Carbon Dating

Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis. Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things. The carbon-14 forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made.

Carbon Dating

Presuming the rate of production of carbon-14 to be constant, the activity of a sample can be directly compared to the equilibrium activity of living matter and the age calculated. Various tests of reliability have confirmed the value of carbon data, and many examples provide an interesting range of application. Carbon-14 decays with a halflife of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute. The low activity of the carbon-14 limits age determinations to the order of 50,000 years by counting techniques. That can be extended to perhaps 100,000 years by accelerator techniques for counting the carbon-14 concentration.

Carbon-14 Equilibrium Activity

Since living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, the ratio of C-14 to C-12 approaches that of the atmosphere.

From the known half-life of carbon-14 and the number of carbon atoms in a gram of carbon, you can calculate the number of radioactive decays to be about 15 decays per minute per gram of carbon in a living organism.

Radioactive carbon is being created by this process at the rate of about two atoms per second for every square centimeter of the earth's surface." Levin

The rate of production of carbon-14 in the atmosphere seems to be fairly constant. Carbon dating of ancient bristlecone pine trees of ages around 6000 years have provided general corroboration of carbon dating and have provided some corrections to the data.

Reliability of Carbon Dating

From the dating of ancient bristlecone pine trees from the western U.S., a correction curve for the carbon dating over the range back to 5000 BC has been developed. Trees dated at 4000 BC show the maximum deviation of between 600 and 700 years too young by carbon dating.

Prior to carbon dating methods, the age of sediments deposited by the last ice age was surmised to be about 25000 years. "Radiocarbon dates of a layer of peat beneath the glacial sediments provided an age of only 11,400 years."

These examples are from The Earth Through Time, 2nd Ed. by Harold L. Levin

Krane points out that future carbon dating will not be so reliable because of changes in the carbon isotopic mix. Fossil fuels have no carbon-14 content, and the burning of those fuels over the past 100 years has diluted the carbon-14 content. On the other hand, atmospheric testing of nuclear weapons in the 1950s and 1960s increased the carbon-14 content of the atmosphere. Krane suggests that this might have doubled the concentration compared to the carbon-14 from cosmic ray production.

Accelerator Techniques for Carbon Dating

Accelerator techniques for carbon dating have extended its range back to about 100,000 years, compared to less than half that for direct counting techniques. One can count atoms of different masses with a mass spectrometer, but that is problematic for carbon dating because of the low concentration of carbon-14 and the existence of nitrogen-14 and CH₂ which have essentially the same mass. Cyclotrons and tandem accelerators have both been used to fashion sensitive new mass spectrometer analyses. The tandem accelerator has been effective in removing the nitrogen-14 and CH₂, and can be followed by a conventional mass spectrometer to separate the C-12 and C-13. A sensitivity of 10^-15 in the ¹⁴C/¹²C ratio has been achieved. These techniques can be applied with a sample as small as a milligram.