The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results. Sometimes only one method is possible, reducing the confidence researchers have in the results. Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.
Methods fall into one of two categories: Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating. These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating. One of the first and most basic scientific dating methods is also one of the easiest to understand. Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology. A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.
The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity. Paleomagnetism is often used as a rough check of results from another dating method. Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint. Researchers can first apply an absolute dating method to the layer. They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer.
Anything below the Taupo tephra is earlier than ; anything above it is later. Generally speaking, the more complex a poem or piece of pottery is, the more advanced it is and the later it falls in the chronology. Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites. Sometimes called carbon dating, this method works on organic material.
Both plants and animals exchange carbon with their environment until they die. Afterward, the amount of the radioactive isotope carbon in their remains decreases. Measuring carbon in bones or a piece of wood provides an accurate date, but only within a limited range. It would be like having a watch that told you day and night. Also called single crystal argon or argon-argon Ar-Ar dating, this method is a refinement of an older approach known as potassium-argon K-Ar dating, which is still sometimes used.
Both methods date rock instead of organic material. As potassium decays, it turns into argon. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least , years old. An open system is where all the products of a uranium decay series remain in the system, or when uranium itself has migrated to or from the sample during burial.
A closed system is where there is has been no migration and all the decay products of uranium remain in material. A key assumption of U-series dating is that the system is closed, or the open system behavior can be accounted for. This requires detailed analysis of the geological history, taphonomy, and stratigraphy of a particular site, as well as complementary isotope measurements on the material under examination.
Closed system behavior is essential for U-Th dating of speleothems, tufa, and calcrete. If possible, 14 C should rather be used for shell, teeth, bones, etc. U-series dating requires specialist clean facilities, to avoid contamination with lead and other metals. The technique is often highly accurate, achieving 1 percent precision in many cases during the last interglacial period.
U-Th dating is especially accurate over this period, particularly in speleothem contexts. The introduction of multiple-collector inductively coupled plasma mass spectrometer MC-ICPMS techniques, which can simultaneously measure several isotopes in the uranium series, and require small sample mass between milligrams and grams, have greatly improved the precision and practicality of the technique. Luminescence dating encompasses a suite of related techniques that rely on similar principles to calculate the time since common minerals such as quartz and feldspar were last exposed to heat or light.
These minerals absorb radiation emitted from natural environmental sources at a quantifiable rate. The technique is broadly split into two categories: Thermoluminescence methods, where the stored energy is released at high temperatures, are widely applied to fired ceramics and other heat-treated material, such as burnt flints and iron-smelting furnaces. Moreover, poor preservation conditions for organic materials in humid tropical regions make luminescence a valuable alternative to radiocarbon dating, even in comparatively recent contexts. Various laboratory procedures, such as repeated heating steps in conjunction with exposure to variable light intensities, can be employed to facilitate the release of stored energy.
The environmental dose rate D R comprises all sources of radiation, including alpha, beta, and gamma radiation and cosmic rays, which have affected the sample since the last emptying of the electron traps. This radiation largely derives from radioisotopes of uranium, thorium, and potassium in soils and underlying geology, and it can be very challenging to accurately estimate the various radiation sources at a sufficiently resolved spatial scale. These sources are measured using different techniques, for example, laboratory measurement of a second sediment sample, or field measurements using portable radiation detectors or buried dosimeters, which are placed in the original sample location.
Various radiation sources affect parts of a mineral grain differently, and environmental fluctuations may have altered the dose rate through time e. Changes to the water content of the sediments through time also need to be estimated, and all of these calculations require meaningful error estimates that can be propagated through to the uncertainty in the luminescence age. A working rule is that the uncertainty provided by OSL techniques is usually about 10 percent of the age and is often higher for TL measurements , but uncertainties in dose rate can mean that greater uncertainties are more realistic.
Quartz grains, the most commonly targeted mineral for OSL dating, are susceptible to low saturation thresholds and low sensitivity. This means that they are typically useful as a dating tool back to only about ka. Methods have been developed to correct for this problem only back to c. Recent development of infrared radiofluorescence IR-RF techniques applied to potassium-rich feldspar minerals is particularly promising for contexts older than c. Early OSL techniques measured the bulk luminescence signal across multiple subsamples, or aliquots, altogether comprising many thousands of grains, and combined these measurements to derive an equivalent dose.
However, this approach assumes that each grain is equally saturated upon last exposure. If the sample is mixed, and grains were not equally saturated upon last exposure, then the OSL age will be an amalgamation of ages over which the grains were last exposed to light. A major breakthrough was the development of the single aliquot regenerative dose SAR protocol for quartz and feldspar, which incorporates sensitivity tests that can correct for these effects. This procedure allows for the dating of sediments with more complex formation histories that may include grains from different sources.
