Age determination (archeology)
When determining the age of archaeological finds, there are different dating methods that can be divided into two large groups, relative and absolute age determination.
Relative dating methods
Relative dating through stratigraphy
As a rule, the guiding principle is that lower layers are more likely to have been deposited than upper layers, and therefore older layers can be found under younger ones ( stratigraphic principle ). Similar to relative age determinations in geology , only the sequence of the layers is determined without measuring the actual age.
Exceptions occur, for example, in the case of relocations or thrusts: For example, in an archaeological finding, older material could have been deposited over younger material through relocation during construction work or through erosion on a slope. Similar events occur in geology: A rock block is lifted by tectonic processes and pushed over a (not lifted) younger one. Such processes are rare and can be identified by taking an overall picture.
Relative dating through find compositions
The temporal change in the shape of the objects, the materials used or the craft techniques means that the composition of the found objects in closed finds such as graves , rubbish pits and depot finds changes in a characteristic way. Such a relative chronological sequence can be represented with combination statistics or in a seriation .
Changing percentages of artifact types can also be decisive for the relative chronology of entire archaeological cultures or time horizons. For relative dating, therefore, it is not the presence or absence of an individual object that is decisive, but its relative frequency. This can be justified as a model that a form is invented in one time stage but is still rare, in the next stage it is then generally known and is used a lot and in the next stage it is slowly replaced by a new form.
A chorological method that can be successful in evaluating burial fields is so-called horizontal stratigraphy. If different regions were used at different times in a burial site, chronologically relevant grave goods from a certain time can only be found in the associated region of the burial ground. The mapping of the offerings on the burial ground plan allows then to recognize different occupation phases in the map image.
Archeology and geology
If finds of early humans ( hominids ) or their products are embedded in geological layers, they can be dated via these. Remnants from fossil-bearing layers can therefore be classified more precisely with the help of index fossils . Major events that create characteristic supra-regional features can also enable layers or rocks to be compared. For example, an iridium layer that was formed when a large meteorite hit was embedded in all rocks around the world at that time. Deposits of volcanic ash can sometimes be assigned to a specific eruption such as that of the Laacher See volcano on a large scale .
Absolute archaeological-geological dating methods
Numerous absolute dating methods are used in archeology. These are based on different approaches. Which of these approaches are applicable and useful is decided on a case-by-case basis for the respective finding.
Geochronological dating by means of isotope decay
Many methods of age determination in geology only provide very rough absolute data. For example, the radionuclides 26 Al or 53 Mn, which are now extinct, were still present when the solar system was formed. With these methods z. B. the age of formation of meteorites or individual components of meteorites can be determined relative to each other. Only by calibrating these relative dating methods with absolute dating methods such as uranium-lead dating can absolute ages be given. The radiometric methods with radionuclides that are not extinct measure how high the proportion of naturally occurring radioactive elements and possibly their decay products is. Since the half-life of the radioactive elements is known, the age can be calculated from this.
For the age of rocks one needs elements with very long half-lives. The following methods, among others, are suitable for this ( half-life in brackets, see also geochronology ):
- Uranium 238 U → lead 206 Pb (4.5 billion years, uranium-lead dating )
- Uranium 235 U → lead 207 Pb (704 million years, uranium-lead dating)
- Thorium 232 Th → lead 208 Pb (14 billion years)
- Rubidium 87 Rb → Strontium 87 Sr (48.8 billion years)
- Samarium 147 Sm → Neodymium 143 Nd (106 billion years)
- Potassium 40 K → Argon 40 Ar (1.25 billion years, potassium-argon dating )
The aluminum-beryllium method is a special feature, as it uses the decay of two radioisotopes that are not in the daughter / mother isotope ratio for comparison. This method of surface exposure dating is also used to determine the age of fossil hominid bones. The age is determined via the aluminum isotope 26 Al and the beryllium isotope 10 Be in the mineral quartz is based on the (known) ratio of 26 Al and 10 Be, both of which are caused by cosmic radiation (neutron spallation , muon capture) on the surface of stones / Minerals arise. The ratio depends u. a. on the altitude, the geomagnetic latitude, the radiation geometry and a possible weakening of the radiation through shielding (e.g. covering).
As a rule, so-called isochrone diagrams are used for geological dating . The advantage of this technique is that the initial concentration and isotope ratios of the daughter elements do not have to be known, rather they are obtained as a further result, in addition to the age of the sample. Furthermore, the isochronous technology has the advantage that it can be reliably excluded that any disturbances caused by environmental influences could have falsified the measured age.
The real advantage of radiometric dating methods is based on the fact that the binding energies of the atomic nuclei are several orders of magnitude larger than the thermal energies of the environment in which potential samples (mostly rocks) can exist at all. An influence on the decay rates (half-lives) by environmental influences can therefore be excluded, so that the radiometric ages - especially if they were obtained using the isochronous method - are considered to be very reliable.
Another absolute dating method is the fission track method. Here the high-energy decay products caused by the radioactive decay (e.g. spontaneous decay of uranium or decay of 40 K to 40 Ar) are made visible along their trajectories by etching under the microscope and counted.
Other geochronological methods
When Warvenchronologie be varves , annual sediment in lakes, counted. These deposits give the floor a striped pattern. This method is particularly suitable for areas with strong snowmelt . For the Eifel region there is a chronology of the last 23,000 years, for a Japanese lake for 45,000 years and for the Lago Grande di Monticchio in southern Italy even for the last 76,000 years.
