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8.1: Introduction to Dating Methods

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    Relative Dating

    After excavating a site, one of the first questions to answer relates to time. Much of the meaning that can be inferred from a site comes from the context—when the site was used and when the various artifacts collected were made, used, and left behind. It is a straightforward question to ask, but one that has long been difficult to answer.

    Newer, more advanced dating techniques now allow archaeologists to establish when sites were occupied and artifacts were made. We can determine when items were discarded, plants were harvested, wood and other items were burned, and tools were made. How specific these dates can be depends on the technique used. Most provide dates as ranges of time, and the ranges are subject to a margin of error (e.g., 10,000–20,000 years ago +/– 2,000 years). Archaeologists combine multiple techniques to further narrow these time frames and increase their accuracy.

    Direct dating tests the archaeological evidence with techniques such as radiocarbon measurements while indirect dating estimates the age of archaeological evidence by dating something else, such the matrix in which the evidence was found. Dating techniques are also categorized by the kind of dates they provide. Relative dating estimates are based on associations and comparisons of the item with other things found at the site and describe an object as being older or younger than the comparison objects. Absolute dating determines an age range (and sometimes a margin of error) for the objects themselves.

    Another way archaeologists date objects relatively is from the stratigraphy in which they were found. This method relies on the Law of Horizontality (the assumption that soil layers accumulate on top of one another) and the Law of Superposition (the assumption that younger soils are found above older soils), which form the basis of stratigraphic dating or stratigraphy, in which archaeologists construct a relative chronological sequence of the soil layers from earliest (at the bottom) to youngest (at the top). This technique provides relative dates not just for the layers in a deposit but also for objects found within them—in this case, the date of discard rather than the date of creation or use. As long as the layer has remained sealed and there has been no intrusion from other layers, stratigraphy tells archaeologists that anything in that layer is at least as old as the soil in which it was found.

    Classifying artifacts using seriation, ordering objects chronologically, can also assist us in dating. When using seriation, the artifacts often are categorized or “typed” based on their qualities and attributes, such as the material from which they were made and their shapes and decorations. Changes in style are particularly useful. Artifacts produced at the same time (and by the same group) will resemble each other in style, but stylistic changes occur gradually over time and small differences accrue. As a result, artifacts from different time periods can look quite different from one another. Consider television sets. You could probably easily put a collection of television sets from their invention nearly 100 years ago to today in correct chronological order based on a few basic characteristics such as screen size, screen depth, and features such as knobs, buttons, and antennas. This is a modern example of stylistic seriation in which dating relies on placing artifact assemblages in serial order based on stylistic changes in their features. Archaeologists frequently use stylistic seriation to date pottery, baskets, and projectile points.

    Frequency seriation places artifact assemblages in serial order by examining the relative frequency of different types of artifacts. It is based on our understanding that stylistic differences in objects often follow similar patterns in terms of popularity—new styles are at first used in small numbers and then, if they become popular, are used more than older styles so more of them show up, at the time and in the archaeological record. A new style can eventually replace previous styles altogether. Charting the frequency of artifacts that have stylistic variations results in “battleship shaped” curves (think about what the deck of a battleship looks like from above) that are narrow at first (reflecting the artifact’s limited use), become wider as the item is adopted and used more frequently, and narrow again as it is displaced by newer styles. However, objects that are purely utilitarian typically have linear curves that appear as straight columns in frequency graphs. Frequency seriations are created for multiple sites in an area using stratigraphy to identify the periods of time to compare. Decorated pottery styles are often dated using frequency seriation. The colors and decorations of the ancestral Puebloans, for example, were sequenced by examining changes in their pottery styles.

    Absolute Dating

    While relative dating techniques offer many benefits, including use of techniques such as stratigraphy for virtually any type of material, they also have limitations. Relative dating techniques can be used to determine what is older and younger than something else but not how many years, decades, or millennia ago the item was made and used. Absolute dating techniques that can assign a range of years to an artifact were developed only in the past century and dramatically expanded archaeologists’ knowledge of the past and ability to classify objects.

    Even historical records such as hieroglyphs in Egypt and Mayan ruler lists recorded on stelae (inscribed upright stone markers) must have some basic information to be dated. Establishing a chronology requires conscientious work to link their dates to our own calendar.

