This award is in support of the second Greenland Ice Sheet Project (GISP II). Large meteoritic impacts probably exert important modifications of global climates and ecosystems, but the size and frequency of these events is not well established. Polar ice cores preserve a record of the accretion of materials from the solar system. The element iridium which is highly enriched in extraterrestrial debris relative to crustal materials, has been determined in Antarctic ice as a measure of the steady state cosmic debris influx, and a pulse of cosmic iridium has been reported to coincide with the 1908 Tunguska impact event. It is likely that many events of Tunguska magnitude should be preserved in a 250,000 year ice core. A complete record of Ir in a long polar ice core would allow determination of the size-frequency history of previous cosmic impacts. It would also determine whether the more uniform background influx of small particles is truly constant. Variation of the osmium isotope ratio will be used to distinguish cosmic (or mantle-derived) Os from meteorite Os. These studies will be undertaken as an ancillary part of the ongoing GISP II ice core program.
