How to date archaeological sites
Relative & absolute dating
The date of an archaeological artifact or other object is either relative (younger or older than something else), or absolute (specific to one point in time).
Relative and absolute dating methods are two distinct approaches used in archaeology to establish the chronological sequence of events, and to date artifacts and fossils, at a site.
There are three man relative dating methods:
Stratigraphy dating is based on the law of superposition, first popularized by Egyptologist Flinders Petrie (1853-1942), that in undisturbed layers (strata) of sediment, the deeper you go, the older the layers are.
When archaeologists excavate, they document the contents of the site layer by layer.
By studying the vertical layout of these strata, researchers can build up a relative chronology of events.
Unfortunately, stratigraphy works less well when the site has been disturbed or the layers are not well-preserved.
Typology (or Seriation) is a relative dating method that compares artifacts and other objects in order to classify them according to their similarity or dissimilarity and thus link them them to a specific period.
The method relies on the proven assumption that the style of a particular artifact tends to change over time.
Stone tools, for instance, evolved in many different stages over time, as did pottery vessels.
By studying the stylistic similarities between artifacts, and comparing them across different layers, archaeologists can establish relative chronological sequences.
This method is particularly useful for dating ceramic assemblages. Unfortunately, seriation is less useful where a site has a limited number of artifacts.
Fluorine dating is a relative dating method is commonly used for bones and other organic items that have been interred in the same soil.
It's based on the fact that, over time, groundwater containing fluorine ions tends to seep into buried materials, where it gradually accumulates.
By measuring the fluorine content of different samples, archaeologists can fix their relative age.
Absolute dating - the provision of specific dates for specific samples - is the foundation of all archaeological and geological timelines.
Without absolute dating there can be no indirect dating, but even one absolute date can validate a whole chronology of relative dates.
For example, the presumed antiquity of lots of prehistoric art in the French Dordogne, is dependent upon a tiny handful of absolute dates.
In addition, evidence is mounting that cave painting did not evolve in linear fashion, but rather developed in isolated surges.
The charcoal drawings in Chauvet Cave, for example, were way ahead of their time. Indeed, some experts still dispute their age, although a raft of absolute dates has now put the matter beyond doubt.
Today, thanks to absolute dates from Mandrin Cave, we know that Cro-Magnon moderns were in France by 54,000 BC, and in Europe perhaps much earlier still. This makes a huge difference to how the relationship between the two species developed.
Without absolute dating it would be almost impossible.
Radiocarbon dating, also known as carbon-14 dating, is a widely used dating method in archaeology, which was developed by Nobel laureate Willard Libby in the late 1940s.
Radiocarbon dating works by measuring the decay of carbon-14, a radioactive isotope of carbon.
Carbon-14 (14C) is formed in the Earth's upper atmosphere through the interaction of cosmic rays with atmospheric nitrogen.
Once formed, carbon-14 combines with oxygen to create carbon dioxide, which is then incorporated into the carbon cycle and taken up by living organisms during photosynthesis.
Consequently, all living plants and animals maintain a constant ratio of carbon-14 to stable carbon isotopes (carbon-12 and carbon-13) during their lifetimes.
However, upon death, the intake of carbon-14 ceases, and the carbon-14 within the organic material begins to decay at a fixed rate.
This decay process is known as the half-life of carbon-14, which is approximately 5,730 years. Specifically, after 5,730 years, half of the original carbon-14 within a sample will have decayed into nitrogen-14.
By measuring the remaining amount of carbon-14 in a sample and comparing it to the initial amount, archaeologists can calculate the age of the sample.
The dating method involves the measurement of the carbon-14 isotope in the sample, through a process called accelerator mass spectrometry (AMS).
Using accelerator mass spectrometry (AMS) or beta counting, researchers measure the remaining carbon-14 in the sample and compare it to the known initial amount of carbon-14.
By determining the ratio of carbon-14 to stable carbon-12 (12C), scientists can calculate the age of the sample.
AMS is highly sensitive and allows for the analysis of very small samples, making it ideal for dating precious archaeological artifacts or limited samples.
