DID A GAMMA-RAY BURST INITIATE THE LATE ORDOVICIAN MASS EXTINCTION?

Gamma-ray bursts (GRB) produce a flux of radiation detectable across the observable Universe. They are probably associated with some supernovae, producing narrow beams of radiation during gravitational collapse. A GRB within our own galaxy could do considerable damage to the Earth's biosphere by ionizing and dissociating molecules in the atmosphere. Conservative rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. It is well know that there are five mass extinctions in the history of life. Many causes have been documented, and GRB may have contributed. The late Ordovician mass extinction has been attributed to glaciation, but this mass extinction may be the result of a GRB, though these hypotheses are not mutually incompatible. A nearby GRB could severely deplete the ozone layer. Thus, intense solar ultraviolet radiation would result from a nearby GRB. In addition, GRB produce opaque nitrogen dioxide in the atmosphere which would trigger global cooling. This is important because climate models suggest that the glaciation at the end of the Ordovician, which follows an interval of relatively warm climate, is hard to explain without some external impulse: a GRB could have provided that impulse. The cooling at the end Ordovician is short lived, which also would be predicted with a GRB. In short, a one two punch for life on the planet is postulated: a GRB leading to increased UV radiation, followed by global cooling. Generalizations about extinction patterns during the end Ordovician are difficult, but patterns in trilobites (a diverse Ordovician group) involving the differential extinction of pelagic trilobites and of those trilobite taxa with a planktonic larval type may be compatible with a GRB and its subsequent effects. Notably, it has been suggested that end Ordovician extinctions began before global cooling, indicating the possibility of a precursor event to the glaciation, and supporting the notion that global cooling alone may not explain the mass extinction. At present there are reasons for associating a GRB with the end Ordovician mass extinction, but additional tests are required.