Page 139 - Encyclopedia Of World History

dating methods 489

The true revolutions, those which transform the destinies of people, are most frequently
accomplished so slowly that the historians can hardly point to their beginnings.
Scientific revolutions are by far the most important. • Gustave LeBon (1841–1931)

seeds, fruit pits, roots, hair, and hide.The half-life of radio- age of the artifact can be determined. Dating techniques
carbon also places restrictions on the application of car- that are based on the rate of energy capture in archaeo-
bon dating. Any specimen less than a few hundred years logical materials are applicable to raw materials that pos-
old usually is too young to date, as not enough of the sess a structure that, in fact, acts to trap such energy.These
radioactive isotope has decayed for an accurate age deter- materials include ceramics, as well as some minerals,
mination. In practice, any specimen much older than coral, teeth, and shell. Luminescence dating had a wide
about seven half-lives (approximately forty thousand range of applicability; it has been used successfully to
years) has so little of its original carbon 14 remaining, date ceramic objects just a few hundred years old, calcite
dating accuracy diminishes dramatically and the tech- deposits more than 250,000 years old, and heated flints
nique becomes unreliable. up to 500,000 years old.
Finally, it should be added that the accuracy of carbon Electron spin resonance, which measures the amount
dating depends on there being a constant amount of car- of energy trapped in shells, corals, volcanic rock, and
bon 14 in Earth’s atmosphere through time. This turns tooth enamel, is another radiation damage technique.
out not to be precisely the case. In a workaround that Here again, the amount of trapped energy present in an
depends on another dating procedure (dendrochronol- archaeological specimen is a function of time; once the
ogy) discussed below, carbon dates can be calibrated to amount of trapped energy is measured and the back-
better reflect the actual age of an object. ground radiation calculated, the amount of time since the
rock, shell, coral, or tooth formed can be computed. Elec-
Radiation Damage Techniques tron spin resonance has been applied successfully to
Another class of radiometric techniques relies on the sta- objects as young as 1,000 years and as old as one mil-
tistically predictable nature of radioactive decay in an indi- lion years of age.
rect way. These procedures fall into a subcategory of Fission track dating, a third radiation damage tech-
radiometric dating called radiation damage techniques. nique, makes use of the fact that radioactive decay may
These techniques, which first were used in the 1950s, take leave microscopic tracks in a material. These tracks may
advantage of the fact that radioactive decay may leave accumulate at a regular rate through time.When this rate
measurable damage on archaeological specimens, dam- can be estimated and the number of tracks counted, an
age that accumulates regularly through time.The amount estimate of the age of an object can be calculated. Fission
of damage, therefore, is proportional to the amount of track dating has been used to date materials as young as
time that has passed. For example, the fired clay in a a few thousand years old; from a human standpoint,
ceramic object deposited in the ground is buffeted by there is no effective upper limit to the applicability of this
radioactive decay that originates in the surrounding soil. procedure.
The energy released through that decay is captured by the
ceramic object, trapped in the atomic lattice of the object Dendrochronology
in an amount proportional to the amount of time the Biology also supplies the archaeologist with an important
object has been in the soil. This trapped energy can be natural calendar in the form of the annual growth rings
released by heat in a procedure called thermolumines- of trees. It is commonly the case that trees add a single
cence or by light in the application called optically stim- growth ring for each year that they are alive. In many
ulated luminescence, and its amount measured. Once the cases, the thickness of a growth ring is a factor of a mea-
rate at which the radioactive material in the soil is releas- surable aspect of the environment during the year the ring
ing energy—the so-called background radiation—is cal- was added; for example, mean temperature or amount of
ibrated, the rate at which energy is trapped by a ceramic precipitation. This correlation between tree ring width
buried in that soil can be calculated and, by inference, the and yearly climate fluctuations actually was recognized in