Archaeology

A

Relative Dating

Relative dating relies on the principle of superposition. This principle states that deeper layers in a stratified sequence of naturally or humanly deposited earth are older than shallower layers. In other words, the uppermost layer is the most recent, and each deeper layer is somewhat older. Relative chronologies come from two sources: (1) careful stratigraphic excavation in the field, noting the precise location of every artifact and remain within layers of earth; and (2) close study of the characteristics of artifacts themselves.

Archaeologists commonly use clay potsherds to develop chronological sequences for cultures of the Neolithic and later periods. Pottery was invented during the last Stone Age period, known as the Neolithic, which began about 10,000 years ago in the Middle East. Potsherds occur in such large numbers in most sites that researchers can collect only small samples of them in a single excavation. Using statistical sampling methods, archaeologists can use smaller numbers of artifacts, such as potsherds, to make accurate estimations of the total numbers of each type of artifact. Archaeologists use such statistical estimates to reconstruct sequences of past cultural change, as follows.

When studying potsherds or other artifacts, archaeologists record variations in characteristics such as material composition, form, style, and decoration. This information forms the basis for developing seriations (artifact sequences), which chronicle artifact evolution over hundreds or thousands of years. Pottery characteristics, like modern automobile designs and clothing fashions, changed over time, growing and then diminishing in popularity. By noting these changes, archaeologists can establish long sequences of artifact styles.

In the 1960s on an ancient village site in the Tehuacán Valley of present-day Mexico, Richard MacNeish examined hundreds of broken potsherds from dozens of sites. From these fragments, MacNeish documented a shift from plain to richly decorated vessels over a period of several occupations of the village. He developed a complete sequence of pottery styles across the entire valley from before 3000 bc to recent times. MacNeish also gave absolute dates to his sequence using radiocarbon analysis from charcoal found near and around potsherds.

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B

Absolute Dating

Absolute dating, sometimes called chronometric dating, refers to the assignment of calendar year dates to artifacts, fossils, and other remains. Obtaining such dates is one of archaeology’s greatest challenges. Archaeologists who specialize in prehistoric periods use a variety of both well-established and experimental methods for absolute dating of ancient cultures.

B

1

Dating to Objects of Known Age

One of the simpler ways to determine the absolute age of an object is to find historical documents or objects of known age that confirm the date, or both. The earliest recordings of dates, documented in writing or some other form of decipherable notation, come from about 3000 bc in southwestern Asia. In other areas, people did not begin to record dates until far more recently. In the Americas, for instance, writing may not have existed until around 650 bc; the civilizations of Mesoamerica, such as the Olmec, Zapotec, Aztec, and Maya, are the only civilizations in the Americas known to have writing—the Inca of South America left no evidence of writing.

Artifacts with known dates, such as coins or pottery of a well-known period, provide archaeologists with comparisons that allow them to assign dates to other sites and cultures that did not have writing. For example, during his excavations of Knossos in the early 1900s Sir Arthur Evans also studied pottery vessels found in Egypt that were made by the Minoan inhabitants of Bronze Age Crete. Knowing the dates of the sites in Egypt where the vessels were found, Evans determined that the Minoan civilization, one of several to rule the island of Crete, flourished between 2000 and 1250 bc (see Aegean Civilization). Because of its dependence on writing, the method of using historically dated artifacts to date new finds can only be used on archaeological sites that existed after the advent of written records.

B

2

Tree-Ring Dating

Dendrochronology, or tree-ring dating, was originally developed in the Southwest United States using the annual growth rings on long-lived trees, such as bristlecone pine. These growth rings fluctuate in width from year to year, depending on annual rainfall. By studying the growth patterns of many ancient trees that lived for long periods of time, researchers can create so-called master tree-ring patterns. These master patterns can be compared with pieces of wood found in archaeological sites. Thus, archaeologists can use wooden objects, such as house posts, to determine the age of artifacts and other remains. Since the 1920s, archaeologists doing research in the Southwest have used dendrochronology to date wooden beams from pueblos. The wooden beams have been well preserved in the dry heat of the area and have been used to precisely date sites such as Mesa Verde, Colorado, and Pueblo Bonito in Chaco Canyon, New Mexico.

In recent years, researchers have applied dendrochronology to European oaks and a variety of Mediterranean trees. Dendrochronologists have established tree-ring chronologies that extend to as early as 6600 bc in Germany. Using these tree-ring chronologies, archaeologists have been able to date the earliest farming in central Europe to between 6000 and 5000 bc. Tree-ring dating has also allowed scientists to date drought cycles that may have been important in the rise and fall of cultures in the Mediterranean and Aegean regions. At the site of one of the world’s earliest farming villages, Çatal Hüyük in Turkey, British archaeologist Ian Hodder used a tree-ring sequence to date individual houses within the settlement that existed in about 7000 bc.

B

3

Radiocarbon Dating

Radiocarbon dating was developed by American chemist Willard Libby and his colleagues in 1949, and it quickly became one of the most widely used tools in archaeology. Radiation from space produces neutrons that enter the earth’s atmosphere and react with nitrogen to produce the carbon isotope C-14 (carbon 14). All living organisms accumulate this isotope through their metabolism until it is in balance with levels in the atmosphere, but when they die they absorb no more. Because the nucleus of C-14 decays at a known rate, scientists can determine the age of organic substances such as bones, plant matter, shells, and charcoal by measuring the amount on C-14 that remains in them. See also Dating Methods: Carbon-14 Method.

Radiocarbon methods can date sites that are up to 40,000 or 50,000 years old. These methods have revolutionized archaeology over the past half-century. For instance, radiocarbon testing of materials from early farming settlements at Jericho, in what is now Jordan, dated these settlements to as early as 7800 bc, indicating that they are more than 3500 years older than was once thought.

In recent years, scientists have developed a new approach to radiocarbon dating using a device called an accelerator mass spectrometer. This device directly counts C-14 atoms, rather than counting rates of disintegration. Accelerator mass spectrometry (AMS) can date a sample as small as a single kernel of grain or a fleck of wood preserved inside a bronze axe socket. This method can date items that are up to 90,000 years old.

Since AMS dates can come from very small, isolated objects, the resulting chronologies can be much more accurate than those from standard radiocarbon dating. For example, American archaeologist Bruce Smith used AMS to date individual maize cobs from caves in the Tehuacán Valley. His results indicated that domesticated corn was grown there by about 2500 bc, much later than earlier radiocarbon dates had suggested.

Radiocarbon dates are approximations, and they are published with statistical margins of error. For instance, a date may be given as 30,000 bc ± 2000 years. However, the radiocarbon dates of objects less than about 8000 years old are also compared with and calibrated to dates from tree-ring analysis. These estimates can pinpoint the age of an object with great precision, often to within 100 years.

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