Methane efflux from high-latitude lakes during spring ice melt

Ice cores removed from shallow ice-covered tundra lakes near Barrow, Alaska, and taiga lakes near Anchorage, Alaska, exhibit increasing concentrations of methane with depth. Methane concentrations in the ice cores increased from 0 mu M in the top 15 cm sections to a maximum of 23 mu M in the lowest 15 cm sections of tundra lake ice and to a maximum of 147 mu M in taiga lake ice. Methane concentrations in the water beneath the ice reflect a similar pattern, with values near 5 mu M early in the ice-covered season, increasing up to 42 mu M in the tundra lakes, and up to 730 mu M in the taiga lakes. Methane levels increase in the water beneath the ice during the course of the winter due to decreasing water volume, exclusion from growing ice, and continued methane production in thawed sediments. Since the ice layer prohibits gas exchange with the atmosphere, the methane is not oxidized, as it would be during the summer months, allowing the winter accumulation and storage of methane in the ice and lake waters. Efflux measurements, taken with floating chambers on the taiga lakes, indicated a large pulse of methane released during the period of ice melt and spring turnover. The efflux from one lake ranged from 2.07 g CH4 m(-2) in 1995 to 1.49 g CH4 m(-2) in 1996 for the 10 day period immediately after ice melt. Estimation of methane efflux using a boundary layer diffusion model and surface water concentrations during the entire ice-free period in 1996 predicted an efflux of 1.79 g CH4 m(-2) during the same 10 day period, compared with 2.28 g CH4 m(-2) for the remainder of the summer season. This observation suggests that almost as much methane efflux can occur during a brief period immediately after ice melt as occurs during the remainder of the ice-free season. Since measurements of methane efflux from high-latitude-lakes are generally made after this breakup period, the overall contribution to atmospheric methane from high-latitude lakes may be twice that of current estimates.