If the sun had a large scale activity fluctuation in a chaotic period on the same time scale, that could also be a factor here.

The large scale magnetic field of the solar system is carried essentially by the solar wind as the charged particles flow outwards from active regions of the sun. The flux of cosmic rays in the inner solar system is modulated by the relatively weak 11 year period of the solar cycle.

If there are periods when the sun becomes dormant enough (i.e. like the Maunder Minimum but probably a much longer and quieter period) to precipitate an ice age for a significant amount of time, it could concievably have the same effect of letting in much more GCR flux.

Just a thought.....

et