C14 is actually produced from cosmic rays hitting nitrogen, not carbon. The impact turns a proton into a neutron. Both CO2 and CH4, once the chemical bond is ma

C14 is actually produced from cosmic rays hitting nitrogen, not carbon. The impact turns a proton into a neutron. Both CO2 and CH4, once the chemical bond is made, only decrease in C14 fraction as it spontaneously decays. Astronomers are pretty good at deducing things from the few, imprecise measurements they can make from millions of miles away, so you should never rule out them being able to figure out something. But it'll depend on a lot of assumptions.