Peer review of: Influence of tectonics on global scale distribution of geological methane emissions (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
Congratulations to you and your coauthors on a well written, timely, and significant research paper 
on a complex topic..... one that is of interest not only to climate scientists and petroleum 
explorationists, but also to earth historians more interested in past land-sea-continent 
configurations and consequences for climate through the ages. 
 
I saw no grammatical or technical errors, but I should note that I did not see reference number 
22, Hunt, cited in the text (but may have overlooked it); please check that ... 
 
It was a pleasure to have had the opportunity to review this manuscript. 
 
Dietmar Schumacher 
 
 
Reviewer #2 (Remarks to the Author): 
 
General points 
This paper seeks to investigate the relationship between tectonic setting and global methane 
emissions using a geospatial analysis approach. The authors compile worldwide datasets for CH4 
seeps, hydrocarbon fields, tectonic setting, faults and heat flow. The authors use a geospatial 
decision analysis approach that enables a hierarchical consideration of the input parameters to 
create a global CH4 favourability model plotted as a map output. 
The study is potentially interesting and to my knowledge has not been attempted before so in that 
sense the work is novel. The authors find that Global geo-CH4 seems to be preferentially 
associated with petroleum basins – not really surprising, with a preference for convergent systems 
– initially slightly surprising until one realises the connection with mud volcanoes which are also 
prevalent in this setting. The results confirm that geo-CH4 seems to behave differently from geo-
C02 on global scales. 
However, it’s not clear what the main applicability of the results from this particular study is. Will it 
stimulate more research into microseepage detection, or help with CH4 budgets for climate models 
or is it to develop methodologies for methane study? The overall aim and impact could be made 
more explicit in the abstract, opening sections etc. This vagueness translates into discussion of the 
limitations of the approach - one obvious limitation is this analysis only considers onshore seeps – 
and there is no discussion of the effect of not considering the very many large hydrocarbon-rich 
basins that are located offshore. Can the onshore results be applied offshore? This limitation is less 
important for method development but more important for global budgets. 
The study mainly focuses on Earth but makes some inferences that the same approach could be 
used on Mars. The problem with this is as it is written, this point just seems out of context – for 
example - it appears abruptly in the abstract (lines 18 & 19) and there it’s not clear why we have 
we gone from discussing CH4 seepage on Earth to Mars methane. There needs to be some context 
for this shift in perspective. The same applies to the section at the end of the paper. It appears to 
be just added on because one of the co-authors has worked on Mars to give a planetary science 
feel to the paper. There is no discussion of what parts of the model could be applied to Mars and 
what could not. In this sense, the manuscript doesn’t contribute much as a method development 
paper. 
In terms of the input datasets for this analysis, the one I am most concerned, an familiar with, is 
the global fault data set. Given the absence of such compilations, I was initially impressed that the 
authors have tried to compile a global fault dataset. The dataset they compiled comes from many 
sources and some of these are global and others national compilations. These sources are listed in 
the supplementary datafile. A map of the faults coloured for fault type is given in the 
supplementary material, however this is very low resolution and its difficult to see the quality of 
the fault data it contains. There does seem to be some unevenness in the coverage. The fault 
mapping feeds into 2 of the parameters in the geospatial analysis – iv) fault density and v) faulting 
style so it is important to make sure the data compiled are robust. (see comments below). 
 
Specific points 
Lines 107 and 113 – it would be good to plot these mean values on Fig 2b as it takes a while to 
convince yourself these are correct because of all the different data that are included on this plot. 
Lines 111-113 – sentence is a bit odd … ‘divergent basins are controlled by tectonic thinning and 
convergent margins are controlled by tectonic loading’. Surely thinning and loading are the 
manifestation of the tectonic activity in each case. Stretching in divergent margins produces 
tectonic thinning and convergence produces tectonic loading? Moreover this sentence seems to be 
in an odd place as it discusses tectonic setting in the middle of a section on faults and seeps. 
Line 174-177 –microseepage is ‘the major geo-CH4 source to the atmosphere’. This seems like a 
pretty important unsupported assertion. If microseepage is so important- who made the 
estimation, what is the magnitude – how has it been measured? Does it make this studies’ results 
insignificant? How does the authors' model enable an estimation of microseepage? If the main 
input are seeps not microseeps then how is this possible – its not explained. 
Lines 183-185 – this type of model verification is important. Here it is ad-hoc and ancecdotal 
whereas it could be greatly expanded to give a statistical verification. Something like XX% of 
seeps where structural setting (etc) is known, show agreement with the model. 
Line 186 –187. I don’t agree that this is intuitive at all given that normal faults are shear 
displacements. There may be a tendency to produce tensile fractures in their shallowest parts 
where they may become tensile fractures. These may also exist in the damage zones but normal 
faults in mnay (most?) sedimentary basins might be expected to be sealing faults. 
 
Supplementary Files 
The supplementary files generally contain a good outline of the method and the underlying data 
compilations. 
Fault compilation comments 
Unevenness in coverage. There are comparatively few structures plotted in Africa, for example. In 
other places the predominant fault type doesn’t appear to be correct. I do not see N-S normal 
faults in Tibet which are commonly developed. The maps here show strike-slip faults instead. The 
map is of tow low a resolution to be able to check these faults properly. 
There needs to be more discussion in the supplementary file of :- 
Fault resolution –what is the smallest fault resolved – is a uniform cut-off in size applied ? There is 
mention in the uncertainty section of the GEM map accuracy but nothing mentioned for the 
national databases that have been used. 
Data gaps – where are the main data gaps – such as Africa, Antarctica etc. 
Active versus inactive faults – more discussion of active versus inactive faults and their 
relationship to seep activity. Are seeps mainly found on active structures? 
QA and verification: Its not clear how much verification and QA was carried out in areas where the 
global and regional compilations overlap. For example, I know that in central Italy the GEM model 
gives a small number of continuous fault traces but regional maps show many more, shorter 
segmented faults. How has this been dealt with? Has there been a QA process to remove 
duplicated fault traces that appear in more than one compilation – if not the density values 
calculated will be meaningless. 
The authors need to show in the supplementary data regional maps from their compilation of a few 
areas such as Turkey, Italy, Tibet, W. US, UK? etc. Alternatively, can the authors make their fault 
compilation available so that it can be verified against known fault maps? 
 
Ken McCaffrey 
December 2019