Radiocarbon dating techniques, first developed by the American chemist
Willard F. Libby and his associates at the University of Chicago in 1947,
are frequently useful in deciphering time-related problems in archaeology,
anthropology, oceanography, pedology, climatology, and recent geology.
Through metabolic activity, the level of carbon-14 in a living organism
remains in constant balance with the level in the atmosphere or some other
portion of the earth's dynamic reservoir, such as the ocean. Upon the organism's
death, carbon-14 begins to disintegrate at a known rate, and no further
replacement of carbon from atmospheric carbon dioxide can take place. The
rapid disintegration of carbon-14 generally limits the dating period to
approximately 50,000 years, although the method is sometimes extended to
70,000 years. Uncertainty in measurement increases with the age of the
sample.
Although the method is suited to a variety of organic materials, accuracy
depends on the half-life to be used, variations in levels of atmospheric
carbon-14, and contamination. (The half-life of radiocarbon was redefined
from 5570 ± 30 years to 5730 ± 40 years in 1962, so some
dates determined earlier required adjustment; and due to radioactivity
more recently introduced into the atmosphere, radiocarbon dates are calculated
from AD 1950.) The radiocarbon time scale contains other uncertainties,
as well, and errors as great as 2000 to 5000 years may occur. Postdepositional
contamination, which is the most serious problem, may be caused by percolating
groundwater, incorporation of older or younger carbon, and contamination
in the field or laboratory.