Geoscience Reference
In-Depth Information
to settle to the bottom of the sea. There it is absorbed into the
muds that accumulate on the seafloor and that eventually harden
into rock.
But over geologic time, the rate of accumulation varies greatly.
Since one component, the meteoritic, is arriving at a constant rate
from space and the other, the terrestrial, is accumulating at a vary-
ing rate, the percentage of meteoritic material in a deep-sea sedi-
mentary rock provides a gauge of how fast the terrestrial compo-
nent built up: The greater the percentage of meteoritic debris in a
given thickness of rock, the slower the sediment accumulated, and
vice versa. The rate at which meteorites fall on the earth is known,
as is the amount of iridium in meteorites, so that the iridium con-
tent of sediments can be used as proxy for the total amount of
meteoritic material they contain. Luis Alvarez later discovered that
two scientists from the University of Chicago had tried to use irid-
ium in this way to measure sedimentation rates but without suc-
cess. "Fortunately, I hadn't heard of their work," Luis commented of
the Chicago scientists, "If I had, I'm sure we wouldn't have both-
ered to look for iridium at the K-T boundary."
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The samples necessary for testing the iridium clock were read-
ily available from the Gubbio clay layer, but measuring the ex-
pected low levels of iridium required a research nuclear reactor.
Fortunately, the Alvarezes and their Berkeley colleagues Frank Asaro
and Helen Michel had access to one. The reactor allows neutrons to
bombard a rock sample and cause atoms of an isotope of iridium to
become radioactive and to emit gamma rays of a distinctive energy.
The number of such rays emitted per second is counted and is pro-
portional to the amount of iridium in the original material. When
analyzing at the parts per trillion level, however, it is extremely dif-
ficult to eliminate contamination (from the iridium that is always
present in platinum jewelry, for example).
Walter Alvarez selected samples ranging over the Gubbio sec-
tion—above, below, and at the K-T boundary—and brought them
back to Berkeley for analysis. Samples from above and from below
the boundary had the predicted amounts of iridium, about the
same as had been measured by others in deep-sea clays—300 parts
per trillion (ppt) or so. The samples from the boundary clay, how-
ever, revealed an earthshaking surprise—iridium levels 30 times
higher than those in the limestones on either side] Back-of-the-
envelope calculations showed that Walter's original idea—that the
clay had built up when the limestone for some reason ceased to be
deposited—could not be correct because then the clay would have
taken an impossibly long time to form. Thus the attempt to use
