Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways and ages

This study reports an atmospheric methane (CH₄) source term previously uncharacterized regarding strength and isotopic composition. Methane emissions from 14 Siberian lakes and 9 Alaskan lakes were characterized using stable isotopes (¹³C and D) and radiocarbon (¹⁴C) analyses. We classified ebullition (bubbling) into three categories (background, point sources, and hot spots) on the basis of fluxes, major gas concentrations, and isotopic composition. Point sources and hot spots had a strong association with thermokarst (thaw) erosion because permafrost degradation along lake margins releases ancient organic matter into anaerobic lake bottoms, fueling methanogenesis. With increasing ebullition rate, we observed increasing CH ₄ concentration of greater radiocarbon age, depletion of ¹³C_CH4, and decreasing bubble N₂ content. Microbial oxidation of methane was observed in bubbles that became trapped below and later within winter lake ice; however, oxidation appeared insignificant in bubbles sampled immediately after release from sediments. Methanogenic pathways differed among the bubble sources: CO₂ reduction supported point source and hot spot ebullition to a large degree, while acetate fermentation appeared to contribute to background bubbling. To provide annual whole-lake and regional CH₄ isofluxes for the Siberian lakes, we combined maps of bubble source distributions with long-term, continuous flux measurements and isotopic composition. In contrast to typical values used in inverse models of atmospheric CH₄ for northern wetland sources (δ¹³ C_CH4 = -58%, ¹⁴C age modern), which have not included northern lake ebullition as a source, we show that this large, new source of high-latitude CH₄ from lakes is isotopically distinct (δ¹³C_CH4 = -70%, ¹⁴C age 16,500 years, for North Siberian lakes).