The stable hydrogen isotopic composition of methane emitted from biomass burning and removed by oxic soils: application to the atmospheric methane budget

Abstract

The stable hydrogen isotopic composition (deltaD) of CH4 was developed for use as a constraint for the atmospheric CH4 budget by characterizing the deltaD of CH4 emitted from biomass burning and removed by oxic soils and the deltaD of atmospheric CH4 in tropospheric background air. These measurements were combined with literature values for the strength and deltaD of the other CH4 sources and sinks to develop regional, hemispheric and global CH4- deltaD budgets.The hydrogen kinetic isotope effect (KIE) during soil uptake of atmospheric methane was alphaDsoil = k(CH4)/k(CH 3D) = 1.099 +/- 0.030 and 1.067 +/- 0.007 for a native grassland and a temperate forest, respectively. This is significantly less than the KIEs associated with the other CH4 sinks. The interhemispheric asymmetry in the soil sink strength suggests a ∼5‰ difference between the overall KIEs during atmospheric CH4 loss in the two hemispheres.The deltaD of methane emitted from biomass burning (deltaDCH4(bb) ) measured in large-scale laboratory combustion experiments and the Brazilian Amazon was -233 +/- 2‰ and -210 +/- 16‰, respectively. These measurements suggest that deltaDCH4(bb) may have a relatively narrow deltaD range globally. Measurements of the fuel biomass deltaD content indicated a significant hydrogen KIE, of -130‰ to -180‰, during combustion.The mean deltaD of atmospheric CH4 at Cheeka Peak, Washington (48°N) was -92.6 +/- 0.5‰ with a seasonal cycle of amplitude 8.6 +/- 2.9‰ between 1991 and 1996. The seasonal cycle was controlled by the balance between CH4 emissions from bogs and removal by OH. The mean deltaD's for the northern and southern hemispheres were -93.0 +/- 1.9‰ and -83.3 +/- 1.5‰, respectively, determined from air samples collected at ∼150°W between 55°N and 65°S during 1989 to 1995. The southward deltaD increase resulted from the higher strength and deltaD of CH4 loss compared to the CH4 source in the southern hemisphere. The deltaD of the global CH4 source derived from the atmospheric measurements and the total KIE during CH4 loss was -280 +/- 37‰, in good agreement with -279 +/- 6‰ estimated from the strength and deltaD of the individual CH4 sources. This indicates that current understanding of the CH4-deltaD budget, including the determinations of deltaD CH4(bb) and alphaDsoil presented here, is robust.