Article reprinted from the Fall 1979 LBL
News Magazine
BERKELEY, CA — A scientific whodunit unfolding at Lawrence
Berkeley Laboratory (LBL) and on the UC Berkeley campus since early summer
has stimulated spirited debate among geologists, paleontologists,
biologists and physicists around the world.
The story revolves around a piece of pink and white limestone in whose
layers are buried evidence for what paleontologists call the "great
dying," the period some sixty-five million years ago when fifty
percent of the earth’s animals and plants were mysteriously wiped out.
The rock is a sample from the sedimentary deposits near Gubbio, Italy,
which were once buried beneath the sea and have since been lifted and
exposed in roadcuts.
When you look closely at the rock, you see a band of brown clay at the
boundary between the limestone which was laid down during the Cretaceous
period sixty-five to one hundred million years ago, and the red limestone
from the more recent Tertiary period. The brown clay was deposited about
sixty-five million years ago, at the time of the great extinctions.
Looking through a magnifying glass, you see that the Cretaceous
limestone is crowded with fossils of a tiny crustacean called Forminifera
globutrucana. They are entirely absent from the clay layer. In the
Tertiary limestone, a few of their relatives reappear.
Though scientists have known for more than a hundred years about the
extinctions from fossil records like these, no one knows what event might
have caused such a world-wide wipeout.
What brought the rock to LBL were conversations between physicist Luis
Alvarez and his son Walter, an associate professor of geology on the UC
Berkeley campus. Walter Alvarez has spent the last several years dating
similar rock samples from Gubbio by a method called magnetic stratigraphy,
using the reversals of the earth’s magnetic field, which are recorded in
the rocks, to date deposits.
Luis Alvarez wanted to know how long the forminifera extinction
lasted before the species began to reestablish itself. One way to find out
was to measure the sedimentary rate -- how long it took for the clay to be
deposited on the ocean floor. He suggested using iridium as the yardstick.
Iridium is a stable, silver-gray metal, rare in the earth’s crust
because it is bound up with iron in the earth’s core. But out in the
rest of the solar system, it is a thousand times more abundant. It is
brought to us by such solar system visitors as meteorites, which burn up
or fragment and drift as fine particles through the atmosphere.
"Because we know the rate at which extraterrestrial matter rains
down on the earth each year," says Alvarez, "it occurred to me
that by measuring the abundance of iridium in the sedimentary deposits, we
might be able to tell how long a period of time was represented by that
clay layer.
"I later learned that this idea was twenty-five years old, and it
had been implemented ten years ago," adds Alvarez parenthetically,
but fortunately he didn’t know that at the time.
The Alvarezes went to see Frank Asaro and Helen Michel, LBL nuclear
chemists who have developed a technique, called neutron activation
analysis (NAA), for making precise measurements of very low elemental
abundances in various materials. Their NAA facility is one of only a few
in the world that could undertake the difficult analytical tasks required
by this problem.
Taking their samples from the clay layer and from the younger and older
Tertiary and Cretaceous sediments on either side, Asaro and Michel
measured iridium and thirty other elements. They discovered a sudden
increase in the abundance of iridium in the clay twenty-five times more
than normal, in comparison with other elements. After the initial jump in
the clay layer, the iridium tapered down, more or less exponentially to
the background, forming a spike. This unexpected increase, called an
anomaly, coincides with the extinctions.
"The anomaly could have been caused by an increase in the amount
of iridium which was deposited along with the clay," says Helen
Michel, "or else the iridium might have remained constant, and the
rate at which clay was deposited slowed down."
"That doesn’t seem likely," she adds, because Walter
Alvarez’s study of the reversals of the earth’s magnetic poles does
not show any significant decrease in the sedimentation rate during that
period."
Finding The Source
"If there was an increase in the deposition rate of iridium-rich
material, a source on the earth would be unlikely, since iridium is one of
the least abundant elements in the earth’s crust," comments Asaro.
"If, on the other hand, the source is extraterrestrial, it would
be naturally enriched in iridium and would be compatible with our
measurements. So we believe the extraterrestrial hypothesis is more
likely."
It wasn’t the first time that an extraterrestrial source has been
considered as the cause of the great dying, because a satisfactory
explanation has never been proposed for such wide extinctions. No sweeping
climate changes, such as the ice ages which would follow, are recorded in
the rocks, and the theory that dinosaurs simply became obsolete after
ruling the earth 150 million years doesn’t account for the demise of so
many other species. But until the discovery of the iridium anomaly at LBL,
there was no direct evidence to support an extraterrestrial theory.
