Tori M Saneda. 21st Century Anthropology: A Reference Handbook. Editor: H James Birx. Volume 1. Thousand Oaks, CA: Sage Reference, 2010.
Situating things in time and space is critical for archaeology and paleoanthropology. Knowing when something happened helps us to understand how humans and cultures evolved. From John Lightfoot and Bishop James Ussher, who calculated the age of the earth using genealogies in the Bible, to Willard Libby, who developed radiocarbon dating and beyond, researchers have been working to establish a chronology of the past.
Absolute, or chronometric, dating techniques provide us with measurable dates—day, year, millennia, for example. Relative techniques provide a basic order to material recovered from a site; they give us an idea of its age—how old something is in comparison to something else. This chapter will cover a few of the techniques from each category. It is not meant to be a comprehensive overview of all dating techniques nor provide an in-depth discussion of the techniques. It is meant to be an introduction to some of the dating techniques available to archaeologists and paleoanthropologists.
Relative Dating Techniques
Relative dating techniques provide the researcher with an order of occurrence but not an absolute date—it is an age in relation to something else. Even though it does not provide a calendar date, it does not mean that the techniques are not important or useful. For sites where it is impossible to recover the appropriate material for absolute dating techniques, relative dating is critical. It can also be used to make connections between sites and artifacts across time and space, as well as to examine site formation processes. Stratigraphy and the index fossil concept are the cornerstones of relative dating, providing a foundation for subsequently developed relative methods.
In geology, stratigraphy deals with the classification and mapping of observable units that form the earth’s crust, using rock description, classification, and interpretation. Archaeologists use stratigraphy to establish relationships in time between artifacts and features. As such, stratigraphy is the cornerstone of archaeology.
Stratigraphy, or stratigraphic dating, is based on the assumption of the law of superposition. First noted by Nicolas Steno (1638-1686), the law of superposition states that geologic strata are progressively older the deeper one goes. Steno observed a layer of shell beneath ancient Rome and posited that it must be older then the ancient city since it was beneath the city. He reasoned that particles in a fluid would be laid down in distinct horizontal layers (strata), an effect he called the principle of original horizontality. For example, let us say that a layer of sediment, or strata, heavy in clay content is laid down (Strata 1). Perhaps through a flood, a layer of organically rich sediment with pebbles is laid down on top of it (Strata 2), and then perhaps another layer of sediment (Strata 3) is laid down, and so on. In this example, Strata 1 is the oldest, Strata 2 is younger than Strata 1 but older then Strata 3, and so on. This may seem common sense to us today; however, it was a new idea in the 1600s.
The principles of stratigraphy established by Steno provided the first relative dating technique. Using the earth’s own strata, it became possible to place things in the order they were deposited. Early attempts at establishing strati-graphic chronologies centered on stratigraphic observation—the noting of artifacts in relation to strata. John Frere (1740-1807) used the method to order stone tools found with extinct animal fossils, and Christian Thomsen (1788-1865) developed the three age system for Stone Age Europe. Boucher de Perthe (1788-1868) was the first archaeologist to employ stratigraphic observation in conjunction with archaeological excavations, ensuring stratigraphy’s vital role in archaeology.
Stratigraphic observation and stratigraphic excavation differ in that stratigraphic observation is simply noting the occurrence of artifacts within geologic strata. Stratigraphic excavation requires the archaeologist to excavate a site using stratigraphic levels. This can be done in two ways: (1) arbitrary levels and (2) natural levels. An archaeologist may excavate a site in arbitrary levels: 10-cm- or 20-cm-deep levels, for example. Once sterile soil, or a level without artifacts, is reached, archaeologists note the various strati-graphic levels on the wall(s) of the excavated unit, generally in the form of a profile. A profile is created by drawing the observed strata using a line level and tape measure. Excavation using natural levels can be accomplished in a couple of ways. One way is to dig until you hit a change in the soil type. The other way is to divide the excavation unit into smaller segments—in half or quarters, for example. One segment is excavated using arbitrary levels. The next segment is excavated using natural strata based on the profile exposed when digging by arbitrary levels.
