Absolute dating

Last updated August 28, 2023

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.^[1]^[2] Absolute dating provides a numerical age or range, in contrast with relative dating, which places events in order without any measure of the age between events.

In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (such as coins and historical records). For example, coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year. Absolute dating techniques include radiocarbon dating of wood or bones, potassium-argon dating, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.^[3]

In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from younger organic remains (radiocarbon dating with ¹⁴
C) to systems such as uranium–lead dating that allow determination of absolute ages for some of the oldest rocks on Earth.

Radiometric techniques

Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half-lives in the thousands of years, such as carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments.

Radiocarbon dating

One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Cosmic radiation entering Earth's atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. With death, the uptake of carbon-14 stops.

It takes 5,730 years for half the carbon-14 to decay to nitrogen; this is the half-life of carbon-14. After another 5,730 years, only one-quarter of the original carbon-14 will remain. After yet another 5,730 years, only one-eighth will be left.

By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.

Limitations

The relatively short half-life of carbon-14, 5,730 years, makes dating reliable only up to about 60,000 years. The technique often cannot pinpoint the date of an archeological site better than historic records but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.

An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem. It is possible, particularly in dry, desert climates, for organic materials such as dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus, dating that particular tree does not necessarily indicate when the fire burned or the structure was built.

For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The development of accelerator mass spectrometry (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.

Potassium-argon dating

Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium–argon dating (K–Ar dating). Potassium-40 is a radioactive isotope of potassium that decays into argon-40. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.

Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K–Ar dating was used to calibrate the geomagnetic polarity time scale.

Luminescence dating

Thermoluminescence

Thermoluminescence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item.

Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light. This light can be measured to determine the last time the item was heated.

Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.

Because of these and other factors, Thermoluminescence is at the most about 15% accurate. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item.

Optically stimulated luminescence (OSL)

Optically stimulated luminescence (OSL) dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to sunlight 'zeros' the luminescence signal. Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains.

Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. The amount of luminescence released is used to calculate the equivalent dose (De) that the sediment has acquired since deposition, which can be used in combination with the dose rate (Dr) to calculate the age.

Dendrochronology

The growth rings of a tree at Bristol Zoo, England. Each ring represents one year; the outside rings, near the bark, are the youngest. Tree.ring.arp.jpg — The growth rings of a tree at Bristol Zoo, England. Each ring represents one year; the outside rings, near the bark, are the youngest.

Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year.

Dendrochronology has three main areas of application: paleoecology, where it is used to determine certain aspects of past ecologies (most prominently climate); archaeology, where it is used to date old buildings, etc.; and radiocarbon dating, where it is used to calibrate radiocarbon ages (see below).

In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11,000 years from present.^[4]

Amino acid dating

Amino acid dating is a dating technique ^[5]^[6]^[7]^[8]^[9] used to estimate the age of a specimen in paleobiology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed. All biological tissues contain amino acids. All amino acids except glycine (the simplest one) are optically active, having an asymmetric carbon atom. This means that the amino acid can have two different configurations, "D" or "L" which are mirror images of each other.

With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.^[10]

Related Research Articles

Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.

Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of Earth itself, and can also be used to date a wide range of natural and man-made materials.

<span class="mw-page-title-main">Carbon-14</span> Isotope of carbon

Carbon-14, C-14, ¹⁴
C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, California. Its existence had been suggested by Franz Kurie in 1934.

Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as paleomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.

Thermoluminescence dating (TL) is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated or exposed to sunlight (sediments). As a crystalline material is heated during measurements, the process of thermoluminescence starts. Thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating.

Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as feldspars, micas, clay minerals, tephra, and evaporites. In these materials, the decay product ⁴⁰
Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that the final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure or open air. Time since recrystallization is calculated by measuring the ratio of the amount of ⁴⁰
Ar accumulated to the amount of ⁴⁰
K remaining. The long half-life of ⁴⁰
K allows the method to be used to calculate the absolute age of samples older than a few thousand years.

Archaeological science, also known as archaeometry, consists of the application of scientific techniques to the analysis of archaeological materials and sites. It is related to methodologies of archaeology. Martinón-Torres and Killick distinguish ‘scientific archaeology’ from ‘archaeological science’. Martinón-Torres and Killick claim that ‘archaeological science’ has promoted the development of high-level theory in archaeology. However, Smith rejects both concepts of archaeological science because neither emphasize falsification or a search for causality.

