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Using Satellite-Based Synthetic Aperture Radar to Evaluate Winter Methane Storage and Efflux During Ice-Off in High-Latitude Lakes

by Allan Phelps, Department of Geology and Geophysics, UAF, and Department of Biological Sciences, UAA

 Changes in the gas composition of our atmosphere is a driving force for concerns about global change. One significant change is the doubling of the radiatively active gas of methane in the last 200 years. Northern ecosystems represent a large source of atmospheric methane. Most work has been done on terrestrial ecosystems, but tundra and taiga lakes have been shown to be sources of methane efflux in the summer. Much less work, however, has been done on winter methane accumulation in ice-covered lakes and the subsequent release during ice breakup. During the summer, surface waters are oxygenated, and much of the methane produced in the sediments is microbially oxidized before reaching the atmosphere. Since lake ice forms a barrier to gas exchange between the water and the atmosphere, dis- solved oxygen is often depleted. The lake sediments from which the methane emanates can remain thawed under the ice, providing a potential methane source all year-round. We found that high concentrations of methane can accumulate in the ice itself and in the water beneath the ice, and that during spring ice breakup and lake turnover, a large pulse of methane is released to the atmosphere during the course of only a few days.

Methane gas is included within bubbles in the seasonal ice cover of shallow high-latitude lakes, with concentrations increasing later in the season at greater ice depth ( Figure 1A). Initial freezing produces clear ice, but about midway through the ice growth season (~January), distinct layers of tubular bubbles (diameter ~1mm, lengths 1?20 cm) form simultaneously across shallow lakes. These bubbles can contain high levels of methane. Through the winter of 1996-97, ice cores were taken from Mosquito Lake in Anchorage (61°11.45'N, 149°48.67'W) at two-week intervals. Water samples and water chemistry data were obtained simultaneously. A temporal plot of the methane data (Figure 1A) shows an increase in methane concentrations in the lowest sections of the ice during the period of January, which coincided with the appearance of the cylindrical bubbles. A smaller increase in methane concentrations can also be observed in the top layer of ice due to periodic overflows of supersaturated water from below onto the ice surface. The middle layers, which are almost free of bubbles, contain negligible quantities of methane. A plot of dissolved methane concentrations in the water beneath the ice (Figure 1B) shows a pattern similar to the ice methane concentrations (though an order of magnitude higher). The drop at the end of the season was probably due to venting of the water meth- ane through thawed moats at the edges of the lake. A plot of the oxidation-reduction potential (Figure 1B) indicates an inverse relationship with the observed methane concentrations. Low (negative) oxidation-reduction potentials force the microbial metabolic pathway to methanogenesis, and the low levels of dissolved oxygen prohibit methane consumption by methanotrophs. This enables dissolved methane to reach much higher concentrations under the ice cover, compared to ice-free periods.

Ice core data provide a history of the lake processes during the winter period. In cores obtained from over 40 lakes, similar patterns of increasing methane concentrations were observed. Although maximum methane concentrations in ice cores from different lakes varied over several orders of magnitude, the sharp increase in methane ice inclusions occurred midwinter in almost all cores. Synthetic aperture radar (SAR) data can be used to detect the formation of the tubular bubbles in the ice and the grounding of ice which would indicate that methane production in the sediments has diminished. This holds promise for developing a model to estimate regional fluxes, but difficulties arise with the variations in bubble gas concentrations from lake to lake.

Floating flux chambers placed on the ice surface prior to ice breakup measured a large pulse of methane released to the atmosphere within a few days of ice-melt (Figure 1C). Since the ice breakup occurs next to the shore first, the date at which methane efflux first appears is also the date at which the ice beneath the chamber has melted and the chamber is free floating. The maximum efflux from the chambers was about 400 mg CH4 m-2 d-1, and the average total methane efflux for the 5 chambers during the 10-day period immediately following ice-melt was 1830 ± 120 mg CH4 m-2. During the breakup period, the dissolved methane peaks preceded the efflux peaks by a few days (Figure 1D). During lake turnover, dissolved oxygen increased, and the oxidation-reduction potential moved to positive values.

Mosquito Lake appears to be representative of similar shallow tundra and taiga lakes and ponds numbering in the millions in Alaska. A large pool of methane accumulates under the ice layer in many of these shallow high-latitude lakes, and during a brief period of ice breakup and spring turnover, much of this methane is released to the atmosphere. Previous studies, conducted well into the summer months, have omitted this breakup pulse. Our results suggest that this spring pulse of methane may be comparable in quantity to the remaining entire summer methane efflux, implying that the contribution of methane to the atmosphere from these lakes could be double the current estimates.

Figs. 1A, 1B, 1C, 1D
Figures 1A,1B, 1C, & 1D

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