The article addresses that. Recent (biologically) produced methane has a different carbon isotope ratio than the ancient gas released from shale deposits.
Asking about the difference between permafrost methane and shale gas produced methane is interesting tho. Reading the article now, lmk if you find a good answer.
Agreed! I wrote the author of the paper to get his input. If you want a copy of his response when I get it, shoot me an email tito@impossiblelabs.io (tho you may be attempting anonymity here...I can post his response to this thread in a few days if of interest.
It is a fair point, but I suspect the methane from shale is way way older than the permafrost methane. My understanding is that the carbon isotope half life is ~5000 years, so after 50,000 years it is 0.1% of its starting level and becomes useless for determining age.
the first 460m of permafrost depth have formed over the past 35,000 years. The permafrost that is thawing now and releasing CO2 has been formed during that period. There is deeper permafrost which may be much older, but that isn't thawing now and releasing CO2.
