N14 goes to C14 under neutron irradiation, and I thought the neutrons came from solar protons and some galactic cosmic ray interactions.
Would a gamma burst (photons) generate enough neutrons to increase C14 from N14 or O17?
I think a good alternative would be a series of extremely large solar flares.
I believe neutrons can also come from “star formation” which occurs when a nucleus is hit by a gamma photon having more energy than the binding energy of said nucleus. The nucleus blows up (leaving a star pattern on cloud-chamber photographs, hence the name) and not shown on those photographs, some of the remnants are neutrons. This only happens at gamma energies well above 5 or 10 MeV. Even if the gammas didn’t penetrate far into the atmosphere, they wouldn’t need to as there are nitrogens up there, and anyway the neutrons can penetrate.