Note that the atmosphere is more likely to loose 12C over time than 13C. (It's the lighter molecule and will have larger speeds at equal temperatures, making Je

Note that the atmosphere is more likely to loose 12C over time than 13C. (It's the lighter molecule and will have larger speeds at equal temperatures, making Jeans escape more likely). The paper itself says "the δ13C composition of the atmosphere may have been less 13C enriched when Gale crater sediments were deposited" (compared to now). I am not sure how they exclude that as an explanation why deposits are more 12C rich then the atmosphere is now.