While several hundred grains are typically measured, only a relatively small number are particularly suitable for OSL dating, because of the unique properties of each grain, and a large proportion of grains will be excluded from analysis. Statistical models are sometimes deployed to distinguish and combine individual single-grain measurements, but correct implementation of such approaches requires some understanding of the site formation and depositional processes. Radiocarbon, U-series, and luminescence are the most widely used dating methods in African archaeology, but there are a number of analytical dating methods that are less often used.
This may change in the future as techniques develop and their practicality and accuracy are improved. In some cases e. Materials made of clay such as pottery, kiln structures, and burnt hearths often acquire a magnetization at the time they are fired.
Vincent J. Hare and Emma Loftus
This magnetization is usually carried by magnetic minerals within the fired clay matrix, such as magnetite or hematite. When these minerals are heated to above their respective Curie points around — o C , and allowed to cool back to ambient temperatures, they acquire a remanent magnetization that is proportional to both the direction and strength of the local magnetic field at the time of firing.
Changes in direction and intensity of the geomagnetic field on timescales from tens to thousands of years are called secular variation. The directional signature of the geomagnetic field at a particular point is measured as declination and inclination. Declination is the angle between geographical north and the geomagnetic field. Inclination , or dip, is the angle made with the horizontal by geomagnetic field lines. To perform archaeomagnetic dating one first needs to have a good understanding of past secular changes in the geomagnetic field.
A regional secular variation curve is required to date archaeological objects from a particular area, because these changes will be different in each region. To build up the required temporal resolution for a regional secular variation curve, it is first necessary to perform archaeomagnetic analysis on many features, independently dated by 14 C or other stratigraphic constraints.
Only once the secular variation curve is constructed can the archaeologist use the declination and inclination to date a burnt feature of unknown age.
Everything Worth Knowing About Scientific Dating Methods | blucocoon.com
Another requirement for this dating method is that the burnt feature is preserved in situ without significant post-depositional disturbance that can change the direction of the preserved magnetic signal, leading to incorrect ages. Archaeomagnetic dating is a highly interdisciplinary field, with archaeologists and geophysicists teaming up to ensure that each burnt feature is properly preserved, sampled, and measured. To reconstruct ancient magnetic directions preserved by burnt materials, both extensive field measurements and laboratory experiments are required.
In the field, the samples are orientated in situ, and a record of this orientation is preserved on each fragment. This orientation allows the geophysicist to properly prepare several small 1—4 cm 3 orientated samples for subsequent measurement in a laboratory. Furthermore, to improve the accuracy of the technique it is common to collect several large fragments, from which multiple subsamples are measured.
The results are then averaged into single values for declination and inclination, using Fisher statistics. In the laboratory, directions are measured by progressive heating until the Curie point. The remanent magnetization of these materials is relatively weak, and the directions are sometimes difficult to measure. It is therefore common to use sensitive cryogenic magnetometers, located in special magnetically shielded rooms. To ensure that the magnetic remanence is properly preserved, tests including low-field susceptibility versus temperature and magnetic hysteresis experiments will often be performed to identify the mineral carrier.
Despite the large, costly, and time-consuming requirements, archaeomagnetic dating can yield very precise chronologies e. Such curves have been refined in Europe for several decades, Western Europe in particular. The curve is constructed from archaeomagnetic analysis of ten independently dated Iron Age burnt structures Hare et al. Direction of green arrows shows the evolution of the field.
Ninety-five percent confidence ellipses for each site direction. However, archaeomagnetism in Africa has recently been rapidly expanding see figure 3. Archaeomagnetic dating should now in principle be possible for the southern African Iron Age from c. The rehydroxylation RHX dating method is based on the phenomenon of mass gain and moisture expansion in fired clay ceramics. When fired to moderate temperatures — o C , common clay minerals, such as those found in pottery, begin to lose structural hydroxyl OH groups.
Upon removal from the kiln, the ceramic begins to chemically recombine with atmospheric moisture, and structural hydroxyl groups are once again chemically bonded to vacant sites in the ceramic matrix. This slow rehydroxylation reaction continues over the lifetime of the ceramic, causing the material to expand and gain mass. The older the ceramic, the more hydroxyl groups are chemically bonded to sites within the clay mineral matrix. Additionally, there is uncertainty over the underlying mechanisms of rehydroxylation, and how this reaction varies with underlying clay mineralogy.