Radiocarbon dating is especially effective for dating organic materials such as wood, charcoal, bone, shell, and other plant and animal remains.
These materials are often found at archaeological sites and provide direct evidence of human activity and past environmental conditions. By dating these organic remains, archaeologists can establish the age of the site and the artifacts found within it.
Radiocarbon dating was used to date charcoal pictures in the shaft at Lascaux Cave (15,000 BC) and the charcoal images discovered at the Apollo 11 Cave (25,500 BC) in Namibia, as well as those at the Chauvet Pont D'Arc Cave in the Ardèche.
In Australia, carbon dating (as well as optically-stimulated luminescence) was used to date fossilized wasp's nests on the famous Kangaroo Painting (15,300 BC), at Balanggarra in the Kimberley.
Despite its widespread use and reliability, radiocarbon dating does have limitations.
The method is most accurate for dating materials up to about 50,000 years old, beyond which the amount of remaining carbon-14 becomes too small to measure accurately.
Additionally, the presence of modern carbon contamination can affect the dating results of relatively young samples.
To mitigate potential problems, archaeologists carefully select samples for radiocarbon dating and often use other dating methods, such as stratigraphy or dendrochronology (tree-ring dating), in conjunction with radiocarbon dating, to corroborate the chronological interpretations.
Optically Stimulated Luminescence (OSL) dating relies on the principle that natural minerals, such as quartz and feldspar, accumulate energy from ionizing radiation in the environment over time.
When these minerals are exposed to sunlight or artificial light during the dating process, the accumulated energy is released in the form of luminescence.
By measuring this luminescence emitted by (say) a sample of sub-surface sediment, scientists are able to determine the date it was last exposed to light.
And once we know how old this sediment is, we can indirectly date objects (like human bones) that are buried in it.
The OSL dating process begins by extracting sediment samples from archaeological sites. These samples contain tiny mineral grains, and their ages are determined by measuring the luminescence signal they emit when stimulated by light.
The process is usually conducted in a laboratory, where specialized equipment is used to measure the intensity of luminescence.
As sediments are buried over time, they accumulate ionizing radiation from surrounding radioactive elements, such as uranium, thorium, and potassium. This radiation traps electrons within the mineral lattice.
Once the sediment samples are collected, they are exposed to a controlled light source, and the emitted luminescence is measured.
This provides an estimate of the time since the sample was last exposed to sunlight or heat, indicating when it was last buried.
OSL dating has proved especially useful for the dating of sediment layers in archaeological sites, such as those connected with human evolution, the timing of human migration, and the dating of ancient landscapes.
It is also used to date buildings and walls. The OSL date obtained from a foundation wall is the last time that foundation was exposed to light before being incorporated into a building: that is, when the building was first built.
OSL dating has been used in a wide variety of archaeological sites. In Australia it was used to date tiny sand grains found within fossilized mud wasp nests overlying Kimberley rock art in the remote northwest.
In China, it was used to date layers of sediment under the Lingjing Bird Figurine (11,300 BC), at Henan.
OSL dating techniques were also used to date stone and bone artifacts recovered from the Xiaogushan cave site - one of the most important prehistoric sites in North China.
The OSL ages obtained revealed the cave was first occupied by humans about 70,000 BC.
One significant advantage of OSL dating is that it allows archaeologists to date sediments directly, without the need for organic material, which is often scarce in certain archaeological settings.
The method is particularly relevant in dating sites where other dating techniques are limited or challenging to apply.
One disadvantage of OSL dating is that the material to be tested must not be exposed to light (which would reset the 'clock'), which can make sampling difficult.
Thermoluminescence was first documented in a paper written by British scientist Robert Boyle (1627-1691) in 1663.
The measurement of thermoluminescence emitted by a mineral or pottery sample was first suggested by the American chemist Farrington Daniels (1889-1972) in the 1950s.
Oxford University pioneered research into TL during the 1960s and 1970s, as a method of dating archaeological material.
TL dating is commonly used to determine the age of ancient pottery - the type of material found most often during archaeological excavations - as well as burnt flints, which have been exposed to extreme heat during their archaeological life.
