外星生命
生物集群灭绝
小行星
白垩纪
地质学
陨石
天体生物学
古生物学
火山
地球科学
消光(光学矿物学)
丰度(生态学)
球粒陨石
航程(航空)
物理
生物扩散
生态学
生物
社会学
人口学
复合材料
材料科学
人口
作者
Luis W. Alvarez,Walter Álvarez,Frank Asaro,Helen V. Michel
出处
期刊:Science
[American Association for the Advancement of Science (AAAS)]
日期:1980-06-06
卷期号:208 (4448): 1095-1108
被引量:3420
标识
DOI:10.1126/science.208.4448.1095
摘要
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.
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