Global Inventory of Fossil and Non-fossil Methane δ13C Source Signature Measurements for Improved Atmospheric Modeling

Studies of the global methane (CH4) budget (including 3D inversion and box model studies) frequently include measurements of atmospheric δ13C-CH4 as an additional constraint for better allocation of different natural and anthropogenic CH4 sources, and this requires information about the δ13C isotopic signature (δ13C-Source) of the different CH4 sources. The δ13C-Source measurement sample sizes used in published global CH4 budgets are either small or unknown, uncertainties are rarely applied, and global representativeness is lacking. We have compiled the most comprehensive database to date including 8,128 original measurements from fossil fuels (FF; including 7,482 conventional and 646 shale gas samples) and 2,068 original measurements from microbial and biomass burning sources, which can be used to define globally-weighted average δ13C-Source with well defined uncertainties. This database shows that previously used global average δ13C-Source are outdated, and particularly FF signatures are significantly lighter than canonical values.