MASS EXTINCTIONS AND THE SUNS ENCOUNTERS WITH SPIRAL ARMS

The terrestrial fossil record shows that the exponential rise in biodi versity since the Precambrian period has been punctuated by large exti nctions, at intervals of 40 to 140 Myr. These mass extinctions represe nt extremes over a background of smaller events and the natural proces s of species extinction. We point out that the non-terrestrial phenome na proposed to explain these events, such as boloidal impacts (a candi date for the end-Cretaceous extinction) and nearby supernovae, are col lectively far more effective during the solar system's traversal of sp iral arms. Using the best available data on the location and kinematic s of the Galactic spiral structure (including distance scale and kinem atic uncertainties), we present evidence that arm crossings provide a viable explanation for the timing of the large extinctions. (C) 1998 E lsevier Science B.V.