When analyzing ice cores , the layers are counted that are formed each year by the snowfall.
The Magnetostratigraphy uses the fact that the Earth's magnetic field has reversed many times over the years. This pattern can be found and counted in the rocks .
Absolute archaeological dating methods
To determine the age of human remains in archeology, starting isotopes with shorter half-lives are usually required than in geology. Radiocarbon dating of organic materials is mainly used here. Radiocarbon dating measures the content of radioactive carbon 14 C, which has a half-life of 5,730 years. This allows age determinations of up to 60,000 years. In older samples, the 14 C content is already too low to be measured. One difficulty with this method is that the proportion of 14 C in the earth's atmosphere is not constant. These fluctuations can be determined with the help of dendrochronology , for example .
Dendrochronology makes it possible to use the characteristic annual rings of some tree species, e.g. B. oak to make dates that can be accurate to the year. Therefore, with good preservation, it is possible to determine the construction time of buildings with preserved wood, such as pile dwellings or wells , the construction time of ships or the manufacture of coffins .
In coin dating, the coins provide a terminus post quem . This means that a find can only have entered the earth after the (youngest) coin has been minted from this find. It is not initially clear how long this happened after the coin was minted.
Archaeological finds (imports)
Archaeological finds from one culture can also be dated using absolutely dated imported objects from other cultures. Examples of this are Greek ceramics or bronze vessels in princely tombs from the late Hallstatt period in eastern France and southwest Germany or Roman imports in the Germania Magna during the imperial era.
Connection with historical events
Occasionally, archaeological find contexts can be dated via known historical events. For example, the cities of Pompeii and Herculaneum were destroyed and buried by the eruption of Vesuvius on August 24, 79, described by Pliny the Younger , so the buildings in these cities must have been built before this volcanic eruption. A terminus ante quem of 79 applies to finds from these locations .
According to historical sources, the immigration of the Lombards into Italy took place in 568. Longobard burial grounds in Italy therefore only date to the period after 568, and immigration here offers a terminus post quem .
Absolute dating by combining archaeological finds and historical data should generally be carried out with great caution, as there is a risk of circularity (so-called mixed interpretation). For example, the historically documented destruction of a settlement could lead one to prematurely relate a layer of fire found there to this destruction and to date it after it, even though the layer of fire actually originates from another fire that has not been historically recorded. Such a circular conclusion can be prevented by an as precise, independent archaeological dating as possible and classification in a larger context. For the above examples, this means that the finds from Pompeii (coins, ceramics, etc.) allow the destruction to be dated around 80 even without knowing the exact date. The dating of the immigration of the Lombards is possible according to the historical date of 568, since the oldest grave goods of the Lombards in Italy come from the last third of the 6th century according to archaeological criteria and special forms, such as fibulae , were not known there before. but according to exact comparisons they were brought by Lombard immigrants from their previous residential areas in Pannonia .
Some methods are suitable for relatively specific areas of application. The Thermolumineszenzdatierung as is used for scientific dating of ceramics. In the meantime, argon-argon dating has also been refined to such an extent that it can be used to absolutely date historical events in certain cases. It was used in 1997 to use it to date the pumice stone from the Vesuvius eruption , which destroyed Pompeii , to the years from 72 to 94 AD. This means that the date given in historical sources (79 AD) is in the range of the quantified error. The independent ages therefore mutually confirm each other. Rehydroxylation represents a new investigation method, namely the degree to which oxygen bridges in ceramics have been broken by the ingress of water. In this way, Moira Wilson from the University of Manchester and her colleagues succeeded in identifying ceramic objects up to 2000 years old with great accuracy.
Merging of relative and absolute chronology
As a rule, a relative chronology sequence should first be created, which is only merged with absolute data in a second step. If this principle is disregarded, this leads to incorrect relative dating if the absolute dating is too imprecise or incorrect.
- Mebus A. Geyh: Handbook of physical and chemical age determination . Scientific Book Society, Darmstadt 2005, ISBN 3-534-17959-5 .
- Heuel-Fabianek, B .: Natural radioisotopes: the “atomic clock” for determining the absolute age of rocks and archaeological finds . Radiation Protection Practice, 1/2017, pp. 31–42.
- Manfred Reitz : On the trail of time. Decipher past puzzles using scientific methods . Wiley-VCH, Weinheim 2003, ISBN 3-527-30711-7 .
- Rolf CA Rottländer: Introduction to the scientific methods in archeology . Archaeologica Venatoria, Tübingen 1983, ISBN 3-921618-19-3 .
- ↑ B. Heuel-Fabianek, in: Radiation protection practice. Cologne 2003,3, p. 69. ISSN 0947-434X
- ↑ A. Brauer: Lake sediments of the Holzmaares from the Vistula Period - varven chronology of the high glacial and evidence of climatic fluctuations. in: documenta naturae. Munich 1994, 85. ISSN 0723-8428
- ^ PR Renne, WD Sharp, AL Deino., G. Orsi, L. Civetta: 40Ar / 39 Ar Dating into the Historical Realm. Calibration Against Pliny the Younger. in: Science. Washington 277.1997, 1279-1280. ISSN 0036-8075
- ↑ scienceticker: Age determination with fire and water