  Based on the intensity and rates of various kinds of intense ionizing
radiation events such as supernovae and gamma-ray bursts, it is likely that the
Earth has been subjected to one or extinction level events during the
Phanerozoic. These induce changes in atmospheric chemistry so that the level of
Solar ultraviolet-B radiation reaching the surface and near-surface waters may
be doubled for up to a decade. This UVB level is known from experiment to be
more than enough to kill off many kinds of organisms, particularly
phytoplankton. It could easily induce a crash of the photosynthetic-based food
chain in the oceans. Regularities in the latitudinal distribution of damage are
apparent in simulations of the atmospheric changes. We previously proposed that
the late Ordovician extinction is a plausible candidate for a contribution from
an ionizing radiation event, based on environmental selectivity in trilobites.
To test a null hypothesis based on this proposal, we confront latitudinal
differential extinction rates predicted from the simulations with data from a
published analysis of latitudinal gradients in the Ordovician extinction. The
pattern of UVB damage always shows a strong maximum at some latitude, with
substantially lower intensity to the north and south of this maximum. We find
that the pattern of damage predicted from our simulations is consistent with
the data assuming a burst approximately over the South Pole, and no further
north than -75 degrees. We predict that any land mass (such as parts of north
China, Laurentia, and New Guinea) which then lay north of the equator should be
a refuge from UVB effects, and show a different pattern of extinction in the
first strike of the end-Ordovician extinction, if induced by such a radiation
event.
