CO2: Mars is not Earth-like
Bizarre chemistry, 2 (Gedunken)
Let us look at the effect of ionizing radiation on long chain hydro-carbons, such as those which comets delivered to the surface of Mars.
Over time, the chains will be broken, and lighter elements such as hydrogen and nitrogen will outgas. The carbon then combines with the oxygen, and you have CO2.
Perhpas some of the nitrogen will absorb neutrons, and be converted to 14C.
One thing for sure, nuclear chemistry on Mars is going to be bizarre, to repeat Hoffman's note.