Methane concentrations in the Beaufort Sea under the water icy canopy, offshore from northern Alaska are 3 to 28 times greater than they are in late summer when the ice is absent in a similar region offshore from northern Canada where methane is in approximate equilibrium with the atmosphere. These observations suggest that methane concentrates in the water under the sea-ice cover during winter and ventilates rapidly in late summer at the ice melts and retreats. Conditions similar to those on the Beaufort Sea shelf likely exist on the much large Siberian shelf, making the Arctic Ocean margin a possible seasonal, high-latitude, marine source of about 0.1 Tg yr-1 atmospheric methane. The small addition of methane likely contributes to the late-summer increase in atmospheric methane that is observed each year particularly in the northern hemisphere. ¿ American Geophysical Union 1993