TL dating works in a similar way to optically stimulated luminescence (OSL). It relies on the principle that inorganic minerals accumulate energy from ionizing radiation in the environment over time.
This energy continues to build up inside the inorganic material until an event (like the application of severe heat) resets the clock to zero.
When a sample of pottery containing these minerals is heated in the dating laboratory, the accumulated energy is released in the form of light, known as thermoluminescence.
Scientists measure the intensity of blue, green or infrared light released, which enables them to pinpoint when the pottery was last subjected to extreme heat - that is, when it was fired in a pit or kiln (i.e. made).
Of course pottery vessels are exposed to heat during cooking, but the cooking process is not hot enough to reset the luminescence clock.
TL dating was developed in the early 1960s as a means of dating fired pottery, and the technique has been used to date late Neolithic vessels from the Indus Valley civilization, as well as geologic formations that are half a million years old.
TL was also used to date rocks at Laugerie-Haute rock shelter, in the Vezere Valley, Dordogne.
TL dating is used to date fired or heated materials, whose luminescence clock was reset when they were last exposed to extreme heat, while Optically Stimulated Luminescence (OSL) dating is mainly used on sediments in which the clock was reset to zero when they were last exposed to sunlight.
Potassium-Argon (K-Ar) dating is a method used in archaeology and geology to determine the age of volcanic rocks and minerals.
K-Ar dating relies on the decay of radioactive potassium isotopes (potassium-40) into argon gas over time.
Potassium-bearing minerals, such as feldspar and mica, are commonly used in this method.
These minerals contain small amounts of radioactive potassium-40, which decays at a known rate into stable argon-40.
During the dating process, the rock samples are first heated in a laboratory to release any trapped gases, effectively resetting the radiogenic clock.
This process releases the accumulated argon gas from the radioactive decay of potassium.
The released argon gas is then measured using specialized mass spectrometers.
By measuring the ratio of potassium-40 to argon-40 in the sample, scientists can calculate the time that has elapsed since the rock or mineral was last heated or formed.
The half-life of potassium-40 is approximately 1.25 billion years, making K-Ar dating particularly suitable for dating rocks that are millions to billions of years old.
K-Ar dating is especially useful for dating volcanic rocks and ash layers, which are present at archaeological sites.
It has been instrumental in establishing the chronology of ancient human habitats.
For example, K-Ar dating has played a key role in unraveling the evolutionary chronology of hominins as far back as the first significant discovery of East African australopithecines at Olduvai Gorge in 1959, as well as Homo habilis fossils in the 1960s.
One limitation of K-Ar dating is that it requires the presence of potassium-bearing minerals in the sample.
Therefore, it is not suitable for dating materials like ceramics or organic remains, which do not contain significant amounts of potassium.
Uranium-Thorium (U-Th) dating, also known as Uranium-Series dating, is a valuable method used in archaeology and geology to determine the age of calcium carbonate materials, such as speleothems (stalactites and stalagmites) and coral.
This dating technique relies on the radioactive decay of uranium isotopes (uranium-234 and uranium-238) into thorium isotopes (thorium-230 and thorium-234) within the structure of the mineral.
By measuring the radioactive decay of uranium isotopes into thorium isotopes, scientists can accurately determine the age of samples with a wide age range.
The U-Th dating process begins with the collection of samples from archaeological sites that contain calcium carbonate materials.
These materials, formed in caves or in the ocean by the precipitation of calcium carbonate, often preserve a record of past environmental conditions and climate changes.
During the dating process, the samples are carefully analyzed in a laboratory to extract the uranium and thorium isotopes.
The radioactive decay of uranium into thorium occurs at known rates, allowing scientists to calculate the age of the sample.
One advantage of U-Th dating is its ability to date materials that are much older than those typically dated by radiocarbon dating.
U-Th dating is applicable to samples with ages ranging from a few hundred years to hundreds of thousands of years.
U-Th dating has been instrumental in dating archaeological sites, such as cave paintings and ancient human occupations found in caves.
