COSMOGENIC NUCLIDES AS A TEST FOR LARGE EXTRATERRESTRIAL IMPACTS

Large extraterrestrial impacts remain a threat to Earth biota on long time scales. Little is known of the frequency of these events, despite past extinction events being tied to impacts. We suggest a new method for testing the geologic record of the last ~1 Myr for large impact events. Cosmogenic nuclides are created when high energy cosmic rays interact with the atmosphere and extraterrestrial matter. The abundance of these nuclides is preserved in ice cores, giving researchers a clue to the cosmic ray flux of the past. Many different mechanisms have been suggested as the cause for variations in these abundances. We estimate the cosmogenic nuclide production on long-period comets as impacts of these objects with the Earth are a possible source of abundance variation. In particular, we examine the isotope variation at the beginning of the Younger Dryas cold period. A controversial hypothesis suggests a large bolide impact is the source of a megafaunal extinction at that time. We provide an additional, definitive test of this hypothesis.