How do carbon 13:12 ratios confirm the biological origin of oldest microfossils?
Via: https://en.wikipedia.org/wiki/Carbon-13#Uses_in_science


  Due to differential uptake in plants as well as marine carbonates of 13C, it is possible to use these isotopic signatures in earth science. Biological processes preferentially take up the lower mass isotope through kinetic fractionation.


Via: https://en.wikipedia.org/wiki/Kinetic_fractionation


  All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of an atmospheric CO2 molecule. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth.