Modeling the impediment of methane ebullition bubbles by seasonal lake ice

Abstract. Microbial methane (CH₄) ebullition (bubbling) from anoxic lake sediments comprises a globally significant flux to the atmosphere, but ebullition bubbles in temperate and polar lakes can be trapped by winter ice cover and later released during spring thaw. This "ice-bubble storage" (IBS) constitutes a novel mode of CH₄ emission. Before bubbles are encapsulated by downward-growing ice, some of their CH₄ dissolves into the lake water, where it may be subject to oxidation. We present field characterization and a model of the annual CH₄ cycle in Goldstream Lake, a thermokarst (thaw) lake in interior Alaska. We find that summertime ebullition dominates annual CH₄ emissions to the atmosphere. Eighty percent of CH₄ in bubbles trapped by ice dissolves into the lake water column in winter, and about half of that is oxidized. The ice growth rate and the magnitude of the CH₄ ebullition flux are important controlling factors of bubble dissolution. Seven percent of annual ebullition CH₄ is trapped as IBS and later emitted as ice melts. In a future warmer climate, there will likely be less seasonal ice cover, less IBS, less CH₄ dissolution from trapped bubbles, and greater CH₄ emissions from northern lakes.