⇒http://www.jstor.org/pss/1683699

Summary
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 1±4 kilometers.

Identification of Extraterrestrial Platinum Metals in Deep-Sea Sediments
Italian Stratigraphic Sections
Results from the Italian Sections
The Danish Section
Results from the Danish Section
The Boundary Layers
A Sudden Influx of Extraterrestrial Material
Negative Results of Tests for the Supernova Hypothesis
The Asteroid Impact Hypothesis
Earth-Crossing Asteroids and Earth Craters
Krakatoa
Size of the Impacting Object
Biological Eflects
Problems in Boundary Clay Composition
Implications
References and Notes