Interpreting Agricultural Find Essay, Research Paper
Ideally, analysis of the materials found on a site begins in the field laboratories while excavation is still in progress. Often, however, reconnaissance and excavation are completed in a relatively brief period of time, and the records and preserved remains are taken back to a museum, university, or laboratory for more analysis. This analysis has many aspects, which include describing and classifying objects by form and use, determining the materials from which they were made, dating the objects, and placing them in environmental and cultural contexts. These aspects may be grouped into two broad categories: chronological analysis and contextual analysis.
: Chronological Analysis
Chronological analysis of archaeological materials identifying their time periods and sequence in time is often done first. Archaeologists use two general kinds of dating methods: relative dating, or establishing when the various materials found at a site were made or used in relation to each other, and absolute dating, or assigning a fairly precise, chronometric date to a find.
The oldest method of establishing relative dates is by analyzing stratigraphy the arrangement of strata in a site. This technique is based on the assumption that the oldest archaeological remains occur in the deepest strata of the excavation, the next oldest in the next deepest strata, and so on. By following this assumption, archaeologists can place the materials collected from the various strata into a rough chronological sequence.
If archaeologists digging in an undated site find a distinctive type of pottery for which the date is known, they may conclude that the other materials found in the site along with the pottery bear the same date as the pottery. This is an example of a relative-dating technique called cross dating.
Similarly, archaeologists may assign a date to an artifact based on the geologic region or strata with which the artifact is associated. For example, archaeologists may conclude that hand axes found in the high terrace of the Thames River in England are older than arrow points and pottery found in the lower terrace because they know that the high terrace was formed earlier than the low one.
The association of artifacts with animal or fossil remains can also be used for relative dating. For example, it is known that superbison became extinct in the Great Plains of what is now the United States and were replaced by modern bison. Thus if archaeologists discover one site in which Folsom fluted points (the distinctive tips of a kind of prehistoric man-made weapon) are found imbedded in superbison remains, and they discover a second site in which a different kind of points, called Bajada points, are sticking in the remains of modern bison, they may conclude that Folsom points were made before Bajada points. This kind of relative dating may also be done using plant remains, particularly plant pollen, which is often preserved in archaeological strata.
If archaeologists know how certain types of artifacts styles of pottery or burial objects, for example evolved over time, they may be able to arrange groups of these artifacts in chronological order simply by comparing them. This method is called seriation.
Archaeologists can judge the relative dates of bones by analyzing their fluorine content, since the amount of fluorine in buried bones increases over time. In the 1840 s Dr. Montroville Dickeson proved that a human pelvis found in Natchez, Miss., dated from the same time as mammoth bones found with it because both had accumulated the same proportions of fluorine.
There are many other methods of relative dating. None of them is as accurate as the absolute-dating methods, however, because the assumptions on which many relative-dating techniques are based can be misleading. Nevertheless, sometimes relative dating is the only method available to the archaeologist.
In absolute, or chronometric, dating, a definite age in numbers of years before the present is assigned to an archaeological specimen. When applied correctly, the methods of absolute dating can yield highly accurate dates. The remains found by classical archaeologists coins or written records, for example may have dates already written on them, but this is not always the case. It is never the case for anthropological archaeologists, who study prehistoric materials.
One system of absolute dating, called varve dating, was developed in the early 20th century by Gerard de Geer, a Swedish geologist. He noted that the mud and clay deposited by glaciers into nearby lakes sank to the lake bottom at different rates throughout the year, forming distinct layers, called varves, on the lake bottom. Because each year s layer was different, the researchers were able to establish dates for artifacts or sites associated with a specific varve.
A similar absolute-dating method dendrochronology, or the dating of trees by counting their annual growth rings was first developed for archaeological purposes in the early 1900 s by the American astronomer Andrew Ellicott Douglass. If an ancient structure has wooden parts, archaeologists can compare the number and widths of the growth rings in those parts with sequences from other samples to find out when that structure was built. Other techniques yield absolute dates based on the thickness of the patina, or residue, that forms over time on certain stone artifacts.
Advances in the physical sciences during the 20th century greatly improved absolute-dating methods. One of the best-known and most valuable techniques is radiocarbon dating (also called radioactive carbon dating, carbon dating, and carbon-14 dating). All living things contain small amounts of carbon-14, a radioactive form of carbon. After death, this carbon-14 changes, or decays, into a more stable form of carbon. Archaeologists can determine the age of once-living things such as bones, wood, and ash by measuring the amount of carbon-14 remaining in the specimen.
- Carbon-14 Production
Radiocarbon dating cannot be used to make accurate age measurements of very old materials materials more than about 70,000 to 100,000 years old. For such objects, archaeologists can use similar techniques involving other chemical elements. Potassium-argon dating, for example, can be used to date rocks millions of years old. A related dating method called fission-track dating can be used on certain stone samples of almost unlimited age. Another modern dating method, thermoluminescence dating, can be used to find out when ancient pieces of pottery or other fired-clay objects were made.
