Gridded maps of geological methane emissions and their isotopic signature
Authors: Etiope Giuseppe , Ciotoli Giancarlo , Schwietzke Stefan , Schoell Martin 
Published: September 2018

Abstract. Methane (CH4) is a powerful greenhouse gas, whose natural and anthropogenic emissions contribute ~20% to global radiative forcing. Its atmospheric budget (sources and sinks), however, has large uncertainties. Inverse modelling, using atmospheric CH4 trends, spatial gradients and isotopic source signatures, has recently improved the major source estimates and their spatial-temporal variation. Nevertheless, isotopic data lack CH4 source representativeness for many sources, and CH4 source attribution is affected by incomplete knowledge of the spatial distribution of some sources, especially those related to fossil (radiocarbon-free) and microbial gas. This gap is particularly wide for geological CH4 seepage, i.e., the natural degassing of hydrocarbons from the Earth’s crust. While geological seepage is widely considered the second most important natural CH4 source after wetlands, it has been mostly neglected in top-down CH4 budget studies, partly given the lack of detailed a priori gridded emission maps. Here, we report for the first time global gridded maps of geological CH4 sources, including emission and isotopic data. The 1°x1° maps include the four main categories of natural geo-CH4 emission: (a) onshore hydrocarbon macro-seeps, including mud volcanoes, (b) submarine (offshore) seepage, (c) diffuse microseepage and (d) geothermal manifestations. An inventory of point sources and area sources was developed for each category, defining areal distribution (activity), CH4 fluxes (emission factors) and its stable C isotope composition (δ13C-CH4). These parameters were determined considering geological factors that control methane origin and seepage (e.g., petroleum fields, sedimentary basins, high heat flow regions, faults, seismicity). The global geo-source map reveals that the regions with the highest CH4 emissions are all located in the northern hemisphere, in North America, the Caspian region, Europe, and in the East Siberian Arctic Shelf. The globally gridded CH4 emission estimate (37 Tg year−1 exclusively based on data and modeling specifically targeted for gridding, and 43–50 Tg year−1 when extrapolated to also account for onshore and submarine seeps with no location specific measurements available) is compatible with published ranges derived by top-down and bottom-up procedures. Improved activity and emission factor data allowed to refine previously published mud volcanoes and microseepage emission estimates. The emission-weighted global mean δ13C-CH4 source signature of all geo-CH4 source categories is −48.5 ‰ to −49.4 ‰. These values are significantly lower than those attributed so far in inverse studies to fossil fuel sources (−44 ‰) and geological seepage (−38 ‰). It is expected that using these updated more 13C-depleted, isotopic signatures in atmospheric modelling will increase the top-down estimate of the geological CH4 source. The geo-CH4 emission grid maps can now be used to improve atmospheric CH4 modeling, thereby improving the accuracy of the fossil fuel and microbial components. Grid csv files are available at https://doi.org/10.25925/4j3f-he27 .