    Coins and other items inscribed with dates are useful for determining the age of a site, though those kinds of items occur only in certain cultures and contexts. Because such items usually were marked when they were created and then were used long afterward, the date stamped on the item tells us only the earliest time of its use rather than when it was actually used at the site. This form of dating is known as terminus post quem, meaning “time after which.”

    Natural annual cycles also provide methods for dating in some contexts. Varves, which are paired layers of outwash gravel and sediment deposited in glacial lakes by retreating ice sheets, allow archaeologists to date the deposits and evidence associated with them. This is possible because melting glaciers deposit coarse silt during summer months via running water and fine clays during winter months when the lakes are covered by ice and fine particles suspended in the water gradually settle to the bottom. Each annually deposited pair of coarse silt and fine clay layers represents one year, allowing archaeologists to establish sequences that count back in time from the most recent layer, which has a known age, to the point at which artifacts were deposited. In Sweden, for example, these glacial sequences have been used to date items back as far as 12,000 years.

    Perhaps one of the most commonly understood means of dating using natural cycles is dendrochronology. Many varieties of trees have one period of growth each year, producing a growth ring that can be seen in the cross-section of the trunk. These rings reflect the environmental conditions of that year’s growing season and are similar in the various trees growing in the same region, often with thick growth rings during wet years and thin rings in years of drought. Archaeologists compare the rings of living and dead trees to create regional sequences that count back from when the first tree in the sequence was cut down to when trees that were used for timber in archaeological sites were felled, such as for support beams for a structure.

    Dendochronology works quite well at sites in which trees were used for building and the environmental conditions preserved the wood over time. Naturally, its use is limited to regions in which trees that produce clearly defined rings grow in climates that have marked summer and winter seasons. It has been applied extensively in the American Southwest, for example.

    The specific conditions needed for absolute dating techniques such as dendrochronology and glacial seriation long limited the ability of archaeologists to provide a specific range of dates for many sites. That changed in the mid-twentieth century when studies of radioactivity led to tools for measuring the natural rate of radioactive decay, the loss of radioactivity, of elements in archaeological deposits. In fact, dates determined using radioactive decay are calculated from 1950, the year in which this dating method was developed. Radioactive materials such as uranium decay at a consistent rate known as a half-life—the number of years it takes for half of that radioactive element to decay (converting it into a non-radioactive element). Each radioactive element has a specific, known half-life, and these dating methods measure the amount of the radioactive element and of its stable decay product, called the daughter element, to determine how many half-lives (years) have passed since the decay process began. These methods are collectively called radiometric dating.

    One of the most widely known radiometric dating techniques is radiocarbon dating, which measures the decay of Carbon‑14 (C‑14). Many elements exist in both stable and unstable (radioactive) forms called isotopes. Carbon, for example, has an atomic number of 6, which is the number of protons, and carbon isotopes vary by the number of neutrons they contain. Carbon‑12 is a stable (non-radioactive) carbon isotope, named for its atomic weight, which is the total number of protons (6) and neutrons (6). Carbon-14 is a radioactive isotope that has 6 protons and 8 neutrons. Its instability leads it to decay, and it has a half-life of 5,730 years.

    Carbon‑14 is significant for archaeology because it is common in archaeological deposits. It is produced when cosmic radiation strikes the atmosphere and is incorporated into molecules of carbon dioxide. As plants naturally absorb the carbon dioxide, they incorporate Carbon‑14 into their structures, and organisms that consume the plants incorporate Carbon‑14 into their tissues. Organic material found in archaeological deposits, including wood, plants, baskets, textiles, and human and animal remains, all contain this carbon. Over time, the Carbon‑14 in the deposits decays at the rate of its half-life of 5,730 years so samples can be taken from organic remains in archaeological deposits to determine how much time has passed since their deaths. The greater the ratio of Carbon‑14 to its non-radioactive carbon by-product, the more recently the organic matter died (there has been less time for decay to occur). Small amounts of Carbon‑14 relative to its non-radioactive by-product indicate that the organic matter died longer ago. Essentially, archaeologists can use anything found in the archaeological record that was once living (and ingesting carbon) to obtain a date using radiocarbon dating.

