Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage

Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage

Adrian L. Melott, Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045. Email: melott@ku.edu

Brian C. Thomas, Department of Physics and Astronomy, Washburn University, Topeka, Kansas 66621. Email: brian.thomas@washburn.edu

Abstrac.t- 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. 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.
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 north than -75°. 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 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.

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Introduction

Our goal is to describe and test a causal agent which has been proposed for one extinction event. There is a wide range of potential causes for Terrestrial mass extinctions. Some of them are external to the Earth, and include bolide impacts (as widely discussed for the K/T boundary) and radiation events. Among radiation events, there are possible large Solar flares, nearby supernovae, gamma-ray bursts (GRBs), and others. These have variable intensity, duration, and probability of occurrence, although some generalizations are possible in understanding their effects (Ejzak et al. 2007). Here we focus on gamma-ray bursts (Thorsett 1995; Scalo and Wheeler 2002).
These are the most remote and infrequent of events, but by virtue of their power, a threat approximately competitive with, for example, that of nearby supernovae. A GRB of the most powerful type (Woosley and Bloom 2004) is thought to result from a supernova at the end of stellar evolution for very massive stars with high rotational speed. Much of their energy is channeled into beams, or jets, which include very high energy electromagnetic energy, i.e. gamma-rays and X-rays. It is a testament to the power of these events that they were first detected when 1969-70 results from monitoring satellites designed to detect nuclear explosions on the surface of the Earth were triggered by events far across the observable Universe. The distance to the events was not known until the 1990’s, but when it was, their power became apparent.
There are several per day in the observable Universe. There are other kinds of events which are also potentially damaging, such as so-called short bursts and Solar flares, but rate information is only now beginning to clarify how much threat is likely from such sources.

Based on the rate of these events in the Universe as a whole, it is possible to estimate the rate and distribution of likely distances to events which irradiate the Earth (Scalo and Wheeler 2002; Melott et al 2004; Thomas et al. 2005; Dermer and Holmes 2005).
These estimates were made as follows: the average rate in the Universe as a whole is scaled to the density of blue light. Blue light is associated with large, hot stars, the kind which are precursors of GRBs. GRBs in galaxies other than our own are too far away to cause damage to the Earth. From the density of blue light in our own Galaxy, we can

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