Thanks for taking the time to consider this. I don't mind being idiotically wrong.

While the production mechanism you outlined is true for Earth, it is not for Mars.

"There will be other elemental transmutations under this rain of cosmic rays, some of which will be more common on Mars than Earth. in this category, I would expect a number of short-lived and a few stable isotopes of medium mass molecules heavier than carbon."

Yep - but you've left out all the other forms of radiation, and most of their products are toxic to life as we know it.

"There might well be more C13 and less C12 than on Earth, due to neutron capture, but the detailed physics is important, and I'm not sufficiently well-grounded to be sure what will and what won't be produced, other than being very confident that due to lack of nitrogen, there will very little C14!"

Being confident without being sufficiently well grounded is the kind of behaviour that leads to loss of crew. The detailed physics of the nuclear chemistry in this case are extremely important, and everyone here keeps forgetting exactly how massive the radiation load is which Mars recieves due to its relative lack of magnetosphere.

For C14 production, take a good look a the atom bomb effect in the recalibration of radiocarbon dates, and the production mechanism set out there.

Since the exact radiation load is important for the calibration of instruments, such as those which detected all of that water, you may want to take a good look at those papers you cited, and then discuss them with someone in your physics department.

Mars is radioactive.

ep