Archaeologists generally use basic stratigraphic methods: the observation and recording of the various strata, particularly noting soil composition and color as well as artifact content. Recently, some archaeologists have employed specialized stratigraphic measures developed by geoscientists in order to create deeper analyses: lithostratigraphy, which examines the composition of strata; biostratigraphy, which examines the fossils contained within strata; and chronostratigraphy, which looks at the age of rocks. Geoarchaeologists suggest a new unit of study: ethnostratigraphy, which is similar to the biostratigraphic unit but is focused on cultural artifacts instead of fossils. Ethnostratigraphy is not widely used because there is no standard of classification as there is with fossils.
Index Fossil Concept/Time Markers
William Smith (1769-1839), a British geologist, observed that fossils found in various strata indicate that life forms changed over time. He demonstrated that by using these differing life forms, it was possible to associate strata from different places with one another. If the same life forms were found in the strata at two different places, then the assumption is that the strata were laid down at the same time. Smith’s ideas are now known as the index fossil concept.
This concept was successfully adapted to archaeology by substituting artifacts for fossils. These artifacts, known as time markers, represent particular time periods. Oscar Montelius (1843-1921), Flinders Petrie (1853-1942), and Nels Nelson (1975-1964) used the principles established by Smith to date a variety of sites in Europe, Greece, and the North American Southwest. Montelius, a Swedish archaeologist, used the method to establish time markers for Neolithic, Bronze, and Iron Age Europe. Petrie used time markers from Egypt on Heinrich Schliemann’s Mycenaean sites in Greece to establish dates. Nelson applied the methodology to pottery found at the San Cristobal site in New Mexico, establishing a master sequence that was used later by Alfred Kidder (1876-1960) in his development of a culture history for the North American Southwest. Nelson’s work is especially important, as he carefully excavated a portion of the site, combining stratigraphic excavation with the time marker concept to create a relative order of occurrence for the pottery. This enabled archaeologists to track culture change over time.
The index fossil concept has become important for paleoanthropology, specifically for sites where no material exists for radiometric dating. In southern Africa and Chad, fossils are compared to fossils from sites in eastern Africa where radiometric dates were acquired.
The index fossil concept is the basis for seriation. Popular in the mid-20th century, today seriation is used when chronometric techniques are not applicable. It is like the index fossil concept in that it uses time markers to establish chronological sequences; however, it differs in that it not only traces stylistic change over time but also examines the frequency of occurrence of the artifacts in question. It can be used on any type of artifact, but it is most commonly used on pottery and ceramics, as those artifacts are ubiquitous in the archaeological record.
Seriation is based on the assumption that cultural styles change over time. A master sequence of the frequency of use can be established to correlate sites with one another. This allows archaeologists to compare occupation sequences among sites in a region and track the popularity of styles across time. The frequency of occurrence, or popularity curve, is charted, creating a graphic representation that resembles the plan of a battleship. These curves are then compared to establish relationships between sites.
The methodology became progressively more quantitative in nature over time, where statistical measures were used to track changing artifact attributes (characteristics), helping to refine seriation sequences. Seriation becomes a particularly robust technique when correlated with chronometric dates. In the North American Southwest, ceramic sequences have been correlated with tree-ring dating, which validates the seriation sequences established.
Amino Acid Racemization
Amino acid racemization was first observed in 1884; however, it was not until the 1950s that the process was recognized as a potential dating technique for fossils. In the 1970s, the technique was applied to archaeological artifacts, such as bone, mollusk shell, teeth, and avian eggs. At first glance, it would appear that amino acid racemization should be a chronometric technique, but for reasons outlined below, it is included with relative dating techniques.