Once the anomaly was discovered, the problem was to find a source that
would deliver the extra iridium and was capable of causing the world-wide
extinctions.
One popular theory discussed at great length in the scientific
literature attributes the extinction to certain effects caused by a nearby
supernova. For example, if a star exploded a light year away, the
increased cosmic ray intensity could have killed the animals by giving
them all an intense dose of radiation. The problem is that the probability
of such a supernova explosion is only about one in a million, in the last
one hundred million years. From the amount of iridium measured in the
rocks, supernova explosion would have had to be a tenth of a light year
away. This upped the odds to a probability of one in a billion.
In spite of the unfavorable betting odds, the supernova theory had the
advantage that it could be checked experimentally in a series of delicate
tests done by neutron activation analyses.
Because it is almost certain that all elements heavier than nickel and
iron are synthesized in the intense neutron fluxes of a supernova, Asaro
and Michel set out to find plutonium-244. This isotope, with a half-life
of about eighty million years, should still be in the clay layer, if the
source of the iridium had been a supernova.
Asaro and Michel brought ideal credentials to this assignment, since
they were both experienced plutonium chemists before becoming NAA
specialists.
From the clay sample, they separated out a few drops of acid solution
which they then evaporated. If the sample contained plutonium, most of it
would be found in the evaporate. Though the procedure sounds simple, Asaro
and Michel in fact separated away almost all known elements, in a real
"tour de force" of chemistry.
When they irradiated the evaporate in the UC Berkeley campus nuclear
reactor, and looked for the gamma ray signature of plutonium-245 (into
which plutonium-244 would have been converted), they came up with "no
signal." They could have detected one tenth of the expected amount,
if it had been present, so the results looked bad for the supernova theory
of the extinction.
Since astrophysicists believe that each supernova should leave its own
fingerprint in the isotope ratios of the different elements made in its
oven, Asaro and Michel embarked on the difficult task of measuring the
ratios of the two iridium isotopes. The tests showed that the iridium from
the spike had the same isotopic ratios as solar system material, so again
the evidence was against the supernova theory.
Maynard Michel, using his mass spectrometer with its supersensitive
electron multiplier detector, is planning on looking further for the
plutonium-244. But with two experimental strikes against it, plus the
fantastically small theoretical probability, it appears the supernova just
doesn’t fit.
Michel and Asaro have widened the search by measuring samples from
similar sedimentary deposits in Denmark. Though the Gubbio samples are
from four sites over a twenty-seven kilometer area, it was important to
determine if the iridium spike occurs in samples from other locations.
"If we couldn’t find an iridium spike in other sedimentary
deposits, we would have a hard time convincing anyone, including
ourselves, that we had made any observations relating to the Cretaceous-Teritiary
extinctions, which we know are world-wide," says Alvarez.
In recent NAA tests on the Denmark sediments, the group found an
iridium spike that is larger by a factor of between five and ten than the
iridium spike in the Gubbio deposits.
"Now that the basic experiments have been done, and it seems clear
that the iridium came from an extraterrestrial source within the solar
system," says Alvarez, "we will spend the next few months trying
to put together a satisfactory theory of the extinction. Our theory must
agree with known properties of the solar system and with documented
evidence from paleontology."
Editor's note: The research team went on to publish a seminal
1980 paper in Science magazine, "Extraterrestrial cause for the
Cretaceous-Tertiary extinction." In the ensuing years, they provided
further evidence to document their theory that a large impact had occurred
at the end of the Cretaceous, and initiated the extinction event. In
1991, scientists found the Chicxulub crater, a 180-kilometer-wide,
20-kilometer-deep structure along the northern coast of the Yucatan
peninsula at the Gulf of Mexico. It is buried beneath up to a kilometer of
Tertiary carbonates. Among scientists, it is now widely believed that the
Chicxulub structure represents a large meteorite impact that was
responsible for the mass extinctions of 65 million years ago.
MASS EXTINCTION/IMPACT RESEARCH GROUP (LEFT TO RIGHT) HELEN MICHEL,
FRANK ASARO, WALTER ALVAREZ AND LUIS ALVAREZ (1969 PHOTO).
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