Analytical chemistry is important in many disciplines and may provide crucial information to a geologist, an atmospheric physicist, or an archeologist. To be useful, however, this analytical information must be accurate, and whether one is measuring the pH of a solution or measuring the age of charcoal from an ancient campfire, the key to accuracy is calibration. Radiocarbon dating is described in most general chemistry textbooks as depending on the production of carbon-14 in the upper atmosphere according the following reaction:

This equation means that ordinary nitrogen is converted by high-energy neutrons (produced by cosmic radiation) into radioactive carbon-14, which decays with a half-life of 5,730 years. In other words, 1.0 g of carbon-14 will decompose to exactly 0.5 g in 5,730 years. The reaction for the beta-particle decay of C-14 is

The individual atoms of carbon produced in the upper atmosphere are highly reactive and combine with oxygen to produce carbon dioxide, which is taken up by plants. Plants are then eaten by animals, and carbon-14 is incorporated into the food chain. Dating is based on the assumption that the percentage of carbon-14 in the atmosphere is constant and that the radiocarbon in all living organisms is in equilibrium with the atmosphere. If these assumptions are correct, the percentage of carbon-14 in living organisms will equal the percentage of carbon-14 in the atmosphere. When the plant or animal dies, equilibration with the atmosphere ceases, and the carbon-14 in the organism begins to decay; the amount of carbon-14 left can be used to estimate the age of the plant or animal remains. One simply needs to measure the ratio of and this can be done very easily using mass spectrometry. A schematic diagram of a simple mass spectrometer is shown below.

The problem with this method of dating is that the proportion of carbon-14 in atmospheric carbon dioxide varies slightly over time because radiocarbon production in the atmosphere is not constant year after year. The rate is affected by changes in ocean ventilation (e.g., a warmer ocean surface releases more dissolved carbon dioxide), or by geomagnetic variation (neutrons have a magnetic moment and will by affected by these cyclic changes in the earth’s magnetic field). Other factors, such as supernova, can cause changes in the flux of cosmic rays (gamma radiation). Cosmic rays produce neutrons and protons when interacting with atoms in the upper atmosphere, and the neutrons produced can then react with nitrogen to form carbon-14. This variation in atmospheric carbon-14 levels means that calibration by dating objects of known age is needed in order to correct the measured ratios for the variable production rate of carbon-14. An elegant way of doing this has been to compare carbon-14 dates with the ages of trees, as determined by counting the annual growth rings of very long-lived trees such as sequoias and certain pines. (Some German pines are 10,000 years old.) Radiocarbon dating gives an accurate age as long as the object being dated lies within this previously calibrated 10,000-year range. In principle, it is possible to date objects as much as 50,000 years old; in practice, for ages much older than 10,000 years, no calibration existed until recently, and errors in dating have been estimated to be as much as ± 3,000 years.

The recent calibration was carried out by Kitagawa from the International Center for Japanese Studies and van der Plicht (1) at the University of Goningen, Netherlands, who analyzed more than 250 samples of fossils taken from annually layered sediment deposits in Lake Suigetsu in Japan. Counting the number of sediment layers is completely analogous to counting seasonal growth rings in trees. New data for younger sediments overlap tree ring data almost perfectly. Using measurements from many different experiments, the authors were able to plot a “calibration curve” comparing the dates from carbon-14 dating and those from other sources. In general, the actual age of an article is slightly below its radiocarbon age. This difference is usually negligible for the period of recorded human history, but can mean a significant correction for earlier periods. The new calibration agrees with attempts to calibrate radiocarbon dating based on much more limited data (2) and with other radioisotope methods (based on uranium and thorium) used to estimate the age of corals (3,4).

The importance of this extended carbon-14 calibration goes beyond ensuring the accurate dating of organic materials, in that it may enable us to obtain a deeper understanding of variations in the earth’s oceans and climate related to the last glacial period, changes in the earth’s magnetic field, and fluctuations in the atmospheric production of radioisotopes.

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