Carbon entering into rock formations has a concentration of elemental Î 13C of about â ' 5.5 , where because of a preferential biotic uptake of 12C , biomass has a Î 13C of between â 20 and â ' 30 . Carbon entering into rock formations has a ratio of Carbon-13 ( 13C ) to Carbon-12 ( 12C ) of about â ' 5.5 ( in units of Î 13C ) , where because of a preferential biotic uptake of 12C , biomass has a Î 13C of between â 20 and â ' 30 .