Proteins are formed from amino acids that occur in one of two molecular patterns: 1-enantiomer (1-types) and D-enantiomer (D-types). In living organisms, the 1-type pattern is the most common, almost to the exclusion of the D-type pattern. This means that the organism is optically active (during the interaction of the amino acid with polarized light, the organism will rotate in the direction in which the light is vibrating as it passes through them) for the 1-type molecules. Having only one type is thermodynamically unstable, so over time the amino acids will change until they are optically inactive, which means that the ratios of 1-types and D-types reach an equilibrium, a process known asracemization. The rate at which the amino acids become optically inactive can be used to calculate an age for the artifact up to several million years. However, calculating an age is dependent on the rate of racemization, which in turn is affected by a number of factors, the most prevalent being the environment. If the temperature history of a site is unknown, then ages cannot be calculated. If items are from the same site, then one can assume that they share a temperature history and can be relatively dated with one another using the degree of racemization. Since it is difficult to determine the temperature history of an item, the relative technique is usually employed.
Fluorine is a reactive chemical found in several elements, most importantly in ground water for archaeological and paleoanthropological purposes. As ground water leaches into materials, especially siliceous rock, bone, and teeth, fluorine ions replace calcium-based minerals. Fluorine ions are extremely reactive with other materials. This means that once it replaces minerals in an object, it is fixed in the material and begins to accumulate over time. The rate of fluorine accumulation can be measured and compared to objects from similar environments to construct a relative chronology.
Fluorine dating is dependent on the amount of fluorine in ground water supplies; therefore, the artifacts being dated must be from similar environments, if not the same, to establish an accurate chronology. The density of the material can also affect fluorine dating. Dense material absorbs fluorine ions more slowly than porous or spongy material. This can be corrected for by comparing the fluorine content to the phosphate content of the object.
Uranium (U) and nitrogen (N) testing can be done in conjunction with fluorine (F) dating to further refine the chronology. This technique, F-U-N dating, was used successfully to identify the Piltdown skull as a hoax. In 1911, fragments of a hominid skull and other vertebrate bones were found in England. Subsequent excavations recovered pieces of a human skull that had both modern and apelike characteristics. Fluorine analysis was applied to the skull bone, as well as to some of the vertebrate bones found associated with the human bone. These tests indicated that the cranial bones were much older than the jaw bones.
Rocks such as flint, basalt, andesite, and other finegrained rocks that are exposed to moisture will develop a weathering rind, or patina. Over time, the rind pushes itself into the unweathered rock. Where the weathering processes are known, the rind may be used to calculate a sequence for a site, from as few as a couple of years up to around 500,000 years. For this method to work accurately, only rocks that have been exposed to the same environmental conditions should be compared. Since this can be difficult to determine, sequences should be verified using another dating technique if possible. In some cases, the patination can be used to calculate an absolute date—for example, obsidian hydration dating (see below for discussion).
Rock varnish is formed when clay elements bond with manganese and/or iron oxides on rock surfaces. The layers of varnish provide environmental information and can be used to estimate when the rock surface was first exposed to weathering. Many different patination techniques have been developed, including trace element trends, metal scavenging, and orange bottom varnish growth; however, they have not been used with much success. The cation-ratio (CR) dating method, on the other hand, has yielded some relative chronologies for rock—stone tools from sites in semiarid and arid conditions, for example.
Cations are positive ions that are impacted by various leaching processes. Cation-carrying clay materials are blown onto the rock surfaces where they chemically react with minerals of varying mobility to form a patina or varnish. It is known that cations of potassium + calcium/ titanium decrease with age; hence, a relative chronology can be established by comparing the ratio of mobile cations to immobile cations. This method cannot provide a manufacturing date for a stone tool; it only tells us about how long ago the rock varnish began to accumulate. CR chronologies need to be calibrated by using absolute dating methods, such as potassium-argon dating or radiocarbon dating. Once a chronology is established for a site, it can be used as a master sequence for relative dating purposes.
Pollen analysis, or palynology, is another relative dating technique, as well as a method of environmental reconstruction. Developed in 1916, pollens found in lake sediments and ice cores can be used for cross-site comparisons to establish a chronology. When comparing two sites, if layers in the pollen diagram are similar, then we can infer that the sites were occupied at the same time. Concomitantly, we can reconstruct what types of plants were available in the local environment as well as develop a picture of the climate. For example, if there is a plethora of tree pollen, then the climate was warm. Pollen analysis is in effect a form of biostratigraphy.
