If you need an accessible version of this item please contact JSTOR User Support

Extraterrestrial Cause for the Cretaceous-Tertiary Extinction

Luis W. Alvarez, Walter Alvarez, Frank Asaro and Helen V. Michel
Science
New Series, Vol. 208, No. 4448 (Jun. 6, 1980), pp. 1095-1108
Stable URL: http://www.jstor.org/stable/1683699
Page Count: 14
  • Download PDF
  • Cite this Item

You are not currently logged in.

Access your personal account or get JSTOR access through your library or other institution:

login

Log in to your personal account or through your institution.

If you need an accessible version of this item please contact JSTOR User Support
Extraterrestrial Cause for the Cretaceous-Tertiary Extinction
Preview not available

Abstract

Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.

Page Thumbnails

  • Thumbnail: Page 
1095
    1095
  • Thumbnail: Page 
1096
    1096
  • Thumbnail: Page 
1097
    1097
  • Thumbnail: Page 
1098
    1098
  • Thumbnail: Page 
1099
    1099
  • Thumbnail: Page 
1100
    1100
  • Thumbnail: Page 
1101
    1101
  • Thumbnail: Page 
1102
    1102
  • Thumbnail: Page 
1103
    1103
  • Thumbnail: Page 
1104
    1104
  • Thumbnail: Page 
1105
    1105
  • Thumbnail: Page 
1106
    1106
  • Thumbnail: Page 
1107
    1107
  • Thumbnail: Page 
1108
    1108