For example, it was used to date the calcite overlying a series of abstract signs and symbols at 11 paleolithic caves in Spain.
These included including the UNESCO World Heritage sites of Altamira Cave (Santillana del Mar), El Castillo Cave (Monte Castillo), and Tito Bustillo Cave (Ribadesella), as well as the Cave of Maltravieso (Extremeadura).
U-Th dating technology was also used to date the Denisovan 'Xiahe mandible', discovered in 1980, in the Baishiya Karst Cave on the Tibetan Plateau, in China.
It has also been used to establish the timing of sea-level changes and the formation of coral reefs, providing valuable information for the study of past climate variations.
Fission track dating is a radiometric dating method used to calculate the age of minerals and rocks.
This technique relies on the damage tracks created by the spontaneous fission of uranium isotopes in mineral crystals over geological time.
It focuses on minerals like apatite, zircon, and sphene, which are commonly found in rocks and have the ability to retain fission tracks.
In the dating lab, the mineral samples are carefully processed to reveal the fission tracks.
This involves etching the mineral surface with acid or using other chemical treatments to make the tracks visible under a microscope.
The density of fission tracks is proportional to the amount of time the mineral has been exposed to the natural decay of uranium isotopes.
By counting the number of tracks per unit area and measuring the uranium content of the mineral, scientists can calculate the age of the sample.
Fission track dating is used for dating volcanic ash layers and other fine-grained sediments that do not contain radioactive isotopes suitable for other dating methods.
It has been instrumental in dating the timing of volcanic eruptions and other geological events.
One of the advantages of fission track dating is its ability to provide age estimates for relatively young materials, ranging from a few thousand to a few million years old.
However, fission track dating also has limitations.
Aside from the costs imposed by the need for specialized equipment, the method may be affected by thermal annealing, where tracks can be partially or fully erased by subsequent heating of the mineral.
The fission track method has been employed to date artifacts at a number of paleolithic sites, and was also used to corroborate the potassium-argon dates for the deposits at several hominin sites in Olduvai Gorge, Tanzania.
Chlorine-36 dating is a radiometric dating method used in archaeology and geology to determine the age of rocks and minerals.
This technique measures the accumulation of the rare isotope chlorine-36 in rocks and minerals over geological time.
Chlorine-36 is a rare isotope that is produced in rocks and minerals by cosmic ray interactions with isotopes of calcium and potassium.
The dating technique may be used where the sample includes granite and basalt, which contain trace amounts of chlorine.
In the laboratory, the sample is carefully processed to extract the chlorine, and separate the chlorine-36 isotope from other isotopes of chlorine.
The concentration of chlorine-36 in the sample is then measured using a technique called accelerator mass spectrometry.
The accumulation of chlorine-36 is proportional to the amount of time it has been exposed to cosmic radiation.
By measuring the concentration of chlorine-36 and comparing it to the known production rate of the isotope, scientists can calculate the age of the sample.
Chlorine-36 dating is particularly useful for dating rocks and minerals that have been exposed to cosmic radiation near the Earth's surface, such as surface exposure dating of glacial deposits and volcanic rocks.
Chlorine-36 dating has been used in a variety of archaeological contexts to do with prehistoric culture.
For example, stone panels at the site of Coa Valley rock art in Portugal, were dated using this technique.
One of the advantages of chlorine-36 dating is its ability to provide age estimates for relatively young materials, ranging from a few thousand to several million years old.
It has been used in the study of glaciations, erosion rates, and the history of landforms.
However, chlorine-36 dating also has limitations. It may be affected by variations in cosmic ray flux over time and space.
Electron Spin Resonance (ESR) is a radiometric dating method used in archaeology to determine the age of fossil teeth and certain types of minerals.
This dating technique relies on the principle that electrons become trapped in defects in the crystal lattice of minerals over time, and their number can be used to estimate the age of the sample.
It is commonly used to date only very old samples - usually older than 500,000 years - who have accumulated a significant number of trapped electrons.
ESR measures the number of trapped electrons in these minerals by exposing them to a magnetic field and measuring the energy released as the electrons return to their ground state.