Varves are annual series of sediments deposited in still bodies of water—a glacial lake, for example. Varves form seasonally. In winter, when glacial meltwater is reduced or stopped altogether, clay material slowly settles to the bottom of the lake, and during spring and summer, silt and sand are deposited, creating a series of dark (clay) and light (silt and sand) layers. This layering effect is sometimes referred to as laminated sediment. A chronology can be established by counting the varves and then correlating the thickness of varves across sites. Long chronologies can be established by following the tract of retreating glacial ice. Varves closest to the glacier are younger than those farther away.
Over the past century, scientists have found that one of the main problems with varve analysis is that it is possible that years can go missing due to natural processes. Therefore, it is necessary to recognize that the dates calculated using this methodology are minimum ages and whenever possible should be correlated with absolute techniques, such as dendrochronology. In addition to chronology building, varve analysis is useful for environmental reconstruction.
Since the 1940s, a plethora of chronometric, or absolute, dating techniques have been developed; these techniques are increasingly more accurate and help us to refine the chronological sequence of the past. In this section, we will look at several different types of chronometric techniques: natural rhythmic and chemical change, radiometric, and trapped charge dating.
Natural Rhythmic and Chemical Change Techniques
As the name implies, natural rhythmic and chemical change techniques measure either an inherent chemical change in an object or natural cycle. It neither relies on radioactive decay, as do several other prominent chronometric techniques, nor does it rely on energy emission. In this section, we will look at astronomical dating, dendrochronology, obsidian hydration, and archaeomagnetic dating techniques.
Archaeomagnetism, a subfield of paleomagnetism, is both a chronometric dating technique and a relative dating technique. It is predicated on the fact that the earth’s magnetic field changes periodically. Archaeomagnetism, which can date sites up to 100,000 years, can be used on baked clay, geological sediments, and igneous rocks. Appropriate samples are the key to getting a good archaeo-magnetic date. In situ features must be used—kilns and hearths, for example—and the orientation retained. This is accomplished by establishing the orientation with a compass or transit, and then encasing the sample in plaster or fastening it to a plastic disk. Once the qualities of the natural remanent (permanent) magnetization are known, the direction of the ancient magnetic field can be determined.
About every 250,000 years, the polarity of the magnetic field flips or reverses. This reversal lasts approximately 10,000 years. In between the times of reversals, magnetic fields change approximately 1° every couple of decades at any given point on earth, with a maximum variation of roughly 20° (as it moves around the geographical north pole). This is called secular variation. This information coupled with the knowledge that some minerals (e.g., ferromagnetic minerals) have a remanent magnetization allows archaeologists and geologists to date sites.
Archaeological data suggest that many ancient societies tracked the movement of the stars—for example, the Maya—and aligned structures with significant events, such as solstices and equinoxes at Stonehenge. Sometimes these data can be used to date buildings. Ancient Egyptian texts, for example, mention astronomical events, enabling Egyptologists to correlate historical events with calculable astronomical events. Based on information in Middle Kingdom and New Kingdom texts, archaeologists have been able to determine that the Egyptian calendar was based on 3 seasons, each consisting of 4, 31-day months, with 5 days left over. These months were anchored to astronomical events, such as when Sirius becomes visible above the eastern horizon, an event known as a heliacal rising, which happens to coincide roughly with the annual flooding of the Nile in July. This event heralded the Egyptian New Year. Using information from Roman histories, we know that the Egyptian New Year and the heliacal rising of Sirius occurred in CE 139. Because we know the length of the period between heliacal risings and how it relates to the Egyptian calendar, it is possible to correlate a date mentioned in an ancient text with a date that is understandable to the modern archaeologist.
But what if no such texts exist, as is the case for the Old Kingdom? A relatively new method of dating, precession dating, has been used to date Old Kingdom features, such as the pyramids of Giza. To understand what precession is, think of a spinning top—If it is spinning fast enough, then it will not fall over and the tip stays in one spot. As it begins to slow down, the tip no longer stays in one spot; it begins to loop outward in a somewhat horizontal circle; that motion, the path of the tip, is called the precession.
