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

Paleobiology, Summer 2009 by Melott, Adrian L, Thomas, Brian C

Introduction

Terrestrial mass extinctions have been attributed to a wide range of causes. Some of them are external to Earth, such as 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), a proposed causal agent for the endOrdovician extinction. 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 2006) 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, far across the observable universe, that they were first detected in the 1969-1970 results from monitoring satellites designed to detect nuclear explosions on Earth's surface. It was not until the 1990s, when the distance to the events became known, that their power became apparent. Several such events occur every day in the observable universe. Other kinds of events 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.

From 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 that irradiate 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 Earth. From the density of blue light in our own galaxy, we can estimate the likely rate in our galaxy - approximately one per 100,000 years. The radiation is known to be beamed, and only those beams pointed at Earth (and any possible enhancement to the general cosmic ray background) will contribute to extinction.

We have made detailed computations of the atmospheric effect of the nearest likely such event in the Phanerozoic, based on the idea that only for this time period do we have any possibility of detecting the effects within the fossil record. Such an event would irradiate Earth's upper atmosphere with approximately ten times the intensity of sunlight (for typically about ten seconds), but all in high-energy radiation such as X-rays and gamma-rays, even though approximately 6000 light years away. There have been arguments that the GRB rate in galaxies like ours may be lower than originally thought (Stanek et al. 2006) as well as counterarguments (Savaglio 2008; Savaglio et al. 2009; Ioka and Meszaros 2008). However, due to nonlinearity in the nature of atmospheric solutions (Thomas et al. 2005; Thomas and Melott 2006; Ejzak et al. 2007) the possible rate difference would not greatly reduce the expected amount of damage to the biota. Much of our analysis is based on our "standard" extinction-level irradiation model, a fluence of 100 kJ/m^sup 2^, which would be the most intense event predicted during the Phanerozoic.

The amount of damage is dependent upon the intensity of the radiation, which declines with distance from the burst. When we combine the correction factor for beaming, the disclike geometry of the Galaxy and the trends we found in our simulations, we find that an event of the intensity described in the previous paragraph is likely every few hundred million years. Furthermore, one can approximate that events of a given damage level (defined as a percentage of ozone depletion) happen at a rate proportional to the inverse square of the damage level. Events with half a given damage level are likely to come four times as often. Other kinds of ionizing events can do similar damage, but they are probably a lower threat. Supernovae may be a comparable threat (Gehrels et al. 2003; Fields et al. 2005, but our simulations would provide only partial understanding of their consequences. Work is in progress to expand the understanding of supernovae (e.g., Thomas et al. 2008).

We include here only the effect of electromagnetic radiation, but it is also possible that the burst may be accompanied by a similar burst of ultra-high-energy cosmic rays, which are high-energy atomic nuclei, mostly protons (Dermer and Holmes 2005; Dermer 2007). This would imply an additional class of damage effects of comparable magnitude and longer duration, which we do not include. We have made a start at understanding and including the atmospheric effects of cosmic rays (Melott et al. 2008).