Late Ordovician Geographic Patterns of Extinction
Thomas, Brian C. and Melott, Adrian L.
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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 more events of potential mass extinction level intensity 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 approximately doubled for up to one decade. The UVB level is know from experiment to be more than enough to kill of many kinds of organisms, particularly phytoplankton. It could easily induce a crash of the photosynthetic-based food chain in the oceans. Certain regularities in the latitudinal distribution of damage are apparent in computational 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. In order 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 characteristic 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 than -75 degrees. We predict that nay land mass (such as parts of north China, Laurentia, and new Guinea) which lay north of the equator should be a refugium from the UVB effects, and show a different pattern of extinction in the "first strike" of the end-Ordovician extinction, if it were induced by such a radiation event. more information on extinction strength versus latitude will help test this hypothesis.