Concentration maxima of methane in the bottom waters over the Chukchi Sea shelf:implication of its biogenic source

Knowledge about the distribution of CH_(4)remains insufficient due to the scarcity of data in the Arctic shelves.We conducted shipboard observations over the Chukchi Sea shelf(CSS)in the western Arctic Ocean in September 2012 to obtain the distribution and source characteristics of dissolved CH_(4)in seawater.The oceanographic data indicated that a salinity gradient generated a pronounced pycnocline at depths of 20-30 m.The vertical diffusion of biogenic elements was restricted,and these elements were trapped in the bottom waters.Furthermore,high CH_(4)concentrations were measured below the pycnocline,and low CH_(4)concentrations were observed in the surface waters.The maximum concentrations of nutrients simultaneously occurred in the dense and cold bottom waters,and significant correlations were observed between CH_(4)and SiO_(3)^(2-),PO_(4)^(3-),NO_(2)^(-),and NH_(4)^(+)(p<0.01,n=44).These results suggest that the production of CH_(4)in the CSS has a similar trend as that of nutrient regeneration and is probably associated with the degradation of organic matter.The high primary productivity and high concentration of organic matter support the formation of biogenic CH_(4)in the CSS and the subsequent release of CH_(4)to the water column.