- Carbon-14 Decay Over Time
: How Archaeologists Estimate the Age of Finds
Prior to about 1950 archaeological sites were dated almost exclusively from inscriptions in the more recent sites or by geological calculations based on soil sequences and the estimated rates of deposit of the soil layers. Tree-ring counts (one ring per year) extended the time scale back a few thousand years in some regions. Analysis of the amounts of fluorine in human and animal bones lying together showed whether or not they were of the same age.
With very ancient specimens, radioactive dating techniques are now used. However, these methods are not absolutely accurate. Age usually must be estimated within wide margins because of variables that cannot be calculated exactly. From time to time, as the techniques become more refined or as sources of error are eliminated, new ages may be assigned to remains.
Two radioactive techniques are frequently used to date the age of fossil humans radiocarbon analysis and potassium-argon analysis. In 1948 physicist Willard F. Libby and his coworkers discovered how to date charcoal and other organic material by carbon-14 testing. Radiocarbon dating traces remains back some 35,000 years. Potassium-argon testing, which utilizes the rate of decay to stable argon-40 of the radioactive potassium-40 found in most rocks, gives dates for sites more than 500,000 years old. Dates can also be obtained by measuring the amount of surface decomposition on certain stone tools or the amount of thermoluminescence seen when ancient pottery is heated.
: Relative & Absolute Dating
Dating is crucial for physical anthropologists, as well as for geologists and archaeologists. It is a method that allows them to determine how old something is whether it be a layer of rocks, a human-like fossil, or a collection of pottery.
There are two kinds of dating: relative and absolute. Relative dating shows the order in which events occurred but does not tell exactly when they occurred. Methods of absolute dating indicate with a fair degree of precision how old something is. Of the two types of dating, the determination of relative age relationships came into use first. Absolute dating depends upon technological advances that have been made in the 20th century.
Geologists and archaeologists have long used relative dating methods to determine the approximate age of the Earth and of fossils and artifacts. Geologists examine the many strata of the Earth s crust to determine the intervals of time from one layer of rock to another. Archaeologists also use the principle of layering to verify the sequence of human cultures.
Another method of determining relative age is fluorine dating. It is based upon the principle that fossil bones absorb the element fluorine from the soil in which they are buried. The longer they are buried, the more fluorine the bones will contain. Determining the amount of fluorine is often not a practical means of relative dating because it requires many samples from an immediate area.
Absolute dating attempts to pinpoint when a given rock, fossil, or other object reached its present condition. The basic method for determining absolute age is called radiometry measuring the rate of radioactive decay of an element. This can be done with a high degree of accuracy, although no method is infallible without a great deal of corroborative testing.
- Radioactive Decay Law
- Half-life
One of the types of absolute dating that has been used by physical anthropologists is potassium-argon dating. It is a method of determining the time of origin of rocks and thereby of the fossils found within them by measuring the amount of decay of potassium-40, a radioactive isotope of the element potassium, into the element argon, one of the rare gases. The half-life of potassium-40, which is the time it will take one half of any quantity of it to decay into potassium, is 1,265,000,000 years. Potassium-argon dating has been used to measure the ages of a wide variety of objects, from meteorites 4,500,000,000 years old to volcanic rocks only 20,000 years old. Such dating techniques applied to the remains and surroundings of ancient human beings have constantly pushed back the estimated age of mankind. By the early 1980 s man was believed to be at least 3 million years old. This is based on the dating of a number of remarkable discoveries of fossil remains made in the Great Rift Valley of Africa, at sites in Ethiopia, Kenya, and Tanzania.
: Radioactivity – Absolute Dating
Late in the 19th century, scientists discovered an amazing activity in certain kinds of matter. Through the ages, atoms of these substances have been shooting off particles and emitting radiations (together called rays) without anyone suspecting that this was happening. Scientists also found that nothing could be done to change the emissions. The application of heat, electricity, or any other force made no difference whatsoever. Emission seemed to be an unchangeable property of the substances.
Many vital uses have come from this discovery. A special use of the element uranium led to the development of nuclear weapons and nuclear energy. Doctors also have found that the rays can penetrate living tissues for short distances and affect the tissue cells. Like x-rays, they can disrupt chemical bonds in the molecules of important chemicals within cells, and so they help in treating cancers and other diseases.