How does this work for archaeology? Earth’s precession can be traced over time. We know that currently the earth’s axis is centered on a celestial pole around the star Polaris. We also know that the celestial pole itself moves around a pole centered in the Draco constellation and approximately how long it takes for this move to occur. Since these are known quantities, it is possible to calculate where the celestial pole was relative to stars at any given time in the past. Using this information, the changes in the alignment of the pyramids of Giza have been correlated with the earth’s precession.
The pyramids at Giza as well as Snofru’s pyramids at Meidum and Dashur are all aligned with the cardinal directions, each with an error of approximately < 1 degree off the previously built pyramid. For example, the Bent pyramid at Dashur, which was built after the pyramid at Meidum, has an alignment that is < 1 degree off of the Meidum pyramid, and so on. Since the Turin papyrus informs us as to how long various kings ruled, it is possible to estimate the duration of the construction of each pyramid, giving us a relative chronology for the pyramids. However, when this information is combined with information on the earth’s precession, it is possible to assign calendar years to the construction of each pyramid.
While more testing is needed to see if other Egyptian pyramids conform to the pattern established by the aforementioned pyramids, the methodology does offer the prospect of dating archaeological deposits for which ancient texts do not provide the necessary information.
The science of tree-ring dating, or dendrochronology, was developed by Andrew Ellicott Douglass in the early 20th century. Douglass discovered that the ring-width patterns of different ponderosa pines were identical. To build a chronology for the North American Southwest, stretching from prehistory to modern times, he compared remains of ponderosa pines across sites based on the assumption that tree rings of the same width were formed in the same year. Counting the number of tree rings provided the number of years that had passed since a tree was cut down. Today, dendrochronological sequences exist for other regions, including the southeastern and temperate United States, Western Europe, the Mediterranean, Australia, New Zealand, parts of Asia, southern Africa, Tasmania, and southern South America.
Dendrochronologists use cross-dating and chronology building. Cross-dating is the process of matching the ring pattern variability between samples. The process of building the dated ring sequences from the samples is called chronology building. Dendrochronology works by using a sample from a living tree and then working backward, overlapping samples until reaching the desired sample, often a piece of dead wood. Then, it is a matter of counting rings to determine the number of years that have passed.
Dendrochronologists use width and density of the rings as well as fire rings and frost rings to cross-date the samples. Dendrochronology has many applications and is useful in multiple disciplines, including oceanography, art history, and botany, to name a few. It has become a staple dating technique in archaeology; however, it is applicable only in areas where appropriate trees are available. Archaeologists also use tree rings to examine human behavior—tree use and environmental reconstruction, for example.
Obsidian Hydration Dating
From the moment it is formed, obsidian, a volcanic glass, will begin to absorb water from the atmosphere. A rind, or adherent hydrated layer, is formed and thickens over time. The density and refractive index of the rind is higher than the original glass, making it easily discernable from the original. The amount of time needed to create the rind’s thickness is calculated using information on the chemical composition of the glass, as well as the relative humidity and temperature of the environment. Because a rind can form in only a few hundred years, obsidian hydration can be used on relatively young samples, as well as samples around one million years old. Thus, we can use the method to determine when an artifact was manufactured.
Errors in the measurement of the rind and determining the rate of hydration can cause errors in age determination. If the relative humidity of the environment of the site where the artifact was found is significantly less than 100%, then an additional correction must be made to calculate a date. This can be done in a couple of ways: (1) Use the rind thickness in conjunction with an artifact dated by using another technique such as radiocarbon dating, and (2) induce hydration experimentally. Most researchers use obsidian hydration and a second or third or more dating method to determine the accuracy of the obsidian hydration dates.
Obsidian hydration can also be used as a relative dating technique if only the chemical composition of the sample is known. The chemical composition can be compared to samples found on other sites in order to determine occupation contemporaneity.
Radiometric techniques rely on the fact that unstable, radioactive isotopes decay over time into a different isotope. Through experimentation, the half-life, or the amount of time it takes for half of the original radioactive isotope to turn into a different isotope, of these unstable isotopes is known and can be used to calculate the age of the material. There are a variety of radiometric techniques; however, we will focus on those commonly used in archaeology and paleoanthropology: radiocarbon dating, fission track dating, potassium-argon/argon-argon dating, and uranium-series dating.
