Geoscience Reference
In-Depth Information
then such a large amount of soot could only have come from global
wildfires in which possibly as much as 90 percent of the total mass
of living matter on the earth burned. Supporters of the impact the-
ory naturally found the presence of the soot highly corroborative.
Opponents pointed out, however, that the conclusion depends on
the assumption that the clay layer was deposited rapidly; if it were
not, the levels of soot would not be extraordinary.
Soot was not the only unexpected substance. Amino acids, the
building blocks of proteins, and ultimately of life, are ubiquitous on
the earth but also occur in a class of meteorites called the carbona-
ceous chondrites. Canadian scientists reported that the boundary
clay contains 18 amino acids not otherwise found on the earth.
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Osmium is a platinum metal almost as rare in crustal rocks as
iridium. Karl Turekian, a geochemist at Yale, noted that the ratio
of two isotopes of osmium, Os 187 and Os 186, in meteorites is
approximately 1:1, but in rocks of the earth's crust it is higher than
10:1. Although chemical and geological processes concentrate some
chemical elements and deplete others, the ratios of the isotopes of
heavy elements such as osmium tend to remain constant. This resis-
tance to alteration is best illustrated by the enormous effort required
in the Manhattan Project to separate fissile U 235 from U 238,
which is 100 times as abundant naturally. Even the heat and shock
of meteorite impact would not change the ratio of isotopes as heavy
as those of osmium, and therefore they can be used as a tracer and
proxy to reveal the origin of the iridium in the K-T layer. If the Os
187:Os 186 ratio in the boundary clay turned out to be close to the
meteorite ratio of 1:1, then the osmium would likely be extraterres-
trial, as would the iridium, an almost identical element. If the
osmium isotopic ratio were much higher, then both the osmium and
the iridium would likely be of crustal origin, weakening the Alvarez
theory. According to David Raup, at Snowbird I, Turekian "made it
abundantly clear that he expected to find ordinary crustal isotope
ratios and that his study would show that the impact theory was nei-
ther necessary nor credible."
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A year and a half later, Turekian and
a colleague reported that the osmium isotope ratios in the bound-
ary clay were closer to meteoritic than to terrestrial levels.
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The
osmium test was less definitive than had been hoped, however, be-
cause an osmium ratio of approximately 1:1 turned out to mark not
only meteorites but volcanic rocks from the earth's mantle. Thus a
low osmium isotope ratio could indicate a mantle source as well as
an extraterrestrial one. A recent study of samples from across the last
80 million years of earth history, however, turned up a low osmium
