Geoscience Reference
In-Depth Information
iridium as a clock failed, but, as often happens in science, no sooner
does one idea fall by the wayside than another springs up.
To check that the startling result was not somehow a bizarre
characteristic of the K-T boundary clay at Gubbio, the team ana-
lyzed two clay layers contained within the limestones above and
below the boundary and found both to have low levels of iridium.
Thus the iridium anomaly was associated with the thin K-T bound-
ary clay, not generally with clays from the Gubbio region.
If not an anomaly of Gubbio clays, perhaps the iridium spike
was merely a local aberration. To find out, the scientists needed to
find another site where the K-T boundary is exposed, collect sam-
ples, and analyze them for iridium. As an indication of just how lit-
tle was known about the K-T boundary in the late 1970s, even a
knowledgeable geologist like Walter had no idea where to look. As
would any intelligent person in a similar quandary, he went to the
library. There he discovered a reference to the sea cliffs south of
Copenhagen, which contain a classical and thoroughly studied K-T
rock section where, as in Italy, a clay layer marks the precise bound-
ary. Measurements of the amount of iridium in the Danish clay by
Frank Asaro showed an even greater enrichment—160 times back-
ground. The Alvarez team was clearly onto something: The iridium
anomaly was not restricted to Italian rocks and might even be a
worldwide phenomenon.
What did the Alvarezes know at this point? That at two widely
separated sites, abnormally high levels of one of the rarest metals in
the earth's crust occur in the exact thin layer that marks the great
K-T extinction and the demise of the dinosaurs. They concluded
that this could hardly be due to coincidence—the high iridium
level must somehow be linked to the extinctions. They knew that
finding how this linkage had occurred was the 65-million-year
question; if they could, they might solve the age-old riddle of dino-
saur extinction.
Since iridium is many times more common in meteorites and in
the solar system in general than in crustal rocks, the Alvarezes began
to consider extraterrestrial sources for the Gubbio iridium. The first
idea they pursued was the one paleontologist Dale Russell favored.
Exploding stars, or supernovae, which generate and then blast cos-
mic material throughout the galaxy, might have implanted the K-T
iridium, suffusing the earth with deadly cosmic rays and thus caus-
ing the extinction. Such nuclear furnaces give birth to a wide vari-
ety of chemical elements, including plutonium. One isotope of plu-
tonium, Pu 244, is a diagnostic marker of supernovae explosions. A