Radiocarbon dating (14C) is the backbone of chronometric archaeological dating. Developed by Willard Libby and a team of scientists at the University of Chicago in the mid-1940s, 14C can be used to date organic material up to around 45,000 years. It provides a date for when something died and stopped taking in carbon.
Carbon-14 is created when cosmic radiation in the upper atmosphere produces a neutron that replaces a nitrogen-14 (14N) proton. From there, 14C is oxidized, which means it is attached to oxygen, to form carbon dioxide, which eventually makes its way into the earth’s oceans and plants. When herbivores or omnivores eat plants, they take in 14C. Carnivores and omnivores also take in 14C when they eat herbivores. Once something dies, it stops taking in carbon and 14C decays back into14N. Based on Libby and others’ work, we know that the half-life of 14C is 5,730 years.
The amount of 14C in a sample is estimated by the amounts of the stable carbon isotopes, carbon-12 (12C) and carbon-13 (13C). A carbon molecule is comprised of around 99%12C and 1% 13C. Only one in a million million atoms are 14C. Once this estimation is made, we can measure the amount of 14C in the sample and determine how much 14C has decayed and how long it took.
When a date comes back from the lab, it tells us how many radiocarbon years old the sample is in relation to 1950 (the year the method was invented). The date returned is associated with ± number, which is the standard deviation for the number of times the lab ran the tests. For example, if the lab returned a date of 4,110 ± 50 BP (before present or rather before 1950), then we would say the sample stopped taking in 14C between 4,160 and 4,060 radiocarbon years BP.
There are a few problems associated with 14C dating. Carbon from the surrounding soil matrix can leach into organic material such as bone, obfuscating the correct date. A second problem involves the reservoir effect. The reservoir effect refers to the problem of samples from aquatic sources, for example, mollusk shells. Radiocarbon dating was developed using atmospheric carbon; in water, carbon disseminates much more slowly, so it builds up in the material (a reservoir of carbon). For marine samples, that means the 14C date could be hundreds of years too young. What the reservoir effect is for riverine samples is as yet unknown. A side effect of the reservoir effect is that dates for the bones of peoples who relied on marine and riverine resources for their primary subsistence could be off. Context of the sample then becomes important. If the lab is acquainted with the context of the sample and any other pertinent background information, then the reservoir effect can be corrected.
Another problem is that organic material takes in carbon in different manners. Not all plants take in carbon in the same way. Plants that live in arid and semiarid regions, with the exception of succulents, convert carbon dioxide into a 4-carbon compound. This means that these C4 plants take in more oxygen then all other types of plants. If plant remains are not identified, then radiocarbon labs cannot make the appropriate corrections, and the plants will appear younger than they really are.
Radiocarbon dating was made more accurate through tree-ring studies where it was noted that 14C dates and dendrochronological dates did not match. It was discovered that one of Libby’s assumptions was wrong, 14C production was not constant. Studies of bristlecone pine indicated that there were over a dozen changes in 14C production over the past 10,000 years, most likely due to sunspot flareups. Corrections can now be made for this phenomenon known as the de Vries effect.
Accelerator mass spectrometry (AMS) links two technologies, particle acceleration and mass spectrometry, to push the date range of 14C back to 55,000 years. This method directly counts carbon ions. One of the primary advantages of AMS over standard14C-dating techniques is that a much smaller sample can be used, thereby destroying a lesser amount of the artifact. Additionally, testing does not take as long as standard 14C tests. While it was hoped that AMS would extend radiocarbon dating to around 100,000 years, the ability to prepare the samples without contamination from modern carbon still prohibits calculation of such a date.
Fission Track Dating
Fission track dating was developed in the 1950s and is based on the fact that as uranium 238 decays, it fissions and leaves tracks as the fragments move through parent material. The track density along with the amount of uranium present in the sample enables an age to be calculated. Thousands of tracks are counted; therefore, a large amount of 238U is needed. The best materials for fission track dating are igneous materials, such as natural glass—obsidian and zircon.