    Radiocarbon dating is performed by chemists, who analyze samples sent to them by archaeologists. The samples must be kept free from contamination so recent sources of carbon (such as paper tags) must not be bagged with anything that will undergo C‑14 analysis. This technique can date objects and materials with a high degree of accuracy but requires calibration as we now know that carbon concentrations in the atmosphere have not remained constant over time. The concentration of C‑14 in the atmosphere at the time affects the amount of C‑14 that is incorporated into the cells of plants and animals. Additionally, our ability to date accurately with this technique is limited to samples that are between 400 and 50,000 years old; accuracy declines beyond that range. There are other issues with C‑14 dating as well, including the marine reservoir effect, which affects radiocarbon dating of shells. Many marine organisms ingest both atmospheric carbon from the environment and older carbon from materials they consume that come from deep within the ocean and are transported to the surface by circulating water and currents. Radiocarbon dating performed on the remains of aquatic life requires calibration to account for these complexities.

    Other radiometric techniques used by archaeologists are summarized in the following table.

    Dating technique Material dated How it works
    Potassium-Argon (K/Ar) Igneous (volcanic) rock, which contains radioactive Potassium‑40

    The ratio of radioactive Potassium‑40 to its daughter product, Argon‑14, is measured in rock samples to determine the number of half-lives that have passed.

    The half-life of Potassium-40 is 1.3 billion years so this method is most accurate for materials that are older than 1 million years.

    Uranium series Travertine (calcium carbonate), which is found in cave walls and floors Provides highly accurate dates for materials that are between 50,000 and 500,000 years old.
    Fission track Obsidian and other glassy volcanic materials Determines age based on the natural splitting (fission) of Uranium‑238, which leaves tracks behind in the surface of the material.

    Many other absolute dating techniques can be used depending on specific conditions and materials at a site. See the following chart for some common examples.

    Dating technique Material dated How it works
    Thermoluminescence (TL) Ceramics and glass

    Over time, ceramics and glass trap electrons that have been released by natural radiation. Heating the material beyond a critical point allows it to release the electrons as light energy, which can be measured. This method is used to determine the last time the material was heated (such as when a ceramic was fired).

    Effectively dates materials that are 100 to 500,000 years old.

    Electron spin resonance (ESR) Materials that decompose when heated, such as tooth enamel

    Similar to TL dating but less sensitive.

    Effective for confirming dates obtained using other methods.

    Archaeomagnetic dating Clay Earth’s magnetic fields have changed over time, causing the location of magnetic north to shift. Magnetic particles in clay record the direction of magnetic north at the time the clay was heated.
    mtDNA Mitochondrial DNA Compares the DNA of individuals and populations found in their cells’ mitochondria (an organelle responsible for energy processing) to establish patterns of migration over time.
    Y chromosome Y chromosomes Compares the DNA from Y chromosomes (male sex chromosomes) of individuals and populations to establish patterns of migration over time.

    Additional space is provided for you to add other absolute dating techniques as directed by your instructor.

    Dating technique Material dated How it works

    Terms You Should Know

    • absolute dating
    • archaeomagnetic dating
    • atomic weight
    • Carbon-12 (C-12)
    • Carbon-14 (C-14)
    • daughter element
    • dendrochronology
    • direct dating
    • electron spin resonance (ESR)
    • fission track dating
    • frequency seriation
    • half-life
    • igneous
    • indirect dating
    • isotopes
    • Law of Horizontality
    • Law of Superposition
    • marine reservoir effect
    • mtDNA dating
    • Potassium-Argon dating (K/Ar)
    • radioactive
    • radioactive decay
    • radiocarbon dating
    • radiometric dating
    • relative dating
    • sedimentary rock
    • seriation
    • stylistic seriation
    • stratigraphic dating
    • stratigraphy
    • terminus post quem
    • thermoluminescence
    • travertine
    • uranium series dating
    • varves
    • Y chromosome dating

    Study Questions

    1. What is the difference between relative dating and absolute dating? Provide an example of each.
    2. Why was discovery of the principle of radioactivity so significant for archaeology? Explain the developments that were made possible and why they are so important.
    3. Using radiocarbon dating as an example, describe how radioactive materials allow for the dating of archaeological evidence.
    4. Your friend observes that you have just learned about many different dating techniques in your archaeology course and wonders why so many methods are needed to figure out how old something is. Describe how you might answer your friend’s question and consider at least two reasons for multiple dating techniques in archaeology.