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CHAPTER 13 • Climate During and Since the Last Deglaciation 233

BOX 13-1 TOOLS OF CLIMATE SCIENCE
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Deglacial C Dates Are Too Young

hen the same Barbados corals that had been dated
14
Wby the C method were dated by the thorium/ura-
20,000
nium technique, the Th/U ages turned out to be older
14
than the C ages by an amount that increased back in
14
time. For samples with a C age of 8000 years, the Th/U
14
age was older by 1000 years, while for samples with a C
age of 18,000 years, the age difference increased to 3500
years. 14 C age (years) 10,000
With both sets of ages giving consistent-looking
trends, scientists faced the problem of deciding which (if
either) was correct. Fortunately, independent evidence
was available from earlier work on the annual rings in
long-lived trees. Individual rings in these trees had been
14
dated both by the C method and by counting backward 0 0 10,000 20,000
year by year from the modern rings. A Th/U age (years)
The ages derived from counting rings turned out to be
14
older than those from the C analyses. Near 8000 to
10,000
9000 years ago, the ages from the tree-ring counts were
14
older than those from the C analyses by 1000 years, the
14
same as the offset of the Th/U coral ages from the C ages. 14 C age
younger
This agreement of two independent methods suggests 9000
14
that the C ages are in error (younger than the actual
ages).
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The main reason the C ages are too young is that the 14 C age (years) 8000
14
rate of production of C atoms in Earth’s atmosphere has
14
varied in the past. The C dating method is based on the
14
assumption that C has been produced in the atmos-
14
phere at a constant rate through time. Atoms of C are 7000
produced when cosmic particles entering our atmosphere
14
from other galaxies transform N atoms into radioactive
14 C atoms, which then slowly decay away. 7000 8000 9000 10,000
Because the rates of cosmic bombardment were B Age from tree-ring count (years)
14
higher during glacial times than they are now, more C Offset of C ages (A) Th/U dates from Barbados corals are
14
14
atoms were produced. Although many of those C atoms
14
older than C ages by as much as 3500 years. (B) Ages derived
have undergone radioactive decay, some of them still by counting tree rings backward in time are offset from C
14
14
remain in the carbon-bearing material used for C dating. dates on the same layers by a similar amount. (A: Adapted
14
14
This surviving excess of C atoms makes it look as if less C from E. Bard et al., “Calibration of the C Time Scale over the
14
has decayed (and so less time has elapsed) than is actually Last 30,000 Years Using Mass Spectrometric U-Th Ages from
the case. Barbados Corals,” Nature 345 [1990]; 405–10. B: Adapted from
M. Stuiver et al., “Climatic, Solar, Oceanic, and Geomagnetic
Why would cosmic bombardment have been higher in
12
14
Influences on Late-Glacial and Holocene Atmospheric C/ C
the past? Earth is partly shielded from cosmic rays by its
Change,” Quaternary Research 35 [1991]: 1–24.)
magnetic field. If this shield has at times been weaker than
it is today, it would have provided less protection against before 7000 years ago, confirming weaker magnetic
cosmic bombardment. Two observations confirm this shielding. Second, direct measurements of the past mag-
explanation. First, other elements known to be the prod- netic field in Earth’s rocks and sediments show that it was
uct of cosmic bombardment were also more abundant weaker at that time.