One problem associated with fission track dating is that if the material is heated to a sufficiently high temperature, then the fission tracks can fade, making the sample appear too young; however, separating the sample into aliquots (equal parts of the original sample), and inducing fission tracks in one of the samples, creates a comparative method that allows fading to be corrected for. The step-heating plateau method is another way to correct for fading. Experiments have demonstrated that the density of tracks in an area (areal track density) of a natural sample is proportional to the track density in its irradiated aliquot. When both a natural sample and its irradiated aliquot are heated to a temperature where the areal track density of the samples reaches a plateau, the ratio value between the two samples increases until it reaches a plateau. The value of the ratio at the plateau can be used to correct for track fading. Similarly, the isothermal plateau technique, whereby a sample is heated at a constant temperature for a long period of time, allows for the correction of track fading.
Potassium-Argon Dating and Argon-Argon Dating
While first used to date hominin fossil deposits, potassium-argon (K-Ar) and argon 39-argon 40 (Ar-Ar) are now used in the natural sciences to date events back to the Precambrian era. K-Ar dating measures the buildup of decay of radioactive potassium 40 (40K) into argon 40 (40Ar) in volcanic and metamorphic rock. Potassium 40 is an unstable isotope. During the decaying process, one of its protons converts to a neutron. This process produces a 40Ar atom, the quantity of which can be measured and a date calculated for when the rock cooled. Radioactive 40K has a half-life of 1.28 billion years, making it more useful to paleoanthropology then radiocarbon dating with its limited dating range; however, K-Ar dating loses accuracy the younger the material. Radiocarbon dating can fill that gap.
Argon 40 is a noble gas, which means that no chemical bonds are formed when it comes in contact with other elements. Since noble gasses can escape from molten lava, it is assumed that when the rock cools, there is no 40Ar left. Any 40Ar present in the sample must then have formed after the rock cooled and became trapped. There are some conditions that can affect the accuracy of the date. Argon-40 could be trapped in the lava flow in unmelted rock. It is also possible that the rock was reheated, allowing 40Ar to escape. Both of these instances would cause the date to be inaccurate.
Argon 39-argon 40 (Ar-Ar) dating can be used to date the same types of material as K-Ar dating. In this method, the measurement of 39Ar is substituted for potassium. The primary difference is that the sample is irradiated at a nuclear reactor, which produces 39Ar, an isotope not produced in nature. The irradiation processes cause 40Ar and 39Ar to be released from the rock sample. The quantity is then measured using mass spectrometry. Ages are calculated by comparing the ratio of 40Ar/39Ar to a standard ratio of a known age that is irradiated at the same time as the sample.
The advantage of Ar-Ar is that it does not have to rely on a separate K measurement; it measures Ar in a single sample. Additionally, a smaller sample can be used. It is more precise because the release of Ar is controlled through the application of increasingly higher temperatures.
Uranium Series Dating
Using the known half-life of several uranium isotopes, the ages of archaeological sites can be determined using theuranium series (U-series) method. Since uranium occurs naturally in material such as marl, caliche, carbonates, speleothems (e.g., stalactites and stalagmites), travertine, mollusk shells, eggshells, bones, teeth, and other materials, the technique is useful for archaeologists, especially for sites older than the upper range of radio-carbon dating.
There are three series of decay associated with U-series dating, only two of which are appropriate for archaeological purposes: (1) uranium series, 238U with its decay products, and (2) actinium series,235U with its decay products. The third series, thorium, or 232Th, and its decay products, is useful for dating geological events. The basic principle of U-series dating is that there is a decay chain that eventually ends in a stable isotope, lead 206, which is associated with238U, and lead 207, which is associated with235U. While the length of time that it takes for uranium to decay into lead is longer than the time frame for human evolution, the length of time for some of the decay products does fall within that time frame, thereby making the dating method useful for anthropology.
U-series dates for archaeological sites are determined in a couple of ways. At some cave sites, for instance, the archaeological material itself is not datable using the U-series method; however, speleothems in the cave are datable. Archaeologists can create an age range for the cultural material by dating speleothems above and below the deposit. Another way is to date the artifact itself such as bone or teeth. One problem with the latter is that bones and teeth not only absorb uranium during their formation but also can absorb uranium from the deposition environment, skewing its age. Reconstructing the U-uptake history of the artifact can mitigate errors that might occur because of the deposition environment.