Argon–argondating is a radiometric dating method invented to supersede potassium–argon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. ⁴⁰Ar/³⁹Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (³⁹K) into the radioactive ³⁹Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the ⁴⁰K/⁴⁰Ar* ratio, and thus to calculate the age of the unknown sample. ⁴⁰Ar* refers to the radiogenic ⁴⁰Ar, i.e. the ⁴⁰Ar produced from radioactive decay of ⁴⁰K. ⁴⁰Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the ³⁶Ar and assuming that ⁴⁰Ar is found in a constant ratio to ³⁶Ar in atmospheric gases.

The environmental isotopes are a subset of isotopes, both stable and radioactive, which are the object of isotope geochemistry. They are primarily used as tracers to see how things move around within the ocean-atmosphere system, within terrestrial biomes, within the Earth's surface, and between these broad domains.

Environmental radioactivity is produced by radioactive materials in the human environment. While some radioisotopes, such as strontium-90 (⁹⁰Sr) and technetium-99 (⁹⁹Tc), are only found on Earth as a result of human activity, and some, like potassium-40 (⁴⁰K), are only present due to natural processes, a few isotopes, e.g. tritium (³H), result from both natural processes and human activities. The concentration and location of some natural isotopes, particularly uranium-238 (²³⁸U), can be affected by human activity.

Luminescence dating refers to a group of methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating. It is useful to geologists and archaeologists who want to know when such an event occurred. It uses various methods to stimulate and measure luminescence.

Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology, molecular paleontology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.

Potassium-40 (⁴⁰K) is a radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% of the total amount of potassium found in nature.

Thermochronology is the study of the thermal evolution of a region of a planet. Thermochronologists use radiometric dating along with the closure temperatures that represent the temperature of the mineral being studied at the time given by the date recorded to understand the thermal history of a specific rock, mineral, or geologic unit. It is a subfield within geology, and is closely associated with geochronology.

Geochronometry is a branch of stratigraphy aimed at the quantitative measurement of geologic time. It is considered a branch of geochronology.

Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault scarps, cave development, and other geological events. It is most useful for rocks which have been exposed for between 10³ and 10⁶ years.

<span class="mw-page-title-main">Radiogenic nuclide</span>

A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive or stable.

Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is commonly known as a "dating method". Several dating methods exist, depending on different criteria and techniques, and some very well known examples of disciplines using such techniques are, for example, history, archaeology, geology, paleontology, astronomy and even forensic science, since in the latter it is sometimes necessary to investigate the moment in the past during which the death of a cadaver occurred. These methods are typically identified as absolute, which involves a specified date or date range, or relative, which refers to dating which places artifacts or events on a timeline relative to other events and/or artifacts. Other markers can help place an artifact or event in a chronology, such as nearby writings and stratigraphic markers.

This page is a glossary of archaeology, the study of the human past from material remains.

Potassium–calcium dating, abbreviated K–Ca dating, is a radiometric dating method used in geochronology. It is based upon measuring the ratio of a parent isotope of potassium to a daughter isotope of calcium. This form of radioactive decay is accomplished through beta decay.

References

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↑ Canoira, L.; García-Martínez, M. J.; Llamas, J. F.; Ortíz, J. E.; Torres, T. D. (2003). "Kinetics of amino acid racemization (epimerization) in the dentine of fossil and modern bear teeth". International Journal of Chemical Kinetics. 35 (11): 576. doi:10.1002/kin.10153.
↑ Bada, J.; McDonald, G. D. (1995). "Amino Acid Racemization on Mars: Implications for the Preservation of Biomolecules from an Extinct Martian Biota" (PDF). Icarus. 114 (1): 139–143. Bibcode:1995Icar..114..139B. doi:10.1006/icar.1995.1049. PMID 11539479.
↑ Johnson, B. J.; Miller, G. H. (1997). "Archaeological Applications of Amino Acid Racemization". Archaeometry. 39 (2): 265. doi:10.1111/j.1475-4754.1997.tb00806.x.
↑ 2008 Archived 2015-01-22 at the Wayback Machine quote: The results provide a compelling case for applicability of amino acid racemization methods as a tool for evaluating changes in depositional dynamics, sedimentation rates, time-averaging, temporal resolution of the fossil record, and taphonomic overprints across sequence stratigraphic cycles.
↑ "Amino Acid Geochronology Laboratory, Northern Arizona University". Archived from the original on 2012-03-14. Retrieved 2012-10-15.