) Arctic Climate Change Research Project. It was also supported financially by JSPS KAKENHI 23224013 and by the Global COE program “From Earth to Earths” of the Ministry of Education, Culture, Sports, Science and Technology, Japan. Figures 1, 2, and 5 were drawn using “Ocean Data View” (http://odv.awi.de/) software. The data used to prepare Figs. 3, 4 and 5 are available from the Data Research System for Whole Cruise Information in JAMSTEC (Darwin; http://www.godac.jamstec.go.jp/darwin/cruise/mirai/mr12-e03/e).
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