These relationships are not yet sufficiently well understood to allow robust dating of several different ceramic types. Efforts are currently under way to address these issues, and this dating method may ultimately move away from gravimetric protocols toward IR spectroscopy or similar techniques.
Because of these difficulties, the rehydroxylation dating technique is currently regarded as under development. Tephrochronology is a method based on using geochemically well-characterized deposits of volcanic ash, correlated with an eruption of known age, to form a powerful chronological framework that links different sites and different regions.
It is therefore not so much an analytical technique based on a rate of decay, or a change in a quantity through time, but rather a technique based on correlation. Ideally, the volcanic ash is released in a single eruption, and transported over an area of thousands of square kilometers. Most tephras have a unique geochemical fingerprint that is associated with a particular eruption. Rather, it is an age-equivalent technique whereby the presence of this particular tephra horizon in other peat sequences, lakes, aeolian sediments, and archaeological sites allows the sequence to be dated indirectly.
And given that the eruption and deposition of the tephra is fairly rapid, within the space of a year or so, tephrochronology is often highly precise. An advantage of the dating technique is that a tephra layer is often relatively easily identified in sediments and other excavation sequences, but its presence depends on the proximity to the volcanic source. In recent years, much research has focused on cryptotephra, which is very fine volcanic glass, invisible to the naked eye: Potential problems with the technique include miscorrelation of one tephra horizon with another geochemically similar eruption, and alteration of tephra chemistry over time.
Tephrochronology is an important dating tool in east African hominin sites, throughout both the Pliocene and the Quaternary, due to its association with the East African Rift System. The site has been excavated, revealing two distinct phases of occupation, the first consistent with habitation of the site before the construction of the stone walling, with pottery stylistically consistent with a pre-Khami group. Vogel obtained a suite of eighteen 14 C dates, which distinguish three chronological phases. The third date, Pta, is on charcoal from a midden that accumulated after the construction of the main walls, and is likely to postdate the construction and habitation of the main enclosure.
Note the reason for the lack in precision in this date is the plateau in the radiocarbon curve between c. Note the large uncertainty of years in the calibrated date Pta, caused by the plateau in the calibration curve c. Predicted secular variation of the geomagnetic field at Mapungubwe Most geomagnetic field models predict large variations in declination and inclination in southern Africa. Large variations are supported by direct measurements Hare et al. Illustrations by the authors. The conclusion drawn from this suite of dates is that the site had been occupied in the 14th century ce , and by the early 15th century the Khami-style settlement had been constructed.
By ce the site was likely abandoned, but this may have happened before. The uncertainty in the radiocarbon date Pta, and others like it from the post-wall midden, shows that it is impossible to say whether Thulamela was abandoned slowly, over years, or whether it was a relatively sudden event.
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Elsewhere, radiocarbon dates from settlements in Zimbabwe show that Khami-style settlements persisted until the end of the 17th century. The period between and ce is an important time in the Late Iron Age of southern Africa, with debate among archaeologists about the collapse or transition of Late Iron Age societies, and therefore the issue of this plateau in the radiocarbon curve limits the ability of archaeologists to answer important questions about this time period.
However, one technique that might be useful in this period is archaeomagnetic dating. In figure 4B we show the predicted secular variation close to the site of Thulamela according to one geomagnetic field model. This model shows strong changes in southern Africa during this particular time period, declination varying by about 5 o , and inclination varying by about 10 o. Direct measurements of other Iron Age sites suggest even higher rates of directional change. These changes could be utilized to provide archaeomagnetic dates for Late Iron Age sites that contain burnt features such as hearths and earthen floors.
The rapidity of these changes suggests that archaeomagnetic dating could provide age uncertainties comparable with, or better than, radiocarbon. Increasingly, different dating techniques are sometimes combined with one another, as a powerful means to corroborate chronology, or to tease apart different aspects of site formation history. The sensible use of Bayesian modeling software designed for chronological analysis can help to compare and combine dates in a rigorous way. The Haua Fteah is a large cave situated in northeast Libya with a long sequence of human occupation, spanning tens of thousands of years.
Given this long sequence, the site is central in discussions of North African prehistory from the original dispersal into the region through to debates about the movement of domesticated plants and animals throughout the Holocene and historical periods. The site was originally excavated and dated in the s and s; recent excavations have focused on improving the chronology with a combination of methods, including radiocarbon dating of multiple materials, single-grain OSL dating of sediments, tephrochronology, and electron spin resonance dating of tooth enamel a trapped charge technique with similar principles to luminescence dating.