In time, scientists learned how to make all other elements give off these rays. These include the major elements that make up our bodies. If such radioactive elements are placed in the body through food or by other methods, the rays can be traced through the body. This use of tracer elements is extremely helpful in expanding knowledge of our life processes. (See Radioactive Decay Law page )
Geologists have learned how to use radioactivity to determine the age of rocks. From this they obtain new checks on the ages of mountain ranges and even the age of the Earth itself. The study of radioactivity continues to contribute to the understanding of the nature of atoms, and from this, scientists are learning how energy and matter interact to bring about everything that happens in the physical universe.
: Units for Measuring Radioactivity
The unit of measurement of the radioactivity of a substance is the curie. One curie equals about 37 billion emissions per second . Matter emitting half of this amount per second would represent 1/2 curie of radiation .
Another unit of measurement used in radioactivity is the radius (rho) of the nucleus. The value of rho for a particular element is approximated by multiplying 1.2 x 10 15 meter by the cube root of the element s atomic number, the number of protons in the nucleus or . (1.2 x 10 15 meter would have to be multiplied by about 20 trillion to equal 1 inch or .) Using this formula and the fact that a nucleus is roughly spherical, it follows that the density of particles is about the same in all nuclei. This density is incredibly high. If a cubic centimeter of matter were as dense as this, it would weigh about 250 million tons. Neutron stars, which are thought to be the result of supernovas, have densities of this magnitude, with an amount of mass roughly equal to that of the sun packed into a star about 12 miles (19 kilometers) in diameter.
: Carbon Dating
Without the element carbon, life as we know it would not exist. Carbon provides the framework for all tissues of plants and animals. These tissues are built of elements grouped around chains or rings made of carbon atoms. Carbon also provides common fuels coal, coke, oil, gasoline, and natural gas. Sugar, starch, and paper are compounds of carbon with hydrogen and oxygen. Proteins such as hair, meat, and silk contain carbon and other elements such as nitrogen, phosphorus, and sulfur.
More than six and a half million compounds of the element carbon, many times more than those of any other element, are known, and more are discovered and synthesized regularly. Hundreds of carbon compounds are commercially important but the element itself in the forms of diamond, graphite, charcoal, carbon black, and fullerene is also indispensable.
Carbon occurs in nature as the sixth most abundant element in the universe and the 19th element in order of mass in the Earth s crust. As the element in the forms of graphite, diamond, and fullerene it is a minor part of the Earth s crust, but compounds of carbon with other elements are very common. The chemical symbol for an atom of carbon is C. Some common natural substances rich in carbon are coal, petroleum, natural gas, oil shale, limestone, coral, oyster shells, marble, dolomite, and magnesite. Limestone, coral, and oyster shells are largely calcium carbonate, CaCO3. Marble, dolomite, and magnesite also contain calcium, magnesium, and carbon. (See Properties of Carbon table page )
- Properties of Carbon
Coal, petroleum, natural gas, and oil shale are mainly compounds of carbon and hydrogen derived from plant and animal sources deposited in the Earth millions of years ago and subjected to high pressure. These deposits were once a part of what is called the carbon cycle, a dynamic system of change still occurring. Through photosynthesis, plants use sunlight to convert carbon dioxide from the air and convert water from the soil into plant tissues such as cellulose and into an energy source such as sugar. Plants release oxygen into the air as the carbohydrates sugar and cellulose are synthesized. Animals eat the plants, breathe in oxygen from the air and oxidize the carbohydrates, or use them as fuel, which releases energy to the animal. Eventually the products of animal metabolism carbon dioxide, water, and other waste products are returned to the atmosphere and the Earth. The cycle repeats itself endlessly.
Besides the wide occurrence of carbon in compounds, two allotropes, or forms, of the element diamond and graphite are deposited in widely scattered locations around the Earth. The third form, fullerene, is unstable in comparison to these two forms and is thus not found widely.
A diamond, no matter what the size, may be considered to be a single molecule of carbon atoms, each joined to four other carbons in regular tetrahedrons, or triangular prisms. The crystal structure is called a face-centered cubic lattice. Diamond is extremely hard but brittle and has a high specific gravity of 3.51. Its high refractive index of 2.42 is a measure of how far diamond can refract, or bend, light. This property gives the diamond brilliance and fire. A diamond can be cleaved, or split, along its crystal faces into smaller pieces with the sides of the cleavage remaining smooth. This property is very important to the diamond cutter and the jeweler.
Graphite, the second allotrope of carbon, was known in antiquity. Natural deposits of graphite have been called black lead, silver lead, and plumbago, which is another name for the lead ore galena. The largest deposits of graphite are in Sri Lanka but the highest quality graphite comes from Madagascar. Other sources are North Korea, Mexico, Canada, Siberia, and New York. In contrast to that of diamond, the structure of graphite consists of layers of carbon atoms joined in regular hexagons by strong bonds. The layers are held together by long-range, relatively weak attractive forces called Van der Waals forces. The layers can slide over each other easily, which accounts in part for the lubricating property of graphite.