Science-based dating

When dealing with objects, an absolute dating can often be arrived at by using scientific methods, among which the best known is radiocarbon dating. This is based on the fact that in the atmosphere there is both 12C (carbon-12, non-radioactive carbon) and 14C (carbon-14, a radioactive isotope formed under the influence of cosmic rays). Both types of carbon occur in the CO2 (carbon dioxide) present in the atmosphere and dissolved in water. CO2 is absorbed by living creatures, and thus the entire biosphere contains 14C. If an organism dies, the absorption of carbon dioxide stops. The levels of carbon are not replenished. The radioactive isotope goes on decaying at the immutable rate at which all 14C is decaying all the time, but the amount of 12C remains constant. In organic material, one can measure how much 14C is left, and as of course the rate of decay, the half-life, of the isotope is known, one can establish the age of the sample being studied; that is,

Figure 2 The dendrochronological method. The dendrochronological method is based on a comparison of year ring patterns in different pieces of wood. If there is a match, such pieces can be fitted into an overlapping sequence. If within a certain area there is enough wood available from trees with overlapping life spans, it will be possible to obtain a reference chronology stretching back for centuries.

If we correct (calibrate) for a number of systematic errors, especially variations in the amount of 14C present in the atmosphere. The result is a dating in years BP (“before present,” understood to be 1950; remember, we need a fixed point for absolute dating). The importance of radiocarbon dating can hardly be overrated: before it arrived on the scene, many archaeological finds were dated only relatively. However, the dates procured are not very precise; there is a wide margin of error.

Radiocarbon dating is supplemented by other techniques, among which the most important is dendrochronology: the study of tree-ring dating. Seasonal variation in the growth of trees leads to observable annual rings. The width of these rings varies under the influence of climate. The resulting patterns of wide and narrow rings are unique. Thus, pieces of the wood of trees felled at different dates can be compared and matched in order to create an overlapping series of tree rings ranging from the present back to as far as a continuous series of timbers can be established. In some parts of Europe, this continuous series now stretches back for 9000 years, which means that every piece of well-preserved wood, not older than 9000 years and of the same or a comparable species, can be dated with exactitude. This is not just a revolutionary way of providing dates that are absolute and exact; dendrochronology has also helped to calibrate the radiocarbon results, and generates data to be used in the research into past climate.

The range of science-based dating techniques is growing all the time. Many are designed for dating prehistoric evidence. Important for historical times is thermoluminescence, which enables one to establish when an inorganic object (that cannot be dated by radiocarbon) was heated to over 300°C for the last time, as, for example, dating when a piece of ceramic was fired.