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

Abstract

This study reports an atmospheric methane (CH 4 ) source term previously uncharacterized regarding strength and isotopic composition. Methane emissions from 14 Siberian lakes and 9 Alaskan lakes were characterized using stable isotopes ( 13 C and D) and radiocarbon ( 14 C) 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 13 C CH4 , and decreasing bubble N 2 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: CO 2 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 CH 4 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 CH 4 for northern wetland sources ( δ 13 C CH4 = −58‰, 14 C age modern), which have not included northern lake ebullition as a source, we show that this large, new source of high‐latitude CH 4 from lakes is isotopically distinct ( δ 13 C CH4 = −70‰, 14 C age 16,500 years, for North Siberian lakes).