VARIATIONS OF ATMOSPHERIC METHANE SUPPLY FROM THE SEA-OF-OKHOTSK INDUCED BY THE SEASONAL ICE COVER

Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and a re used to assess the magnitude of seasonal variations of methane flux es from the ocean to the atmosphere in this area. Methane concentratio ns northeast of Sakhalin were observed to range from 385 nmol L(-1) un der the ice cover in winter to 6 nmol L(-1) in the icefree midsummer s eason. The magnitude of supersaturations indicates that this part of t he Okhotsk Sea is a significant source for atmospheric methane. From t he seasonal variation of the supersaturations in the surface waters it is evident that the air-sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse i nto the atmosphere of the order of 560 mol km(-2) d(-1) can be expecte d when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km(-2) d(-1) w hich is of the order of the average annual flux in the survey area. Th e magnitude of the seasonal CH4 flux variation northeast of Sakhalin c orresponds to an amount of 7.3 x 10(5) g km(-2) whereby 74% or 5.4 x 1 0(5) g km(-2) are supplied to the atmosphere between April and July. F or the whole Sea of Okhotsk the annual methane flux is roughly 0.13 x 10(12) g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long-term data of atmospheric methane which are recorded at the same latitude adjacent to areas with season al ice cover show a regional methane pulse between April and July. The large-scale level of atmospheric methane in the northern hemisphere u ndergoes an amplitudinal variation of about 25 parts per billion by vo lume (ppbv) which translates into approximately 36 Tg. Thus the estima ted 0.6 Tg of ice-induced methane dynamics in northern latitudes can h ardly explain this seasonal signal. However, the effects of seasonal i ce cover on pulsed release of methane appear strong enough to contribu te, in concert with other seasonal sources, to characteristic short-te rm wobbles in the atmospheric methane budget which are observed betwee n 50 degrees N and 60 degrees N.