Earth cratering record and impact energy flux in the last 150 Ma

Although the Earth's cratering record is subject to strong bias (i.e. unknown craters yet to be discovered or obliterated by geological processes, geo-chronologic uncertainties of impact events) a compilation of the 33 best dated large impact craters on Earth with diameters larger than 5 km, and younger than 150 Ma, their diameters, geochronologic ages, and the corresponding uncertainties can be used to construct a diagram summarizing our current knowledge on the influx of impact energy onto the Earth versus time. From the crater diameters, we estimated the corresponding impact energies through suitable scaling laws. Then, we associated with each crater a gaussian (bell) function of time centered at its age, with a half-width consistent with the age uncertainty and a total area proportional to the impact energy. Finally, all the bell functions corresponding to different craters were summed up and the resulting curve (smoothed out by computing running averages over 4 Ma) was plotted on a semilogarithmic scale. From this curve, it is apparent that the recently discovered 144.7 Ma old Morokweng crater in South Africa, which is associated with the Jurassic/Cretaceous boundary and related mass extinction, corresponds to the highest energy influx peak, almost an order of magnitude larger than the Chicxulub crater in Yucatan which, on the other hand, is associated with the Cretaceous/Tertiary; boundary mass extinction, the most severe in the marine record of the last 150 Ma. The third largest impact energy flux peak corresponds to the Late Eocene Popigai (Siberia) and Chesapeake Bay (U.S.A.) giant impact craters, which are also associated with a global biotic crisis. From the overall record examined herein it emerges that there is probably a threshold size of approximately 3 km for the smallest projectile capable of triggering large-scale extinctions. There is no convincing evidence for periodicities in the distribution of crater ages. A few groups of several craters appear to be more closely spaced in time than in a purely random distribution. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.