Trapped Charge Dating
Trapped charge dating involves methodologies that measure trapped charges in a mineral such as feldspar or quartz. Background radiation causes the electrons of some atoms to change in a manner that causes them to become trapped in the flaws of the crystalline structure of minerals. The amount of trapped electrons increases over time and can be used to estimate the radiation dose of the object, which can then be divided by the annual dose of radiation to determine the specimen’s age. In this section, we will examine the three types of trapped charge dating used in archaeology: thermoluminescence, optically stimulated dating, and electron spin resonance.
Luminescence dating techniques, thermoluminescence (TL dating) and optically stimulated dating (OSL dating), measure the changes to the structure of crystals contained in minerals caused by exposure to natural background radiation. Developed during the late 1960s and 1970s, luminescence dating provides a way for archaeologists to date sites that radiocarbon dating cannot, as it is accurate up to 100,000 years, and perhaps as much as 300,000 years.
TL dating can be used on material that contains minerals such as feldspar and quartz that emit a light when heated quickly to 500° C—pottery, bricks, burned stone, flint, and so on. As an artifact is “exposed” to natural background radiation, electrons are released from the crystalline structure and caught in flawed parts of the structure, sometimes referred to as trapping sites or lattice deficiencies. This accumulation of background radiation in natural material must be zeroed for it to be useful for archaeological dating. Zeroing occurs when the natural material is fired. This releases the electrons that had already accumulated in the materials used to construct the object, for instance, a clay pot, so that when the firing is completed light-emitting electrons have been released. Over time, these electrons will begin to accumulate again as the pot is exposed to natural background radiation. When heated in a laboratory, the released electrons emit light that can be measured to calculate when the material was last heated to 500° C. Because the method measures the last time an object was at 500° C, context becomes critical to differentiate between the time of manufacture and possible refiring events.
OSL dating differs from TL dating in that it is used on soil, not fired artifacts. The same types of materials that TL dating relies on—for example, quartz and feldspar—are also used in OSL dating. When these materials are exposed to sunlight, their luminescence clock is zeroed, which is referred to as bleaching. When buried, the material begins to accumulate trapped electrons, which, like TL dating, can be measured. Unlike TL dating, OSL dating uses light, not heating, to release electrons. To arrive at a date, a light of a specific wavelength is passed through a sample. The trapped electrons are released and emit light. To ensure the accuracy of dates, the standard for OSL dating is to date the soil grain-by-grain.
Electron Spin Resonance
Developed for the earth sciences in the mid-1970s, electron spin resonance (ESR) dating is used in archaeology for dating tooth enamel, shell, and burned stone tools, although it is usable on other materials such as speleothems and spring-deposited travertine. Accurate from 10,000 to 100,000 years, ESR dates the time when the artifact was buried. Like the other trapped-charge methods, context is critical.
ESR measures the quantity of trapped charges in an artifact. With this method, a sample is ground up and exposed to electromagnetic radiation. During the process, the sample is exposed to an external magnetic field. At a particular moment in the process, the trapped electron, or paramagnetic center, aligns itself parallel with the external magnetic field. When microwave radiation is introduced into the process, the paramagnetic center absorbs the radiation and flips its magnetic field to the opposite direction. The amount of radiation that is absorbed is proportional to the number of paramagnetic centers, which is proportional to the amount of radiation absorbed in the past. In the 1980s and 1990s, the ESR method was revamped to measure uranium (U) uptake.
The development of accurate dating methods is critical to understanding the evolution of humans, both biologically and culturally. Over the past century, newer and more accurate chronometric methods have been developed—radiocarbon dating, potassium-argon dating, obsidian hydration, and so on. Ideally, multiple dating techniques, including both relative and chronometric, would be employed for any one site, thereby cross-checking dates for accuracy. While many of the chronometric techniques are costly, continued improvements of the methods may help to make dating sites more cost-effective. New methods are also developed and employed in the effort to reconstruct our past.