Dating methods continue to evolve and improve over time…
Prior to the development of radiocarbon dating in the late 1940s, archaeologists relied primarily on historical records and the position of archaeological finds to determine the relative order of past events. Today, there is a whole suite of dating methods derived from chemistry and physics that can determine the numerical age of the dated sample. Among these modern methods are radiometric dating techniques.
Radiometric dating techniques are based on the principle that naturally occurring materials contain variants of particular chemical elements (called isotopes), and some of these variants are unstable and undergo radioactive decay over time as the atoms transform into more stable forms. The time it takes for half of a given amount of an unstable isotope to decay is called half-life. If we can estimate the original amount of certain isotopes in archaeological materials and the associated half-life, it is then possible to deduce the sample’s age from the existing isotope amount. Here are some radiometric dating methods that have been used at Liang Bua:
Carbon-14 (14C) is a radioactive isotope of carbon (stable carbon isotopes are 12C and 13C). Radiocarbon is constantly being produced in the upper atmosphere by cosmic rays, and it is incorporated into organisms in the form of carbon dioxide. When organisms die, they stop exchanging carbon dioxide and, thus, the amount of 14C in an organism begins to decay with a half-life of about 5,730 years. By measuring how much 14C is left in a sample, the time elapsed since the organism died can be estimated. Radiocarbon dating can be used on almost all organic matter and some carbon-bearing inorganic materials. However, because the amount of 14C in the atmosphere has not remained constant over time, scientists have used other dating approaches, such as dendrochronology (dating wooden samples using tree rings), to calibrate radiocarbon dates. The use of radiocarbon dating is effectively limited to samples younger than 50, 000 years, as the amount of 14C is too small to measure in samples older than this.
This method is used to date calcium carbonate materials, such as cave formations that are collectively called speleothems (e.g. stalactites, stalagmites, flowstones), as well as bones and teeth. In these materials, the uranium-234 (234U) isotope decays into thorium-230 (230Th) over time as part of a long decay chain (called the uranium series) that begins with 238U and ends with 206Pb. Because the half-life of 234U (245, 000 years) is much longer than that of 230Th (75, 000 years), the age of a sample can be determined by comparing the existing 234U/230Th ratio to an initial ratio estimated for the time of sample formation.
Instead of tracking time based on the amount of radioactive decay of one or two isotopes, another type of numerical dating method relies on measuring the amount of radiation released during radioactive decay that is then absorbed by mineral grains (e.g. quartz) in the deposit. Specifically, the exposure to radiation causes electrons to move around inside the mineral grains, and some of these electrons become trapped at defects and other imperfections inside the mineral grains. These trapped electrons accumulate over time at a rate determined by the amount of background radiation in the deposit. When these mineral grains are exposed to light or heat, the trapped electrons are provided with enough energy to escape from their traps. This process is called ‘bleaching’ when the electron traps are emptied by light.
Based on this principle, the number of trapped electrons in archaeological samples can be estimated by stimulating the mineral grains using light or heat, and measuring the emission of light (luminescence) when the stored electrons are released. By also estimating the background radiation levels in the dated sample and the surrounding deposit, scientists can determine the length of time that has passed since these grains were last exposed to light or heat. Depending on how electrons are released from their traps, luminescence dating techniques can be separated as follows:
- optically stimulated luminescence (OSL) — visible light is used for stimulation
- infrared stimulated luminescence (IRSL) — infrared photons are used for stimulation
- thermoluminesence (TL) — heat is used for stimulation
All of these techniques can be applied to date sediment samples from archaeological sites, although OSL and IRSL are the methods of choice. TL can also be applied to artefacts, pottery and ceramics that contain mineral grains which have been heated to a high temperature when the artefact was made.
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