Gridded maps of geological methane emissions and their isotopic signature
Authors: Etiope Giuseppe , Ciotoli Giancarlo , Schwietzke Stefan , Schoell Martin 
Published: January 2019

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 their isotopic signatures are 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 (geo-CH4) seepage,
i.e. the natural degassing of hydrocarbons from the Earth's crust. While
geological seepage is widely considered a major source of atmospheric
CH4, it has been largely neglected in 3-D inverse CH4
budget studies 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∘×1∘ maps include the four main categories of natural
geo-CH4 emission: (a) onshore hydrocarbon macro-seeps, including
mud volcanoes, (b) submarine (offshore) seeps, (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, in the Caspian region, in Europe and in the East Siberian
Arctic Shelf. The globally gridded CH4 emission estimate
(37 Tg yr−1 exclusively based on data and modelling specifically
targeted for gridding, and 43–50 Tg yr−1 when extrapolated to also
account for onshore and submarine seeps with no location specific
measurements available) is compatible with published ranges derived using
top-down and bottom-up procedures. Improved activity and emission factor data
allowed previously published mud volcanoes and microseepage emission
estimates to be refined. The emission-weighted global mean
δ13C-CH4 source signature of all geo-CH4
source categories is about −49 ‰. This value
is significantly lower than those attributed so far in inverse studies to
fossil fuel sources (−44 ‰) and geological seepage
(−38 ‰). It is expected that using this updated, more
13C-depleted, isotopic signature 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 modelling, thereby improving the accuracy of the fossil fuel
and microbial components. Grid csv (comma-separated values) files are
available at https://doi.org/10.25925/4j3f-he27.