However, particular care should be taken to ensure that different dating measurements are combined on a correct, common age scale. For example, radiocarbon dates are given in radiocarbon years before bp , while OSL ages are given in years before measurement. The example of Haua Fteah shows that care should be taken when combining dating methods, if the goal is a highly resolved chronology.
Bayesian methods are particularly powerful in such situations, where the archaeologist wishes to analyze groups of dates from a stratigraphic sequence. The mathematics of Bayesian statistics allows for the complicated probability distribution functions that describe the likely age range of a calibrated radiocarbon date to be handled with relative ease. In particular, these methods enable probabilistic ranges to be ascribed to chronological breaks or boundaries in a stratigraphic sequence.
Other advantages are that these methods provide relatively easy quantitative identification of outliers, and different dates can be synchronized on a common age scale. None of these useful analyses would be straightforward with more conventional statistical methods. These models evaluated the correspondence between dates derived from different methods, within alternative sedimentological and cultural stratigraphic frameworks.
The statistical models also formally identified inconsistent dates, thereby highlighting possible stratigraphic ambiguities or inaccurate dates, and discounted such dates in the overall model. The updated chronology can be meaningfully contrasted with local and global climate records to unpick the technological consequences of environmental shifts, including the appearance of microlithic Oranian technologies at the peak of the Last Glacial Maximum, at c.
The new chronology also emphasizes the early development of Upper Palaeolithic Dabban technologies c. Despite some of the limitations of radiocarbon, such as the issue of calibration plateaus, it remains the most useful and accurate dating technique in African contexts. Unfortunately, there has in the past been a shortage of analytical expertise on radiocarbon dating in Africa, largely due to the expense of establishing and operating a radiocarbon laboratory.
However, the only facility capable of making radiocarbon AMS measurements on the continent currently is in South Africa, and it is establishing the laboratory procedures to produce precise radiocarbon measurements. This represents a very promising development for the continent. OSL ages require considerably more instrumental time, are typically several times the cost of a radiocarbon date, and often require the direct involvement of a luminescence dating specialist in project design, sampling and data interpretation.
Consequently, OSL ages are not as common in the archaeological literature as radiocarbon dates, despite their wider age range, and greater applicability to geological and sedimentological materials. However, the development of single-grain methods has seen rapid increases in the application of luminescence dating to archaeological sites, and indeed, many methodological advances have been developed to address archaeological problems. Applications of OSL dating in Africa have frequently demonstrated the great antiquity of archaeological contexts, far older than alternative dating methods had established.
In the Nubian Desert, a novel application to sediment that had blown over and partially covered rock engravings verified that the engravings dated to the terminal Pleistocene, earlier than had been previously accepted. Older evidence for early art, including incised shell beads, ostrich eggshell fragments and ochres, and pigment processing tools, comes from rock shelters and caves in southern Africa and north Africa, extending beyond c.
U-series dating has been extensively applied to African archaeological contexts and the hominin record, with particular success at cave sites, and in combination with palaeomagnetic constraints e.
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It is important to consider that scientific dating methods are an active area of fundamental research. The refinement of techniques is currently enabling new approaches to archaeology and the study of the African past.
Scientific Dating Methods in African Archaeology
Several new developments are particularly exciting. Single-compound radiocarbon dating is being developed to target individual molecules, such as hydroxyproline, a component of the protein collagen found within bone, and new techniques are also allowing the non-destructive extraction of organics for radiocarbon analysis e. On older timescales, archaeologists and palaeoanthropologists are increasingly combining a variety of complementary techniques, including palaeomagnetism and tephrochronology, to place constraints on the complex histories of hominid-bearing localities.
This practice should be welcomed, along with collaboration between archaeologists, geologists, and dating specialists. There is a pressing need for technical skills and facilities on the continent, a challenge that Africa is well-positioned to meet. Another frontier is the development of archaeomagnetic dating in Africa, which has the potential to resolve longstanding and complex questions around the end of the Late Iron Age in southern Africa. A direct dating method applicable to rock art remains the ultimate technical challenge because radiometric techniques are often out of the question but one that probably awaits future generations.
Discovery of Radiocarbon Dating.
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Oxford Radiocarbon Accelerator Unit. Science-Based Dating in Archaeology. Geeraerts, and Simo Spassov. Archaeomagnetism and Archaeomagnetic Dating. New Methods, Models and Implications. Tauxe, Lisa, Subir K. Butler, and Rob van der Voo. Left Coast Press, Many dates for African archaeological sites can be found in the regional date lists, which were semi-regularly compiled and published by various authors in the Journal of African History from the early s to the s.