Carbon-14 Dating

Carbon-14 is produced when cosmic rays collide with atoms in the atmosphere to create an energetic neutron that then collides with a nitrogen-14 atom (seven protons, seven neutrons) to make a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons). Carbon-14 is radioactive, with a half-life of 5,730 years.

These C-14 atoms combine with oxygen to form carbon dioxide, which plants absorb into plant cells through photosynthesis. Animals and people eat the plants and take in the C-14 as well as the normal nonradioactive isotope C-12. The ratio of C-14 to C-12 in the air and in all living things at any given time is assumed constant; about 1 in 10 trillion carbon atoms are C-14. The C-14 atoms are always decaying, so after an organism dies, no new carbon atoms are taken in and this ratio of C-14 to C-12 atoms decreases.

The carbon-14 radiocarbon dating of living and once-living materials began with Willard Libby in the 1940s. Antiquities dated by C-14 agree with other date records until they begin to disagree for dates more than several thousand years ago. Why is there disagreement in the dates between C-14 dating and the written records?

Answer

The ratio of C-14 to C-12 in living organisms will depend on many factors, including the local climate and the amounts of C-14 in the atmosphere, factors that can vary on time scales as short as tens of years. The radiocarbon dating process assumes in its zero eth order approximation no variation in these factors over hundreds and thousands of years. But the cosmic ray intensity reaching the atmosphere may vary considerably, so the amount of C-14 produced will also vary. As the variations in the cosmic rays are determined by other independent methods, they can be incorporated into the C-14 dating as adjustments.

According to research literature, tree ring counts indicate that C-14 dating has fluctuations of the C-14 concentration in the atmosphere between 1400 and 1700 B.C.E. Furthermore, a comparison of radiocarbon-determined ages with ages of archaeological materials accurately established by other methods reveals that for the period from 100 B.C.E. to 1400, radiocarbon dating gives values that are too large, and that prior to 100 B.C.E. the radiocarbon values are too small.

At about 1600 B.C.E., the C-14 date values are about 175 years (5 percent) too small, increasing to about 300 years (6 percent) at 3000 B.C.E. The discrepancy appears to be a result of slight variations in Earth’s magnetic field over the years, which would alter the cosmic ray intensities and hence C-14 production in the atmosphere. These corrections allow C-14 dates to be corrected, and even for 100,000 years ago the radiocarbon dates are good to within 5 percent.