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Uses of Radiocarbon Dating
Climate science required the invention and mastery of many difficult
techniques. These had pitfalls, which could lead to controversy. An example
of the ingenious technical work and hard-fought debates underlying the
main story is the use of radioactive carbon-14 to assign dates to the
distant past. For other examples, see the essays on Temperatures
from Fossil Shells and Arakawa's Computation
| The prodigious mobilization of science that produced nuclear weapons was so
far-reaching that it revolutionized even the study of ancient climates.
Nuclear laboratories, awash with funds and prestige, spun off the
discovery of an amazing new technique radiocarbon dating. The
radioactive isotope carbon-14 is created in the upper atmosphere when
cosmic-ray particles from outer space strike nitrogen atoms and transform
them into radioactive carbon. Some of the carbon-14 might find its
way into living creatures. After a creature's death the isotope would
slowly decay away over millennia at a fixed rate. Thus the less of
it that remained in an object, in proportion to normal carbon, the
older the object was. By 1950, Willard Libby and his group at the
University of Chicago had worked out ways to measure this proportion
precisely. Their exquisitely sensitive instrumentation was originally
developed for studies in entirely different fields including nuclear
physics, biomedicine, and detecting fallout from bomb tests.(1)
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| Much of the initial interest in carbon-14 came from archeology,
for the isotope could assign dates to Egyptian mummies and the like.
As for still earlier periods, carbon-14 dating excited scientists
(including some climate scientists) largely because it might shed
light on human evolution the timing of our development as a
species, and how climate changes had affected that.(2)
It was especially fascinating to discover that our particular species
of humans arose something like 100,000 years ago, no doubt deeply
influenced by the ice ages.(3) A few scientists noticed that the techniques might also be
helpful for the study of climate itself.
| From its origins in Chicago, carbon-14 dating
spread rapidly to other centers, for example the grandly named Geochronometric
Laboratory at Yale University. The best way to transfer the exacting
techniques was in the heads of the scientists themselves, as they
moved to a new job. Tricks also spread through visits between laboratories
and at meetings, and sometimes even through publications. Any contamination
of a sample by outside carbon (even from the researcher's fingerprints)
had to be fanatically excluded, of course, but that was only the beginning.
Delicate operations were needed to extract a microscopic sample and
process it. To get a mass large enough to handle, you needed to embed
your sample in another substance, a "carrier." At first acetylene
was used, but some workers ruefully noted that the gas was "never
entirely free from explosion, as we know from experience."(4) Ways were found to use carbon dioxide
instead. Frustrating uncertainties prevailed until workers understood
that their results had to be adjusted for the room's temperature and
even the barometric pressure.
| This was all the usual
sort of laboratory problem-solving, a matter of sorting out difficulties
by studying one or another detail systematically for months. More
unusual was the need to collaborate with all sorts of people around
the world, to gather organic materials for dating. For example, Hans
Suess relied on a variety of helpers to collect fragments of century-old
trees from various corners of North America. He was looking for the
carbon that human industry had been emitting by burning fossil fuels,
in which all the carbon-14 had long since decayed away. Comparing
the old wood with modern samples, he showed that the fossil carbon
could be detected in the modern atmosphere.(5)
| Through the 1950s and beyond, carbon-14 workers
published detailed tables of dates painstakingly derived from samples
of a wondrous variety of materials, including charcoal, peat, clamshells,
antlers, pine cones, and the stomach contents of an extinct Moa found
buried in New Zealand.(6) The
measurements were correlated with materials of known dates, such as
a well-documented mummy or a log from the roof of an old building
(where tree rings gave an accurate count of years). The results were
then compared with traditional time sequences derived from glacial
deposits, cores of clay from the seabed, and so forth. One application
was a timetable of climate changes for tens of thousands of years
back. Many of the traditional chronologies turned out to be far less
accurate than scientists had believed a bitter blow for some
who had devoted decades of their lives to the work.
| Making the job harder still, baffling anomalies
turned up. The carbon-14 dates published by different researchers
could not be reconciled, leading to confusion and prolonged controversy.
It was an anxious time for scientists whose reputation for accurate
work was on the line. But what looks like unwelcome noise to one specialist
may contain information for another. In 1958, Hessel de Vries in the
Netherlands showed there were systematic anomalies in the carbon-14
dates of tree rings. His explanation was that the concentration of
carbon-14 in the atmosphere had varied over time (by up to one percent).
| De Vries thought the variation might be explained by something
connected with climate, such as episodes of turnover of ocean
waters.(7) Another possible explanation was that,
contrary to what everyone assumed, carbon-14 was not created in the
atmosphere at a uniform rate. Some speculated that such irregularities
might be caused by variations in the Earth's magnetic field. A stronger
field would tend to shield the planet from particles from the Sun,
diverting them before they could reach the atmosphere to create carbon-14.
| Another possibility was that the cause lay
in the Sun itself. De Vries had considered this hypothesis but thought
it ad hoc and "not very attractive."(8)
However, solar specialists knew that the number of particles shot
out by the Sun varies with the eleven-year cycle of sunspots. Also,
the Sun’s own magnetic field varies with the cycle, and that
could change the way cosmic particles bombarded the Earth.. In 1961,
Minze Stuiver suggested that longer-term solar variations might account
for the inconsistent carbon-14 dates. But his data were sketchy. Libby,
for one, cast doubt on the idea, so subversive of the many dates his
team had supposedly established with high accuracy.(9)
| Suess and Stuiver finally
pinned down the answer in 1965 by analyzing hundreds of wood samples
dated from tree rings. The curve of carbon-14 production showed undeniable
variations, "wiggles" of a few percent on a timescale of a century
or so.(10) With this re-calibration
in hand, boosted by steady improvements in instruments and techniques,
carbon-14 became a precise tool for dating ancient organic materials.
(By the 1980s, experts could date a speck almost too small to see
and several tens of thousands of years old.) Tracking carbon-14 also
proved highly useful in historical and contemporary studies of the
global carbon budget, including the movement of carbon in the oceans
and its complex travels within living ecosystems.
| It was particularly interesting that, as
Stuiver had suspected, the carbon-14 wiggles correlated with long-term
changes in the number of sunspots. Turning it around, Suess remarked
that "the variations open up a fascinating opportunity to perceive
changes in the solar activity during the past several thousand years."(11) The anomalies were evidence for something
that many scientists found difficult to believe the surface
activity of the Sun had varied substantially in past millennia. Carbon-14
might not only provide dates for long-term climate changes, but point
to one of their causes.
Biosphere: How Life Alters Climate
Past Cycles: Ice Age Speculations
1. Libby (1946); Arnold and Libby (1949); Libby
2. E.g., Ericson and Wollin
(1964), pp. 6, 12-13; Emiliani (1956).
3. Bowen (1966), p. 216,
drawing on Emiliani.
4. Technique: Suess (1954);
explosion: Barendsen et al. (1957), p. 908.
5. Suess (1955).
6. For example: Kulp et al.
7. de Vries (1958).
8. de Vries (1958), p. 99.
9. Stuiver (1961), said only that
the "evidence suggests some correspondence"; Libby (1963).
10. Suess (1965); Stuiver (1965); Stuiver and Suess
(1966); for a review in midstream, see Lingenfelter
(1963). Further details in e-mail interview
of Paul Damon by Ted Feldman, 1998, copy at AIP. BACK
11. Suess (1965), p. 5949.
© 2003-2004 Spencer Weart & American Institute of Physics