It has been suggested that galactic shock asymmetry induced by our galaxys infall toward the Virgo Cluster may be a source of periodicity in cosmic ray exposure as the solar system oscillates perpendicular to the galactic plane. Here we investigate a mechanism by which cosmic rays might affect terrestrial biodiversity, ionization and dissociation in the atmosphere, resulting in depletion of ozone and a resulting increase in the dangerous solar UVB flux on the ground, with an improved ionization background computation averaged over a massive ensemble (about 7 x 10^5) shower simulations. We study minimal and full exposure to the postulated extragalactic background. The atmospheric effects are greater than with our earlier, simplified ionization model. At the lower end of the range effects are too small to be of serious consequence. At the upper end of the range, ~6 % global average loss of ozone column density exceeds that currently experienced due to effects such as accumulated chlorofluorocarbons. The intensity is less than a nearby supernova or galactic gamma-ray burst, but the duration would be about 10^6 times longer. Present UVB enhancement from current ozone depletion ~3% is a documented stress on the biosphere, but a depletion of the magnitude found at the upper end of our range would double the global average UVB flux. For estimates at the upper end of the range of the cosmic ray variability over geologic time, the mechanism of atmospheric ozone depletion may provide a major biological stress, which could easily bring about major loss of biodiversity. Future high energy astrophysical observations will resolve the question of whether such depletion is likely.
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