Archaeology

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Potassium-Argon Dating

Potassium-argon dating provides approximate dates for sites in early prehistory. Geologists use this method to date volcanic rocks that may be as much as 4 billion to 5 billion years old. Potassium is one of the most abundant elements in the Earth’s crust. Many minerals contain radioactive K-40 (potassium 40) isotopes, which decay at a known rate into Ar-40 (argon 40) gas. Scientists use a device called a spectrometer to measure the accumulation of Ar-40 in relation to amounts of K-40. The ratio of these elements can indicate the age of a geologic layer, generally since it last underwent a metamorphosis, such as melting under the heat of molten lava from a volcanic eruption. Thus, geologic layers rich in volcanic deposits lend themselves to potassium-argon dating.

Prehistoric archaeological sites such as the Koobi Fora area of East Turkana, Kenya, and Olduvai Gorge in Tanzania, both of which formed during periods of intense volcanic activity, have been dated using the potassium-argon method. However, such dates commonly have a high margin of error. For instance, in the 1960s British archaeologist Glynn Isaac studied a site in a layer of Koobi Fora in which it appeared early humans had butchered animal carcasses. Isaac dated the site at 2.6 million years old, with a margin of error of over 250,000 years.

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Other Methods of Absolute Dating

Archaeologists also use more experimental methods of absolute dating. Electron spin resonance (ESR) measures the electrons captured in bone or shell samples up to 2 million years old. ESR testing on human tooth enamel from Skhul Cave in Israel dates some of the earliest anatomically modern humans in southwestern Asia to about 100,000 years ago.

Uranium series dating measures the radioactive decay of uranium isotopes in rocks made up of calcium carbonates, such as limestone and calcite. This technique may be used to date bones and tools embedded in these rocks. For instance, in 1994 archaeologists Allison Brooks and John Yellen used uranium series dating to determine the age of early African fish spears made of animal bone. The spears, which came from Katanda in the present-day Democratic Republic of the Congo, are thought to date to about 20,000 years ago.

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Thermoluminescence is a technique that measures electron emissions from once-heated materials, such as pottery or rocks that were once exposed to solar or volcanic heat. Many thermoluminescence tests have produced unreliable results. Archaeologists are attempting to refine the technique.

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Interpreting the Archaeological Record

Once a site has been documented, mapped, and dated, the archaeologist tries to integrate all the data into a coherent and understandable picture of the past. Archaeologists draw from what is already known of the archaeological record to develop their interpretations. Their new interpretations then add to that body of knowledge.

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Classifying Archaeological Finds

Everyone classifies objects—we know the difference, for example, between eating utensils and automobiles. We also make choices among objects—we choose a spoon to eat soup and a fork for salad, and we use large trucks to carry loads but small cars to save gas. The objects that archaeologists study were all once classified in similar ways by the people who originally made or interacted with those objects. Thus, archaeologists classify their finds to help them understand past cultures.

In archaeology, classification is a research tool that is used to distinguish among different artifacts and other material objects. Archaeologists use various systems of classifying artifacts to organize data into understandable units. Archaeological classifications describe artifact types, such as different forms of pottery, as well as relationships among different objects of a common type, such as clay vessels. Archaeologists call this system typology—a hierarchical classification based on artifact types and groupings.

When studying thousands of stone tools or potsherds, archaeologists search for patterns in them, such as of shape, color, and material composition. These patterns become the variables that define each category of object. For example, the category “containers” may include such objects as shallow bowls and round-based pitchers with curved handles.

After grouping the artifacts from an excavation into specific types, archaeologists determine the sequence in which those artifact types existed in the past. The process of determining this sequence is called seriation. Archaeologists believe that sequences of artifact types, or seriations, illustrate how past cultures changed over long periods of time.

Archaeologists often analyze artifact type sequences from many sites covering large areas of land. The comparison of multiple type sequences can show how particular types of artifacts spread from one group of people to another in the past. For example, during a period of over 1000 years beginning in about 1500 bc, a distinctive shell-ornamented pottery known to archaeologists as Lapita ceramics spread widely from one island to another in the southwestern Pacific. The continual evolution of Lapita pottery and other items across islands shows that the people maintained an extensive canoe trade in volcanic glass and other materials.

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Tracing Cultural Exchange

From the earliest times, human societies have exchanged raw materials and manufactured items with their neighbors and even with people living in other areas. People have traveled particularly far for valued materials—such as the best toolmaking stones, metal ores, and seashells—or for artifacts not manufactured locally, perhaps mirrors or wrought metal tools. When archaeologists find known artifact types far from their place of origin, they can begin to piece together ancient patterns of trade. For example, Celtic tribes in central and western Europe imported wine in Greek vessels from Mediterranean lands sometime around 200 bc. Several archaeological studies have traced the extent of this trade by plotting the distribution of such vases along the Rhine and Rhone river valleys.

Increasingly, archaeologists are turning to techniques that allow them to trace the source of materials in ancient trade. For example, analyses by George Bass and Cemal Pulak of the copper ingots they recovered from the Uluburun shipwreck off southern Turkey showed that the copper came from mines in Cyprus. Numerous analyses of this type have revealed that trade assumed increasing importance over time during the ancient past, especially with the rise of early civilizations in Egypt and Mesopotamia after 3000 bc.

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