The number of trapped electrons is proportional to the time since the mineral was last heated or exposed to sunlight.
This allows scientists to estimate the age of the sample since the last time it was exposed to heat or light.
ESR dating is particularly useful for dating materials that are beyond the range of other dating methods, such as the dating of fossil teeth from early human ancestors and extinct animal species.
Electron spin resonance has been employed for absolute dating of archaeological materials like quartz, flints, carbonate crystals, and fossil remains for nearly 50 years.
ESR dating was also used to date the Divje Babe Flute (c.58,000 BC) from Slovenia, the world's oldest musical instrument.
However, the technique is best and most commonly applied to the dating of tooth enamel from teeth fossils associated with early humans, whose antiquity is beyond the reach of carbon-14 dating.
In fact ESR dating is the only chronometric method that can be applied to date Early Pleistocene fossil teeth from early hominid occupations.
ESR dating was also used to date horse teeth found at the Neanderthal Mousterian site of La Micoque in the French Dordogne.
One of the advantages of ESR dating is its ability to provide age estimates for very old materials, dating back hundreds of thousands to millions of years.
This method has been instrumental in understanding the evolution and migration patterns of early humans, and environmental changes over geological time.
ESR dating also has limitations. It needs special equipment and expertise, making it less accessible than other dating methods.
Additionally, not all materials are suitable for ESR dating, and the technique may be affected by environmental factors such as exposure to radiation or chemical weathering.
Dendrochronology, also known as tree-ring dating, is a popular method used in archaeology and environmental science, to determine the age of wooden artifacts and structures.
This dating technique relies on the analysis of tree rings, which reflect the annual growth patterns of trees.
It can be applied to ancient timbers, wooden beams, or even wooden artifacts like musical instruments, and furniture.
The growth rings in the wood are counted and measured to establish a chronological sequence of rings, known as a tree-ring chronology.
Each ring in a tree's cross-section represents one year of growth. By comparing the patterns of tree rings from different samples, scientists can identify matching sequences and extend the chronology backward in time.
Dendrochronology dating is particularly useful in regions with distinct seasons, as the variation in growth rings is more pronounced in such areas.
The accuracy of dendrochronology dating is exceptional, often allowing for the precise dating of wooden objects to a specific year or season.
Furthermore, the dating can be cross-verified with historical records or other dating methods to enhance its reliability.
Tree-ring dating can look back about as far as 10,000 BC.
Dendrochronology has been instrumental in dating wooden structures and artifacts from various historical periods, including ancient buildings, medieval castles, and Native American dwellings.
It has also provided valuable information about past climate variations and environmental changes.
For example, a large oak well lining found near the town of Ostrov in the Czech Republic was dated to 5256-5255 BC, using dendrochronology, which makes it the oldest wooden construction worldwide to be dated by this method.
One limitation of dendrochronology is that it relies on the availability of well-preserved wooden samples, which may not always be present at archaeological sites.
Also, the development of long tree-ring chronologies requires the collection of samples from living trees and old structures, which can be time-consuming and labour-intensive.
Uranium-lead (U-Pb) dating is a radiometric dating method used to determine the age of Stone Age rocks and minerals, as well as others that are billions of years older.
This dating technique relies on the radioactive decay of uranium isotopes (uranium-235 and uranium-238) into lead isotopes (lead-207 and lead-206) over geological time.
It focuses on samples containing zircon, monazite, and other minerals that are rich in uranium and lead.
In the laboratory, the uranium and lead isotopes in the sample are carefully separated. The ratio of uranium to lead in the sample is then measured using mass spectrometry or other analytical techniques.
The radioactive decay of uranium into lead occurs at known rates, allowing scientists to calculate the age of the sample based on the isotopic ratios.
The age is determined by comparing the amount of lead in the sample to the amount of uranium and using the known decay constants.
U-Pb dating is particularly useful for dating rocks and minerals that are millions to billions of years old.
It has been instrumental in dating ancient volcanic rocks, meteorites, and other geological formations.
