As Methane Concentration Goes Up, Stable Isotopes of Methane Go Down: 13C Implicates a Microbial Source Across Latitudinal Gradients
Tuesday, 16 December 2014: 5:30 PM
The mixing ratio of methane in the atmosphere has increased in recent years for reasons that are not completely understood. Stable isotopes allow us to elucidate sources of methane due to the relatively distinct isotopic signatures from fossil fuel, biomass burning, and microbial sources. The Stable Isotope Laboratory at INSTAAR has been measuring δ13C of CH4 from a subset of the NOAA Earth System Research Laboratory, Global Monitoring Division’s Cooperative Air Sampling Network since 1998. Here we examine our 15 year record and show that since 2007, the global average atmospheric methane δ13C value has decreased by 0.08‰. This is coincident with an increase in the growth rate of atmospheric methane after a period of near steady-state conditions from 1999 to 2006. The decrease in δ13C of CH4 is evident at sites in the Arctic as well as in the northern mid-latitudes and the Southern Hemisphere. There has been no change in the inter-hemispheric difference of atmospheric methane isotopes, suggesting that the increased emissions do not originate predominantly in the Northern Hemisphere. Likewise, Miller-Tans plots, which examine regional, higher-frequency deviations from the background signal, show that sources of methane have become depleted in 13C since 2007 across latitudes, implicating stronger microbial emissions (such as those from wetlands). We use a 3-box model (Northern, Tropical and Southern) to test scenarios of increased methane sources and find that increased microbial emissions from the Northern Hemisphere and tropical regions are necessary to explain the isotopic shift. Furthermore, because there is not a change in the north-south gradient of δ13C of CH4, and because fossil fuel emissions are primarily from northern latitudes, fossil fuel sources alone cannot explain the increase in methane emissions since 2007. This study shows the value of including stable isotopes in long-term, global observation networks to constrain sources of atmospheric methane.