Defining the problem space
"No" nitrogen on Mars, eh, Nick...
Hmmm, C14 usually produced by cosmic radiation striking nitrogen... and no more nitrogen...
The problem is short term half life isotopes of organic chemicals.
From the long lived isotope ratios in the SNCs, some initial estimate can be formed as to Mars annual radiation exposure, at least for some limited areas at the time of ejection. Note that the effects of travel through raw space also have to be included.
Then you have the secondary radiation releases caused by initial isotope decay on Mars. The you have tertiary releases...
Mars radiation is like an std, it just keeps giving and giving.... gravity with no magnetic field... concentration of heavier isotopes by gravity...
ep