Uranium-lead (U-Pb) dating has been used in a variety of archaeological situations relating to ancient fossils outside the scope of C14 dating.
For example, it was used in Africa to date 'Little Foot', a remarkably complete Australopithecus africanus skeleton.
One of the significant advantages of U-Pb dating is its ability to provide precise and reliable age estimates for rocks and minerals that have undergone multiple geological events.
This method has been widely used in reconstructing the geological history of various regions and understanding the timing of tectonic events and mountain-building processes.
However, U-Pb dating also has some limitations. It requires high-quality samples and costly specialized equipment.
Also, the method is not suitable for dating materials that are relatively young, as the amount of lead produced by radioactive decay will be too low to measure accurately.
Samarium-neodymium (Sm-Nd) dating is used to determine the age of rocks and minerals containing samarium and neodymium isotopes.
This dating technique relies on the radioactive decay of samarium isotopes (samarium-147, samarium-148, samarium-149, and samarium-150) into neodymium isotopes (neodymium-143 and neodymium-144) over geological time.
The process focuses on rocks such as granites and gneisses or minerals like monazite and bastnaesite, which are rich in samarium and neodymium.
In the laboratory, the samples are carefully processed to extract the samarium and neodymium isotopes.
The isotopic ratios of samarium and neodymium in the sample are then measured using mass spectrometry or other analytical techniques.
The radioactive decay of samarium into neodymium occurs at set rates, allowing scientists to calculate the age of the sample based on the isotopic ratios.
The age is determined by comparing the amount of neodymium in the sample, to the amount of samarium.
Sm-Nd dating is particularly useful for dating rocks and minerals that are hundreds of millions to billions of years old.
It has been instrumental in dating ancient crustal rocks and determining the timing of geological events such as the formation of mountains and continents.
One of the advantages of Sm-Nd dating is its ability (like that of U-Pb dating) to provide age estimates for rocks and minerals that have experienced multiple geological events.
It has been widely used to study the evolution of the Earth's crust and the formation of continental landmasses.
However, Sm-Nd dating also has limitations. Like U-Pb dating, it needs high-quality samples and highly specialized equipment, which can be challenging to obtain and expensive.
Also, like U-Pb dating, it is not appropriate for dating materials that are relatively young.
Rubidium-strontium (Rb-Sr) dating is used to determine the age of rocks and minerals containing rubidium and strontium isotopes.
This dating technique relies on the radioactive decay of rubidium isotopes (rubidium-87) into strontium isotopes (strontium-87) over geological time.
It works on igneous rocks such as granites and basalts or minerals like feldspar and mica, which are rich in rubidium and strontium.
In the laboratory, the rubidium and strontium isotopes in the sample are carefully separated and measured using mass spectrometry or other analytical techniques.
The radioactive decay of rubidium into strontium occurs at known rates, which enables scientists to determine the age of the sample based on its isotopic ratio of strontium to rubidium.
Rb-Sr dating is particularly useful for dating rocks and minerals that are hundreds of millions to billions of years old.
It has been used to date ancient volcanic rocks, and has helped scientists to understand the timing of geological events, such as mountain-building and tectonic plate movements.
One of the advantages of Rb-Sr dating is its ability to provide age estimates for rocks and minerals that have experienced metamorphic events.
It is widely used in studying the thermal history of rocks and the timing of regional metamorphism.
However, like U-Pb and Sm-Nd dating methods, Rb-Sr dating requires high-quality samples and highly specialized equipment, which makes it costly.
Also, the method is not suitable for dating young materials, since the amount of strontium produced by radioactive decay may be too small for precise measurement.
(1) Scientific Dating in Archaeology. (2022) Seren Griffiths (Editor) Oxbow Books. ISBN: 9781789255621
(2) Radiocarbon Dating: An Archaeological Perspective Hardcover (2014) R.E. Taylor, Ofer Bar-Yosef. Routledge. ISBN-10: 1598745905. ISBN-13: 978-1598745900
(3) Science-Based Dating in Archaeology (1990) M.J. Aitken. Routledge. ISBN 9780582493094