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Thursday, August 28, 2008

Walter, Chanton, Chapin III, Schuur, Zimov: Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways, ages

Journal of Geophysical Research, Vol. 113, 2008, G00A08; doi:10.1029/2007JG000569

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

K. M. Walter (Institute of Northern Engineering and International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.)

J. P. Chanton (Department of Oceanography, Florida State University, Tallahassee, Florida, USA)

F. S. Chapin III (Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska, USA)

E. A. G. Schuur (Department of Botany, University of Florida, Gainesville, Florida, USA)

S. A. Zimov (Northeast Science Station, Cherskii, Republic of Sakha, Russia)


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 CH4 concentration of greater radiocarbon age, depletion of 13CCH4, 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 (δ 13CCH4 = −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 (δ 13CCH4 = −70‰, 14C age 16,500 years, for North Siberian lakes).

(Received 9 August 2007; accepted 11 April 2008; published 2 August 2008.)

Key words: arctic lakes, methane, ebullition, isotopes, thermokarst

Index Terms: 0490 Biogeosciences: Trace gases; 0454 Biogeosciences: Isotopic composition and chemistry (1041, 4870); 0428 Biogeosciences: Carbon cycling (4806); 0458 Biogeosciences: Limnology (1845, 4239, 4942); 0708 Cryosphere: Thermokarst.

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Citation: Walter, K. M., J. P. Chanton, F. S. Chapin III, E. A. G. Schuur, and S. A. Zimov (2008), Methane production and bubble emissions from arctic lakes: Isotopic implications for source pathways and ages, J. Geophys. Res., 113, G00A08, doi:10.1029/2007JG000569.

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