Vol. 10, No. 8 August 2000 INSIDE • Officer and Councilor Nominees, p. 9 GSA TODAY • 2000 Medalists, Awardees, p. 19 • Fellows, Members, p. 29 A Publication of the Geological Society of America • Student Associates, p. 34 Impact Events and Their Effect on the Origin, Evolution, and Distribution of Life David A. Kring, Department of Planetary Sciences, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA, [email protected] ABSTRACT Impact cratering has affected the geologic and biologic evolution of Earth, from the earliest stages of accre- tion to the present. The environmental consequences of impact cratering and their biologic repercussions are illus- trated by the Chicxulub impact event and its link to the Cretaceous-Tertiary (K-T) mass extinction event. While smaller impact events are more com- mon, there were probably four to five additional impact events of this size during the Phanerozoic. These types of large impact events, and even larger ones, occurred more frequently earlier in Earth history. A particularly intense period of bombardment appears to have occurred ~3.8–3.9 Ga, corresponding to the earliest isotopic traces of life on Earth. These impact events may have made it difficult for preexisting life to survive or may have provided the neces- sary environmental crucibles for prebi- otic chemistry and its evolution into life. INTRODUCTION It has become increasingly clear that impact cratering has affected both the geo- logic and biologic evolution of our planet. Although this view has its roots in the Figure 1. Earthrise over Smythii impact basin with Schubert impact crater on horizon. Views like this Apollo era (Fig. 1; McLaren, 1970), it was during Apollo missions made it clear that Earth is part of a planetary system rather than an isolated not widely recognized until studies linked sphere, subject to the same bombardment that battered the surface of the Moon. (Apollo 11 AS11-44- the mass extinction that defines the end 6551) of the Mesozoic Era with the Chicxulub impact event (L.W. Alvarez et al., 1980; Hildebrand et al., 1991). That particular tile, and the ambient conditions on Earth organism’s ability to adapt (Newell, 1962). event also illustrates how a process that at the time of impact. Consequences can When the environmental effect is largely destroys some organisms can create oppor- range from the death of individual organ- regional, the changes must overwhelm the tunities for other organisms—in this case isms to the complete extinction of species. migratory capacity of a species or last leading to distinctly different ecosystems While the former can be the direct result longer than its dormant capacity. When during the Cenozoic Era. This dual pattern of an impact event (e.g., shock wave– the effect transcends geographical bound- of disaster and opportunity has existed induced hemorrhaging and edema in an aries and becomes global, the change must with impact events throughout Earth his- animal’s lungs [Kring, 1997]), the more be rapid relative to the time scale of evolu- tory, even during the earliest development important biological effect, including tionary adaptation or, again, last longer of life. extinction, will be through impact- than the dormant capacity of a species. The biologic consequences of impact generated environmental changes. To be The minimum types of impact events cratering depend on many factors, includ- an effective extinction mechanism, the needed to exceed these extinction thresh- ing the energy of the impact event, the environmental changes need to extend type of target materials, the type of projec- throughout a habitat range and exceed an Impact Events continued on p. 2 GSA TODAY August IN THIS ISSUE Vol. 10, No. 8 2000 Impact Events and Their Effect GSA Names 2000 Medal and Award GSA TODAY (ISSN 1052-5173) is published monthly on the Origin, Evolution, and Recipients ............................. 19 by The Geological Society of America, Inc., with offices at 3300 Distribution of Life .................... 1 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Field Forum Report . ................ 20 9140, Boulder, CO 80301-9140, U.S.A. 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